Pump Pressure

[clivet]

Got a bit of a problem, I am commissioning a pump circ Ammonia system running at -40c, I am struggling to get any sensible pump pressure. We are using Witt pumps, if I dead end the system I get pressure but as soon as I open it up again everyting goes imediately to suction pressure, I have tried increasing surge drum pressure and level neither of which seem to have had any effect, there is also no oil in the pumps (checked that first!) anyone got any ideas????

[Craig Nicholson]

Clivet .

What type of Witt pumps are you using -GP51 ,52 etc.
What is your NPSH and what does your piping layout
to your pump look like .

Please send info and i will assist you with your problem.

Regards
Craig Nicholson

[clivet]

Craig
In short I dont know without checking what my pump model numbers are and what my NPSH is I wont be back on site until the morning. The piping to the pumps is via a common drop leg from the drum into separate supply legs, the discharge from the pumps goes via check valves into a common liquid line, there is a balance line from the liquid line to the top of the drum and that is currently shut.
Iv been reading another thread on this site that relates to problems arising from the pump sucyion leg being too close to the wet return line and suffering from pump cavitation as a result.
I will check things out in the morning and get back to you.

Clive

[US Iceman]

Clivet,

It sounds like the pump is going into cavitation when the discharge valve is opened.

Some questions for you:

1) how much pressure does the pump develop when the discharge valve is closed (dead headed)?

2) Are you running multiple pumps at the same time on the same vessel?

3) An easy one: Is the pump rotation correct?

[Julian]

Have you checked to see that your hand regs on the evaps are not to far open?
If you close all the hand regs down what is the pump pressure?
Hane you got a minium flow line installed?

[clivet]

I am running multiple pumps off the same vessel, and indeed of the same leg, when the pumos are deadheaded I get about 4 bar pressure just prior to the regulating valves at the evaps these valves are almost closed at present. As far as the DOR is concerned both pumps show an increase in discharge pressure before the pump check valve when run.
I think it is quite possible that the pumps are cavitating, I will try runnin with one turned off this morning and see what happens, hanks for your sugestions so far.

Clive

[ecclesk]

there is a balance line from the liquid line to the top of the drum and that is currently shut.

These should be opened, we normally have ours open about 3 or 3 1/2 turns.

3) An easy one: Is the pump rotation correct?

an easy one but check to be sure, also are the pumps brand new?, i have come across a gp 51 before which someone had overhauled and the impeler had been installed the wrong way round,

Have you checked to see that your hand regs on the evaps are not to far open?
If you close all the hand regs down what is the pump pressure?

Have come across this in the past, it's well worth playing about with the settings on these valves.

Have you checked the inlet filters in the pumps? on one of my systems there are no oil separators, oil is returned via oil rectification, these means there is a lot of oil floating around the hole system and did give us problems with cavitation, during comisioning we were constantly changing these, and in the end we used a stanly knife to remove the fine gauze from the filter this made a massive difference but i wouldnt do this until your sure the system is clean.

Also check liquid line strainers.

[US Iceman]

I get about 4 bar pressure just prior to the regulating valves at the evaps these valves are almost closed at present

OK. How much pressure is the pump supposed to generate?

The pressure at the regulating valves (which I assume are hand expansion valves) is the sum of the pump head (pressure that the pump can generate) plus the surge drum vapor pressure minus all of the pressure losses due to friction and elevation.

If you are getting 4 bar of pressure at the -40C evaporators the pumps are indeed pumping.

If the pump discharge pressure goes to the surge drum pressure when the pump discharge valve is opened, it sounds like the hand expansion valves are wide open.

The pump is trying to produce a lot of flow with very little head developed. This can substantially increase the NPSH required by the pump and cause cavitation.

Are multiple pumps required with simultaneous operation, or will one pump provide sufficient liquid flow?

[Tycho]

When you close the discharge valves, does it take a few seconds for the pressure to reach 4 bar?

Can you get a stable pump pressure if you defrost and shut of some of the consumers?


does the pump pressure oscilate?


all of the above indicate that you have to low liquid head on the suction side of the pump.


Unless you have a bypass valve open, or the pump is running the wrong way, I'd say it's indication of a to small ammonia charge.

[Josip]

Hello Clivet,

here are some notes from Witt GP operation&installation manual:

To be sure the pump will operate even at a low-pressure difference resulting in maximum capacity, the diameter of the downleg to the pump should be executed as mentioned in the table below

Required diameter of the downleg to the pump

GP 41 DN80(3”)
GP 42 DN80(3“)
GP 51a DN100(4“)
GP 51 DN 100(4“)
GP 52 DN150(6“)

Discharge pipe is not so critical but liquid velocity up tp 1,5 m/s is normally recomended.

A by-pass valve (adjustable) is required if the system design or the evaporator installation allow closed discharge running. Operating refrigerant pumps partially at or at a fully closed throttled condition is not allowed and will damage the refrigerant pump! The adjustable by-pass valve shall be sized DN 20.

The pump shall be connected using a vertical pipeline to the separator. To prevent interaction between pumps it is advised that each pump be connected individually to the separator.

Only when a stand-by pump is installed may two pumps be connected to a common downleg.

Capacity can be too low due to:

- direction of rotation wrong
- frequency of the pump too low
- stop valves behind the pumps closed
- too much gas in liquid
- liquid level in separator too low
- valves on cooler closed
- regulating valve not opened far enough
- contamination is blocking the piping system
- non return valve of an other pump leakes
- power supply not right, working on only 2 phases
- impellers worn out
- oil in pump at low temperature
- not sufficient suction head

- Intermediate pieces do not align hope your pumps are new and this is not a cause.


Try to run only one pump with discharge valve open 2-3 turns, when you obtain some stable pressure try to open discharge valve more until pressure start to drop. At that point stop and close discharge valve for 1/4 of turn and leave it.

You can repeat this with another pump and see if everything is the same (pressure, consumption..) maybe to switch on both pumps simuntaneously and check what happen with pressure....if pressure remain ok, if not, try to run only one pump and then you can start to play with RV on evaporator/s

Best regards,

Josip

[clivet]

Gentlemen
Thank you for your ideas and help, using your thoughts and think logically I decided that the cavitation route was the best option to follow, thus I turned off one of the pumps from"the common drop leg" and lo, pump pressure was forthcoming.
Thanks chaps
Clive

[Peter_1]

Clivet, where can we send the invoice?