Pump pressure w/ changing S.G., constant mass flow 1

[jwazzu]

It appears to be a simple matter to calculate pump discharge pressure with changing specific gravity using the formula press(psi) = Head(ft)*S.G./2.31, but this if for constant volumetric flow rate, correct? How would one calculate discharge pressure with a constant mass flow rate with changing specific gravity? The range of S.G. I am looking at is about 0.75 to 0.84, and this is for fuel.
Thanks

 

[jwazzu]

I forgot to mention that these are centrifugal, AC-driven pumps.

 

[bulkhandling]

There is no impact on your pump performance. The pump will deliver same head (ft) fi the fluids have the same viscosity. The only difference is the motor horse power - the motor shall be size based on the heavier fluid.

Some people have delusion on pumping different fluid S.G. thinking that when pumping lighter fluid, pump will dicharge more feet of head.

For the same piping, same fluid viscosity, static head will be the same and friction will be the same. Since pump will have the same speed, for heavier liquid, more momentom therefore higher presssure will be generated. As the head measured in feet for both heavier and lighter fluid, The pump will deliver exactly the same rate.

 

[zdas04]

"The same ... fluid viscosity ... static head .... friction will be the same". I have rarely seen two fluids with significantly different specific gravity having the same viscosity.

It is a pretty big leap to say that static head and friction will be unchanged for a 12% change in specific gravity. I have to think that the change in SG will also have an impact on the other flow parameters. I've found the flow rate through centrifigul pumps to be pretty dependent on fluid characteristics.



David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

 

[jwazzu]

bulkhandling, thank you very much for the reply. Let me clarify the situation a bit.

This is for a laboratory, test situation. Let me give an example:

A pump is required to have a mass flow rate no higher than 10 psi at 10,000 pounds per hour with a fluid specific gravity of 0.75. The specific gravity of the test fluid may vary, let's say up to a specifc gravity of 0.8. What pressure would that same pump produce at the same mass flow rate of 10,000 pounds per hour but with a fluid specific gravity of 0.8? The mass flow rate is the same, so I believe the momentum would be the same. The volumetric flow rate would be decreased, so the head would not be the same. The piping on the discharge side of the pump would not be the same, as the volumetric flow would have to be throttled back to maintain the 10,000 pounds per hour flow rate. What I am trying to determine here is what pressure at a specific gravity of 0.8 would correspond to the 10 psi pressure with a lower specific gravity?

A proposal was to calculate the change in volumetric flow rate due to the change in specific gravity, and using a relationship of head vs. volumetric flow rate, calculate a new head for that new flow rate, and then calculate a new pressure using psi = head*SG/2.31. This method seems a bit brute force to me, and I'm not sure if it is correct.

zdas04, thank you for your input. I think for this situation I'll have to assume that the viscosity will remain basically the same.

 

[Artisi]

The discharge head in vertical height from a centrifugal pump remains unchanged irrespective of the S.G. but the pressure at the discharge will change in relationship to the S.G.

But back to your problem -
It appears that the controlling factor is the flow rate in mass not volume - is this corect??

"A proposal was to calculate the change in volumetric flow rate due to the change in specific gravity, and using a relationship of head vs. volumetric flow rate, calculate a new head for that new flow rate, and then calculate a new pressure using psi = head*SG/2.31. This method seems a bit brute force to me, and I'm not sure if it is correct."

Cannot see why this is 'brute force" as it is the only way to do it, you need to correct everything to flowrate as this is what a pump does, it delivers "flow" against what ever the head is that's imposed on it.

Naresuan University
Phitsanulok
Thailand

 

[katmar]

David's comments are correct (as usual) but he was a bit gentle in calling bulkhandling's opinion that the static head and the friction would be the same "a pretty big leap". Bulkhandling has obviously made many useful posts here before, but in this case he has unfortunately got it wrong.

If there is a change in elevation between the pipe suction and the final point of discharge the static head will change. It is directly proportional to the height change :-
Static head = density x gravity x height
So if the SG increases the static head will also increase (gravity and height being unchanged).

On the other hand, as the SG increases the friction head loss will decrease for a fixed MASS flow through the same pipe layout. This is because the friction head is proportional to the product of the friction factor and the (velocity squared). For fully turbulent flow the friction factor is not dependent on the Reynold's Number (and therefore not dependent on the SG or the viscosity or the velocity) and for practical purposes we can assume a constant friction factor. But the velocity decreases proportionally to the increase in SG, and the friction head loss is proportional to the velocity squared, so the friction head decreases quite strongly as the SG increases.

You did not state whether there was a change in elevation so I will ignore the static head and conclude that as the SG increases the pressure drop required to drive the 10,000 lb/h through the line decreases.

Now let's have a look at the pump. If the SG increases you will pump less volume for the same mass. This moves you to the left on the curve, which means the head in feet of pumped liquid increases (depending to the the shape of the pump curve). If the head in feet and the SG increase the delivery pressure in PSI increases by the product of these two increases.

The net effect of increasing the SG is that the pump delivers more pressure but the pipeline requires less pressure. To get them to match you will have to throttle down on a control valve somewhere to get them to match. Alternatively you could use a variable speed drive on the pump and slow it down a bit.

I agree with Artisi in that I would not call this a "brute force" method. This is what engineering is all about and I regard these calculations as somewhat elegant. But maybe we engineers are all just brutes and what appears elegant to us would appear brutish to someone else!

regards
Katmar

 

[bulkhandling]

I was habitually (and wrongly) thinking same flow in volume.

If for same volumetic flow, the principle in my last posting is still true.

To make things simple, you just need to convert your mass flow for both SGs to volume flow and do calc for head in feet. Forget SG for now, you'll get two operating points.

Again, for motor sizing and HPSH check, both SGs shall be considered.

 

[Artisi]

Katmar.

1. ""So if the SG increases the static head will also increase (gravity and height being unchanged).""
SG does not alter the discharge head - head remains unchanged irrespective of the SG - what does change is discharge pressure as read on a gauge at the discharge.

2. Is a steady 'Flow" - mass required throughout the pumping cycle or is the aim to deliver 100,000 in 1 hour of pumping.

Naresuan University
Phitsanulok
Thailand

 

[katmar]

Artisi,

I think we are talking at cross purposes because of confusing terminology. I must confess that when I re-read what I had written I realized that I was a bit loose in my terminology.

When I said that the static head would increase I was not talking of the discharge head of the pump. I was referring to the possibility that the piping system ran upwards. You are correct in that if the static head due to the piping going upwards is measured in metres of the pumped fluid then it does remain the same if the SG changes.

What I meant, but did not say very clearly, was that if the pipe runs upwards and the SG increases then more pressure (in PSI, kPa, mmHg etc) would be required to overcome this static head. I was simply trying to separate the pressure loss behavior due to piping elevation differences from the pressure loss due to friction.

Thanks for pointing out the confusion

Katmar

 

[UmeshMathur]

jwazzu:

The manufacturer's pump head v/s flow curves are typically for suction VOLUMETRIC flow v/s DIFFERENTIAL head. So, as Katmar has noted, the suction volumetric flow will be lower at a given mass flow when the specific gravity increases.

However, some pump curves show a local maximum in the head v/s flow relationship, so it is not necessarily true that differential head will increase as suction volumetric flow decreases. (I assume that you have a discharge mass flow rate controller that is throttling the pump discharge valve to maintain a mass flow rate equal to the set-point and that the tuning of the flow controller is good).

 

[jwazzu]

Thank you all for your comments. I appreciate the time you've taken. I will try to post what the resolution is as soon as I have time.
Thanks again.