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  1. #51
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    Re: Liquid Pump Low Dif



    Quote Originally Posted by Krohnie84 View Post
    The building was built in 1995, the original units were designed to defrost/heat with a regulated hot gas pressure of 70-75 psig. The expansion that took place in 2008 added units that are designed for 105 psig.
    Usually, plan has outlet pressure regulator to keep certain constant(all year around) pressure in hot gas main. This is hot gas main. Evaporator has inlet pressure(or back pressure regulator) regulator to keep certain constant pressure inside the coil during defrosting. What do you have for new and old evaporators? What is the difference?



  2. #52
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by Segei View Post
    Usually, plan has outlet pressure regulator to keep certain constant(all year around) pressure in hot gas main. This is hot gas main. Evaporator has inlet pressure(or back pressure regulator) regulator to keep certain constant pressure inside the coil during defrosting. What do you have for new and old evaporators? What is the difference?

    Each unit has a regulator on the pipestand before the evap.

    Graph key:

    Yellow = Vessel Level

    Green = pump dif

    Red = compressor slide valve %

    Pink/purple = suction pressure

    graph.jpg

    After adjusting suction pressure, unload and load times on the compressor and the liquid line flow valves I got another alarm around 4:30 this morning.

  3. #53
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by Krohnie84 View Post
    Each unit has a regulator on the pipestand before the evap.

    Graph key:

    Yellow = Vessel Level

    Green = pump dif

    Red = compressor slide valve %

    Pink/purple = suction pressure

    graph.jpg

    After adjusting suction pressure, unload and load times on the compressor and the liquid line flow valves I got another alarm around 4:30 this morning.
    It looks like I described before. Sudden change in refrigeration load and compressor try to handle this issue. Increase suction pressure again. Try to determine what happen with evaporators in this moment. During the night load will be lower and probably is higher that this issue will happen during the low refrigeration load.

  4. #54
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    Re: Liquid Pump Low Dif

    Krohnie ,
    For liquid level to suddenly spike there is a sudden load change for this to happen as Sergei suggests .

    - Can you determine pump controls that protect it like timers for pump differential & pressure setting

    - pressure setting that activates cavitation timer
    - time before pump cuts out
    - does pump cut back in automatically , if how long before it does .
    Normally pump cavitation timer would be set for 30-60 seconds so it may have time to recover hickup .
    Auto restart after 2 minutes etc for 6 cycles of this before alarm goes off in a certain time period of say 30 minutes to avoid unnecassary call outs .

    -Have you observed & adjusted liquid makeup hand regulating valve as previously suggested in earlier posts .

    - Can you also monitor liquid makeup on/off as well ?

  5. #55
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    Krohnie ,
    For liquid level to suddenly spike there is a sudden load change for this to happen as Sergei suggests .

    - Can you determine pump controls that protect it like timers for pump differential & pressure setting

    - pressure setting that activates cavitation timer
    - time before pump cuts out
    - does pump cut back in automatically , if how long before it does .
    Normally pump cavitation timer would be set for 30-60 seconds so it may have time to recover hickup .
    Auto restart after 2 minutes etc for 6 cycles of this before alarm goes off in a certain time period of say 30 minutes to avoid unnecassary call outs .

    -Have you observed & adjusted liquid makeup hand regulating valve as previously suggested in earlier posts .

    - Can you also monitor liquid makeup on/off as well ?
    At this time there are controls and I can change the settings within the equipment's parameters. As for protection timers.... the computer system does have a timed reset and delay in place but I am unable to change those settings. < The delays and resets do work, the pumps have turned themselves back on even if the system alarms. I have seen a large number of resets that never alarmed and some that have alarmed but reset before anyone could take a look.

    I have been "playing" with this valve, but at this time there has been no change. < I won't change it until I have had another dif alarm to make sure there is no other issue.

    Liquid on and off to what? From the HPR to the MTV or from the MTV to the LTV? I cannot control the liquid flow from the high pressure receiver to the med temp vessel (float control on receiver< discussed in prev posts) but I am able to control the flow from the Med temp to low temp vessel. At this time I have found no connection between the two but I have tinkered with it to see if it would make a difference. As for the flow, if I decrease it I don't have enough make up for the vessel (not even a half a turn less) so I have found no reason to increase it due to the fact that it makes up the level just fine.

