PDA

View Full Version : Liquid ammonia injection for cooling.



mrsparky
13-01-2007, 09:06 AM
Please excuse my terminology for I come from an electrical background. We have one ammonia compressor that the motor has a hard time starting. The compressor inertial load seems to be greater than designed for the motor. The motor, compressor bearings and slide valve can be eliminated for this problem due to extensive troubleshooting.

The compressor is stopped now and has been off for a couple of days. The liquid ammonia injection for compressor cooling is frosted up. None of the other shut off compressors is displaying any frosting up and are in fact ambient room temperature.

The piping: First is a shut off valve then a check valve then a solenoid then a window then a regulator then shut off valve and then compressor entry. There is heavy frost at the window and between the solenoid and regulator.

I do not believe this frost should be there since the compressor is turned off and not requiring any cooling. Is it possible that liquid ammonia is being injected into the compressor when off thus causing the motor to see a heavy load during start. The motor only sees a too heavy of a load during start and not run. Any ideas of what is going on here? Also, what should one see in the window when the compressor is off or when the compressor is on. Thank you for any and all responses.

Andy
13-01-2007, 12:11 PM
Please excuse my terminology for I come from an electrical background. We have one ammonia compressor that the motor has a hard time starting. The compressor inertial load seems to be greater than designed for the motor. The motor, compressor bearings and slide valve can be eliminated for this problem due to extensive troubleshooting.

The compressor is stopped now and has been off for a couple of days. The liquid ammonia injection for compressor cooling is frosted up. None of the other shut off compressors is displaying any frosting up and are in fact ambient room temperature.

The piping: First is a shut off valve then a check valve then a solenoid then a window then a regulator then shut off valve and then compressor entry. There is heavy frost at the window and between the solenoid and regulator.

I do not believe this frost should be there since the compressor is turned off and not requiring any cooling. Is it possible that liquid ammonia is being injected into the compressor when off thus causing the motor to see a heavy load during start. The motor only sees a too heavy of a load during start and not run. Any ideas of what is going on here? Also, what should one see in the window when the compressor is off or when the compressor is on. Thank you for any and all responses.

Hi Mrsparky:)

Yes your liquid injection solinoide is passing:) To prove this take the compressor off auto start and leave it off with the shut off valve to the liquid injection solinoide shut. When you want to stsrt the compressor put it back on auto start. When the compressor has started again open the liquid injection feed shut-off valve:)

Ammonia causes erosion of the solinoide valve seats by an effect called wire drawing. You could change the valve to a Danfoss AKV which has a peak type plastic used on the static part of the seat and the usual teflon on the slug that moves in the valve when energised.

Danfoss have some new valves available based on the ICM module, these are discussed earlier in this tread.

Happy fault finding:)

Kind Regards Andy:)

NH3LVR
13-01-2007, 03:38 PM
The piping: First is a shut off valve then a check valve then a solenoid then a window then a regulator then shut off valve and then compressor entry. There is heavy frost at the window and between the solenoid and regulator.

Welcome to the Forum Mrsparky!
Andy, as always is correct.
I am very concerned about the piping arrangement you describe. If indeed you have a check valve before the Solenoid, it could lead to a dangerous situation.
I believe it is most likely a bit different than your description. Or I may be not understand you correctly.

If this is an American built package, you most likely have a Sporlan Solenoid for the liquid injection. They do fail and I have replaced many of them for that reason.
The biggest problem here is if the machine is full of liquid. This could be very dangerous if you are able to get it started.
Do you have a local mechanic familiar with these? If so I would encourage you to contact them.
Judging from the time of morning this post was made I think you may be in a breakdown situation.
Send me a private message and I may be able to be of help to you.

US Iceman
13-01-2007, 03:52 PM
FIrst of all welcome to the RE forums mrsparky.



None of the other shut off compressors is displaying any frosting up and are in fact ambient room temperature.


This what you should be seeing on the problem compressor also. It certainly sounds as if the solenoid valve is leaking. Are you sure it's not energized when the compressor motor is OFF?

What is the oil separator sump temperature at start-up?

Do the separator site glasses have a lot of foaming in them during start-up?

Josip
13-01-2007, 10:06 PM
Hi, Mrsparky :)

Welcome to RE




The compressor is stopped now and has been off for a couple of days. The liquid ammonia injection for compressor cooling is frosted up. None of the other shut off compressors is displaying any frosting up and are in fact ambient room temperature.

The piping: First is a shut off valve then a check valve then a solenoid then a window then a regulator then shut off valve and then compressor entry. There is heavy frost at the window and between the solenoid and regulator.

Well, your solenoid valve is damaged and there you have a leak (frost after solenoid valve till compressor entry) even your compressor is colder (you can check it with infrared thermometer) and you must replace solenoid valve with new one as per Andy's advice.


I do not believe this frost should be there since the compressor is turned off and not requiring any cooling. Is it possible that liquid ammonia is being injected into the compressor when off thus causing the motor to see a heavy load during start

Even when compressor is off there is suction pressure within due to equalizing hole in suction check valve. Ammonia for cooling is under condensing pressure and easily can be injected into compressor under suction pressure and partially evaporate and partially remain in oil causing a lot of foam and reduced lubrication during run. But before that with very cold and dense (sticky) oil it is not easy to start compressor.

Best regards, Josip :)

NH3LVR
13-01-2007, 10:36 PM
I have asked Mr Sparky to PM me.
I was not trying to short circuit the Forum, but wanting to get more detail for us to discuss.
It is not uncommon for a small leak in the Solenoid to be present for sometime before it is noticed or causes a problem.
However I am really concerned about the heavy load at startup. It may well be a Electrical problem, but I fear for his safety if the screw is full of Liquid.
All of us could determine this quite easily. But his limited experience with Compressors leaves me feeling very nervous.
I do respect his willingness to tackle this problem.

Josip
13-01-2007, 11:09 PM
Hi, NH3LVR :)


I have asked Mr Sparky to PM me.
I was not trying to short circuit the Forum, but wanting to get more detail for us to discuss.
It is not uncommon for a small leak in the Solenoid to be present for sometime before it is noticed or causes a problem.
However I am really concerned about the heavy load at startup. It may well be a Electrical problem, but I fear for his safety if the screw is full of Liquid.
All of us could determine this quite easily. But his limited experience with Compressors leaves me feeling very nervous.
I do respect his willingness to tackle this problem.

Here in my country in one company they run compressor for almost 2 years with leaking injection solenoid valve. Sometimes compressor was frozen completely:eek: . Nobody ask for service :confused:.

Call for service was to change shaft seal, oil filter and oil with complete check. Rotating compressor by hands there was some resistance and advice was to open and check screws. Found all Ok. Bearings must be replaced no way to check them.

Finally small leak cost as complete overhaul (change of complete set of bearings) SAB 163.

Best regards, Josip :)

mrsparky
14-01-2007, 02:54 AM
I am at home now and tried a "quick reply" but it did not go through so if my post response gets doubled that is the reason.

Thanks all for the inputs. I suspected the solenoid but our experienced refrigeration man is out on sick leave.

I will try and answer all of your questions:
1.) I suspected the soleniod but have not yet checked for energization voltage when the compressor is off. I will do this at the next opportunity.
2.) The pipe fitting just before the solenoid may indeed be a strainer rather than a check valve.
3.) I am sure that the oil is up to temp before start.
4.) I am 99.999% positive that the problem is not electrial.

