Messages

1

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 29, 2022, 03:49:00 PM »

You previously stated that 'But back pressure has nothing to do with energy cost per gallon produced.'  Your own equation (and the example we have hashed through) clearly shows that to be false.  Increasing back pressure = increased energy per gallon produced.

When using a pressure tank to provide 5gpm, the pump is running at 25gpm, but only for 20% of the time, the pump never runs at 5gpm.

Since you keep bringing up VFDs or VSDs, isn't the situation the same as with a tank?    A constant pressure VFD is only pumping against a back pressure of 50psi (assuming that is the setpoint) + the lift pressure.    So even neglecting the impact of being closer to BEP at lower RPM,  just the reduction in back pressure nets the same 35% reduction in energy as with a tank.    If you include the increase in pump efficiency by operating closer to BEP, then the VFD looks even better, no?

If you have physics that shows otherwise, I would be keen to see it.

Hydraulic HP = Head (ft) x Flow Rate (gpm) x (Specific Gravity)
                                                         3956

This is the formula for figuring Brake Horse Power.  But head is irrelevant when the curve already shows the horsepower. 

You are comparing the CSV working at 5 GPM and the system without a CSV working at 25 GPM, which makes no sense.  When running the CSV at 25 GPM the pump will only see 4 PSI or 9' of head more than if there was no CSV, which is 524' instead of 515'.  Do the math or just look at the curve and you won't be able to see any appreciable difference in horsepower with only 9' of head difference.  I can even add a bypass that will keep the CSV from adding 9' of head when wide open if needed.  But it is not worth the effort.

When using 5 GPM with the CSV the curve shows 2.5 HP, or 0.5HP per gallon, no need to do the math. We already know that without a CSV the pump will only be using 0.22HP per gallon, but will be cycling itself to death while that is happening.  Actually it will be a little more than 0.22HP per gallon if you figure the number of cycles and add the inrush currents for each cycle.

I am not confused about VFD's but most people are.  Even though the benefits of using a Cycle Stop Valve can far outweigh any loss of efficiency, of course restricting flow from a pump is less efficient.  But it is the same thing when using a VFD.  I wish others would do the math on VFD's and see that reducing the pumps speed also increases the horsepower per gallon used and is the same as restriction from a CSV.

2

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 29, 2022, 10:04:46 AM »

Pumping into a cistern and using a couple of different size booster pumps fitted with Cycle Stop Valves can be even more efficient and would take a lot of cycling off the well pump either way.  Would be glad to discuss this further if you want to do the math?

This is a good suggestion, and maybe what I am leaning towards now. 

3

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 29, 2022, 09:40:52 AM »

OK well good question.  But back pressure has nothing to do with energy cost per gallon produced.  As you can see from the pump curve, as the back pressure on the pump goes up, the horsepower of the pump, which is the energy used, goes down.

This makes no physical sense.   It takes more energy to move a unit mass of water against a higher back pressure than it does against lower back pressure.   This is the whole premise of the work equation: work = flow*delta_pressure.  The horse power of the pump goes down not because of higher pressure, but because the pump is moving less water and therefor doing less work.  As we demonstrated earlier the amount of energy needed to move the same amount of water goes way up as the head pressure increases.

To convince your self of this, think about it in terms of vertical head instead of pressure (the pump doesn't know the difference), and let's use SI so that the units make sense.  In this case we are then thinking about moving a certain amount of mass per second up a certain distance against gravity.     To make it obvious, let's go back to the 5 GPM case:

With the CSV the pump sees 740' of head, or 225m.   We are pumping at 5 gpm, or 0.32 kg/s of water.   So the pump is lifting 0.32kg of water 225m every second.   The best case scenario, the power requirement is  (Height*mass*gravity):  225*0.32*9.8 =  705J/s or 705W or about 0.95hp.

Without the CSV valve, the pump sees 515' of head or 157m.  The pump is still providing 5 gpm average by cycling on and off.   So the pump is lifting 0.32kg of water 157m every second.    Now the power requirement is 157*0.32*9.8 = 492 J/2 or 492W or 0.64hp. 

Regardless of pump efficiency, the increase in head pressure from the CSV increases the load on the pump from 0.64hp to 0.95hp.    In reality, because pump efficiency also goes down at higher head pressures, the situation is way worse - the pump actually needs 2.5hp at 5gpm, so the overall efficiency is 0.9/2.5 = 36%.