  6. #56
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    Re: Liquid Pump Low Dif

    I was talkig about liquid makeup from HPV to MTV .
    What type of valves & controls do you have on this line ?

    How is liquid makeup achieved from MTV to LTV ?
    Is it off MTV liquid pump ?
    Last edited by RANGER1; 15-10-2010 at 11:57 AM.

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    I was talkig about liquid makeup from HPV to MTV .
    What type of valves & controls do you have on this line ?

    How is liquid makeup achieved from MTV to LTV ?
    Is it off MTV liquid pump ?

    From HPR to MTV is a mechanical float. The HPR also feeds the thermosyphons for the compressors.

    There is a solenoid valve and a hand expansion right behind that. The solenoid is controlled via hanson controller.

  8. #58
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    I was talkig about liquid makeup from HPV to MTV .
    What type of valves & controls do you have on this line ?

    How is liquid makeup achieved from MTV to LTV ?
    Is it off MTV liquid pump ?

    What about MTV to LTV , what is the setup .

    Is the pump cavitation same , better or worse ?

  9. #59
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    What about MTV to LTV , what is the setup .

    Is the pump cavitation same , better or worse ?
    Quote Originally Posted by Krohnie84 View Post
    From HPR to MTV is a mechanical float. The HPR also feeds the thermosyphons for the compressors.

    There is a solenoid valve (AKA "King Valve") and a hand expansion right behind that. The solenoid is controlled via hanson controller.
    Low temp pumps have had little to no problems with cavitation (knock on wood). Med temp I am still having problems with the pump pressure falling out. I have increased the suction pressure more than a few times and messed with the expansions on the liquid return (off of the header) and the vent lines for the pumps.

  10. #60
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    Re: Liquid Pump Low Dif

    The TS-Pilot Receiver: otherwise HPR...most likely has an internal weir at +/- 50% and once it clears the weir, a float valve at the "high" vessel delivers liquid to the medium temp vessel: That about right?

    Any chance this little condition you got follow a compressor shutting itself off?

    When a compressor shuts down on a T-Sypon arrangement: The liquid in the pilot receiver (reservoir) drops quickly...That because on standby the liquid in the Thermosyphon Return Line rises all the way to the weir inside that vessel...Whereas while there was oil flow that vertical would be near-dry.

    So a compressor shutting down causes you float to close...Your suction pressure was low already or the machine would have stayed on: if it shut down on low load...SO

    Float valve closed. No liquid to MT vessel. MT vessel pressure drops further. Liquid in level column froths and Hansen probe sees it as quick rise in liquid level. With no flash gas to vessel compressor simply pull vessel down to lower pressure. Liquid in vessel starts to cool and that generates "submerged vapor" like what happens when you snap the cap off a bottle of warm beer.

    Pump pumps froth. No differential pressure. No flow through cooling line or bypass...Pump stops on Diff Pr Control.

    5-seconds later: Stability resumes and pump will start and hold....

    This can also happen when a condenser pump kicks in while the outdoors is cool...the thermosyphon vessel looses its continuity of liquid cause this new condenser sucks back on the liquid delivery and the float closes...And the whole thing follows through as described. I expect your intermediate vessel sits high and the inlet piping to the pumps is not quite clean; but can't tell that from here.

  11. #61
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by sterl View Post
    The TS-Pilot Receiver: otherwise HPR...most likely has an internal weir at +/- 50% and once it clears the weir, a float valve at the "high" vessel delivers liquid to the medium temp vessel: That about right?

    Any chance this little condition you got follow a compressor shutting itself off?

    When a compressor shuts down on a T-Sypon arrangement: The liquid in the pilot receiver (reservoir) drops quickly...That because on standby the liquid in the Thermosyphon Return Line rises all the way to the weir inside that vessel...Whereas while there was oil flow that vertical would be near-dry.

