Pump Priming Instrumentation

[Andrew ONeill]

Hi Team,
I went down a rabbit hole a little while ago about using instrumentation to prove a centrifugal pump is primed. Intent is to have interlocks or alarms to prevent dry running of said pump after a maintenance changeout and / or suction piping lineup changes. I'll note the question for this topic is about instrumentation, not about changing seal / pump types, piping layout etc.

The specific scenario is shown in the attacked sketch.

I won't go into the issues of why this is being looked at, other than to say it involves competence / knowledge of the people operating the equipment.

1) Traditional (somewhat dodgy) way to prove priming is to use a suction pressure transmitter. If P >0, must have liquid, therefore primed. In practice not true but, does work reasonably well if the reservoir is above the pump, which in this case it isnt.
2) Monitor flow meter, if no flow for XX seconds, trip pump -> This works, but typically results in pump damage bring done during the dry run period. Also the nature the pump system is that it pumps uphill, so the pump will dead head on startup until the speed is sufficient to overcome the existing system discharge head. (so a "no flow trip" needs some amount of time delay to avoid spurious tripping, typically around 30 seconds)
3) A flow switch, same as 2) above

4) led me down that last pathway of a level switch / level transmitter. Install a level switch / transmitter in a suitable location, if level detected, then pump is primed. for me this seems to be to only reliable way to detect pump prime prior to starting the pump.

5) Another idea was to monitor VSD power, if no fluid in the pump, power draw should be very low, might be able to setup a fairly simple equation and trip pump if power is very low vs pump speed. again this if after startup, but conceivable could have a short delay timer of say 5 seconds vs a typical delay of 30 seconds for the flow meter example, which would likely be a tolerable dry run time.

I'm curious if anyone has experience in this sort of arrangement, has any comments, or other ideas along this line.

 

[pierreick]

I think what you describe (& 4) is called Liquiphant (Endress &Hauser) or fork to stop or to forbid the start of an equipment in case of no liquid.

Liquiphant | Endress+Hauser

Note : A foot valve + priming pot will help to pump from a pond, the fork being there to prevent the pump to start (no liquid)

Nothing new here
my 2 cents
Pierre

 

[georgeverghese]

Presume that inverted U on pump discharge is for priming. If so, wouldnt it be better to mount an LT ( dp cell ) on this U leg to feed into pump start permissive logic ? When level is above a given point, logic permits pump start.

 

[LittleInch]

You have a few ways of doing this and I note that's a pretty reasonable sized pump (220kW)

The best way to ensure you have flooded suction is to instigate a pre start routine within the pump starter which energises a small vacuum pump inside your upturned tee piece and then only give a permissive to start when the level gets to the top of the up stand pipe. That way is virtually foolproof until the operators learn how to give a false reading from the level switch....

Next best is to incorporate into the stop routine that you close the outlet valve first, let the pump dead head for a few seconds whilst starting to close the inlet valve which should be as far away from the pump as possible. Hence two actuated valves.

Install a "foot valve" aka non return valve in the end of the inlet pipe.

I don't think your solution looks workable as there is no way to fill the chamber with water and hence your LT / LS would need to be lower than the top of the inside of the pipe

Make the inlet pipe fleixble or jointed, fit a foot valve and then when the pump stops raise the end of the pipe to be level with the pump centreline.

 

[goutam_freelance]

Intent is to have interlocks or alarms to prevent dry running of said pump

The foolproof way will be to have a priming tank with monitoring of liquid level (presence) in the tank. A vacuum device can also be used as suggested by @LittleInch.

 

[Andrew ONeill]

Hey everyone, Thanks for the replies.

 

[Artisi]

A better approach would be to look at a positive prime system, this will eliminate the worry of not being primed and running dry, although a simple not primed alarm can be a fitted - just in case.