How does a pump trade flow rate for head, i.e., pressure? 6

[Robert Clark]

A pump’s power is a product of its pressure rise times volume flow rate. Commonly, a pump is specified by giving the pressure rise, or head, it can provide and the flow rate, such as in gallons per minute(GPM).

But some pumps provide the user with a chart that shows how the pressure rise can be varied with a corresponding change in flow rate, inversely related.

This is what I need for my application. I need a higher head than what the specs say for the pump, allowing for the reduced flow rate. The specs don’t say whether or not these values can be varied. So is there some common method by which this is done for pumps with this capability?

I thought they just reduce the inlet size to change the flow rate, with an associated change in the size of the pressure rise. But then I thought this would just mean the pump would just suck harder on the water input source, making the flow rate stay the same.

So how do pumps with this variable capability do it, and can other pumps be adapted to also do it?

Robert Clark

 

[3DDave]

It depends on the pump, which you have neglected to give any information about. Higher pressures than specified usually means the pump will not do it or the pump will break. Which is more important in your application?

 

[Artisi]

centrifugal or positive displacement?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[TugboatEng]

Centrifugal pumps are easy. The impeller diameter and rotation speed impart a certain velocity to the fluid. Couple that with the fluid density and you have a kinetic energy as the fluid enters the volute or diffuser. With no flow, the fluid nearly comes to a stop and the kinetic energy all becomes pressure. As flow increases, the velocity in the volute does as well and there is not as much kinetic to potential conversion and the discharge pressure drops. Also subtract friction losses due to higher flow. With high enough flow cavitation starts which dramatically reduces the density of the fluid which leads to a rapid decline in pump performance.

 

[Robert Clark]

For my application, maximizing head is more important, even at the sacrifice of flow rate. I’m referring to pumps used in pressure washers, such as:

SIMPSON Cleaning MS60763-S MegaShot Gas Pressure Washer Powered by Kohler RH265, 3100 PSI at 2.4 GPM.

https://www.amazon.com/Simpson-Cleaning-MS60763S-Megashot-Pressure/dp/B00SH904YS

For my application, I want to get even higher pressure rise. The reduced flow rate is OK.

Robert Clark

 

[Artisi]

No idea on what you are trying to achieve - but the maximum discharge pressure is 3100 psi, there is probably a pressure relief valve in the system, maybe you can reset it to a higher pressure and suffer the consequences of a pressure failure.
If you require a higher pressure purchase a pump suitable for your requirements.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[3DDave]

Like this:

https://www.youtube.com/watch?v=lbyjRT_xkpc

 

[1503-44]

I will assume you have a centrifugal pump. Positive displacement pumps, recip cylinders, etc. can
achieve a head that is more limited by power and mechanical design strength. Centrifugal pumps proportion total energy available between kinetic and potential energy to achieve a given head for a given flow rate. Less energy spent lifting to any particular head means that there is more energy available to increase the flow rate. Low flows can be lifted to high heads, but as flow increases there is more resistance to flow, so less energy is available for the lift. That is generally true within the limits of volumetric size, friction and power limitations of the pumps design.

"I need a higher head than what the specs say for the pump"
As mentioned, the pump motor provides the total energy available, so a given pump is always limited to proportioning its total energy between flow and head such that the power used is always less than the power available. Basically Q x H must always be less than P, so you will only be able to increase head, if you can accept a lower flow rate. Within that basic limitation, some things can be done to change the curve and improve efficiency, but there are still those volumetric size and friction and power constraints, so changes to an existing pump are limited and must be done within those basic limits. Anything else requires an increase of motor power, so power is an important limit, if you want higher head at the same, or a higher flow. Otherwise you are always going to operate somewhere on, or inside, the existing curve.

Changing the pump curve can be done by operating at a different speed, or changing the impeller. Running faster increases both flow and head, but you soon run out of power. Increasing impeller diameter increases head, but you can only increase size a small percentage before the impeller does not fit inside the pump casing, plus you again soon run out of power available from the motor. Thus most changes you can easily accomplish are focused on improving efficiency at lower Q x H limits shown on the existing pump's curve, such as installing a VFD, turning down the speed to operate at both a reduced head and flow, but using less power to do it than if you reduced flow using just a flow control valve. Changing the impeller or speed changes the pump curve. It allows improvements in efficient power use for flows and heads on the "new curve". Restricting flow with a valve only moves your operating point between points on the existing pump curve. That often comes at reduced power, but lower efficiency.

 

[TugboatEng]

Positive displacement? Your problem is even simpler. The pressure relief valve/unloader sets the system pressure. If your pump has high flow, it may exceed the motor power rating at the relief/unloader setting. Reducing the displacement of the pump will allow you to achieve high pressure while remaining within your power constraints.

 

[1503-44]

Oh well.

Yes. Positive displacement. Super high pressures at small flow rates.

 

[LittleInch]

Robert,

This is an engineering forum and works best if you can describe and post engineering data.

So "some pumps provide the user with a chart that shows how the pressure rise can be varied with a corresponding change in flow rate, inversely related." really needs this chart so we can see what you can see. otherwise we're guessing.

"... I need for my application.". What is your application? - pressure, flow, temperature, pumped fluid would help us understand what you're trying to find.
"For my application, I want to get even higher pressure rise. The reduced flow rate is OK." - How much higher? How much less flow? Be specific.