This has been an on going problem with this compressor for at least two years. The motor groans loudly while in star that only stops once it switches to delta and comes up to speed. The same problem showed up with different motors. The refrigeration man warned me about liquid ammonia in the compressor screw and what can happen. I will check for solenoid energization voltage and if absent will ask refrigeration to order a new solenoid of the type that Andy mentioned. Thanks again, Robert

US Iceman
14-01-2007, 02:58 AM
And we wonder why ammonia gets a bad rap!!!!!


I'm sure you have seen this type of maintenance practice before. It's very common in HVAC and commercial refrigeration systems. Unfortunately, it is also getting to be more common in industrial refrigeration too.

Too many accountant types thinking they are saving money!:mad:

US Iceman
14-01-2007, 03:01 AM
The motor groans loudly while in star that only stops once it switches to delta and comes up to speed.


Are you also sure the slide valve is all the way unloaded before the compressor tries to start?

One more question... Are the other compressors wye-delta start also? no problems on them I assume?

mrsparky
14-01-2007, 03:34 AM
Since we have solved the other problem I would like to ask one more that I am curious about. We have two new compressors for flash freezing at a new freezer. The booster for the extreamly low temperatures has had several problems. It burned up its motor connections in the peckerhead and also had a bad oil level sensor. These problems were repaired. What I am curious about is that in the upper window of the oil separater tank, sometimes I see foaming and sometimes I don't. I only see the foaming on the booster low temp compressor and not any of our other regular compressors. What would cause the foaming to appear in the upper window and then to disappear. I woud say that an hour or two is enough to cause this reversal. Again thank you, Robert

mrsparky
14-01-2007, 03:38 AM
In answer to US Iceman, It was proven that the slide valve is working correctly. Our compressors are all star/delta start except for one I am currently changing to soft start and the two new ones I talked about in my last post. I convinced the boss to change to soft start after reading and communicating with an electrical engineering forum.

US Iceman
14-01-2007, 03:57 AM
It burned up its motor connections in the peckerhead...


What caused this? Were the split-bolt connectors loose, or did the insulation pads chafe against the peckerhead?

By the way, it's been a long time since I heard anyone use the term peckerhead. If someone has not heard of this before, they might think you are calling them names!:D



What would cause the foaming to appear in the upper window and then to disappear.


It could be caused by cool oil at start-up, or some liquid refrigerant getting into the oil during pull down.

Does this occur only at certain times?

And, when the oil is foaming...what's the discharge temperature?

nh3wizard
14-01-2007, 03:59 AM
Is the machine liquid injected also? Could be overfeeding

mrsparky
14-01-2007, 04:03 AM
The reason for the motor connection to burn up in the peckerhead was the "extra flex steel" motor taps locked down in a connection block. The electrical engineering forum advised a "stud block". With all the "political correctness" I should have said motor junction box but that was the term I was taught.

US Iceman
14-01-2007, 04:45 AM
With all the "political correctness" I should have said motor junction box but that was the term I was taught.


It's the term I was taught also. It seems ever time I have used it, everyone just looks at me funny.

My comment was not meant to enforce political correctness, but only to highlight a term most may not be familiar with.

Josip
14-01-2007, 11:32 AM
Hi, Mrsparky :)


Are you also sure the slide valve is all the way unloaded before the compressor tries to start?


In answer to US Iceman, It was proven that the slide valve is working correctly. Our compressors are all star/delta start except for one I am currently changing to soft start and the two new ones I talked about in my last post. I convinced the boss to change to soft start after reading and communicating with an electrical engineering forum.

Before compressor start slide valve must be in minimum position, but that does not mean during inrush there is no movement of slide valve toward max position causing heavy load to electric motor.

Sometimes this happened with Stal screw compressors due to some problems with unload/load solenoid valves or check valves or relief valve. It is not common but sh*t happens;) .

I'm sorry we are speaking about some problems but still not know anything about your compressors (all screws are almost the same but not exactly;) )

Please can you tell us something about?


It's the term I was taught also. It seems ever time I have used it, everyone just looks at me funny.

Regarding "peckerhead" in Webster's dictionary found only "pecker" - no need to say anything more;)


What I am curious about is that in the upper window of the oil separater tank, sometimes I see foaming and sometimes I don't. I only see the foaming on the booster low temp compressor and not any of our other regular compressors. What would cause the foaming to appear in the upper window and then to disappear. I woud say that an hour or two is enough to cause this reversal.

It is normal to see some foam due to injection cooling. If there is not any change in oil pressure and compressor's sound no problem. Maybe to see is it possible to reduce amount of injected ammonia even if oil temperature rise a little.

Best regards, Josip :)

Andy
14-01-2007, 12:00 PM
Since we have solved the other problem I would like to ask one more that I am curious about. We have two new compressors for flash freezing at a new freezer. The booster for the extreamly low temperatures has had several problems. It burned up its motor connections in the peckerhead and also had a bad oil level sensor. These problems were repaired. What I am curious about is that in the upper window of the oil separater tank, sometimes I see foaming and sometimes I don't. I only see the foaming on the booster low temp compressor and not any of our other regular compressors. What would cause the foaming to appear in the upper window and then to disappear. I woud say that an hour or two is enough to cause this reversal. Again thank you, Robert

Hi Robert:)

Oil foaming is very common:confused: can be a whole host of things:eek:

Oil too warm

Oil with large quantities of refrigerant traped in it boiling off

The design of the oil separator, lots of inpingment

The selection of the oil separator, in particular low temperature compressors during pulldown (the oil separator size when the suction pressure is up needs to be bigger than when at or near design temperature)

As Josip and NH3 Wizard indicates, an over injecting injection system is another possibility.


The best cure for liquid injection problems, is none. Use a refrigerant(thermosyphon) or water cooled oil cooler instead.
The water cooled oil coller can be used to preheat boiler water or factory wash down water, saving money. Your compressors will have addtional capacity and in effect (with high stage directly or low stage indirectly) you will be increasing the size of the system condensers:)

Kind Regards Andy:)

mrsparky
14-01-2007, 06:15 PM
Thank you for the informative comments. On any future installations I can make reccomendations about compressor cooling. I already knew from glancing at literature that liquid injection is not as efficient.

In re to US Iceman and Josip. The reason I know the slide valve is at minimum position at start is that the dial indicator is pointing to zero. The refrigeration man had shown me in the past how the dial indicator is connected mechanically to the slide valve so for it to indicate zero the slide valve must be at minimum. Please correct me if I am wrong.

In re to Josip, inrush has a different meaning in the electrical world. Are you talking about the inrush of gas?

Josip
14-01-2007, 07:42 PM
Thank you for the informative comments. On any future installations I can make reccomendations about compressor cooling. I already knew from glancing at literature that liquid injection is not as efficient.

In re to US Iceman and Josip. The reason I know the slide valve is at minimum position at start is that the dial indicator is pointing to zero. The refrigeration man had shown me in the past how the dial indicator is connected mechanically to the slide valve so for it to indicate zero the slide valve must be at minimum. Please correct me if I am wrong.

In re to Josip, inrush has a different meaning in the electrical world. Are you talking about the inrush of gas?

Slide valve must be at zero position otherwise you have not starting permission;), but that not prove there is not any slide valve move during start time.

Due to my, not so good, english, here inrush has meaning from the moment when we push start button (we are in star -with very high amperes-low speed, waiting couple of seconds from 3-4 up to 10-12 (usually 7-8) coming to nominal speed and reduced amperes til here, then switch to delta.

No need to say that good electrical motors coming to nominal speed within 1,5 sec after start with star/delta combination.

