Some pictures and curves would be good here. You don't have unpredictable inlet conditions, you seem to have varying outlet conditions.

I think you could do all of this with a fixed speed pump and a control valve controlled in part by inlet pressure.

Be aware that NPSHR doesn't mean you won't get cavitation of the pump.

Normal practice is to allow a1-2m (4-6ft head above NPSHA to provide this comfort level.

But in answer to your question the clue is in the C part of MCSF - continuous. Now how this is defined is often rather wooly or simply not stated.

SO it could be 30 mins, 6 hours, one day - No one really knows. What time scale are you looking at to operate below this magic curve line ( which is actually a thick fuzzy line).

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

I agree with LittleInch's suggestion of using a constant speed pump.
See this thread that I just answered for typical use guidelines for VFD. thread407-484994: retrofit of old condenser pump: 70 meter head to 30 meter head

Your head requirements are basically constant and your demand should really be filled by the highest flow rate possible while maintaining NPSH. You should be able to set that by fixing only one safe minimum suction pressure, as the filling rate of trucks is not exactly a critical application that requires VFD flow control. Any reasonable flow rate is OK. Its something determined by suction pressure, with or without VFD. You apparently need suction pressure control.

Suction pressure control for NPSH by VFD is difficult and probably not necessary. You only have a few psi to use as your control signal. And really, you are interested in only one value, NPSHR. One value pressure control of NPSH boils down to On/Off. You can pump when you have it and don't pump when you don't. You could easily do this with a pressure switch to cut power to pump on low suction pressure. Realistically, I dont know why you would even need a control valve. With min NPSHR covered by PSL, excess NPSHA provides its own flow control.

A PSL is not as kool as a VFD, but it is far more efficient.

Hello, thanks for the replies. I have a sketch I'll try to scan and upload to clarify. We are going to try this with the full diameter impeller, will see what happens at the low speed where MCSF is indicated, maybe set a minimum to tune out any problems.

There's a typo in my post, "7.63 inch to avoid MCSF: 1557rpm 365gpm@45'/16.5' NPSHr" (should say 265gpm, not 365). I was trying to emphasize the dramatic difference in NPSHr for identical flow and head points at the respective impeller diameters (16.5' at 7.63 vs. 6.9' at 9.0).

I did consider a flow control valve (200gpm) and just an inlet side pressure switch. At 1750rpm 9", 200gpm is 87' (so 42' of throttling required at our 45' operating point). NPSHr is not shown at that point on the curve (leaves off to the right at 270gpm, 11' - appears flat though, not dropping even if I hand draw an extension) - takes 7.2hp. Given ample NPSHa a standard 1750rpm selection would be 7", 200gpm @ 48', NPSHr = 25', 3.6hp. It does seem magical that using the 9" with a VFD, we can get 200gpm at 40+ ' at 1225rpm, NPSHr = 5.8, 2.6hp.

Truck filling is a big deal, takes 30 minutes or more now (2000 gallons, so "push" from pipeline through BFP amounts to around 67gpm). It appears we can pull 266gpm (7.5 minute fill) with the 9" impeller at 45' & 6.9' NPSHr (vs. the roughly 10' NPSHa at that flow rate). Everything 1750rpm, whether or not flow is restricted, appears to require more suction head for similar volume. I frequently "unsell" VFDs, but seems like the answer here.

It will be interesting to see how well the PID loop works (do it all the time for discharge pressure, 1st attempt here controlling for suction). I'm hoping it will be smooth, take maximum advantage of available water supply . . .

Nothing there changed my opinion.

If your problem is suction pressure and NPSH, no VFD will cure that.

If fast truck loading is the problem, you need to do that at the max rate. It will never be more than what your suction pressure will supply. A VFD will not change maximum rate.

The ONLY thing a VFD will let you do is pump a wide variation of flow rates efficiently. If money is the problem, get a vfd. You will be able to pump low flow rates, <60% BEP, cheaply. Higher flows will cost about the same.

I suggest your money is better spent on curing your suction pressure troubles.

Build a tank. Fill it at whatever maximum rate that supply pressure allows. When a truck comes, turn on a high speed, high capacity pump and fill the truck in 60 seconds or whatever. Like Formula 1. (They gravity fill, no pump needed). Do that until the tank runs dry, or no more trucks come.

