Pump suction low causes tripping

[Thanos93]

Hello all,

New to the forum and new to the field of enginnering.

So i'm experiencing a situation very confusing in a potable water pump. It seems that everytime we start the pump, you see the manometer on the suction side going below zero and coming to 0.5 bar after some minutes. The discharge stays on the 70 head pressure and the recirculation valve opens about 60 to 70 percent. If you open fully a 3/4 inches drain in the network you see that the suction going down and tripping the pump and the recirculation valve of course closes.
For the record the suction line and discharge line are both 2" inches and the optimal NPSH is 2.55m with 15m3/h.

My question is why the suction side has so much pressure dropping and making it tripping?

Thank you.

 

[Artisi]

Have you measured the flow thru'the pump when it is running.
Plus a lot more detail would help
Pump curve
Total head across the pump
Flow rate
Power input
Diagram of the installation showing levels, pipe lengths and fittings etc.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[mk3223]

IMO, it could be the issue that the pump has a suction line too small or not enough NPSHa to provide a proper flow as the pump in the start-up stage. Once started up, the pump flow circulates back to suction line and provides the proper NPHS as the pressure raises up.

May recheck the pump hydraulic, and consider to increase the pump suction line to increase the NPSHa to the pump.

 

[TugboatEng]

What kind of trip? Is the pump tripping on overload? Overcurrent? What is the trip mechanism? Is it magnetic? Bimetallic? Soldered link? The last case, despite being resettable, never quite performs the same after a trip.

The slow rise in suction pressure may have something to do with the inertia of the system. It takes a minute for all of the fluid to come up to speed. The trip when you open a drain indicates the pump is getting overloaded. Measure the motor current and compare it to the safe full amps of the motor and compare that to.tbe overload relay settings.

 

[LittleInch]

Thanos93,

you might be new, but please look at your post again and think, if someone else posted that question how can I work anything out from the information provided?

Provide the information requested by artisi and we might be able to understand what the hell is going on. At the moment no one has a clue.

E.g. where is this "drain valve"?

The fact your recirc valve is virtually fully open implies very little forward flow.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Mech5656]

I am going to focus on a specific part of your question. You said when you start the pump, the suction pressure goes below zero and goes up after few minutes. This means the flow goes into pump with low pressure, then the pressure increases and increases and eventually flow has enough pressure to overcome the pump resistance and pump runs. This may be a case of high friction at pump inlet or just now enough pressure at suction.

 

[Thanos93]

Hello and good morning all!

Sorry for not answering straight away, i didn't have access to the internet.
Well, to begin i'll answer as much as i can. First of all, the potable water network system is in a power plant. The drain that i'm talking about is whatever drain you open, for example even if you open an eyewash the pressure drops in the discharge and the suction sides. The pump trips of Low Low alarm suction pressure in 0.3bar. The suction pressure without any consumer open stays at 0.5 bar.

Furthermore you see the pump curve in the image below:

and the NPSH:

 

[Artisi]

First piece of the puzzle to hand, we now have an idea of what is going on.
What is the recirculation valve / line, when does it operate and why, where does it discharge?

Sketch of the system will help.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[razookm]

It looks like the pump is going to the end of the curve flow when any consumer is opened and additional flow is causing the pressure drop in the suction line. The same may be the case when pump is started with no pressure in the discharge header. Try operating the pump with throttled discharge valve and see whether there is any improvement.

 

[LittleInch]

We still know nothing about this system, how it's controlled, what pressures or flows there are at different times, and why the pressure trip is set so close to its static pressure.

For the pressure to go down then up on start indicates that there is a high flow followed by a lower flow.

Normally systems like this include an accumulator to allow flow to start and stop without creating a shock to the system.

Only by supplying a proper diagram and information can we figure anything out.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Thanos93]

Well, the recirculation valve (its a pneumatic valve) opens when we dont have any consumer open, so it recirculates back to the water tank adjusting accordingly the discharge pressure to be at 7 bar.
It adjust itself by closing completely when you have a consumer open, like an eyewash or a drain anywhere in the network system.

 

[LittleInch]

OK, I suspected as much.

It sounds a bit like your recirculation valve isn't closing fast enough, hence the flow through the pump when you suddenly open up a demand is too high and hence the pressures everywhere start to fall.

Hence why most of these systems incorporate an expansion tank to even out the flow and pressure to prevent this from happening.

Does your system have an expansion tank on the downstream section from the pump at 7 bar?
If so is it working properly - these usints often fail after a few years / loose air pressure etc

If not why not?
HOw fast does your re-circ valve close?
What is it driven by? - Pressure of forward flow d/s the recirc line?
Why is your trip set so high?
And so close to the operating point?

Is this a new system or has something changed over time or has it always been a PITA?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Thanos93]

We have an expansion tank but its only two years that its operating now so i guess it hasnt lose the air inside. Also the recic. valve takes measurement on the discharge side and not on the suction side. Regarding the closing of the recic. valve happens very fast so i believe that's not a problem. The high alarm trip is set to 8.5 bar so the pump always run. The normal operation is to stop when reaching the 7 bar after some minutes. There are two options im considering. Either the pipeline is small thus the pump doesn't have enough flow but i don't think so cause the company designing and calculating the pipeline for the pump must have made huge mistakes or the recic. valve have to be adjusted with new setpoints.

I'll try what @razookm said by throttling the discharge valve to see how the pump behave.

Thanks for your answers.

 

[LittleInch]

Expansion tanks can fail very quickly and / or air leaks out to the point where the expansion tank is mainly water and hence very little expansion takes place. It's the first place to go looking.

The second place is the pressure set point of the trip on the inlet side.

Why is is so high?
The pump will operate well down to about 0.5 bara, so why is it set at 1.3 bara when the static pressure (presumably from your tank) is only 1.5 bara?

Also check the inlet line for any partly closed valves, blocked filters or pipes.

Throttling the output is a very temporary measure and doesn't really solve your issue.

Also when your pump stops does the outlet pressure die away? or does the pump restart when the pressure reduces to a certain point.

Pre packaged potable water pumps are usually reliable, so not sure why this one is designed as it is.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.