Best regards, Josip :)

US Iceman
14-01-2007, 08:36 PM
The refrigeration man had shown me in the past how the dial indicator is connected mechanically to the slide valve so for it to indicate zero the slide valve must be at minimum.


That's quite true, as long as the mechanism is adjusted and claibrated properly.

If for some reason the slide valve is partially loaded, the inrush current will be dramatically higher and the motor could stall, especially with wye-delta.

My questions come from one of your earlier comments about several motors being tried and that was not the problem.



This has been an on going problem with this compressor for at least two years. The motor groans loudly while in star that only stops once it switches to delta and comes up to speed. The same problem showed up with different motors.


Therefore, if you have tried changing motors with the same end result, I would tend to want to absolutley rule out anything with the compressor.

mrsparky
15-01-2007, 01:06 AM
This thread has certainly grown in length and I appreciate the interest it has generated.

My original thought was the leaking liquid injection solenoid with the subsequent ammonia wet screw is the source of the hard starts. The test suggested by Andy should prove if the leaking solenoid is causing this problem. At the least, repairing the leaking solenoid will make for a safer installation.

If the hard start problem still persists, then if the slide valve is wandering during start would not the dial indicator give me a visual test? As long as it stayed zero would it not be at minimum? I have watched the slide valve dial indicator during start and it remains at zero. The refrigeration assured me in the past that the slide valve is adjusted for minimum when the dial indicator points to zero.

I also thought about the check valve after the compressor discharge but if I remember correctly (I am still at home because of the holiday), there is a gas pressure gauge located before it and it did not show an abnormal reading. If the solenoid is not the source of the hard start then I am out of ideas again. The refrigeration man and I am at an impasse because he maintains that the screw is not the problem and I have proven that electrical is not the problem. I have probably missed some suggestions from other posts on this thread and will read it all carefully again if the injection solenoid is not the problem. Thanks, Robert

Josip
15-01-2007, 01:35 AM
Hi, Robert :)




If the hard start problem still persists, .........................

The refrigeration man and I am at an impasse because he maintains that the screw is not the problem and I have proven that electrical is not the problem. I have probably missed some suggestions from other posts on this thread and will read it all carefully again if the injection solenoid is not the problem. Thanks, Robert

There is one more possible test. Remove coupling and then start your motor without compressor (it will run at least for 30-60 seconds without any jumper. Check amperes, noise...

Also try to rotate compressor by hands is it hard or smooth.

Another possibility (not easy) to install another motor (if available-seems it is) to that compressor and then try to run it. I did not understand from your posts if you already did that.

Anyhow still no information about type of compressor,.....etc

Best regards, Josip :)

NH3LVR
15-01-2007, 01:37 AM
Mr Sparky;
It occurred to me that the first test might be the simplest.
Disconnect the coupling between the Compressor and Motor and attempt to start the Compressor.
If the hard starting is caused by a poor contact it might start anyway under the reduced load. But it is a simple test that is easily accomplished.

US Iceman
15-01-2007, 01:52 AM
The refrigeration man and I am at an impasse because he maintains that the screw is not the problem and I have proven that electrical is not the problem.


Something is not right.

What kind of screw compressor is it?

You mentioned the discharge check valve and the pressure upstream of the valve. There should be a bypass of some sort on the suction check valve. Sometimes a hole is drilled in the check valve, most of the time there is a bleed line around the check valve.

The purpose of this equalizing or bleeding to suction pressure is to reduce the pressure the compressor is starting against.

NH3LVR
15-01-2007, 02:30 AM
I have never had a problem starting a Screw against head pressure. That is not to say it could not be a problem in some cases, but when it starts the pressure is equal on both sides. The Suction pressure is measured upstream of the suction check valve. The compressor equalizes quite quickly when it is shut off.
It would be of great help if Mr Sparky would tell us which Screw Package he has.

mrsparky
15-01-2007, 03:24 AM
If it is an electrical problem with the hard start, the installed motor would have to have a defective squirrel cage rotor and the spare motor would have to have exactly the same problem. These are both re-wound motors probably more than once. While this is a probability it is highly unlikely.

The uncoupled motor displays no problems with start and run. Only when it is coupled to the compressor does the groaning occur. I have taken RPM measurements during start and compared to a working system. My RPM meter is slow to read out but the RPM for the defective system is lower than a working system at transition to delta. Increasing the transistion time has only the negative effect of keeping the motor at high amps longer than it should.

The electrical side of this problem has been hashed out in an electrical engineering forum some time ago and unless the rotor is spun then the problem lies with the compressor. This is why I am seeking guidance in this refrigeration forum.

I think it is an FES compressor but I seem to remember seeing the word "Hartford" or something like that on the manufacturers name plate. I don't know the size of the compressor but the motor is 500HP and this compressor is being used for high stage and not booster.

US Iceman
15-01-2007, 03:34 AM
It could be an FES package with a Hartford compressor on the package. Hartford compressors ar ethe old Dunham-Bush screws.



My RPM meter is slow to read out but the RPM for the defective system is lower than a working system at transition to delta.


How much lower is the RPM on the problem compresssor. What you are describing sounds like a stall condition.

Has any checked the speed-torque curves for the compressor and motor yet? The motor may be perfectly fine, but it may not develop sufficient torque during start-up. I've seen this happen on a brand new motors (several of them in fact).

We are going on a little bit of a witch hunt here, but we do not have access to the compressor or anything else, so please be patient with us.;)

mrsparky
15-01-2007, 03:42 AM
I forgot to address one other issue previously mentioned. The motor is coupled to the compressor now and I will not be able to perform the following test in the near future. I seem to recall from earlier tests that the compressor shaft will turn by hand but it takes some muscle. How much force should be needed to turn a uncoupled compressor shaft?

mrsparky
15-01-2007, 03:54 AM
I am pulling from an old memory board (my head) for the RPM data. All of our star/delta compressors have a factory (RAM Co.) transition time of 6 seconds. The RPM in a working system is app. 1/2 of motor data plate RPM after 6 seconds. So the RPM is around 1600 RPM in a working system at transition. Increasing transition time does not significantly increase star RPM at transition to delta. In the defective system the RPM is app. 1200 at transition. I would have to check my records for more accurate data.

I will add that the working system transition to delta at 1/2 data plate RPM surprized many electrial engineers who maintained that the star configuration should be close to data plate RPM at transition. This arguement was discounted by observing many other working systems.

NH3LVR
15-01-2007, 04:13 AM
It should take very little force to turn the Compressor by hand. They are a bit sticky after sitting for a bit. But after you get it to turn you should be able to turn it by hand with a little bit of leverage. The motor should not add anything appreciable except inertia when attempting this. It should turn without any binding. Have you checked the end play?
I am suspecting a problem in the starter. It sounds like it almost, but not quite single phasing. I may be wrong, but a connection or contact could be breaking down under the higher Amperage when the motor is coupled to the compressor.
If it is an FES machine it will say so on the control panel, or at least have an FES contract number on the inside of the door. Share that information with us, if you would.
The one reason I doubt you have a compressor problem is that it would most likely failed in the length of time you have had the problem, if it was mechanical.

mrsparky
15-01-2007, 04:43 AM
In re NH3LVR, The compressor shaft did not turn freely by hand when uncoupled from the motor, if memory serves me correctly. I remember wondering if there was enough oil on the screws or if they were to tightly bound. As I mentioned earlier, once up to speed the compressor performs like the other compressors, although I think it pulls 5%-10% more amps. than the other compressors at different slide valve positions.