I think you just want to play with a VFD toy.

What would be more useful than a sketch is an estimate of the times in h:m that you will operate the pump at various flow rates on a typical operating day (when not limited by suction pressure) and a system curve.

If you can't estimate that, you should probably not be thinking about vfd.

Since neither vfd or cv will cure suction pressure issues, you should also have an estimate of typical suction pressures and their h:m per day. As you can only have a valid solution of any kind for times when suction pressure is above minimum.

Thanks again for the replies, sorry for responding late here.

I do like playing with my VFD toys. This part still seems magical to me: "1750rpm selection would be 7", 200gpm @ 48', NPSHr = 25', 3.6hp . . . using the 9" with a VFD, we can get 200gpm at 40+ ' at 1225rpm, NPSHr = 5.8, 2.6hp"

Our system here ("900ft 4" PVC underground pipe fed by a water district turnout with a "normal operating range" of 17.5-20psi") is unpredictable, that "normal range" is all over the map at different times (often way below normal), beyond anyone's control. A friend of mine makes fun of me for misunderstanding NPSHr & NPSHa, says they must be corrected for atmospheric (so 25' is really 25-14.7= 10.3', 4.5psi if monitored by gauge), says for testing they use a vacuum gauge at pump inlets to find problems. I definitely need to understand that part better - seems unfair that NPSH must be corrected, but NOT the standard flow/head output curve - but for now I'll take his word for it.

Any case, we set this one up as described, but used a 7" instead of 9" impeller (cause already built that way), found we could overspeed the pump without overloading the motor and with no evidence of cavitation (on the day of the test, at least) - filled up trucks in 5-6 minutes, everybody happy. The discharge side pressure switch is set for On at around 15, Off around 25 psi, seemed to cycle well (no oscillating/short cycling). I set the PID loop at 5psi, reverse action (so VFD runs full blast, then slows down/sleeps when inlet pressure drops) - didn't get to test that part as they installed no local valve on the supply side, said they would test it later by throttling the supply line valve at the other end.

All in all I think it's a reasonable design, providing automatic response to highly variable NPSHa while seeking maximum flow rate without pump damage or tripping. We did consider the gravity storage tank, also worried a hydrotank might be needed on the discharge side - seemed cheaper/easier to just try the VFD first. I wanted to understand the 9" impeller thing (so then our VFD current limit/speed foldback would kick in below, rather than above nominal 60hz), might do that later if they do have problems with poor supply pressure. Meantime it seems to work OK.

How much of the 900' of line is yours?
Putting the pump closer to the source will greatly improve the NPSH situation.
Much easier to push water than suck it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

I'll make fun of both you and friend. You correct for NPSHa by adding atmospheric pressure.
NPSHR is given in "absolute ft" or when converted to pressure, it is absolute pressure.
"(so 25' is really 25-14.7= 10.3', 4.5psi, if monitored by gauge"
You cannot subtract psi from feet. Convert the psi to feet and then you can.
If it is psi, rather than 25', you add 25psi to 14.7 psi =40psi, that's 92.3 feet of water.

NPSHA is calculated by summing the following

+Ft of Head equivalent of applied pressure, if drawing from a pressurized tank or pipeline)
+Ft of elevation (+ is elevation of suction water surface above centerline of pump)
..... If drawing from a pressurized pipeline, use pipe centerline elevation,
..... by converting gage pressure from gage elevation to pipe centerline, or
..... simply use the elevation of the gage.
-Ft of Friction loss in suction pipe, valves, fittings and tank exit coefficients
-Ft Head equivalent of vapor pressure
+Velocity head (often ignored)

Thanks for that 44.

Keith Cress
kcress - http://www.flaminsystems.com

Keith, Others
Please notice the correction in the edited post above. "If drawing from a pressured pipeline, use the indicated pressure with the gage's elevation by converting indicated pressure to pipe centerline elevation, or simply use gage's elevation."