The follow on question, where is that 0.3hp going?  It has to be dissipated in the CSV. 

So yes, back pressure absolutely changes the amount of energy per gallon produced.  This is fundamental to Bernoulli's equation:
https://en.wikipedia.org/wiki/Pump#Pumping_power

This may explain some of the confusion about Variable Speed Pumps.

4

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 28, 2022, 04:45:29 PM »

Thanks for the detailed discussion, I think we are agreed on the efficiency issues w.r.t to operating away from the pumps best efficiency point.

However I think we are talking past each other on the issue of back pressure.   To make sure I understand the process - in the scenario we are discussing, what is the pressure that the pump is working against at a flow of 5, 10, 15, 20, 25 gpm? 

At 25 GPM the CSV3A2T has only 4 PSI friction loss, which is so negligible you wouldn't even be able to measure a difference in power consumption of the pump.  The CSV only puts back pressure on the pump when less than 25 GPM is being used and back pressure is needed.
 

5

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 28, 2022, 01:48:12 PM »

Interesting analysis and graph - it allows us to add some efficiency numbers.   But I am still not following.   

The power requirement of the pump pumping against 346 psi of head pressure is always going to be about 50% higher than pumping against 223psi of head pressure assuming the same flow (Q) (work = Q*dP).   So as a baseline in this configuration the CSV will require ~50% more power than using a pressure tank at all flows except the absolute maximum flow rate of the pump and the CSV is no longer regulating and providing back pressure.

This is further compounded by the pump efficiency issue at low flow.  The figure you are showing lets me put some numbers on that, at 5gpm the pump is consuming 2.5 HP or about 0.5hp per gallon pumped per minute [again excuse the terrible units].   At 25GPM the pump draws 5.5hp or 0.22hp/gallon/min.   So the pump ~250% more efficient at 25gpm than at  5gpm. 

So at flows close to BEP, the CSV would use 50% more energy to pump the same amount of water as a tank.  At the minimum flow (5gpm) the CSV would use 50% more energy due to back pressure on top of 250% more due to pump efficiency for 375% more energy in total.

I can see an argument that the CSV would cycle less (if the flow stays above 5gpm, below that the CSV would actually cycle more, unless you use a large pressure tank).  But the extra energy cost is pretty major.   

I appreciate the offer to try it out, but do you think the numbers will be far off what we are estimating here?

6

Frequently Asked Questions / Re: Increased back pressure and pump efficiency

« on: September 27, 2022, 04:26:01 PM »

I was not considering a VFD as an alternative option, but a large pressure tank.   

In the pressure tank case the pump would be running against a back pressure 173 + 50 psi= 223psi as opposed to 173 + 173 = 346psi with the CSV.    The physics here is the part that is bothering me.   Regardless of the efficiency of the pump, the work the pump is doing is equal to flow * delta_Pressure.  So in the case with the valve it is doing 5x346 = 1730 units of work, whereas operating against a pressure tank it is doing (on average) 5x223 = 1115 units of work (the units don't make sense here, but they are the same).    There is really no way around the fact that a pressure reducing valve is wasting energy, it is reducing the energy of the fluid, without doing useful work, that energy has to come from the pump.

Also with the pressure tank option, the pump will cycle at a rate depending on the draw down of the pressure tank, but when it is on, it will be running close to the BEP, so it will be more efficient.    I am trying to balance increased cycling vs reduced efficiency.   Is running at ~65% efficiency worth the decrease in the cycling?

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Frequently Asked Questions / Increased back pressure and pump efficiency

« on: September 27, 2022, 12:34:07 PM »

I am looking at using a cycle stop valve in an irrigation well system.   It is a deep well (well depth 700', guessing the pump is around 650', static water level is 250' above the pump or ~400' below ground) with an unknown 5hp pump down the well and will be used for irrigating a few different properties. 

Ideally we would match the pump flow to the size of the irrigation zones and have the irrigation controller switch on the pump.  However as we will have several different irrigation controllers attached (and want the use of spigots as well) this doesn't work.   So now I am considering a pressure regulator + bypass or a cycle stop valve.

The part I am having difficulty with is the efficiency of the system, particularly if we use lower flow drip irrigation zones.    The pressure up stream of the valve will always be what ever the ultimate pressure of the pump is (at the bypass flow).   Doesn't this force the pump run at the left most edge of its performance curve, which is way outside the optimum efficiency of the pump?  Does this cause issues with pump cooling?