I take it from the posts that the consensus must be that the leaking ammonia cooling injection solenoid is not causing my hard start.

Please believe me when I say there is nothing wrong with the electrical side except maybe (very slim possibility) we have two motors with rotors spun the same way. A couple of years ago we even bought a brand new motor for this compressor. Luck had it that another compressor had a motor go bad when the new motor arrived so it got the new motor. All this compressor has seen during my employment is re-wound motors, that the re-wind shop say are good.

NH3LVR
15-01-2007, 05:19 AM
This is turning into quite a mystery!
I do have one thought, a little far, out but bear with me.
I am wondering if there could be a problem with the compressor suction check valve, or the bypass around the check being open too far. This will cause the oil to move back up the suction line on shut down. It would work it's way back down over time, but could cause a problem in starting.
I have seen it happen before, although the only symptom I noticed at the time was a loud noise on startup.

mrsparky
15-01-2007, 06:15 AM
In re NH3VLR, The last symptoms you talked about are exactly what I am witnessing during start. It would satisfy my contention that the electrical is in good shape and the experienced refrigeration mechanic that the compressor is in good shape.

Is there a simplified way for an electrician to check the condition of the suction check valve and the bypass around it? The refrigeration man is supposed to be back this coming week and I will go over this thread with him to try and find a solution.

NH3LVR
15-01-2007, 03:14 PM
Is there a simplified way for an electrician to check the condition of the suction check valve and the bypass around it?

I cannot be too specific without know which panel you have, but here are the basics.
Remember first that the suction pressure indicated on the panel is measured in the suction line upstream of the suction check valve-not the pressure inside the machine itself.
After the machine has been sitting for awhile the discharge pressure indicated on the compressor (not on a discharge gauge somewhere else in the system) should have come down to suction pressure.
If it has not you do not have a problem with the suction check-you have a different problem, so do not continue.
Start the machine and let it run until it is up to temperature. Note the oil level.
Shut the machine off. Watch to see how quickly the indicated suction pressure falls off. Normally this will take at least twenty minutes or more. If the check valve is defective it will fall very rapidly. If the bypass around the check is open too far it will fall but not as rapidly.
Also watch the oil level. If it goes down very quickly after shut down it may indicate the oil is going back into the compressor.
This is a simple explanation. The results of this test are subject to interpretation. I would not encourage you to hastily make adjustments. The bypass valve I am talking about is very easy to confuse with all the other small valves on the machine.
A failed suction check can cause oil loss after shutdown, but it may not happen in your case. If the suction line is of a sufficient length before it goes into the suction header it may well not go back into the system, but sit above the machine waiting for the next start. How long it will stay there before going back into the machine is a question I do not have the answer to.
Again do not be too hasty to try to make adjustments.
NH3LVR

Good Luck

US Iceman
15-01-2007, 05:08 PM
If the suction line is of a sufficient length before it goes into the suction header it may well not go back into the system, but sit above the machine waiting for the next start.


That's another possibility. If the oil is sitting in the suction line, it could be similar to a flooded start which does cause higher than normal current draw.

I would also agree with the comments about not adjusting anything. Strongly agree in fact. This is what usually happens when troubles appear. All sorts of adjustments are made, and finally someone finds the problem after a lot of guessing.

It's less trouble to try to spend the time solving the problem, than it is to correct all of the changes performed. This is like someone wanting to adjust superheat on TXV's at the first sigh of a perceived problem. It's almost always the wrong thing to do.

Josip
15-01-2007, 09:04 PM
Something is not right.



For sure!

We still do not know all about compressor and electric motor, i.e. complete unit history, just thinking out loud...

Have feeling we all wandering around like blind chickens trying to find corn seed...

...reading all posts couple of times I cannot put away that there is some problem with electric motor. This big noise and decrease of rotation during inrush shows that motor torque is just above needed for that compressor:confused: .

N.B. I had a problem with 5 Stal screws where we had to evacuate complete unit down to suction pressure to make start, otherwise there was the same story what we have here. There was not bypass valve installed. (This was a good school for all of us for another plant where we installed those valves to solve the same problem.)

Problem persist from early beginning due to ignorance of electrical engineer because he ordered electrical motors like 3000 rpm/250kW. Later on, speaking with electrical engineers from Stal, I found out there is a list with about 40+ points of data needed to choose the right motor. As I am not el. engineer that is a Science Fiction for me:eek:

What can cause that:

1. motor maybe was not good from early beginning

2. motor was rewind....

3. If there is a bypass valve which must be open during inrush it is possible that one is not working properly due to mechanical malfunction or burned coil thus no internal leaking for easy start.

4. Problem regarding oil is also possible but then there should be a squeezing sound...

Maybe I am not right, but this is my experience and just hope to help.

One more question is that maybe 2 speed motor???

Best regards, Josip :)

Andy
15-01-2007, 09:11 PM
[QUOTE=mrsparky;57918] valve positions.

I take it from the posts that the consensus must be that the leaking ammonia cooling injection solenoid is not causing my hard start.
QUOTE]

I would say that an injection valve fault is your most likly fault.

Frosting accross the valve whilst off.

Compressor hard to turnover by hand.

What else could it be;)

Kind Regards Andy:)

US Iceman
15-01-2007, 10:12 PM
Have feeling we all wandering around like blind chickens trying to find corn seed...


That's my feeling too. I don't want to start a finger pointing contest, but this problem does not pass the "smell test".



Later on, speaking with electrical engineers from Stal, I found out there is a list with about 40+ points of data needed to choose the right motor.


I have seen something like this sheet before also. A motor is not just a motor. This is why I mentioned the speed-torque curves earlier.

mrsparky, have you contacted FES yet? They should be able to help you.

mrsparky
16-01-2007, 03:31 AM
The holiday is over and I am back on my third shift at work and I saw the experienced refrigeration mans car in the parking lot. We should now be able to tackle this thing in earnest. I'll get back after a little while.

US Iceman
16-01-2007, 03:38 AM
I have some more questions... does the wye-delta starter have an open or closed transition?

One last one; you mentioned the problem had been going on for about 2 years. What changed two years ago?

mrsparky
16-01-2007, 04:41 AM
Two years ago I became the de facto ammonia electrician. Eight years ago when I hired on this compressor was down and being worked on and has been a pain ever since. All of our star/delta starters are closed transition. I spoke to the refrigeration man about the injection solenoid. After feeling it he agrees that it needs replaced and that we have a box full of new ones. I will replace this solenoid after I take care of my other plant duties.

I looked at the slide valve dial indicator and it is pointing at 4% while the compressor is off. I will pay close attention to this after the solenoid is replaced. The way things work around here it may be a day or two before I have anything to report about the progress being made. I will provide feedback as things unfold. Thanks again to all. Robert

mrsparky
16-01-2007, 08:03 AM
:) Replaced the injection solenoid and now the motor is quieter during start.

Josip
16-01-2007, 10:31 AM
Hi, Robert :)


valve positions.

I take it from the posts that the consensus must be that the leaking ammonia cooling injection solenoid is not causing my hard start.



It certainly sounds as if the solenoid valve is leaking.


Well, your solenoid valve is damaged and there you have a leak (frost after solenoid valve till compressor entry) even your compressor is colder (you can check it with infrared thermometer) and you must replace solenoid valve with new one as per Andy's advice.




I would say that an injection valve fault is your most likly fault.
Frosting accross the valve whilst off.
Compressor hard to turnover by hand.
What else could it be

Kind Regards Andy



Replaced the injection solenoid and now the motor is quieter during start.