---------- Revised Text------------
NPSHA is calculated by summing the following

+Ft of Head equivalent of applied pressure, if drawing from a pressurized tank or pipeline)
+Ft of elevation (+ is elevation of suction water surface above centerline of pump)
..... If drawing from a pressurized pipeline, use pipe centerline elevation,
..... by converting gage pressure from gage elevation to pipe centerline, or
..... simply use the elevation of the gage.
-Ft of Friction loss in suction pipe, valves, fittings and tank exit coefficients
-Ft Head equivalent of vapor pressure
+Velocity head (often ignored)
-------------------------------------

And be sure that friction loss in the suction line is calculated using the gage to pump length of pipe starting from the point where the gage connects to the pipeline.

Bentov, Ed Stainless asked a good question that is still unanswered - re pipeline owernership. Locating the pump towards the town main supply overcomes many problems but of course doesn't help you getting your kick out of using VFD.

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.)

If the NPSHA is calculated correctly, there could be no problem. NPSHA is apparently around 25ft (dependent on height of the gage).

With a only a 900ft run, its going to take a long distance to recover head lost to friction. Its probably cheaper to increase the pipe diameter, maybe for only the first half of the run, than to drag the pump station and power cables off the plot plan to perhaps some location 400ft down the street.

Prior to going any further with how do I overcome my problem, the very first question to be answered is, do the local authorities allow direct pumping from their main?

My experience is, no they don't.

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.)

Some Plumbing Codes require that all booster pumps be equipped with a low-pressure cutoff switch, which prevents the creation of a vacuum when the pressure on the suction side reaches values of 10 psi or less. When negative pressure isn’t accounted for, a phenomenon called cavitation occurs, where small bubbles rapidly form and collapse in the flowing water. Cavitation can cause localized shockwaves, damaging the booster pump impeller and inducing mechanical vibrations, which affect the motor driving the pump as well.

Quote (bentov (Electrical)(OP))

Our system here ("900ft 4" PVC underground pipe fed by a water district turnout with a "normal operating range" of 17.5-20psi") is unpredictable, that "normal range" is all over the map at different times (often way below normal), beyond anyone's control. A friend of mine makes fun of me for misunderstanding NPSHr & NPSHa, says they must be corrected for atmospheric (so 25' is really 25-14.7= 10.3', 4.5psi if monitored by gauge), says for testing they use a vacuum gauge at pump inlets to find problems. I definitely need to understand that part better - seems unfair that NPSH must be corrected, but NOT the standard flow/head output curve - but for now I'll take his word for it.

Atmospheric pressure is added to the NPSHa when you have an open tank, but is not added to the pressure from a pressurized pipeline.

Per your stated pressures, you don't have adequate NPSHa to make this system work. With a flow of 300 gpm, you have 17.5 psi pipeline headloss. That would leave you with 0 - 2.5 psi NPSHa.

As others have stated, you need a bigger pipe diameter or relocation of the pump.

bimr: my experience is NO direct pumping from the main as it can lead to putting the main under vacuum (vacuum breakers can fail).
The quick cals. I did, probably 200GPM is not achievable.

Gravity storage tank at the discharge seems the best solution and could even negate the need for a pump - not unlike refilling the water tanks in the railway steams engines of years ago.

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.)

There is no difference between the calculation for a closed pressurized tank, or a closed pressurized pipeline.

NPSHa calculation is the same for all. The same for an open or a closed tank, or a pressurized pipeline. The object being to obtain total head, which always includes atmospheric pressure. As gage pressures do not ever include Patm, whenever you have a gage pressure, be it from a gage placed on a pipeline, or on a tank, it needs to be added.

This link has several examples of the NPSHA calculation, showing both that for open and closed tanks. Note that the pressure given for the closed tank example is already in units of PSIA, so Patm has already been added to the tank's gage pressure reading.
https://pumpsdesign.com/npsha/

There is no difference at all between the calculation for any open, or closed pressurized tank, or a closed pressurized pipeline.

What follows however is somewhat different between tanks and pipelines.

Including Velocity Head
If you include velocity head, rather than ignore it, you must use the velocity at the point where the pressure reading is taken. If the gage is on a tank, the velocity in the tank is probably zero, or very close to zero, as a tank's area of flow is usually very large and velocities within very, very small. If the gage is on a pipeline, the velocity will probably be small, but not zero and, if there is a velocity, the pressure gage will not include any velocity head in its reading of static pressure head alone, therefore you can add the velocity head at that point where the gage is attached, if you want to account for velocities heads. It is conservative to ignore it and they usually don't amount to much anyway.