    So a compressor shutting down causes you float to close...Your suction pressure was low already or the machine would have stayed on: if it shut down on low load...SO

    Float valve closed. No liquid to MT vessel. MT vessel pressure drops further. Liquid in level column froths and Hansen probe sees it as quick rise in liquid level. With no flash gas to vessel compressor simply pull vessel down to lower pressure. Liquid in vessel starts to cool and that generates "submerged vapor" like what happens when you snap the cap off a bottle of warm beer.

    Pump pumps froth. No differential pressure. No flow through cooling line or bypass...Pump stops on Diff Pr Control.

    5-seconds later: Stability resumes and pump will start and hold....

    This can also happen when a condenser pump kicks in while the outdoors is cool...the thermosyphon vessel looses its continuity of liquid cause this new condenser sucks back on the liquid delivery and the float closes...And the whole thing follows through as described. I expect your intermediate vessel sits high and the inlet piping to the pumps is not quite clean; but can't tell that from here.
    As far as I remember nobody mention that this issue happen when compressor turn off. Even when it off, it takes time to cool oil in the oil cooler. So it will take time for liquid to fill up the return line.
    I believe in sudden change of the load. It can happen if several evaporators off or make up solenoid close.

  12. #62
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    Re: Liquid Pump Low Dif

    The pressure from the pump drops out almost at random. The compressors don't shut down and I have checked the discharge pressure and evap run times to see if there is a group shutting down all at once and so far nothing is coinciding. If you like I could provide another graph with some more details.

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by Krohnie84 View Post
    The pressure from the pump drops out almost at random. The compressors don't shut down and I have checked the discharge pressure and evap run times to see if there is a group shutting down all at once and so far nothing is coinciding. If you like I could provide another graph with some more details.
    What is your mid. temp. suction pressure now?

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    Re: Liquid Pump Low Dif

    Is there a spare pump next to this one you can turn on ?
    Has pump direction been checked ?

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    The vent line dosn't have to be open to much as its only for cooling & lubrication of bearings .
    Still surprised that it would cause cavitation though if opened to much .

    Liquid trunk , is that the main discharge line off liquid pumps back into wet return of vessel ?
    the vent return into the reciever may be causing the cavitation, depending on how it is piped inside the reciever. I have seen some really poor designs lately regarding that vent return causing a "whirlpool" action inside the vessel.

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    Re: Liquid Pump Low Dif

    There is alot of talk for a pump cavataion problem. By now, has your issue been resloved? I would like to ask what type of compressor oil or oils are you using?

  17. #67
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    Re: Liquid Pump Low Dif

    The problem still is occurring, at this time we have switched to a smaller compressor with economizers on the low compressors.

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    Re: Liquid Pump Low Dif

    1. I presume this is a low side recirulating vessel. If your having a cavitation problems with the sound of rocks in the pump, it is frozen oil or other frozen liquid solids. This will partily block flow through the pump and cavitate. When the pump is off for awhile, then frozen solids will melt and back to normal operation. These unwanted liquids or oils may be the pipe fitters using thread cutting oils and other residue coated pipe oils never being clean during original installation. I dealt with this problem for awhile. My final solution to get rid of it was to switch to another compressor oil. We had Frick #3 oil, this was a very bad oil choice for low temp -30F. After switching to a good low temp synthetic oil and flushing the system with oil changes that problem went completely away. We are using "Petro/Canada Reflo68". Based on the design of the vessel internal piping the pump should not cavitate. Liquid ammonia should fill the pump suction inlet and prime itself.
    2. Let me know if this looks like a solution to the problem, if not post another reply problem...

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    Re: Liquid Pump Low Dif

    The problem is on the Med temp pumps, the suction pressure is 36 psig. We do use Frick # 3 oil, but even our low temp only runs at 3psig.

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    Re: Liquid Pump Low Dif

    After reading posts: #3,5,7,9,10.

    1. From the graph in post #3 the pump shuts down (1st) from dif loss, the vessel fills (2nd), and suction drops alittle?
    2. Once the dif is satisfied, the pump restarts and they go into cavitation?
    3. What is the pump dif set point?
    Last edited by That's too cold; 11-11-2010 at 09:42 PM.

