Pump discharge pressure vs performance curve

[PBtoAU]

In the past, I've always converted pump discharge head to pressure using the outlet temperature/specific gravity. But with centrifugal compressors, I've used inlet temperature to convert head to pressure. Now I have a case where's there's significant temperature rise across a pump under low-flow conditions. Setting aside the issue of whether this pump is poorly suited to my application, is it proper to use the inlet temp to convert head to pressure, or the estimated outlet temp? Shouldn't the pump calcs be handled the same way as the compressor calcs?

 

[QualityTime]

Hi:

I mainly deal with centrifugal pumps. Pumps pump water (or liquid)and compressors compress gas...two different things...

Pressure gauges are used to measure pump suction and discharge pressures and a flow meter is used to measure pumped flow. From there the numbers are plotted against them manufacturers pump performance curve.

 

[TenPenny]

That's a really interesting question.

My instinct would be that you want to use the discharge temperature, because the energy/velocity at the discharge is what is converted to pressure, so it would be the sg at the discharge temperature that would give you the correct conversion.

However, as I said, that's my instinct, and I've never thought about this somewhat obscure question before.

 

[PBtoAU]

Thanks for the input everyone. Interestingly, the process simulator that I typically work with (Aspen Hysys) uses inlet SG to convert pump TDH into pressure. It might be instructive to simulate this in Pro2 or Aspen+ to see if they agree.

 

[BigInch]

You must use the temperature of the fluid at the point of interest.

If you are interested at points inside the pump, you must first calculate the temperature profile of the fluid along its path, then the SG.

It makes no sense to me to calculate the discharge head based on the inlet temperature, unless you do not have enough temperature change between suction and discharge to make a difference in the SG and the resulting head calc.

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[ione]

Head is not affected by the fluid specific gravity, whilst pressure is, and this is why it is advisable to use head instead of pressure. Specific gravity is intrinsically a “relative” value. What I mean is that SG is the ratio of the density of a substance to the density of a referenced substance.
In your case you’re converting head to pressure and you are on the discharge side of the pump, so I would use the fluid SG considering discharge side conditions (i.e. temperature).

By the way could you tell us what fluid are you talking about and what is the temperature rise detected? Water SG decreases of approx 4 % when its temperature passes from 4 °C to 99 °C

 

[PBtoAU]

The fluid is a light hydrocarbon mixture, with a pump delta-T of about 50F degrees. Though the SG decrease is small in percentage terms, the pump TDH is on the order of 2000 ft. Therefore the absolute pressure difference is significant for our application.

 

[ione]

PBtoAU,

Still stay with my previous post, I would use SG at discharge to convert head to pressure.
I’d like to add that the use of the SG at suction is advisable to calculate the brake horsepower.

 

[PBtoAU]

I asked AspenTech why they use inlet SG to calculate discharge pressure in Hysys. Here's what they said:

"The ideal head, h, can easily be defined as a function of the isentropic or polytropic work. The relationship is:

(1) W = MW*F*g*h

where:
W = ideal pump power
MW = molecular weight of the gas
F = molar flow rate of the inlet stream
g = gravity acceleration

The ideal power required, W, to increase the pressure of an incompressible fluid is:

(2) W = (P2-P1)*F*MW/ñ

where:
P1 = pressure of the inlet stream
P2 = pressure of the exit stream
ñ= density of the inlet stream
F = molar flow rate of the stream
MW = molecular weight of the fluid

Substituting (2) in (1) gives us a relationship between head and discharge pressure:

(3) h = (P2-P1)/(ñ*g)

This is why the inlet data are used to calculate the discharge pressure."

 

[BigInch]

Fine, except you used the word "ideal" 2 times.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[BigInch]

Not to mention using "incompressible" once too many.

"We have a leadership style that is too directive and doesn't listen sufficiently well. The top of the organisation doesn't listen sufficiently to what the bottom is saying." Tony Hayward CEO BP
"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.liv

 

[ione]

PBtoAU,

As already noticed by BigInch, you’ve (they’ve) used the term “ideal”.
Part of the ideal power (work) performed is lost in form of heat. Part of heat is dissipated through the pump casing and part is used to heat up the fluid as it passes throughout the pump. This is why in my previous post I have indicated as advisable to use at SG at suction condition in order to calculate the brake horsepower. In fact using SG at discharge will lead to an underrated value.