Note: Following reference specifically says P_suction = "stagnation pressure"
Stagnation pressure includes any velocity head
https://www.pumpsandsystems.com/sites/default/file...
https://www.pumpsandsystems.com/topics/understandi...
https://www.engineeringtoolbox.com/npsh-net-positi...
http://www.pumped101.com/npsh.pdf

There is no difference at all between the calculation for any open, or closed pressurized tank, or a closed pressurized pipeline.even when including Velocity head, but you must remember to use the velocity at the pressure gage's location, whenever you choose to include velocity head.

Lastly

The acceleration head of a reciprocating pump is another potential loss, which must be subtracted from NPSHA, but only if a recip pump is used.
https://www.ruhrpumpen.com/en/downloads/135-rdp-te...

Not forgetting 10 psi pressure drop back flow eliminater + plus fill valves etc.

Requires a thorough hydraulic calc to establish the facts.

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.)

Artisi, Others,
I made some additions while Artisi was posting.
Please recomment as you see fit.

Atmospheric pressure is not added unless the tank is open to the atmosphere:



In the OP post, the system is not open to the atmosphere.

Artisi, I don't understand your comment regarding booster pumps. Every high-rise, hotel, stadium as well as many commercial and industrial facilities use direct connected booster pumps. Putting tanks on building rooftops is no longer done. Would expect the connection to have a backflow preventer as well.

Link

Here it is mentioned in the building code.

"Per the IBC, a low pressure cutoff was and still is required on all booster systems to prevent negative pressure on the suction side of the pump when a positive pressure of 10 psi or less occurs on that side. Before the days of building automation system connections, alarms consisting of a light or horn would alert the building engineer on-site to any issues requiring attention."

Link

bimr; my only comment re booster pump was it may not be needed, as usual we are not privy to the overall scheme of things - maybe the fill point is out in the never never - how many times a day is the tanker or fire pump filled and at what rate of fill.

The next thing we will be asked is because the deliver head will be 12ft or 16ft - why does my pump keep tripping out on overload - maybe because its capable of 40 or 50 ft and its operating way right on its curve.

my comment on gravity tank was based on the high head loss thru the 900 ft line etc at the rate of fill nominated - if little water is required per hour / day etc, a header tank will more than likely be the best move - it has time to constantly fill direct from the pipe line -

But, we are crystal ball gazing at the moment - nothing unusual .

Nearly enough for me this year on this one - lets see if we understand more next year.

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.)

Bimr.

NPSH is always measured in absolute terms as head of liquid.

Your claim only works if the pressure measured in the pipe is expressed in absolute pressure units, bara or psia. Most guages and transmitters provide pressure relative to local atmospheric pressure, bar(g) or psi(g). Then you need to add local atmospheric pressure to gauge pressure when you do NPSH calculations to get absolute pressure.

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

NPSHa
Several NPSHa calculation examples here,
https://pumpsdesign.com/npsha/
https://www.pumpsandsystems.com/topics/understandi...

LI,

Of course, you are referring to the a in psia means that the pressure is measured above absolute zero, a perfect vacuum.

An absolute vacuum. What better reference point is there. Same one the atmosphere uses.

Hi,
A few basics about pump hydraulics.
Good luck
Pierre

• https://files.engineering.com/getfile.aspx?folder=b35a4f50-bf7d-4347-aa3c-6

Hi,
Another good reference
Note : You can download on Internet a reference book written by the author of the document attached .
Enjoy the reading.
Pierre

• https://files.engineering.com/getfile.aspx?folder=65710357-65b8-4397-91dc-7

Bentov,

It would be useful if you could supply the following information:

1) Is your back flow preventer before (suction side) or after (discharge side) of your pump
2) Is there any elevation change from your tapping point on the main to the pump?
3) what is you calculate or measured pressure drop at 200 and 300 gpm?

You seem not to have grasped that NPSH is measure din absolute terms, so feet above an absolute vacuum. SO a gauge reading of say 5psi (11.55 ft) in absolute terms is actually 5 psi + local atmospheric pressure (14.5 at sea level), so 19.5 psia or 45 feet to compare to your NPSH figure.