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by Krohnie84 View Post
    The problem is on the Med temp pumps, the suction pressure is 36 psig. We do use Frick # 3 oil, but even our low temp only runs at 3psig.
    As I mentioned you can increase pressure to 40 psig. You can switch some evaporators off. If you have 5 evap. run #1,3,5. 2,4 off. If concerns about air circulation, run fans for 2,4 on hand mode. Problem is that capacity of your evaporators significantly greater than refrigeration load.

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by That's too cold View Post
    After reading posts: #3,5,7,9,10.

    1. From the graph in post #3 the pump shuts down (1st) from dif loss, the vessel fills (2nd), and suction drops alittle?
    2. Once the dif is satisfied, the pump restarts and they go into cavitation?
    3. What is the pump dif set point?
    Your right on, I also check the liquid valves to see if anything turned off or on close to that time and I have yet to find anything coinciding.

    The pump seldom shuts down. It stays running and will only cut out if the computer times out at twenty minutes.

    The pump can't see a dif of twenty psi or lower.

  23. #73
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    Re: Liquid Pump Low Dif

    The fun of ammonia nuisance problems...

    1. The liquid line valves and evap coils will not have anything to do with this problem.
    2. Will the software allow you to input 5 to 8 psi dif? 20 psi is too high.
    3. Question: what starts the software "time out" at twenty minutes? When the dif switch is open/close? Upon new or restart-up of pump? Can the pump start/stop be controlled by suction pressure?

  24. #74
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by That's too cold View Post
    The fun of ammonia nuisance problems...

    1. The liquid line valves and evap coils will not have anything to do with this problem.
    2. Will the software allow you to input 5 to 8 psi dif? 20 psi is too high.
    3. Question: what starts the software "time out" at twenty minutes? When the dif switch is open/close? Upon new or restart-up of pump? Can the pump start/stop be controlled by suction pressure?
    Disagree with 20 psi to high as even with pump off the liquid head on dicharge of pump could be higher than 5-8 psi .
    As long as pump has some sort of delay before shuts down & several auto restarts before alarm it should look after itself unless something is really wrong like low level etc as well as the hundred other scenarios we have offered .

    Krohnie could also try restricting pump discharge for awhile & see what happens to keep pump in pump curve . If it works go from there .
    There seems to be alot of advise given & info asked for but not carried out .

  25. #75
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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    Disagree with 20 psi to high as even with pump off the liquid head on dicharge of pump could be higher than 5-8 psi
    The low dif of 5 to 8 will for sure be off the low side of pump curve and in cavitation.

    Yes, there will need to be "time delays" added or input changes into the software to avoid nuisance shut downs.

    We can only do one task at a time until it is fixed.

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by RANGER1 View Post
    As long as pump has some sort of delay before shuts down & several auto restarts before alarm it should look after itself unless something is really wrong like low level etc as well as the hundred other scenarios we have offered .

    Krohnie could also try restricting pump discharge for awhile & see what happens to keep pump in pump curve . If it works go from there .
    There seems to be alot of advise given & info asked for but not carried out .
    I have tried to adjust the pump dif alarm setting but the system will not let me change it, nor will it let me change the twenty minute delay on the alarm. I have been in the room and at the computer when this happens and the pump continues to run with the low dif until it runs under 20psi for a period of 20 minutes.
    I have yet to try restricting the discharge, I will start shutting it off in small increments starting Sunday (Beginning of my work week).
    Don't assume that the info I have been getting from all of you hasn't gone into this problem. I have been bringing these ideas up and we have gone through and either eliminated it as the possible problem or we have been slowly changing things and watching (over periods of days) if there is any effect. Truthfully the only idea we have yet to execute is bringing the suction pressure all the way up to 40psi.

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    Re: Liquid Pump Low Dif

    krohnie84 ,
    Can you also tell exactly whats on the pump nameplate & what the nett pump pressure is ie pump pressure minus suction pressure .

    What is measurement from centre line of suction of pump to working level in HT vessel ?

    Also do you have much oil in system , as in adding oil & draining oil ?
    Last edited by RANGER1; 13-11-2010 at 09:45 AM.

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    Re: Liquid Pump Low Dif

    I will get that info tomorrow when i go in.

