sucking water a lonnnnnngggggg way! doable?

[Waramanga]

Hi all,

I know I need to maintain the min NPSH at the back of a pump (HHW in this case, 80deg C). Could I just pump the system pressure up to a value that will allow me to keep the suction above the min NPSH of the pump? Has anyone seen this done before? I am verifiying a design and the designer assures me that there will be no issues, I agree in theory but I dont like the idea of having unneccesarily high system pressure just to keep the pumps out of a crowded plantroom (thats the reason they are so far away). The set-up is a decoupled primary secondary system with secondary pumps in remote plantrooms (the bypass is in the main boiler plantroom). We have some runs to the back of the pump close to 80kpa dp, others are much lower (< 10kpa). These secondary circuits are going to be interacting like crazy in my opinion.

What do people think?

thank you

 

[willard3]

Is this an open or a closed system? I wonder why you are worrying about NPSH on a heating system.

 

[TrippL]

One method for preventing cavitation is to provide one or more booster pumps in front of each high NPSHR pump. Booster pumps should be single-stage, double suction pumps that run at low speed.

Can also, increase the pipe size or reduce flow-rate / pump speed.

Not sure how the secondary circuits are piped in the second part of your question. Maybe provide a rough sketch.

 

[317069]

http://www.eng-tips.com/viewthread.cfm?qid=199822

also pump munufucturer is a good reference

 

[walkes]

It seems to be a closed loop with the secondary loop pumps in question. If your primary pumps are okay, then the suction side of the secondary pumps will be the system pressure (likely inlet pressure at primary/expansion tank) plus differential from the primary less head loss in between. That should be fine.

 

[chicopee]

If you can get pump performance curves which should include NPSH curves from manufacturers, you should be able to make a decision.

 

[Waramanga]

I think my description needs a bit of clarification. The set-up is a fully de-coupled primary secondary system, that is, the pump for the boilers just moves the water around a plantroom. The remote secondary pumps 'pull' the water from the plantroom loop out to the field plantrooms, sometimes with 80kPa dP in the suction pipe. Atmpspheric less 80kPa is 20kpa absolute if there fill pressure is atmospheric, not realistic I know!) so most pumps should handle this arrangement.

BTW, what is a typical fill pressure for a system, assuming the system is all at the same level??

thanks guys

 

[carlosgw]

The fill pressure should be above the pressure needed to fill the system to the top. Without trying to follow the above. The pressure at various points depends on the location of the expansion tanks. It can be figured out from there.

 

[walkes]

Typical fill pressure is the difference in feet from the fill point to the highest point in the system converted to psi, then add 5 psi.

 

[Waramanga]

height of the system plus 5 psi. Ok so if I have to pull water from the fill point through pipe with (for arguments sake) 150kpa pressure drop then the pressure in the tank will need to be at least 150kpa absolute. Has anyone set-up a system like this?

 

[walkes]

No, the static fill pressure and friction loss in your piping are separate considerations in this closed loop. See the attached schematic from Bell & Gossett's air control design booklet. It illustrates the system pressure when the pump is running and when it is not.

When the primary pump is not running the system will be at the fill pressure. (The suction side to the secondary pump will be at the same fill pressure). If your fill pressure is sufficient for the secondary pumps NPSHR then you should have no problem. When the primary pump is running the suction pressure to the secondary will be higher.