HOWEVER, the points made above by others are valid. If you are transporting potable water, you really don't want to go below 0psi(g) or really less than 5 psi. Apart from any regulations that the water company has, this leaves open the potential to allow unfiltered ground water into your pipe which would contaminate your supply.

Therefore your best bet if this is the case is to set your PID controller to 5sig at the pump inlet and maximise flow based on that.

Very few pumps of this size have an NPSH higher than 25 ft and most much less, so any pressure above 0 psig at the inlet will be good enough.

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

If you are attempting to set up a system like this you need to understand the principles of cross connection control and booster pump systems. This a a good reference EPA Cross-Connection Control Manual - also attached.

Your pump must be controlled so it will not operate when the pump suction pressure is less than your jurisdiction's criteria. With the pipe size, length, flow rate, and the initial pressure range provided I suspect that most of the time the pump would short cycle, as the low pressure switch would turn the pump off as soon as it is started.

This makes the entire question about NPSH moot, as your pump suction is not allowed to be below the criteria pressure for this pump.

You will need either an airgap at the truck fill point (there are rules about air gaps) or another type of cross connection control device, appropriate for your piping arrangement. The installation may be subject to codes compliance AHJ plans review / approval, you should check.

• https://files.engineering.com/getfile.aspx?folder=517cfd59-2281-40fd-85d3-c

Well this got busy, been away, sorry . . . this thread was kind of inactive, just noticed while checking for replies to my other one (hoping for a rule of thumb on downthrust k factor, no luck there yet) that I left it hanging, felt guilty so followed up (while sleepy, short on time). For the record, I do know the difference between hd-ft & psi (and my pump buddy certainly does), but I deserve to be made fun of for my brain fart.

Some answers: the "jurisdiction" is a closed irrigation district pipeline supplying non-potable water to farmers; the backflow device is after the new pump; the water truck and firewater tank fills are air gapped; there is no elevation change between tap and pump; calculated pressure drop at 200gpm = 21.6ft (5.11fps), at 300gpm = 46ft (7.7fps)

44, thanks for all the input. I read those links (had seen Evans before but not Henshaw and the "desire" of fluids to boil). It's comforting to recall how widely misunderstood are NPSH and cavitation, and that I'm not the only one.

To rephrase the dumb part, my friend made fun of me for not knowing to add atmospheric to the NPSHA (so at sea level we start with 14.7x2.31= 34ft). So to monitor the suction inlet for inadequate NPSHA on that pump with psig (gauge, corrected for atmosphere) vs psia (absolute) devices you'd need to calibrate for 25-34 = -9ft (or -3.9psi) - thus a vacuum (rather than pressure) gauge or transducer. As I recall he said "no problem, you'll be plenty safe at 5psi" while shaking his head at my ignorance.

To be clear: NPSH (Required and Available) in pump specs is always expressed in absolute, yes? In our closed pipeline, the NPSHA at the pump inlet will be whatever is leftover after the pipeline friction loss? I used the applied pressure stated by the supplier, 20psi (46ft), subtracted the 21.6ft friction losses at 200gpm, leaving (46-21.6=) 24.4ft NPSHA for the pump. Was I supposed to add 34ft at the beginning, so then it's 46+34= 80ft applied, 80-21.6= 58.4ft NPSHA at the pump inlet? That seems way too high (since by all accounts this system is in trouble due to inadequate supply), must be missing something still . . . getting sleepy again, go ahead and make fun of me, I can take it.

I have found that anytime you do hydraulics it is more convenient to always work with absolute pressure. Convert EVERY pressure you have TO ABSOLUTE pressure ... immediately.

NPSHr is given in ABS head.

Supplied pressure is usually given in gage pressure, but obviously there is no standard convention. When any given pressures are not specified as psiG, or psiA to be either gage or absolute, I assume gage pressure for preliminary work, but VERIFY that it is gage. It is also good practice to verify the msl elevation of the pump installation site, because they are not always going to be at sea level with 14.7 psia atmospheric pressure (33.9ft) and 60°F. I subtract 0.5 psi (and maybe) 3°F for each 1000ft above sea level, although using the highest site temperature is usually more conservative.