Best regards, Josip :)

nh3wizard
16-01-2007, 04:59 PM
Does the solenoid have a set point for the Oil temp?
If so what is it?

karelsdr
17-01-2007, 02:54 AM
We all agree that your solenoid is leaking by. But if it is frosting after being off for a few days , it is not a big leak and is not your starting problem.To get the frosting only at the valve means your oil sump heaters are boiling off the liquid in the serarator. It is possible your motor has been sized at full load running conditions(e.g. -20 deg evap and 95 degree con) If this is the case you need to throttle the suction valve from near closed to start. Then check your oil sump for frosting and oil temp. Any abnormal noises just shut if off and reasses

US Iceman
17-01-2007, 03:06 AM
Hi karelsdr,

Welcome to the discussion. You raise a good point about the liquid injection solenoid. I'm not convinced the replacement of the solenoid valve solved the underlying problem.

It may have minimized the problem to an extent that the compressor now starts without any perceived problems.

I think it would be prudent to continue to check into the details more to ensure everything is OK. As I said before, something about this does not pass my smell test.

Wye-Delta starters on large motors can be fickle. Since you only have 33% of the current, the starting torque developed by the motor is also lower. Any amount of voltage imbalance can also reduce this and also increase the motor winding temperatures.

It would be interesting to see any motor teardown report that might have been done.

karelsdr
17-01-2007, 03:07 AM
How much time from the wye contactor to the delta contactor

US Iceman
17-01-2007, 03:16 AM
I think six seconds was the value set for the transition timer. For some reason I want to say the value I remember being told to set this should be around 3-4 seconds on a Westinghouse starter many years ago.

mrsparky
17-01-2007, 03:37 AM
in re nh3wizard, quote the experienced refrigeration mechanic, "The solenoid on this machine does not control oil temperature. The solenoid comes on when the machine starts and turns off when the machine stops. Oil temperature is controled by the thermo expansion valve that is set to 70-80PSI and this gives you an oil temperature of 125-130 degrees F."

in re karlsdr, all of our star/delta starters transition time is factory default to 6 seconds.

in re US Iceman, take my word for it there is nothing wrong with the electrical side of the machine unless the installed re-built motor has a spun out rotor and the spare has a spun out rotor in exactly the same manner. While this is a possibility it is highly unlikely. Unless the motor re-wind experts are incompetant, again highly unlikely.

The loud noise during start has been significantly reduced after the injection solenoid replacement. I will explore the earlier post from karelsdr with the experienced refrigeration mechanic about the suction valve after he gets back down from the roof.

US Iceman
17-01-2007, 04:10 AM
in re US Iceman, take my word for it there is nothing wrong with the electrical side of the machine unless the installed re-built motor has a spun out rotor and the spare has a spun out rotor in exactly the same manner. While this is a possibility it is highly unlikely. Unless the motor re-wind experts are incompetant, again highly unlikely.


Please be assured I'm not calling anyone incompetent, or in any way pointing fingers. I've seen brand new compressor packages (w/new motors and starters) exhibit similar problems.

I may be reading more into the situation than warranted, but my only point is, there are a lot of things to review (as evidenced by Josips commment on the motor check-off sheet).

mrsparky
17-01-2007, 04:31 AM
US Iceman, I am very competent in electrical and checked out the electrical side some time back with direction from a electrical engineering forum. I picked through it like looking for a needle in a haystack. I disassembled the hay stack straw by straw and did not find a needle. That is how closely I checked it. There are non-invasive ways to check for a spun out rotor without disassembling the motor. These are pretty complicated and may be beyond my capability. I did not pursue this test because both motors gave the same symptom of a loud motor noise during start. My gut feeling was that the motor load i.e. compressor had a higher than designed inertial load during start, albiet this could be caused by a partially spun out rotor. While a motor running at 3550RPM is more more prone to rotor spin outs than one running at 1760RPM, this spin out is usually caused by frequent starts and stops. Our compressors are run for months at a time and off for months at a time while their work duty is rotated. Thanks, Robert

karelsdr
18-01-2007, 01:44 AM
The timing in wye delta starters varies from site to site(load,voltage,coupling etc.)But 6 seconds sounds like a long time to me.

mrsparky
19-01-2007, 12:27 PM
in re karelsdr, your last post sounded like a question.

Star/Delta is old school. Some call it Wye/Delta but it is the same thing. The idea is that at transition to Delta is that you want the least amount of slip between the rotor field and the stator field. The less amount of slip the lower the current at transition. The refrigeration star/delta starters where I work at are designed to cross over at one half of motor RPM. With smaller motors, configured in Star/Delta the inertial load is less and this can be accomplished at a shorter transition time. In my case any reduction of transition time would result in a large over current that trips the 1200A circuit breaker. Extending the time further does not do any good because at 6 seconds the motor configured in Star is turning about as fast as it can and lengthing it only stresses the motor thus the shift to Delta at max Star RPM. Hope this helps. Robert

Andy
20-01-2007, 01:28 AM
Hi Mr sparky:)

what about the transition between star and delta, is there possibly a tranistion spike. This happens if the phase sequence is wrong and you are skipping a set of coils in the motor and spinning it another 90 deg before going to delta, causing a addtional peak of amps called a transition spike.

Kind Regards Andy:)

mrsparky
20-01-2007, 04:44 AM
Hi Andy, We have closed transition starters so the so the phasing problem at transition is not a problem like open transition, if this is what you are talking about. I am positive that that there is absolutly nothing wrong with the electrical side unless both motors are have partially spun out rotors. I would bet my house on it. Replacing the injection valve really improved my machine start. The failure of this injection valve when the machine was stopped was odd in that once in a while it would be OK. Sometimes it would only have frost at the window, sometimes it would only be cold at the window and sometimes it would be so bad that the compressor would be frosting up. This problem was resolved in my original post and the first reply which was yours. Thanks, Robert

mrsparky
20-01-2007, 12:43 PM
I have re-read this thread again and hope that I did not sound condensending from any of my remarks. It was not meant that way.

One thing intrigues me about an earlier post from NH3LVR. He mentions a time that he had a problem with a suction valve that caused similar symptoms. He specifically mentions about the bypass being too far open and that to me inplies that it should be open a little bit. Our refrigeration mechanic informed me that our bypass valve is kept closed all the time unless there is a pump down (think that was his words). From reading this thread I asked if the suction valve has a internal bypass. He said that it did not, that it was a older style.

He is my friend and a respected refrigeration mechanic and I am not checking his work so I know we are doing it right. The thing in the back of my mind is like US Iceman mentioned in a earlier post. The injectior valve replacement only masked the real problem and I think he is right. The motor now starts a lot easier than it used to but it still does not sound like the others during start. Like its inertial load is greater than all the others.

nh3wizard
20-01-2007, 05:36 PM
I didnt read the whole thread, but is it possiable for the discharge check be stuck open? Maybe causing the hard start?

Just throwing it out there

Andy
21-01-2007, 02:05 PM
Hi Andy, We have closed transition starters so the so the phasing problem at transition is not a problem like open transition, if this is what you are talking about. I am positive that that there is absolutly nothing wrong with the electrical side unless both motors are have partially spun out rotors. I would bet my house on it. Replacing the injection valve really improved my machine start. The failure of this injection valve when the machine was stopped was odd in that once in a while it would be OK. Sometimes it would only have frost at the window, sometimes it would only be cold at the window and sometimes it would be so bad that the compressor would be frosting up. This problem was resolved in my original post and the first reply which was yours. Thanks, Robert

Robert:)

I have never come accross closed transition starters, what way does this work, can you post a link please.