    As for oil, we seldom have to add and it might go as long as month or two before the oil pod has to be drained.

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    Re: Liquid Pump Low Dif

    The pump ideal dif press is 30psi minimum??

    1. When the pump operates normal for a period of time and then falls to 20 psi for longer than twenty minutes, the shut down occurs.
    2. Then I would point my finger at oil slugging on the suction side of the pump. ???

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    Re: Liquid Pump Low Dif

    First I would like to apologize for the time between my posts. Lots of projects going on around here; including the installation of a ammonia air handler for the production area. J

    First off I was asked to get info on the pump/s that are having this issue. It is as follows:

    TEIKOKU
    Type: R42-21704DM
    Head 124ft
    Capa 90 gal per min

    And the pump runs at 80 psi with a 38psi suction pressure. So a diff of 42psi.

    Since the last time I talked with you guys we have changed the sequence of the Compressors and I have been noting some things.
    First, the sequence change was on the med temp side (where the issue is located). We turned off the 300 hp compressor and switched to the 125 hp. Each of our low temp compressors have economizers on them so there is no worries about load issues. Since it is so nice and cold here in Minnesota we heat our dock areas from Nov to Mar.

    Since I changed the compressors over at first we were still having issues with the dif pressure. I started to question the use of the economizers because they are always on when the low temp compressors are above 80%. I decided to test out just having the one 125hp comp run without any “aid” from the low side. What I found is that even though the suction pressure goes up for a longer period of time (When there is load on the Med Temp) the smaller compressor can make due just fine. Since the compressor is much smaller than the one I had running before it really isn’t slamming on to where it draws a ton of ammonia back to the vessel all at once. I had not had an alarm for two weeks (sweet!) until we switched low temp compressors and my co-worker forgot about the economizer and it is the lead compressor being at 100% all the time. The suction pressure was below 30psi until I found what he had done/forgotten to do.

    So right now I am not worried about the pump, but come summer when I am running the 300hp comp (due to load) I will be having the same issues again?

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    Re: Liquid Pump Low Dif

    Quote Originally Posted by Krohnie84 View Post
    First I would like to apologize for the time between my posts. Lots of projects going on around here; including the installation of a ammonia air handler for the production area. J

    First off I was asked to get info on the pump/s that are having this issue. It is as follows:

    TEIKOKU
    Type: R42-21704DM
    Head 124ft
    Capa 90 gal per min

    And the pump runs at 80 psi with a 38psi suction pressure. So a diff of 42psi.

    Since the last time I talked with you guys we have changed the sequence of the Compressors and I have been noting some things.
    First, the sequence change was on the med temp side (where the issue is located). We turned off the 300 hp compressor and switched to the 125 hp. Each of our low temp compressors have economizers on them so there is no worries about load issues. Since it is so nice and cold here in Minnesota we heat our dock areas from Nov to Mar.

    Since I changed the compressors over at first we were still having issues with the dif pressure. I started to question the use of the economizers because they are always on when the low temp compressors are above 80%. I decided to test out just having the one 125hp comp run without any “aid” from the low side. What I found is that even though the suction pressure goes up for a longer period of time (When there is load on the Med Temp) the smaller compressor can make due just fine. Since the compressor is much smaller than the one I had running before it really isn’t slamming on to where it draws a ton of ammonia back to the vessel all at once. I had not had an alarm for two weeks (sweet!) until we switched low temp compressors and my co-worker forgot about the economizer and it is the lead compressor being at 100% all the time. The suction pressure was below 30psi until I found what he had done/forgotten to do.

