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New Engineer - Pump Pressure/ Flow Basic Question

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Fooooks

Mechanical
Sep 15, 2016
48
Hi all, I just joined and am a new hire at a bulk liquid storage facility. No previous experience outside of college.

I have a basic question that I'm trying to understand physically. I cannot grasp what is happening to a fluid throughout a pipe and how it relates to the discharge pressure of a pump. Does it mean that if the pump requires more pressure (such as going uphill, discharging to atmosphere), the flow rate will be less? Does this apply to Bernoulli's Equation, which says a higher pressure decreases flow? But then if I run the pump at a higher pressure, my mind thinks the flow will increase, as there is a larger difference between discharge pressure of the pump and the atmosphere.

For example, if there is a pump with a discharge pressure of 100 psi, the fluid has a certain flow rate through 100' of pipe, assuming releasing to atmosphere. However, if I add a valve that has a high pressure drop into the pipeline, doesn't that mean the pressure drops? But this pressure decrease does not result in a higher flow, instead you would need a higher pressure from the pump to maintain the original flow rate. But a higher pressure is supposed to mean a slower flow rate if I look at a pump curve, I thought.

I must be missing something for this example. But I am having trouble understanding how losses and pressure drops downstream affect the pump-driven flow rate. I hope my thoughts are coming out appropriately.
 
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Look at the pump curve.
For a standard centrifugal pump.
As flow increases, typically moving to the right along the curve the pressure or head decreases.
Conversely if head increases the flow decreases. Generally the flow/head relationship is a curve so the relationship is not a straight line but in the middle point of the curve its often pretty close.

So if a pump is pumping to atmosphere and you suddenly close the discharge valve somewhat , the pressure applied to the pump increases and the flow drops. Because the flow has dropped the pipeline loss diminishes a little(less friction loss) but this is outweighed by the loss across the valve. In other words the total head on the pump has increased and the flow drops.

A positive displacement pump is different in that up to its limits the flow drops only marginally with increasing head, however the input power required , if flow is to be maintained increases proportionally.

Regards
Ashtree
"Any water can be made potable if you filter it through enough money"
 
First you must keep in mind that pressure is the result of flow of a certain liquid through a certain pipe (material, diameter, length) and not vice versa. Simply speaking (and physically not correct) it is friction which leads to a certain pressure. The attached tutorial is a lot to read but very suitable to understand the basics.

"Does it mean that if the pump requires more pressure (such as going uphill, discharging to atmosphere), the flow rate will be less?" Yes, that is correct for centrifugal pumps but not for most positive displacement pumps.
"But then if I run the pump at a higher pressure, my mind thinks the flow will increase, as there is a larger difference between discharge pressure of the pump and the atmosphere." That will happen with a positive displacement pump if you speed it up. But again: You will then inrease the flow and the higher pressure is only the result.

"For example, if there is a pump with a discharge pressure of 100 psi, the fluid has a certain flow rate through 100' of pipe, assuming releasing to atmosphere. However, if I add a valve that has a high pressure drop into the pipeline, doesn't that mean the pressure drops?" The pressure drops across the valve but not at the pump's outlet.

But this pressure decrease does not result in a higher flow, instead you would need a higher pressure from the pump to maintain the original flow rate. But a higher pressure is supposed to mean a slower flow rate if I look at a pump curve, I thought.
 
 http://files.engineering.com/getfile.aspx?folder=0aede5b4-c0f2-4f56-a8b6-8acf77d17de7&file=tutorial.pdf
Fooooks,

The answer from micalbrch and the excellent looking tutorial will probably help you a lot, but my go at your questions are below.

One thing you need to get a hold of is that any pumped system is a system - the key is matching the pump to your particular piping system with all its valves, static heights, pipe sizes, branches etc. You need to think of a centrifugal pump as essentially a constant pressure unit with variable flow ( Ok the pressure changes from one end of the curve to the other but normally by no more than 20%) and a PD pump as essentially a constant flow unit with variable pressure ( again not exactly true, but flow doesn't vary by more than 20%)

pump_serie_ezmtud.png


Does it mean that if the pump requires more pressure (such as going uphill, discharging to atmosphere), the flow rate will be less? If nothing else changes ( i.e. you don't change the pump or anything else and you're using a centrifugal pump then yes - the flow rate will reduce. Increase the pressure required too much and you might get zero flow.

Does this apply to Bernoulli's Equation, which says a higher pressure decreases flow? Forget Bernoulli's equation when it comes to pressure drop and pumping systems. This is more for orifices and other temporary flow restrictions, not piping friction.

But then if I run the pump at a higher pressure, my mind thinks the flow will increase, as there is a larger difference between discharge pressure of the pump and the atmosphere. Correct on the basis that nothing else has changed.

For example, if there is a pump with a discharge pressure of 100 psi, the fluid has a certain flow rate through 100' of pipe, assuming releasing to atmosphere. However, if I add a valve that has a high pressure drop into the pipeline, doesn't that mean the pressure drops? Yes, but only on the downstream side of the valve.

But this pressure decrease does not result in a higher flow, instead you would need a higher pressure from the pump to maintain the original flow rate. Correct for the overall system, but within the valve itself there will be a small section where the flow velocity gets much higher.

But a higher pressure is supposed to mean a slower flow rate if I look at a pump curve, I thought. Correct. What has happened to your system is that the system curve has changed and moved to the left as you've introduced more friction or head loss into your system so the intersection point between pump and system curve has moved to the left as well.

I must be missing something for this example. But I am having trouble understanding how losses and pressure drops downstream affect the pump-driven flow rate. I hope my thoughts are coming out appropriately. As said, understand that anything you change on the downstream system will change your system curve. The effect will be to change the flow rate as it will change where the two curves intersect.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
 
Thank you Ashtree, LittleInch, and Micalbrch. The answers have helped me immensely, I did not consider the system curve moving and was fixed on the idea of Bernoulli with the relationship between velocity and pressure. I am going to read the tutorial that was posted. It was really helpful that you all answered my questions I specifically asked, so I was able to follow along easily. I hope it was not too much trouble. Thanks, again.
 
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