Constant Pressure VFD on well tankfill application

[Open Neutral]

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.

I'm trying to understand what makes this Goulds-3AS30 Aquavar SOLO VFD so fancy, and expensive. 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?

I calculate:

380psi at the pump [0.432psi/ft * 880 ft]
the dynamic head @ 7GPM - [0.2psi/100ft * 9 hundred ft = 1.8psi]


So why does he need or want a "constant pressure" VFD? How will the pressure vary, and why would it help to adjust the motor speed in a start-stop tank fill application?

Other things disturb me, too.

380 psi is near the [normal] Schedule 80 pressure rating per one source [470 psi@73F - http://www.harvel.com/technical-support-center/product-specifications/pvc-pipe-schedule-80];
they will hang the pump from the PVC, not a stainless wirerope, etc.
What keeps the pump from torque twisting on startup?

But that's all for later.

Given the world class VFD gurus here, I'm sure I'll learn something on this topic....

 

[GoldDigger]

Constant pressure = the static head corresponding to the well depth plus the average tank pressure? The pressure range in the tank is small compared to that.

 

[Open Neutral]

Constant pressure = the static head corresponding to the well depth plus the average tank pressure? The pressure range in the tank is small compared to that.

Why would the tank be pressurized at all? That just makes life harder on the well pump....

The tank feeds the irrigation pump [not shown...].

The VFD has a pressure sensor to sense the line pressure.....

 

[ActionDave]

Why would the tank be pressurized at all? That just makes life harder on the well pump....

You need a pressure tank to get water pressure in the house so the water comes out of the pipes and the pressure stays constant. It is also easier on the pump. Without a pressure tank the pump will short cycle.

Back pressure on the well pump or irrigation pump does not make them work harder because the pump is moving less water. Takes a some concentration to get your head around it. Think about bailing water a gallon at a time per second then switch to a three gallon bucket and keep the same pace.

The tank feeds the irrigation pump [not shown...].

The VFD has a pressure sensor to sense the line pressure.....

Those Gould controllers are all the rage these days. They are a good way to regulate the pump and protect it from running dry. Very reliable from what I have seen. I think it is a good install for your situation.

 

[ptonsparky]

The soft start witht the VFD will prevent the pump from twisting the pipe at startup. The 1500 ft depth will take care of a good portion of that anyway.

 

[templdl]

The soft start witht the VFD will prevent the pump from twisting the pipe at startup. The 1500 ft depth will take care of a good portion of that anyway.

That reminds of a pump OEM of mine that provided pumping systems for municipalities. They took my soft starter and accessed a point in the circuitboard so that they could accelerate the centrifugal pump just to the point where it starts to pump water at which point it applies full voltage.
The reduced voltage caused the motor to slip to prrvent the shocknof water hammering before applying full voltage to the motor. I can't recall but doesn't a centrifugal actually start to pump at around 75% rpm?
It worked excellent and avaind the cost of using a vfd.

 

[kwired]

Why would the tank be pressurized at all? That just makes life harder on the well pump....

The tank feeds the irrigation pump [not shown...].

The VFD has a pressure sensor to sense the line pressure.....

How is tank and supply/outlet arranged? Where would the pressure switch be located? It is common with municipal pumping stations to control the tank level via monitoring pressure in the pump house where they have an elevated tank.

Is your tank elevated to provide pressure or is it a pressurized tank?

 

[Jraef]

...
I'm trying to understand what makes this Goulds-3AS30 Aquavar SOLO VFD so fancy, and expensive. 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? ....

If, as you say, the outlet of the well pump is going to the TOP of the tank and "dumping" into it, then I think you have a valid concern.

But it's more likely not set up that way. The new paradigm is to have the pump connected to the same pipe feeding the house FROM the tank, which eliminates the added static head of having to get to the top of the tank. Then the pressure transducer is maintaining a constant pressure on THAT feed pipe by going into a PID loop in the VFD to modulate the pump speed. So as the user draws water from the feed pipe, the tank level drops, which lowers the pressure and the drive speeds up the pump, based on the flow, to maintain that pressure. The tank then serves as a "surge capacity" against the recovery rate of the well. As the well level drops, the pump has to work harder to maintain the pressure (because static head is increasing), so the drive may end up slowing itself down to prevent the pump sucking air. The tank then supplies continued flow, at maybe less pressure, until the well can recover.