Kind Regards Andy:)

US Iceman
21-01-2007, 03:46 PM
Our refrigeration mechanic informed me that our bypass valve is kept closed all the time unless there is a pump down...


If this bypass valve is the one that bypasses the suction check valve, it is not a bypass valve, but rather an equalizing line (which happens to have a valve in it).

This valve is normally opened just a little bit to equalize the discharge pressure down to suction pressure BETWEEN THE TWO CHECK VALVES (SUCTION AND DISCHARGE).

When the compressor motor is de-energized , the suction check valve closes to prevent excessive backspin on the rotors. Once the rotors have come to a complete stop, the gas pressure is equalized down to suction through this valve.

If the valve is closed, the pressure cannot equalize. This means the compressor would be starting against discharge pressure, which would be the same as what nh3wizard was hinting at.

The purpose of this equalizing valve is to allow the compressor to start at low pressure, and could also be adding to the problems experienced.

US Iceman
21-01-2007, 03:52 PM
Andy,

Have a look at this link.
http://www.mastercontrols.com/EngInfo/Articles/Wooddall/TA_RWood.htm

The problem you mentioned is due to the use of an open transition starter. The closed transition style uses a third contactor to keep the circuit energized.

All of the wye-delta starters used for screw compressors here in the US are the closed transition style (at least every one I have seen).

Andy
21-01-2007, 05:13 PM
Andy,

Have a look at this link.
http://www.mastercontrols.com/EngInfo/Articles/Wooddall/TA_RWood.htm

The problem you mentioned is due to the use of an open transition starter. The closed transition style uses a third contactor to keep the circuit energized.

All of the wye-delta starters used for screw compressors here in the US are the closed transition style (at least every one I have seen).

Hi Iceman:)

thanks for the link:)

yes I can remember seeing this on some older plant, the extra contactor is linked to a set of resistors to keep the magnetic fields going at Y Delta change over.

To be honest I haven't seen this in years. We have two 415volt 355kW motors starting on LV, but they are soft start, quite a lot of 200kW motors starting on open transition Y Delta without any problems:) I think we have a 250kW on the star delta start, but that is as big as it goes. After that 3300 volts and even one at 11kv on a double tapped trasformer for a two speed start (now that is scary):eek:

I wonder why we don't use the closed transition over this side of the pond now a days, is it our voltage:confused:
I was taught to look for transition spikes when commissioning, and would change the leads on the motors to minimise this (means changing four leads), this worked OK for me. Does anyone else do this:confused:

Kind Regards Andy:)

NH3LVR
21-01-2007, 09:47 PM
This means the compressor would be starting against discharge pressure, which would be the same as what nh3wizard was hinting at.

The purpose of this equalizing valve is to allow the compressor to start at low pressure, and could also be adding to the problems experienced.

This has not been a problem in my experience. I will concede that most Screw Compressors I work on these days have Soft Starts. Starting problems have not been a major issue for me.
When the Screw is started the pressure is equal on both sides. It is unloaded and I do not think it pumps much in the few seconds it takes to come up to speed.
I do have two machines that use Wye Start/Delta Run. Up to this point I have not noticed any starting difference between full Head Pressure starts and pumped out. Others may have had different experiences. Some machines start differently than others for many reasons.
I do work on machines occasionally without the Suction Bypass Valve. The largest problem I have experienced is that under some conditions they will fill with liquid. It does not take much of a leak when in a Discharge Check or Liquid Injection Valve over a long down period for this to occur.

mrsparky
22-01-2007, 03:50 AM
Hi All, Andy, with a closed transition starter the motor is never disconnected from the line during start therefore there are no phasing problems at transition. If you ask me it is a ramshackle design fraught with electrical problems. The contactor connected to the resistors is only pulled in for app. one half of a second at the moment of transition. Timing is crutial and any mishap causes large over currents. I will now be using soft starters when ever we have a closed transition starter blow apart.

I have wondered about the discharge check valve in the past. It was explained to me that when the slide valve is at minimum the gas ony circulates inside the compressor. Please correct me if I am wrong. Something about this last statement seems wrong to me because I have read that the compressor is a positive displacement pump. I have wondered in the past and now if the slide valve is truly at minimum when the dial indicator points to zero. Then again this would not make a difference if it is off much since the gas that is not being discharged is only circulating. Please remember that I am in new territory here but have a good physics understanding.

TXiceman
22-01-2007, 04:33 AM
First you need to be sure the slide valve is properly calibrated and the compressor will not start unless the slide valve is at minimum position.

Next, is this a high stage compressor or is it a booster compressor? If it is a booster and a smaller motor, it could have problems using a wye-delta starter.

Also, has anyone made a check on the motor starting torque to see if it can start the compressor in wye windings?

Until you can get the liquid solenoid repaired, if the machine will be shut off, manually close the block valve to the liquid injection and tag the unit out of service. As long as it is running the solenoid is open and the leak is not a problem.

What is the separator temperature prior to starting?

Ken

mrsparky
22-01-2007, 04:56 AM
The oil temperature in the separater tank was 109 degrees F at start.

This is a high stage compressor. Our high stage compressor motors are 500HP and the booster motors are 400HP.

The motor starts the compressor in wye but the RPM was app. 20% lower than a working compressor at transition when I made the measurement some months back. This machine does not have the high pitched whine during start that the other compressors have.

The liquid injection solenoid was replaced and this helped out during start (less noise).

I do not know how to measure motor torque during start. I am a electrican and not an electrical engineer. If I could make this measurement with an uncoupled motor during start this would tell me if the rotor has problems. If the problem is electrical it would have to be a partially spun out rotor and the spare would have an identical problem. This is a possibility because both motors are old and rebuilt except for the rotor. I have been over the electrical with a fine toothed comb except for the rotor. There is a test that energizes the motor winding with a reduced voltage. Then as you rotate the motor, current measurements are taken to check the condition of the rotor. I think that the currents should remain stable. I may read up on this better and try this approach also.

TXiceman
22-01-2007, 04:20 PM
mrsparky, to check the motor torque, you will need to get the speed-torque data and the starting torque of the compressor from the manufacturer. It should be in the data in the technical manual. Now you need the speed-torque data from the motor manufacturer to overlay with the compressor data. The motor manufacturer should be able to help with this for you.

The compressor curve should be below the motor curve for motor to pull the compressor up on start. With a Wye-delta starter, you need to be sure you have the torque to get the compressor rolling and accelerate while in the wye configuration.

While the soft start starters are gaining in popularity, the wye-delta is still a viable method of starting a motor to limit inrush.

You stated that you are in wye windings for about 6 seconds. Is the motor still accelerating at this point or has it peaked out. Have you tried to shorten the time for transition? I ahve seen a number of applications where they performed better by shortening the time for transition. The motor vendor should be able to help you with the time.

Ken

US Iceman
22-01-2007, 08:50 PM
When the Screw is started the pressure is equal on both sides. It is unloaded and I do not think it pumps much in the few seconds it takes to come up to speed.


With screw compressors, unloaded and slide valve position are two different animals. The slide valve can be at 0%, but the compressor still has some pumping capacity. After reaching full speed (at 0% slide valve position) the compressor typically has somehwere between 10% and 20% of the full load displacement trying to pump.

The critical thing in this thread is not the problem after reaching full speed, it's the problem of reaching full speed.

Once the rotors reach about 50% speed they start to pump more gas. This is also somewhere around the RPM where the starter (in question) begins to have problems.

