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squidward
21-11-2008, 09:48 PM
Hi Everybody! I'm working on an ammonia system that we use to condense CO2 vapors. Two years ago I put a new compressor and motor on, been running great since then. Til now, that is. The plant operator called me because of low suction pressure shutting him down. Went down there and verified the suction valve opened on startup, ran to the computer and saw suction pressure dropping into a vacuum. Evaporator showed 45% level. Impossible, right? Right! there was in fact no NH3 in the evaporater because oil from the harvester had screwed up the level controller. So I cleaned that up, got a level in the evap. and running again. Plant guy going to drain oil harvester more frequently. While I was out of town, he called to say we're losing alot of oil. Told him I can't get back so call in service guy and change out coalescent filters, which they did.and added oil. One week later all the oil from the reservior is in the evaporater.Very little oil in the siphon pot, so I don't think it went out the discharge. Got a look at the piping diagram and figured that the oil is going out the economizer port thru the (bad)check valve every time he shuts down(twice a day)right into his receiver. Check valve looks OK but I got a new one coming monday. There was a bunch of oil in it too.Anybody out there ever have this problem? My boss is not convinced.

US Iceman
21-11-2008, 10:06 PM
If an economizer is used, the refrigerant and oil will both back-flow if the side port check fails. It is not a simple matter of just a little oil as the entire oil separator can equalize pressure with the side port when this happens.

So... it can and does happen. Sorry for the boss, one for the employee!:D

nh3wizard
24-11-2008, 02:55 PM
If an economizer is used, the refrigerant and oil will both back-flow if the side port check fails. It is not a simple matter of just a little oil as the entire oil separator can equalize pressure with the side port when this happens.

So... it can and does happen. Sorry for the boss, one for the employee!:D

I have seen this happen in the past, one way to double check is valve it off and see if your levels stay the same, if they do then you know its the economizer, if it continues there are other issues.

Not poking fun at your boss but has he been in the industry long?

squidward
24-11-2008, 06:59 PM
Thanks, I'm new to refrigeration. My Boss has been around our business for a long time.(air separation)Very smart guy, several hundred miles away. I just got my EPA 608 certification. The more this stuff breaks down, the more I learn. However, chillers are a small part of my total workload. It's great to be able to ask more experienced people if I'm on the right track. Thank you!

CHIEF DELPAC
24-11-2008, 10:23 PM
In my small NH3 plant the operators always shut the suction valves and if the compressor has the econ port in use they also shut the valve at the port. However I have also lost the oil from the separater when pumping down the comp. and oil separator prior to doing mantenence. Lastly what is a harvestor.

squidward
25-11-2008, 12:48 PM
Hi Chief! This system has an automatic valve on the suction line and the economizer line. The suction valve closes immediatly, but the economizer is driven by the pressure in the receiver, so it stays open.On shutdown it takes about two minutes for discharge pressure vessel and receiver to equalize, during which time I believe hot oil is crossing over. Oil harvester scavenges oil from evaporator for removal by operator. He used to do it quarterly-about 4-5 gallons

TXiceman
25-11-2008, 04:46 PM
Seen many systems with a leaking or a valve as you desscribed, blow oil out the economizer port on shut down. SO it is not new and your boss may be smart, but he has seen something new to add to his experience.

Ken

squidward
25-11-2008, 07:25 PM
My new valve showed up today( 1 day late)It's in and system is running. Almost up to temp (oil) when it is we're gonna shut it down and watch economizer line with temp gun. I am confident it is fixed. Also, we're gonna get our computer guru to give auto valve on that same line orders to close asap on a shutdown. Thanks to all!

squidward
25-11-2008, 09:02 PM
Wow! I was out at the compressor at shutdown and that pipe leading up to the checkvalve went from 30.8 deg. F. to 84 deg F. as soon as it shut down. After the check valve stayed at 31 deg. F. Success!

nh3wizard
25-11-2008, 09:26 PM
Wow! I was out at the compressor at shutdown and that pipe leading up to the checkvalve went from 30.8 deg. F. to 84 deg F. as soon as it shut down. After the check valve stayed at 31 deg. F. Success!