Pressures above 3-5000 psig you're into small metering type injection pumps and water lances.

"So how do pumps with this variable capability do it, and can other pumps be adapted to also do it?" - Far too vague a question to get a good answer. See the posts above and answer the questions and we might get somewhere...



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

 

[Robert Clark]

I’m thinking of performance curves like this:

https://www.enggcyclopedia.com/wp-content/uploads/2011/09/pump-curve2.jpg

From the web page:

https://www.enggcyclopedia.com/2011/09/pump-performance-curves/

It is notable that the relation between head and volume flow rate isn’t exactly an inverse relationship. That is, cutting the flow rate by 1/2 doesn’t improve head by 2.

Robert Clark

 

[Artisi]

2 points,
These are centrifugal pump curves and you are trying to discuss positive displacement pumps.
The first curve can't even spell brake horsepower correctly.

Suggest you find a basic book on pump hydraulics.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1503-44]

The centrifugal pump head curve is nonlinear. The head curve represents how much energy is needed to lift an amount of fluid equal to the flow rate to the height indicated on the curve. Energy is Head.

The curve can be seen to be the result of Total Pump Energy minus the energy needed to move the fluid through the pump, which is the amount of energy that can be used for lifting "H". Total Energy - Friction Energy = Lifting Energy

The pump head curve in your link,

Has a total energy of 81ft
Lifting energy at the following flow rates are,
At 1000 gpm it can lift 79.5ft
At 9000 gpm it can lift 65ft
At 16000 gpm it can lift 20ft

The Friction energy curve
Plotting those points we can see the equation for how much energy is needed to ovecome friction and nove the fluid.
Plotting on XL and trending that Friction Energy Line, we see that it is a polynominal with degree 2. Friction energy is proportional to the flow rate squared, according to the equation of the trend line shown on the chart here. That's logical because we know that drag force is proportional to velocity squared. Friction = drag and flow rate gpm is proportional to velocity going through the pump.

So if we take total energy as a horizontal line H=81 and subtract the values we get from the friction equation trend equation, we get the pump head curve. The Pump Head curve = 81- the Friction Energy trend line equation.

While there are similar energy usage parallels with positive displacement pumps, their high pressures and limited operating range of flow rates makes their "curves" look like a nearly vertical and almost straight line.

 

[LittleInch]

Robert,

You're doing this all wrong.

What you need to do is define what differential head you want at whatever flow you want and then go find a pump that will do this at its duty point.

What you're trying to do is say - I've got a car which will do 70 MPH in 5th Gear quite efficiently. What I'm now looking at is what sort of slope will it go up in 1st gear.

Centrifugal pumps will generally follow the sort of curve you posted, noting that your first curve is rather brutal - most pumps have a fairly flat line with maybe a 10 to 15% drop from shut in head to BEP / duty point and then stop at about 60% of shut in head. Also centrifugal pumps don't like operating below about 30-40% of BEP on a continuous basis.

Your other posts though seem to talk about very high pressure (>5Kpsi) at very low flow rates. That is normally the remit of small piston type pumps which are very different to centrifugal pumps.

But first you need to define your duty, then you can find a suitable pump, not try to throttle down a pump which for centrifugal units will only give you 15 to 20% extra differential head and use about the same amount of energy as the efficiency falls dramatically.

And as my colleague mr 44 says, the reason the head flow curve isn't a straight line is that you have some squared terms in there for friction effects within the pump itself.

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

 

[Artisi]

Not forgetting poor flow approach to the impeller eye either side of the ideal, internal leakage, recirculation - just to highlight a few other points.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[hydtools]

The system load determines where a given pump will operate.

Ted

 

[Robert Clark]

As LittleInch suggested, perhaps I should say where this is coming from. Take a look at this video:

Drone Could Help Firefighters By Putting Out Fires.

https://www.youtube.com/watch?v=Bm2BVTTir4c

Now imagine this with much, MUCH longer hoses, and going both vertically and horizontally. I think you can see where I'm going with this.

For such an application, you need both high head and high flow rate. Such pumps do exist, such as the 14-P-220 mud pump. It is capable of 7,500 psi at 375 gpm:

https://www.nov.com/Segments/Rig_Te...plex_Mud_Pumps/14-P-220_Triplex_Mud_Pump.aspx

And such pumps do allow variation in head and flow rate:

These pumps cost tens of millions of dollars. You're not just going to get them to be used for such an application.

Then my goal is to get proof-of-concept examples to be done first, even if done with the low flow rates possible with pressure washers and the demonstrations done by amateurs would be sufficient.

For such low cost initial demos conducted by amateurs, rather than buying expensive pumps with the required very high head, it would be better to adapt the low cost pumps available to get the high head needed.

Robert Clark

 

[1503-44]

Better fitted with spray paint cans. I can't see it putting out enough volume to actually extinguish anything more than a fat Cuban cigar. Even a real helicopter needs a few trips to fight a real fire. If you ditch the thrusters and turn up the water jet velocity, you might even get enough lift to fly it and put out the fire at the same time.

On the other hand, Banksy could do murals 984ft tall

 

[TugboatEng]

High pressures are like Viagra for hoses. The reaction forces placed on the drone would be immense, both from the nozzle and the hose trying to straighten itself.