Constant Pressure VFD on well tankfill application

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GoldDigger said:
Well, some constant pressure pumps work best with a small (~2 gallon) pressure tank attached to prevent chatter.
With the whole building plumbed in, the sum of all the air chambers at each of the fittings would add up to a large enough "pressure tank".
Maybe not the reason, but a potential problem.
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It is a balancing act to get the right size pump, pressure tank, etc to make the system work together successfully. Easier to run your finger across a chart to select the recommended parts than to build from scratch each time.
 
Besoeker said:
I'm with you and, TBH, I've never hears of a constant pressure VFD. Or at least not until I stumbled across this threads.
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I believe they are generally being marketed by the pump makers as a constant pressure controller, but is essentially a VFD, tailored to controlling water pumps, instead of just a general purpose drive. Most will power either a three phase or a single phase pump motor as well, you just change a setting, and discontinue using the starting capacitor/potential relay control box and hook the motor directly to the drive.

Constant pressure name comes from its ability to maintain a fairly constant pressure in the system as compared to just a simple system controlled by a pressure switch which will vary pressure depending on cut-in, cut-out set points. With those set ups pressure easily varies up to 20 or 30 PSI in many systems.
 
kwired said:
I believe they are generally being marketed by the pump makers as a constant pressure controller, but is essentially a VFD, tailored to controlling water pumps, instead of just a general purpose drive.
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How do you tailor a VFD specifically to control water pumps other than by having external transducers? If it is controlling pressure, the pressure transducer has to where the pressure is being measured - i.e. not in the drive.
The drive just does what it is told to do.
 
Besoeker said:
How do you tailor a VFD specifically to control water pumps other than by having external transducers? If it is controlling pressure, the pressure transducer has to where the pressure is being measured - i.e. not in the drive.
The drive just does what it is told to do.
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The parameters are already set when the installer gets them. We may have to change a few but for the most part it is plug and play or the menu is limited to just those needed for centrifugal pumps.
 
ptonsparky said:
The parameters are already set when the installer gets them. We may have to change a few but for the most part it is plug and play or the menu is limited to just those needed for centrifugal pumps.
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But how does the drive control pressure without an external transducer?
 
Jraef said:
The drive in question here comes pre-packaged with an outdoor rated pressure transducer, already wired to the drive with I think 100 ft. of signal wire.
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So, as I said, external to the actual drive.
I think calling it a "Constant Pressure VFD" is a bit of misleading sales speak.
 
Open Neutral said:
A friend in the Sandy Eggo area just had a well drilled. Gulp, 1500 ft.

The well driller wants to sell him a:

GOULDS 7GS30 7GPM 3HP 4IN PUMP END
CENTRIPRO 3HP 230V 3PH 4" SUB MOTOR
3HP AQUAVAR SOLO CONTROLLER 1PH IN/3PHASE OUT CONSTANT PRESSURE DRIVE

and 880 ft of
1.25IN DEEP SET PVC DROP PIPE W/SS COUPLING,
to fill a 5000 gal tank @ 7 gpm.

.... Given the well dumps into a tank, there should be zero head pressure at the tank, so why does he need or want a "constant pressure" VFD? How will the output pressure vary, and why would it help to adjust the motor speed in a start-stop tank fill application? ....
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Disclaimer: I am out of my element with well pump controls. But I have been around a few.

1500' well - pump set at 880'. I wonder what is with that. Maybe they didn't hit the aquifer until 1500' and the water level came way up. Just curious - Probably doesn't matter much.

Back to your question;
7gpm is pretty low production. The 5000gal tank says the required flow rate is more than the well can deliver. Since the water is for irrigation, I suspect the pump is running pretty close to 24/7. That translates to: They need water. They need all they can get.

I'd guess, the pump controls are set up to limit the well flow rate to where the well does not get pumped down. As noted by others, the "constant pressure" doesn't make much sense. I'm guessing the "constant pressure" controls are the same as is used to dial down the well flow rate to just under all the well can do.

You might consider asking the supplier what is the purpose for the controls? And ask the all important question, where are they measuring the pressure? As you noted, the tank is atmospheric, and once you are out of the well head, there is very little pressure drop to the tank.

ice
 
ptonsparky said:
Given your knowledge and experience level vs mine, I am a bit hesitant, but Yes, unless some other mechanical means was employed which would negate the reason for the drive.
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Well, we've been supplying VSD pump systems into the water industry for quite a few years. Most of these have been for water companies/authorities supplying potable water. Pumps in pumping stations pumping treated mains water - lots of it and quite often over long distances. The London Water Ring Main is 50 miles for example. One of the more important parameters they have to control is pressure - you don't want too many burst water mains under the streets of London......

Pressure is measured remotely from the drive. In the early days we had external PID controllers. Nowadays, a PID loop is often part of the drive internal package. The PID loop can be used to any measurable parameter including, but not exclusively, pressure.

Perhaps that goes some way to explaining why I think calling it a Constant Pressure VFD is a bit.........odd.
 
Provide an analog water level sensor and you now have a constant water depth pump!

PS: The dry running prevention is often done simply by sensing load current versus pump speed and would not need to involve a pressure switch at all.
Low current, low power factor with full voltage applied means that the pump is sucking air.
 
iceworm said:
Disclaimer: I am out of my element with well pump controls. But I have been around a few.

1500' well - pump set at 880'. I wonder what is with that. Maybe they didn't hit the aquifer until 1500' and the water level came way up. Just curious - Probably doesn't matter much.
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I too wonder about that. Pumping from a 1200 ft water level means > 500psi load on the pump and the piping...so it's a good thing....

Other parts of the quote bother me as well.
  1. The quote includes 1IN SCH 40 PVC PIPE- SOUNDING TUBE. What do you do with that in this application? Drop a tape measure down?
  2. There's no stainless support cable; in my limited experience with wells, there always was same. Relying on PVC for supporting the pump is.....interesting.
  3. What is the pressure rating for their 1.25IN DEEP SET PVC DROP PIPE W/SS COUPLING

Back to your question;
7gpm is pretty low production. The 5000gal tank says the required flow rate is more than the well can deliver. Since the water is for irrigation, I suspect the pump is running pretty close to 24/7. That translates to: They need water. They need all they can get.
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I don't think he has specific goals. He wanted a well to save some of the orange trees there now. [He purchased the house and orchid this year.] 7gpm => 10,000 g/day.

I'd guess, the pump controls are set up to limit the well flow rate to where the well does not get pumped down. As noted by others, the "constant pressure" doesn't make much sense. I'm guessing the "constant pressure" controls are the same as is used to dial down the well flow rate to just under all the well can do.
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That makes little sense to me; the driller was claiming it delivered 30+ gpm but the pumps costs would escalate significantly.

You might consider asking the supplier what is the purpose for the controls? And ask the all important question, where are they measuring the pressure? As you noted, the tank is atmospheric, and once you are out of the well head, there is very little pressure drop to the tank.
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There's an external pressure sensor. My questions start at "Why even care about the outlet pressure?" I would have a pump control based on tank level: the level drops, the pump starts; the tank is full, it stops.

I plan to ask Goulds what flavor control they would spec for same. Wading through their literature now....
 
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