Whats the pressure after the check?

US Iceman
25-11-2008, 09:50 PM
...that pipe leading up to the check valve went from 30.8 deg. F. to 84 deg F. as soon as it shut down.


Now show that to your boss! This shows you that the back-flow does indeed occur and why the check valve is required.

squidward
26-11-2008, 01:19 PM
NH3wiz, pressures are 170psi discharge, 2psi suction and 50 psi in the receiver.. that auto valve in economizer line WAS closing on shutdown, so pressure after check probably stayed near 50... after seeing oil get sucked up into line I'm guessing that the pipe mostly filled up on shutdown, then drained into the receiver when that valve opened on startup. I took that check valve apart,crazy thing looks fine!(glad I ordered one in spite of that inspection) Plant guy says everything is runnuing good this morning, oil levels are right where we left them, oil harvester nice and frosty. Oh and we changed settings in computer to leave condenser fans on for a while to bring down discharge pressure after shutdown. Thank you!

nh3wizard
26-11-2008, 02:20 PM
:DGlad to see it worked out

NH3LVR
26-11-2008, 03:13 PM
I have also seen this happen if the Suction Check failed to close, or if the bypass around the Suction Check was open too far.
One of my customers had a old Frick and the Suction Check did not open. They took out the disc and started it up. It worked fine until they shut it off.
Another time we installed a used Stahl Screw, started it up and went to lunch. It shut down while we were gone and all the oil went back up the suction line. We were a bit puzzled when we got back, to say the least.

sterl
07-04-2009, 05:51 PM
Any pilot operated solenoid or regulator for refrigerant is going to permit reverse flow....Closing the side port regulator or solenoid won't prevent the reverse flow. It will prevent the refrig from the economizer heat exchanger from heading for the compressor should the HP warm the economizer at all.

Though some don't: This condition often accompanied by the machine spinning in reverse once motor de-energized, and the whole thing comes to a halt.

squidward
05-06-2009, 07:29 PM
Latest update! It was not the economizer check valve! One cold morning it emptied the oil back up into the evaporator. Luckily this time I was there to see it happen. It seems that whoever designed this skid decided to use a pnuematic valve on the suction line, meant to close on shutdown.(no check valve!) I have seen it operate normally, so didn't think it was the problem. Wrong! found a piston o-ring half rolled out of it groove and binding up the whole works. Fixed that up system runs great again. Until that little control relay stuck and kept the motor/compressor running all nite w/ no ammonia, fans or anything else. When the operator got there in the morning the oil temp was 455 deg. F. The whole skid shut down 12 hours earlier, computer thought it was off. Had to pull the main breaker to shut it down. New compressor went in no problem, had to replace oil level probe, melted. Also the little balls in the sight glasses, put in new sight glasses, coalescent filters, oil and oil filter. Started up and ran good but shut down on high oil temp. Forgot about the thermostatic controlled oil valve- can you believe that was $800!

Magoo
06-06-2009, 01:40 AM
At 400 + F oil temp, carbonized oil is going to a long term issue in system.
A new compressor as well, is the plant jinxed or what.
And I can believe the cost of amot oil reg vav, imagine what they cost down here, on the other side of the globe.
Keep us all up-dated on progress with system.

squidward
08-06-2009, 01:40 PM
Thanks! So far so good. I had to tighten a fan belt last week, otherwise it is running fine. I don't think too much oil circulated into the system during the overheat, and we drained every bit that we could while doing repairs.

Frickman
29-07-2009, 07:49 AM
tell you plant guy o be close to the compressors when they shut down next. you can and will hear the screws spinning backwards if you have a bad suction check or bad valve assembly.