NPSH and temperature

[MaRrKeRZ]

Can a self-priming pump in a sump application (ie 6ft of suction lift and 20ft of discharge head) handle hot water? I know that as pressure decreases, boiling point of water drops as well. What do I need to know to figure out at what temperature my pump will boil the water it is lifting (and lose its prime)?

 

[LittleInch]

Search vapour pressure water v temperature. When it equals the pressure you have then that's the temperature. NPSH is not the same thing.

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

 

[MaRrKeRZ]

I have a graph of vapor pressure of water and temps. I know that the water going into this pump won't exceed 180F during normal operation. Water will boil at 180F at about 9PSIA (or so my graph says). Mfg cannot tell me what pressure my pump inlet will be at during operation with a 6ft suction lift. How can I calculate and see if the pump will lift the water or boil it?

My concern is that as temp goes up and vapor pressure increases, available NPSH may be less than required NPSH

 

[LittleInch]

Convert everything into feet head in absolute pressure.

Head of water atmospheric pressure is about 33ft. Subtract the vapour pressure at 180 about 16ft. Subtract your lift (pump cl minus water level) 6 ft.Subtract inlet pipe losses say 2 ft What's left is NPSHA - about 8 ft. Bit low and pump might be cavitating.

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

 

[georgeverghese]

Typically, the NPSH required value for a pump quoted is based on cold water at ambient temp. There are 2 possible ways to extrapolate to operating temp for NPSHr:

a) Get the pump vendor to run a witnessed NPSH test at the operating temp range.

b) Use the graph in fig 10-25 in Perry Chem Engg Handbook - 7th edition, which is abstracted from the Hydraulics Inst Standard.

 

[MaRrKeRZ]

@George I will check the graph. May have to run my own test as I am the vendor and buy directly from mfg.

 

[LittleInch]

What's your quoted NPSH?

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

 

[TenPenny]

"Mfg cannot tell me what pressure my pump inlet will be at during operation with a 6ft suction lift."

Well, that seems a bit strange. While it's unlikely they will give you an exact figure for the pressure at the eye of the impeller, they do tell you NPSHr by the pump, right?

If you can't follow LittleInch's post and draw conclusions from that, you have no business selling a pump to anyone.

 

[MaRrKeRZ]

Mfg gives npsh requirements on the pump curve, yes. If the water was at 1.0SG and 70°F, I would be fine. Well within required NPSH. When water temp is 180°F though...

I was able to convert everything into absolute pressure and figure out what my static inlet pressure would be, then cross reference vapor pressure of water chart to find my boiling point at that temp. Seems im ok and will not boil water on my suction side until about 198°F. Still not sure if that will change when the pump is running (especially when starting a cycle).

@tenpenny keep your opinions to yourself. If you don't have anything productive to add to a discussion, don't post. The whole purpose of a forum is to share information and ask questions. I want to know if my installation will work before bidding a job. So I ask to educate myself. Nothing wrong with that.

 

[LittleInch]

Mark,

The boiling point issue is a red herring. The pump will start to cavitiate badly at a temperature / vapour pressure below the actual boiling point.

The NPSHR figure from the pump vendor is your first point of call. You need to have enough NPSH to be at or above that number. A key part of the calculation for you is the vapour pressure of the water at your pumping temperature (180F)

However that isn't the end of the story. NPSH is based on a head loss of 3% from normal operation. Cavitation can often start at a level higher than NPSH, so the normal rule is to allow at least 1m (3ft) above the NPSHR to allow for this.

Please advise what the pump NPSHR figure is.

I think your system is right on the limit and possibly below for many pumps.

Please don't respond to posts you don't like in that fashion. It's frowned on in these forums and your post might end up being red flagged and deleted.

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