This is the reason why the slide valve has to be at 0% slide valve position and the pressures are equalized in the compressor. Anything that can cause the toque load to be higher during start can affect the starting ability.

On full voltage starters you hardly if ever see these problems. On wye-delta start and auto-transformer it's a different problem and the speed torque curves are essential in determining what the problem is, or is not.

If the compressor requires more torque than the motor can develop during this time, strange noises and things happen.;)

When TXiceman comes back, maybe he can confirm (or deny) these points, but I believe this is true.

NH3LVR
22-01-2007, 09:08 PM
When TXiceman comes back, maybe he can confirm (or deny) these points, but I believe this is true.

I never doubt you Iceman!:)
I am due to do some service on a Wye Start/Delta Run 400HP Screw this spring. I have been after them to install a 1/4" bypass but it will require some welding, so it has not been done. I bleed the machine off with a hose before I work on it. Perhaps I will take some Amperage readings just to compare.

US Iceman
22-01-2007, 09:27 PM
I have been after them to install a 1/4" bypass but it will require some welding, so it has not been done.


That must be one of the older packages as I was under the impression almost all of the newer ones had the equalizing line around the suction check.

Unless... it is one of those that have a small hole drilled in the check valve plate(s) for equalizing the pressures.

NH3LVR
22-01-2007, 10:09 PM
It is an old Mccormack Screw. We replaced the panels with FES. Unfortunately there is no hole through the check valve. Still run good though.

US Iceman
22-01-2007, 11:50 PM
Well... at least I was half right.:D

mrsparky
23-01-2007, 02:10 AM
Tonight I will try and perfrom the "Single Phase Test" to check the motor rotor. If I can get the right voltage, 10% of line, for the test, and it passes then I will be focusing on the slide valve.

How do I prove that the slide valve is at minimum when the dial indicator points to zero without disassembling the compressor or using x-ray.

This motor sounds like it has trouble from the git go. The high picthed whine is absent (heard on all the other compressors) and sounds to be struggling close to the beginning of start and before it reaches one half motor plate RPM which should also be the transition point. As compared to working machines. This machine can not reach one half motor data plate RPM even if you left it in wye for as long as you wanted. Of course even in a working machine the motor cannot speed up much over one half data plate RPM before it ceases to accelerate while in wye after you have extended the transition time. The fault sounds like it is occuring only in wye. Once delta is connected you can hear that the motor has enough torque to wind out.

I of course have checked electrical connections all the way back to the main switch gear. The motor does not have a current imbalance any different from a working machine. The voltage is less than 1% imbalance.

I can manually operate the slide valve at the control station. If I load up the machine to 100% by the digital readout at the control station, the mechanical dial indicator is pointing to 100% and motor current is at FLA. If I manually unload the machine from the control station to 0% then the dial indicator is pointing to zero and the motor current is app. one half of FLA and this measurement is in agreement with a working machine by less than a factor of 10%. I can supply more accurate data when I get into work tonight. This seems to steer me away from the slide valve. Is my thinking correct?

US Iceman
23-01-2007, 02:37 AM
The voltage is less than 1% imbalance.


That's not bad, but it can affect the torque output of the motor slightly. If the motor torque available is marginal, then any other factor affecting it adds to the starting problem.

Since you have ruled out the electrical side of things, I think you need to ask FES and the motor manufacturer for their speed torque curves and verify the combination is correct.



If I load up the machine to 100% by the digital readout at the control station, the mechanical dial indicator is pointing to 100% and motor current is at FLA. If I manually unload the machine from the control station to 0% then the dial indicator is pointing to zero and the motor current is app. one half of FLA...



Which is exactly why you do not want to see screw compressors operating in an unloaded state. They use a lot of energy very inefficiently in this condition. just a tip to look for this if you are trying to save energy.:cool:

TXiceman
23-01-2007, 04:57 AM
US Iceman was correct. The Compressor starts with pressure equalized across it at minimum slide valve position.

Which compressor is on the package, is it a D/B, Mycom, Howden, Kobe or a GEA. FES has used them all. All of these use hydraulic pressure to load and unload. It is simple enough to manually drive the slide valve to minimum position and check calibration to show minimum position on the control panel dispaly.

As the machine starts, watch the manual slide valve indicator to see it it tries to load.

I am beginning to think that the motor may not have enough torque in the wye configuration to get the compressor up to speed. Either due to not enough motor torque or too much compressor load.

Is this a conventional design with the compressor mounted on top of the separator or is the compressor mounted on a frame with a vertical separator on the end. In this case, the compressor is "submerged" and can have problems starting if you run the oil pump an excessive amount of time prior to starting. The compressor is logged with oil and the motor has to push the oil out of the casing while starting. You can also get this if the machine does not go to minimum load at shut down.

I like to manually unload the compressor to miminum load and then shut it down with a submerged design.

As I had noted earlier, 6 seconds may be too long in wye windings. Try to shorten the time to transition to 4 seconds or maybe even 2 seconds.

As a last resort, you may need to install a bypass around the compressor to keep the pressure down and time it out once started and close the bypass. The bypass will have to be pretty large to move the gas and keep the discharge pressure from building.

Excuse my rambling, but I tend to solve problems by talking out loud and see what if anything makes sense. I try not to answer myself too often when other people are watching.

If you can provide the compressor model, I maybe able to come up with the speed torque data you need. You will have to contact the motor vendor for the data in the wye and delta configurations.

Ken

mrsparky
23-01-2007, 05:18 AM
Sorry I took a little time to get back to the forum. I am preparing to do the single phase test with guidance from an electrical engineering forum. If the motor passes the test then I will be almost positive that the electrical side is not causing the problem

The compressor is a conventional package with the motor and compressor mounted on top of the tank. The compressor is a "Hartford" model number 2512DLR3VDFOEM0010. The motor is 500HP/3550RPM. We have nine compressors here and one other is exactly the same as this one. It does not have any problems so I do not see a need to verify that the motor and compressor are compatable, you may know a reason that eludes me.

That is why I think that this is a real fault and not a design problem.

US Iceman
23-01-2007, 05:19 AM
As a last resort, you may need to install a bypass around the compressor to keep the pressure down and time it out once started and close the bypass. The bypass will have to be pretty large to move the gas and keep the discharge pressure from building.


Yep, I've seen this done before on some large screw boosters with wye-delta starters and the "submerged" compressor installation you mentioned. That site had experienced a similar problem to what we are discussing here.



Excuse my rambling, but I tend to solve problems by talking out loud and see what if anything makes sense. I try not to answer myself too often when other people are watching.


I've found this to be a good tool for analyzing problems too, but people do tend to look at you funny when you start to answer yourself.:D

US Iceman
23-01-2007, 05:51 AM
I looked up a model 2512DLR3VDFOEM0010 on the Hartford compressor website. The minimum torque values are:

Breakaway torque: 130 ft-lbs
Starting torque: 182 ft-lbs

The motor would have to provide at least 182 ft-lbs in the wye configuration to get the rotors up to speed.



We have nine compressors here and one other is exactly the same as this one. It does not have any problems so I do not see a need to verify that the motor and compressor are compatable...


Having others of the same compressor does not mean anything. If the motor on the package in question has ever been changed from it's original, or any other changes made to suction pressure, etc. anything can happen.

From earlier information it appeared this problem had been occuring for several years, so something has been going on here for a long time. It was also mentioned the motor had been in the repair shop several times (maybe less, I don't recall now). What necessitated the motor rebuild(s)?