20psiG + 14.7 psi = 34.7 psiA
34.7 psiA × 144/62.37 = 80.1ft
Friction loss Hf = 21.6ft
Friction loss 21.6ft x 62.37/144 = 9.4 psi
Head at suction 80.1ft -21.6ft = 59.5ft
Suction Pressure 59.5ft x 62.37/144 = 25.8 psiA -14.7psi = 11.1 psiG
You mentioned that low pressure supply is 17.5 psiG, so min suction pressure might be 2.5 psi less
MIN Suction Pressure = 11.1-2.5 = 8.6 psiG (23.3 psiA)
MIN Suction Head = 23.3 psiA x 144/62.37 = 53.8 ft

Since vapor pressure is very small and supply pressures of 17.5 to 20psi? (the pipeline pressures are gage pressures, right?), then it looks like lots of suction head is available. 20 psiG, even 900ft away is quite a lot of head. If the typical house had a 6" pipe connection to the mains, we would all have a good sense of how much head that really is, but we'd get too beat up and battered around in the shower. What we usually see is the few psiG available at the end of a 25ft x 1/2" garden hose. If I can get 20 psiG pressure at the inlet to a large pump station, that's usually more than what I need to keep all the monsters well feed.

Pierreick (Chemical)3 Jan 22 06:48 2nd reference is great. Its very complete with lots of additional references included in one pdf that will save much hunting around for all the other stuff you usually need to do pump system design.

That's ok - it took 6 months last time

So your questions

NPSH (Required and Available) in pump specs is always expressed in absolute, yes? YES

In our closed pipeline, the NPSHA at the pump inlet will be whatever is leftover after the pipeline friction loss? - More or less. Is your pipeline truly horizontal? - Not even a few feet going from underground to the pump inlet? Whilst it is fairly small, don't forget vapour pressure of the water - ~1ft at 20C, but can get significant at higher temps.

I used the applied pressure stated by the supplier, 20psi (46ft), subtracted the 21.6ft friction losses at 200gpm, leaving (46-21.6=) 24.4ft NPSHA for the pump. Was I supposed to add 34ft at the beginning, so then it's 46+34= 80ft applied, 80-21.6= 58.4ft NPSHA at the pump inlet? That seems way too high (since by all accounts this system is in trouble due to inadequate supply), must be missing something still - Yes.

SO other question - Is there anything in the line between the tap off point ( valves, bends, elbows, filters, connections with smaller ID etc)
Also what exactly is your pipe? Is 4" the ID or OD? even a few mm smaller ID makes a big difference at this level.

Also don't forget that NPSH is not onset of cavitation. The way they measure this is to slowly throttle the inlet until the head difference between inlet and outlet drops by 3% for the same flow rate. As shown in the graph below, you can get a divergence at higher flows between cavitation and NPSH. The usual action is to provide somewhere between 1 to 3m ( 4 to 10 ft) above the NPSH value to avoid cavitation.

Your value of NPPSHR at 25ft is quite high and indicated you are at a high speed with a small impellor.

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

It's a "will be supplied by a 900ft 4" PVC underground pipe".
This calculator gives a 9.2 psi loss for 200gpm
BUT it also gives 19.6 psi loss for 300gpm

https://inventory.powerzone.com/resources/friction...

20psiG + 14.7 psi = 34.7 psiA
34.7 psiA × 144/62.37 = 80.1ft

Friction loss @ 300gpm 19.6psi x 144/62.37 = 45.25ft
Head at suction 80.1ft -45.25 = 34.8ft
Suction Pressure 34.8ft x 62.37/144 = 15.1 psiA -14.7psi = 0.4 psiG
You mentioned that low pressure supply is 17.5 psiG, so min suction pressure might be 2.5 psi less
MIN Suction Pressure = 0.4-2.5 = -2.1 psiG (12.6 psiA)
MIN Suction Head = 12.6 psiA x 144/62.37 = 29.1 ft
-Vapor Press/Head = est 0.82ft @ 70°F
NPSHa = 28.3ft
NPSHr = ?

Now back to the VFD to control flow problem.

A VFD should not be used as flow control based on suction pressure.
Supply pressure is natural flow control. As supply pressure increases, so does flow. As supply pressure decreases, so does flow. Furthermore, you cannot change the supply flow rate from a pipeline to be more, or less than what the supply delivers at any given supply pressure. What you do downstream does not change that. That's a bit different than pumping from a tank that will usually supply whatever you want until it runs out. Any change you make to the flow rate or pressure delivered by a pipeline should be considered extremely temporary, or you will probably begin to affect the pipeline's operation upstream, something that usually causes trouble.

