How to measure 'head' on a submersible pump

[CLB]

I thought I Understood this until I looked into it further, I am now more confused than ever.

If a pump manufacturer provides a performance table for a submersible pump, is the 'head' measured from the pump, or from the water surface. ie, would it be beneficial to raise the pump from the bottom and closer to the surface for improved performance. I initially thought it would make no difference as the pump is only lifting water from the surface, but now I'm not so sure.

Also would the same apply if the pump was mounted outside of the tank but level with the bottom of the tank so that water in the outlet pipe was level with the surface when the pump is switched off.

Cheers

 

[lw395]

All that matters is hydrostatic pressure, and pressure loss due to flow through the pipes.
The lift is measured from the surface, but the 'drag' from lengths of pipe matters too.

 

[thinwater]

No, the head is the differential the pump can develop. More inlet pressure = more outlet pressure.

Head = velocity head + pressure head. You can have either maximum flow or maximum height, but not both.

Centrifugal pumps add energy to the water, not pressure or flow.

 

[VicS]

Straight answers:

I thought I Understood this until I looked into it further, I am now more confused than ever.

If a pump manufacturer provides a performance table for a submersible pump, is the 'head' measured from the pump, or from the water surface.

From the surface of the water

ie, would it be beneficial to raise the pump from the bottom and closer to the surface for improved performance .............

No, (although it would reduce the resistance to flow if the discharge hose length was reduced)

Also would the same apply if the pump was mounted outside of the tank but level with the bottom of the tank so that water in the outlet pipe was level with the surface when the pump is switched off.

Yes. The head would still be measured from the water surface

Cheers

 

[ghostlymoron]

The total head is the sum of static head and friction head as I assume Thinwater meant. Static head is the difference in height between the water level in the sump and the centreline of the discharge in the case of a bilge pump. Friction head is the pressure required to overcome friction in the pipeline and can be significant.

 

[CLB]

All that matters is hydrostatic pressure, and pressure loss due to flow through the pipes.
The lift is measured from the surface, but the 'drag' from lengths of pipe matters too.

Ok, think I understand this

No, the head is the differential the pump can develop. More inlet pressure = more outlet pressure.

Head = velocity head pressure head. You can have either maximum flow or maximum height, but not both.

Centrifugal pumps add energy to the water, not pressure or flow.

Nope, definitely lost me on this one

Straight answers:

From the surface of the water

No, (although it would reduce the resistance to flow if the discharge hose length was reduced)

Yes. The head would still be measured from the water surface

Great, simple answers I can understand

The total head is the sum of static head and friction head as I assume Thinwater meant. Static head is the difference in height between the water level in the sump and the centreline of the discharge in the case of a bilge pump. Friction head is the pressure required to overcome friction in the pipeline and can be significant.

Ok, got it.

So friction needs to be taken into account, ie keep the hoses as short as possible, but when pump manufacturers provide performance tables with LPH at certain head heights, it is probably fom the water surface.
This relates to pond pumps and not some ultra scientific lab equipment, so it is not too critical, just trying to work out what size I need for a certain water flow at around 1m head from the water surface, but 2m from where the pump will be located (1m below the surface)

Cheers all

 

[ghostlymoron]

If the discharge is submerged, there will be pressure at the end of the pipe so the total head is measure to the water surface not the end of the pipe.

 

[thinwater]

No, the head is the differential the pump can develop. More inlet pressure = more outlet pressure.

Head = velocity head + pressure head. You can have either maximum flow or maximum height, but not both.

Centrifugal pumps add energy to the water, not pressure or flow.

What I said was correct.