    So right now I am not worried about the pump, but come summer when I am running the 300hp comp (due to load) I will be having the same issues again?
    Your economizers should have back pressure regulators(BPR) to keep certain pressure in med. pressure receiver. Probably, they don't adjusted properly.
    Most likely, you don't have med. tem. refrigeration load at all, because it is cold outside. However, you have load of flash gas that comes from HPR. This load can fluctuate due to modulating of the HPR float.
    Assume that total capacity of all economizers is 30TR. This is constant capacity. Med. tem. load of flash gas will fluctuate from 0TR to 60TR. When load is above 30TR, mid. tem. suction pressure will gradually increases. This won't harm plant operation, because this flash gas load(not cooler load). When load is lower than 30TR, mid. tem. suction pressure will be pulled down and pulling will affect the pump operation and prime will be lost.
    I suggest you the following sequence of operation. Right now, run 125HP compressors(suction 38 psig)and adjust economizers operate in the range of 40-45 psig(open 45 psig, close 40 psig). they will help to this compressor to handle fluctuation load of flash gas. During spring time you can switch from 125HP compressor to 300HP compressor. During summer operation(significant mid. temp. load) you can lower set points for economizers and they will be able to run constantly and compressor(s) will load and unload to handle the load fluctuation.

  32. #82
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    Re: Liquid Pump Low Dif

    Well, it has been a while but I would like to give an update to the situation a lot of you have been helping me with.


    Last I had told you was that we were running a 125hp compressor with no economizers. There were very few issues, but the pump still would cut out once or twice a week. We found that our lead low temp compressor was not turning it's economizer off. Once we discovered it we manually turned it off and trouble shot the controls for that particular valve.

    After that it seemed like smooth sailing except for our med temp cooler couldn't maintain it's set point with us running 38 suction pressure (keep in mind that this "cooler" was once a dock and the units were only cooling about 1/3 the load they are now).

    Another issue was once we had a cold spell (below 20 degrees F outside temp) the dock wanted to heat. If you recall in a previous post I informed everyone that the dock units heat and defrost with 105psi of hot gas. Once any one unit goes into defrost or heat mode the 125 hp compressor can't really keep up. The suction pressure would rise to 43ish and it will take some time before the compressor can bring it back down.

    At this time we are now running our 300hp compressor again (seal was leaking a bit, decided to run it to get her to close up). With this compressor running the suction pressure no longer climbs like before but the issue is that this compressor loads up so fast that it causes the vsl to bounce around and then the pump kicks out. At this time I am working at the control set points to slow the compressor down but no luck as of yet.

    Any comments or ideas would be welcome.

    Thank you

  33. #83
    Join Date
    Oct 2010
    Location
    Minnesota
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    Re: Liquid Pump Low Dif


    There was two issues that were "plaguing" us.

    1. The computer controls were taking too long to load the compressor up and by the time it would the controls would load the compressor past what was necessary to recover the suction pressure. If you had looked at the graph the pressure might go three maybe four psi over the set point, the graph only show system pressure, the pressure at the compressor was much higher than the set point. At times I found it above by almost 10psi. The controls via the computer won't let me change the set points, but the compressor controls will. Right now we have the med temp compressor's slide valve set to auto. This makes it follow the compressors program. The company that programmed the computer is trying to bend us over to change the littlest thing in their programs so we make due.

    2. Our newest penthouse units, and their running a 105psi hot gas heat/defrost. I couldn't trend the times where the units would go into heat/defrost (Nothing tells me specifically when). Our other system (with much older controls and from another company) will let us set up the defrost times, this new system goes by hours of cooling. If it is set for defrost every 8 hours, it means that it needs to cool for 8 hours before. Not continues hours, just cooling time. Hell, it could take 8 days of only cooling for 1 hr a day before a defrost(exaggeration), but it doesn't seem the right way to go.
    Well, we started to force the units into defrost/heat modes and we were able to create the issue. What kind of sucks is that the system is designed to where the low temp units will dump their hot gas into the med temp suction lines. So, we have adjusted the dock units to a lower hot gas with out any issues. We cant' do that to the low temp coolers, they do cool for long periods of time and the defrost time ends up being too long if the gas pressure is too low.

    Another change is that we are running our 450hp compressor for the med temp. It is running at less amps then the 150 (Which might have blown out a gear, another story) and rarly gets above 20% capacity.

    Fun times, fun times.

    Any questions, love sharing crap like this.

  34. #84
    Join Date
    Sep 2008
    Location
    Canada
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    Re: Liquid Pump Low Dif

    You need to adjust hot gas defrost. Three parts of defrost process are important. Hot gas supply, ammonia condensation and ammonia condensate draining. If they balanced, you will have good defrost with minimum blow-by gas.

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