So, IF YOU CANNOT CHANGE THE SUPPLY PIPELINE FLOW RATE & PRESSURES, as it is a property of the supply pipeline, how will adding a vfd help?

A vfd cannot make more water, nor can it eliminate water. It can only change the pressure of the flow it is given. If it can change the flow rate of the supply pipeline, then the vfd is either drawing down the pressure of the supply pipeline by taking more water than the pipeline can deliver, or slowing down the pipeline water delivery and increasing the pipeline pressure. This is the paradox of hydraulic analysis. You must begin your calculations at a point where the flowrate and pressure characteristics are very well known, and in fact, they must be constant, unless you are doing a dynamic analysis. Dynamic analysis still requires that constant flow and pressure be maintained somewhere in the system for the short period during each time step, however they are allowed to vary between time steps.

It does often feel like I'm battered by the need for some attempt at dynamic analysis despite not fully understanding the regular kind, oh well! Regarding use of VFDs for flow control based on suction pressure, I do have a local example. A canal company captures drainage water in large ditches (with highly variable flows) using large pumps that start to cavitate when the level is low. We supply VFDs with submersible level transmitters, setup PID so pumps slow down as level drops. The efficiency is horrible but they don't care, happy to capture the available water that was otherwise lost when pumps were turned off to avoid self destructing.

This case seemed similar, problem was unpredictability of the supply beyond our control. Without actually monitoring all conditions carefully, we can say that the "push" through the pipeline (based on known truck fill rates) was averaging 60-70gpm, which seemed about right given stated applied pressure and BFP loss plus calculated friction loss (seemed a little high actually, probably because I was leaving out atmospheric). Filling the (very large) yard dust control trucks was a constant irritant/inefficiency, but fighting a recent fire and seeing the storage tank nearly emptied while watching the pathetically slow refill rate really got some attention. The goal here is similar to the drainage capture: stepless pump speed response to available supply (whatever it happens to be) at maximum possible flow, automatic full time till tanks are full. While admitting my lack of qualification for proper hydraulic analysis, I offered to setup the VFD thing as an initial problem solving attempt (because it's cheap compared to other alternatives, and yes, kind of fun I guess).

When I first started talking about this with my pump friend (over beer, after I tried to give him the analysis job for pay, which right away sounded to him like more trouble than it was worth), his reaction was "it'll never work, need to put the pump at the other end, or make the pipeline a lot bigger, or at the very least install a vented storage tank at the pump". I sense that was the first impression on this forum as well, but now it seems there's consensus that we do have adequate NPSHA for at least 200gpm - maybe more, though I expect new problems arise (air entrainment?) at higher pipeline velocities. And yes LI, the high NPSHR is due to a small impeller at high speed, which brings me back to my original question (looking at forcing our pump to be a 1200rpm design with a 9" impeller using a VFD, taking advantage of the dramatic reduction in NPSHR for the same flow rate, but flirting with "full reject" on the MCSF curve).

This one's quite fun though.

I don't agree with my respected friend mr 44 - you can control any VFD on anything you want and inlet pressure is one parameter for sure to maximise flow. You can then set it not to go too fast if the inlet pressure rises more than you anticipate.

Basically though looking at some pump curves, your chosen pump is WAY TOO BIG. See https://www.absolutewaterpumps.com/media/blfa_file... page 46 for your pump.

You want something more like the pump on page 41 - where 300 gpm is closer to the right hand end of the curve. A large diameter slow running pump will give you better NPSHr. However once you go below 0psig other things might happen. If you've already got that pump then so long as you don't go below the MCSF for too long I wouldn't worry. These are not large pumps just bigger than you need. As you say efficiency falls off a cliff, but if it maxes out flowrate for a short while who's worried.

Your velocities are a bit higher than "normal" but you won't get any air entrainment off a pipeline supply unless you're got a lot of air then you might need to avoid going below 0 psig.

Oh and I keep reading BFP as Boiler Feed Pump and not Back Flow Preventer....