A pump imparts energy. The discharge head has 2 parts: velocity and pressure.

a. The head is expressed as the difference between the inlet and the out let. If the pump is 1' deeper, the inlet pressure and the outlet pressure are increased by 1'. Thus, changing the elevation does not change the total height it can pump. However, lifting it closer to the surface does increase the chance it will gulp some air, which will destroy the performance. If a pump is above the surface (lift) there are also limitations.

b. There is no friction head in the pump rating. That depends on the piping, obviously, so the manufacture cannot know it. There is no pressure head at the discharge, if discharging to air. If you were pumping into the bottom of a tank there would be. So the head you need to calculate is height plus friction plus velocity.

c. There is always a velocity head when there is good flow. As a conceptual estimate, if the end of the pipe is flowing fast enough to throw the water 2' straight up, there is a 2' velocity head. Make sense?

d. Friction head requires looking it up in tables. However, it is NOT always about minimizing distance. Keeping it straight and avoiding sharp turns matters too. Hose bends are not bad, but a sharp 90 costs a few feet of straight run.

 

[JumbleDuck]

A pump imparts energy.

How much energy does a pump impart when maintaining a stationary column of water, do you suppose?

 

[thinwater]

How much energy does a pump impart when maintaining a stationary column of water, do you suppose?

Centrifugal pumps come with a chart showing the acceptable operating envelope. Since you are operating outside of the envelope, the question is both pedantic and irrelevant. That's like asking the glide ratio of a plane that is dropped flat and stalled.

 

[penberth3]

How much energy does a pump impart when maintaining a stationary column of water, do you suppose?

The input power to the pump minus motor efficiency and friction losses. What's left will heat the water.

 

[JumbleDuck]

Centrifugal pumps come with a chart showing the acceptable operating envelope. Since you are operating outside of the envelope, the question is both pedantic and irrelevant.

You can find the performance curve for a Stuart-Turner centrifugal shower pump at https://www.showers-direct2u.co.uk/images/product/full/46587-Performance-Jet-Curve.jpg. (They explicitly prohibit reuse, which is why I haven't used it as an image.) You'll notice that, as you;d expect for this application, it's rated for use at zero flow. How much mechanical energy do you think it is imparting to the water then?

That's like asking the glide ratio of a plane that is dropped flat and stalled.

Planes don't drop flat to stall. They stall first (almost always at an angle of attack of around 18^o, then drop flat.

That aside, I am interested by your notion that "centrifugal pumps add energy to the water, not pressure or flow". The Steady Flow Energy Equation would seem to disagree. In what way do you think a fluid can hold energy if not by pressure or flow?

 

[JumbleDuck]

The input power to the pump minus motor efficiency and friction losses. What's left will heat the water.

Indeed. And if it's a reciprocating pump ... ?

 

[thinwater]

You can find the performance curve for a Stuart-Turner centrifugal shower pump at https://www.showers-direct2u.co.uk/images/product/full/46587-Performance-Jet-Curve.jpg. (They explicitly prohibit reuse, which is why I haven't used it as an image.) You'll notice that, as you;d expect for this application, it's rated for use at zero flow. How much mechanical energy do you think it is imparting to the water then?



Planes don't drop flat to stall. They stall first (almost always at an angle of attack of around 18^o, then drop flat.

That aside, I am interested by your notion that "centrifugal pumps add energy to the water, not pressure or flow". The Steady Flow Energy Equation would seem to disagree. In what way do you think a fluid can hold energy if not by pressure or flow?

I have no interest in empty-headed jousting. You know the answers to your own questions and I know pump engineering. If you or the readers really want to learn about centrifugal pumps, there is Google.

My point was that they do not function like a PD pump.

 

[JumbleDuck]

I have no interest in empty-headed jousting. You know the answers to your own questions and I know pump engineering. If you or the readers really want to learn about centrifugal pumps, there is Google.

My point was that they do not function like a PD pump.

Your claim was that "centrifugal pumps add energy to the water, not pressure or flow", so I think it is reasonable to ask how you think they can add energy without either increasing the pressure or flow. Heating aside, of course, but that's another matter. On the face of it your claim is up against the First Law of Thermodynamics, and that's generally not a good position to be in.

Without wishing to argue from authority, I'll point out that I have been teaching fluid mechanics in universities for over thirty years.