It's beginning to sound as if you do not want to check the information provided....:(

mrsparky
23-01-2007, 08:51 AM
Quite the contrary, I want to get to the bottom of this. Whatever the solution is, it will seem obvious after all that we have been through.

I am assuming that this compressor is designed to be powered by a 500HP refrigeration motor. The spare motors we have here have been swapped around with different compressors without any problems. If there is nothing wrong with the motor or compressor itself then I should look at external ancillary equipment, i.e. discharge valve, bypass etc.

There is bypass tubing with shut off valve around the suction check valve. This is kept shut off and the suction check valve does not have an internal bleed. I told the refrigeration man that this should be cracked open to equalize pressure when turned off. He told me that FES does not do it that way. From reading other posts this may not make any difference anyway.

We have had motors burn up due to starters blowing up, bad bearings and swelled rotors. We are closely monitoring motor bearings now to prevent this easily preventable fault. When a closed transition starter blows up most of the time the fault cause was never determined (but is most likely caused by welded contacts that can't release when they should) and after replacing contactor contacts, fuses, resistors, etc. it works for years. That is why I sought permission to change the last starter blow up to a soft starter. That project is almost completed and I am only waiting for stud blocks.

I hope this facility does not sound like a hopeless case but I am condensing 8 years of my observation so please don't think we are incompetent.

I am at the point of pulling my hair out on this one, what little hair I have left.

mrsparky
23-01-2007, 12:00 PM
I performed the single phase test on the motor to check condition of the rotor. It passed with flying colors. I can now focus on the compressor side.

nh3wizard
23-01-2007, 05:29 PM
mrsparky, what part of Georgia are you located?

What kind of liquid levels do you have, and how far from the recirc is the compressor? I have seen where the level has caused hard starting due to the liquid feed not close all of the way and raising the level higher than normal but not high enough to cause a high level shut down, and getting some wet back in the suction on the compressor, plus making it sound differnt on starts.

Just more to throw out there:rolleyes:

US Iceman
23-01-2007, 05:38 PM
...raising the level higher than normal but not high enough to cause a high level shut down, and getting some wet back in the suction on the compressor, plus making it sound differnt on starts.


Sounds reasonable. That's essentially a flooding problem on start-up caused by the liquid foaming in the vessel.

A quick way to check if this is the culprit is to monitor the discharge temperature. If the discharge temperature goes down during start-up, the problem is due to liquid ammonia.

The discharge temperature should slowly increase to it's normal temperature as the compressor package warms up.

nh3wizard
23-01-2007, 08:32 PM
It didnt take long to figure out due to liquid causing the oil filters to plug up.

US Iceman
23-01-2007, 09:15 PM
It didnt take long to figure out due to liquid causing the oil filters to plug up.


Did you cut any of the filters apart? The ones I have seen are filled full with a lot of junk that came out of the system.

It's like the liquid flushes all of the debris right of the pipes and vessels and deposit it in the compressor where the oil filters are then left to try to filter the enormous amount of junk out of the oil.

I have ran into this on large system start-ups also. Not much fun...

Andy
23-01-2007, 11:01 PM
Hi Mrsparky:)

most slides have an end stop. 0% is not 0 capacity, usually a machined ring is fitted to the slide piston cylinder limiting how low the slide goes. Time to pull down the screw I think:D
Check the slide stop and check with Harford what should be in there.

Also do a bearing movement/play check as per manufacturers requirements and check your rotor outlet clearance.

I suppose you could check the VI against your other compressor, probably 3 in this case by the code you quoted and check with the manufacture what the VI should be.

Wish you luck:)

Kind Regards Andy:)

TXiceman
24-01-2007, 03:34 AM
VI should have minimal effect at start up. Also, has this compressor had this particular motor on it since start up or just since you noticed the starting problem. If you have had the motor rewound, it is possible the motor shop altered the windings enough to have less torque during start up. Do you have another motor to swap out and try on the compressor?

How many hours are on the machine and does it roll freely (with sleeve bearings, it will not roll as freely as a rolling element machine) when disconnected from the motor? Are the rotors meshing smoothly or do they drag.

Have you checked for phase imbalance when under starting load? It may check OK at rest or once running, but one leg may drop while under load.

Does the motor start OK in delta winding? Set your transition time down to 1 or 2 seconds and see how it starts.

A normal torque motor, 500 HP motor should have enough torque to start a 2512 compressor.

I can see that being of the electrical nature, your inclination will be to check electrical and blame mechanical for any problems that may not be explained. The electrical problems are easier to check than tearing into a compressor, so let's exhaust all of the electrical items before we start opening up a compressor that may not need to be opened.

The 2512 compressor has replacable angular contact ball bearings for thrust, but the sleeve journal bearings are pressed in and line bored in place when needing replacement. If you get this far into the machine, you will need a good machine shop to go on from this point.

Oh and Andy, If you can not fix it, remove all traces of evidence that you tried to fix it.

Ken

US Iceman
24-01-2007, 04:57 AM
I have a question...

On the oil pre-lube operation before the compressor starts; how long does it take to develop sufficient oil pressure to prove out the safety circuit before the starter engages?

The longer it takes to develop oil pressure with a pre-lube pump, the more time you might have of pumping oil into the compressor, which could also create additional torque loads that have to be overcome.:confused:

mrsparky
24-01-2007, 12:04 PM
nh3wizard, I work in "Hotlanta" and reside in Forest Park although I have to admit that I am a yankee from the Buckeye State. I am an electricain with limited knowledge about refrigeration but the hi temp recirculator tank is only ten feet away from this compressor. There are two windows on the oil separter tank. The bottom widow is full and the upper window is empty.

I did not work on it any last night but will check the discharge temperature at my next opportunity, if there is a temperature sensor. I am still familiarizing myself with the mysteries of ammonia refrigeration. Once I get through all of this I may try and get certified. Lots of jobs for this trade.

If I set the transition time too low then at transition there is too much slip and the circuit breaker trips.

When I performed the single phase test I had to remove the coupling guard to slowly rotate the motor. The motor was still connected to the compressor and it hand rotates with a force (guess only) of five pounds. It turned relatively easy by hand contrary to what I stated in a much earlier post.

Being the only third shift electrician at the facility, I am called for everything and anything. I derive some pleasure of chastising mechanics when it is a mechanical problem. This problem seems mechanical to me but I can't prove what the cause is yet except that it is not electrical from the motor back to the mains disconnect.

The oil pumps for app. ten seconds before the motor receives a start command.

mrsparky
26-01-2007, 11:59 AM
Thanks to all for the moral support and technical support. This project has been put on the back burner for a little while. During the cold months we don't need as many high stage compressors running and other projects are taking precident. I am at an age where I have to forget something in order to learn something new and I have been learning a lot of new stuff lately. I'll resume contact when we start working on this problem again. Thanks again to all. Robert

mrsparky
26-01-2007, 01:05 PM
One more tidbit, A manager, a friend of mine, asked me this morning about a propane refrigerator in a camper. I told him that it is an absorbtion machine and told him to rent the Harrison Ford movie "Mosquito Coast". Has anyone every watched this movie and is this technique in the jungle possible? Only watch the first three quarters of the movie and then turn it off.

Andy
26-01-2007, 11:42 PM
Oh and Andy, If you can not fix it, remove all traces of evidence that you tried to fix it.

Ken

I feel your pain:D

Kind Regards Andy:)

wolfie
19-04-2007, 09:44 PM
Is there A bursting disc fitted in the manafold system
My be wrong without more info