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LI, I agree you can control it on suction pressure or flow, but why would you. This system is limited by available supply. You can never, never maximize flow to be greater than what the supply can give you, no matter how you control it. Please think of some scenario that might prove how you could do it. This problem is a good enough example. The object is to fill the trucks as fast as possible. Here I think the range we can pump from with a vfd or cs pump is 200gpm at 20psig to 300gpm at 17.5psig. Assume supply parameters are linear in between those two points. Control that, or any other parameter and it will not increase system flow. The best you can do is pump at max rate. A cs does that with no control, until it runs out of NPSH. That is a on/off low pressure suction switch. Don't need vfd to do that. What would be your maximum flow? How can you get more flow and stay within the supply limits. To fill a truck fast, run at max until suction drops, turn off, wait until suction returns, do again. With vfd, when suction drops, go to idle. That's the only difference. Otherwise run as fast as possible. Tthat may also mean you will always operate very close to min NPSHr. The cs will start, run a bit, then stop. The vfd will run constantly, but very slow or at idle when near lo P limit. Neither has appreciable advantage there, vfd will be more economic, if suction p returns slowly and it can spend a lot of time pumping at low rates. So is it better to run all day at low rates, or just shut off for awhile and wait until suction p builds higher and shoot it out quickly at constant speed, then shut down again. I dont know. More info required.

The only thing you can do with a pump, a vfd controlled pump, or a constant speed pump, is pump at some flow rate that is within the available supply rates. With a vfd pump you may be able to do that more efficiently when you want to pump at rates that are 30% less than the Constant Speed, CS, BEP rate, maybe not. The CS BEP - 30% is an approximate economic break even point for many VFD-CS decisions. Other of the economic advantage, you can do exactly the same thing with a CS pump and a control valve. Neither option will flow any more, or any less than the other. Neither option will increase pressure any more than the other.

The ONLY advantage of a VFD is efficiency in pumping at rates of 10 to 70% of CS flow. If you pump at very low rates a lot of the time, VFDs are great. If you want to pump at max all the time CS benefits are greater. Sometimes a VFD can eliminate bypass piping and save some additional money, but bypasses are not needed for filling trucks at max rates. Neither are control valves. A simple On/Off valve works. With all the extra controls and wires, if I can avoid a VFD, I prefer to avoid a VFD.

I dont say that because I don't like variable speed pumps. Most all of the oil pipeline pumps I work with are variable speed .. just driven by variable speed diesels. I do it without the "F" ... VSDs.

I wouldn't worry about any air entrainment. You are not pumping from a tank or sump that might vortex.

Why you would control on inlet / suction pressure is that maximises the available flow possible from your system without starving the pump of pressure.

Now there are other things which mean operating below 0 psig may create air bubbles as entrained air starts to come out of the water and you end up with "fizzy" water or maybe air pockets, but that's not the key issue here.

The inlet pressure to the inlet line is seemingly more variable than we are aware and also there may be some other flow restrictions we don't know about.

I don't think efficiency counts here, but I do agree with you most of the time on VSDs, it's certainly not a universal best method of ding it and even in this case, a fixed speed unit with control valve operated by inlet pressure would do just as good a job. The salesmen for VFD have done a great job on most non technical people.

The rea key is that our OP is just using too big a pump. End of.

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Also: If you get a response it's polite to respond to it.

I would indeed "control" suction pressure, but with an on/off switch. No pid/vfd needed. No energized circuits. No fans running.
Just like a level switch.

Why would you run a vfd on idle,
or run the pump at 1cm3/h for an unknown length of time?
Just to keep suction pressure low?

I think we agree, but maybe you're trying too hard not to. LOL I don't disagree with anything you said,

Yeah, that's exactly what I said in the beginning. OP sold himself on VFD before he ever got here and still won't let go, but that's not entirely unexpected. He's electrical and can apparently do whatever makes him happy. That's a killer combination.

Bentov, we fixed your NPSH problem, at least until you start trying to pump the pipeline down too much, so just slow down, or shut off or go to idle, or whatever, no matter how big an impeller you have, whenever the pressure drops below NPSHr. All is kool.

Itsmoked has a nice trick to eliminate the PSL if you're using a vfd, but close the tank fill nozzle when not in use.