 

[JumbleDuck]

If a pump manufacturer provides a performance table for a submersible pump, is the 'head' measured from the pump, or from the water surface. ie, would it be beneficial to raise the pump from the bottom and closer to the surface for improved performance. I initially thought it would make no difference as the pump is only lifting water from the surface, but now I'm not so sure.

I'll try my explanation ...

"Head" is just another word for pressure, but instead of expressing it as force/area (Pa, psi) you express it as the height of a column of fluid which would exert the same additional pressure or, to put it another way, the height of a column of fluid which could be supported by the pressure in question. It's a useful way of expressing things because we're often concerned with getting fluid to a particular height (in pumps) or taking water from a particular height (in turbines).

So when pump manufacturers give a head-flow curve, like this

they are basically saying "you can move a lot of fluid with a low pressure difference, less fluid with a higher pressure difference or none at some maximum pressure difference". For water and other liquid pumps it makes sense to express the pressure as a head, because that relates directly to the configuration of the pump.

So ... let's think about your submersible pump, and let's imagine that on the surface it can move 500l/minute at a head of 2m - ie, to an outlet 2m above its inlet. Now keep the outlet in the same place and submerge the pump by 1m. The outlet is now 3m above the pump, so the pressure/head at the outlet has to be 1m higher. But that's OK, because the pressure at the inlet is also 1m higher, so the 2m difference across the pump is still all you need.

Also would the same apply if the pump was mounted outside of the tank but level with the bottom of the tank so that water in the outlet pipe was level with the surface when the pump is switched off.

Yup. You can put the pump where you like, more or less. In fact you could take a hose from the surface to the pump inlet and another from the pump outlet to the discharge point and move the pump anywhere you liked - up, down or sideways - and it would be fine. Well, subject to the fact that water starts boiling if you try to put ~10m of suction on it, but that's a different issue.

In other words, all that matters is the height difference between the water surface (in the bilge) and the discharge point (in the hull). That difference is the head and the pump's performance curve will give you the flow rate. So, for example, my Rule 500 has to pump about four feet from bilge to discharge which means ...

... that it can do about 380 gallons per hour.

As other have said, you get pressure drops because of friction in pipes and fittings, but for the typical size of pipe and flow rates in yacht pumps these drops are absolutely tiny and can be safely ignored.

The great advantage of centrifugal pumps is that when the discharge head increases too much, they'll spin happily in the water, warming it up a bit but not otherwise coming to any harm. That's why they are used for power showers, for example, or for inflating rubber dinghies. Displacement pumps either have to stall the mechanical system driving them or have some sort of pressure relief system to allow them to continue running. Displacement pumps have head-flow curves just as centrifugal ones do.

 

[thinwater]

^^ And I've been doing refinery design for just as long. It's trite when academics trot out the sheepskin.

 

[penberth3]

Indeed. And if it's a reciprocating pump ... ?

It will heat the water, as I said earlier.

 

[VicS]

I thought I Understood this until I looked into it further, I am now more confused than ever.

If a pump manufacturer provides a performance table for a submersible pump, is the 'head' measured from the pump, or from the water surface. ie, would it be beneficial to raise the pump from the bottom and closer to the surface for improved performance. I initially thought it would make no difference as the pump is only lifting water from the surface, but now I'm not so sure.

Also would the same apply if the pump was mounted outside of the tank but level with the bottom of the tank so that water in the outlet pipe was level with the surface when the pump is switched off.

Cheers

I think you can be excused.

Even the Academics and Engineers don't seem to be able to figure this out between them .

It's all physics really .............. nobody understands physics!

 

[VicS]

Indeed. And if it's a reciprocating pump ... ?

It will heat the water, as I said earlier.

No!

Eventually if you keep increasing the head the pump will stall .... then the motor will burn out and it wont work at all,

1st Law of Science. If it moves it's biology, if it smells it's chemistry if it doesn't work it's physics!

( 2nd law of science. If its broken it's technology)