Secondary issues: the VFD is allowing them to use a 3 phase pump when they only have a single phase source, which lowers the current in each conductor going down the well and that translates to less wire size, less weight, less VD etc. Also the soft start capability prevents the twist stress you mentioned, and then soft STOPPING can prevent water hammer if they have a foot valve or check valve in the pump. It also eliminates the need for starting capacitors and potential relays in the control panels for single phase submersibles, which gets very problematic on 1500' wells.

That Goulds drive is not really doing anything that cannot be done with any other VFD, except they tailor the user interface to make it very simple for people using it to understand, mainly by hiding all of the internal complexity from them. Plus they extend the warranty on the entire system when used as a package, and there is no finger pointing when something goes wrong, you make one phone call to Goulds. When people use their own VFDs on Goulds pumps, they get the basic warranty on the pump and as a rule, any issues are immediately blamed on the VFD first (even if it isn't the problem). It's just good marketing really, and several of the major pump companies are following suit now.

 

[GoldDigger]

As I understand the OP's description the well pump is feeding an open (atmospheric pressure) storage tank, so there is no pressure in the tank other than the depth of the water. There is a separate pump from that tank to pressurize the irrigation system.
If the tank is empty the extra few feet of head to dump into the top of the tank is not going to matter much.
But if it is an atmospheric tank, the OP will probably need to use a float switch rather than a pressure switch to control the pump.

 

[ActionDave]

As I understand the OP's description the well pump is feeding an open (atmospheric pressure) storage tank, so there is no pressure in the tank other than the depth of the water. There is a separate pump from that tank to pressurize the irrigation system.
If the tank is empty the extra few feet of head to dump into the top of the tank is not going to matter much.
But if it is an atmospheric tank, the OP will probably need to use a float switch rather than a pressure switch to control the pump.

Even so he is still going to need some kind of pump starter and a way to protect the well pump from low or no water. The Gould drive does both.

 

[Besoeker]

How is tank and supply/outlet arranged? Where would the pressure switch be located? It is common with municipal pumping stations to control the tank level via monitoring pressure in the pump house where they have an elevated tank.

Is your tank elevated to provide pressure or is it a pressurized tank?

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.
Sure, we have done systems where the pressure, at the point where it is measured and taken as a feedback signal, is maintained at a constant level.
But that's a system design issue /requirement.
A VFD is a VFD and an element in that system. It has to be told what to do to maintain constant pressure. It isn't an inherent characteristic. At least not one that I've come across.

 

[Jraef]

As I understand the OP's description the well pump is feeding an open (atmospheric pressure) storage tank, so there is no pressure in the tank other than the depth of the water. There is a separate pump from that tank to pressurize the irrigation system.
If the tank is empty the extra few feet of head to dump into the top of the tank is not going to matter much.
But if it is an atmospheric tank, the OP will probably need to use a float switch rather than a pressure switch to control the pump.

That's why I said, IF it is as he depicted, his concern is valid in that a pressure control setup on a tank fill from the top is pointless.

The reason I posted that diatribe was to suggest that, if he was ASSUMING that to be the case, then that might explain why they sold them the constant pressure system. I have been seeing them pushing that a lot around here, and many of the people who were used to seeing the top fill atmospheric systems don't have experience with this new concept yet.

But now that you mention it, the irrigation pump down stream (assuming that's what he meant) is not fitting that scenario, it would seem pointless. So maybe they sold him the packaged system for the well pump just because it's a 3 phase pump and a single phase supply so the VFD solves that issue. In that case, the pressure transducer system on the well pump might end up as superfluous and they will need something else to tell that pump when to come on, i.e. level control.

It is also possible (albeit not likely because I know the Goulds people I have encountered don't have that kind of depth) that someone is ADAPTING the intended use of the pressure control AS a form of level control. If you can put the pressure transducer into the bottom of the tank, the level of the tank will correspond to the pressure on that transducer and with the proper math, can be used to maintain a level in the tank.

 

[Cow]

It is also possible (albeit not likely because I know the Goulds people I have encountered don't have that kind of depth) that someone is ADAPTING the intended use of the pressure control AS a form of level control. If you can put the pressure transducer into the bottom of the tank, the level of the tank will correspond to the pressure on that transducer and with the proper math, can be used to maintain a level in the tank.

This is what I would bet on.

I just gave a quote to a customer for wiring a similar setup. I believe the pump company here uses these same Gould VFD's. They are planning on using a pressure transmitter to monitor water level in a ground water sump.

 

[Jraef]

I just looked at a manual. This is intended to be used with a pressurized diaphragm tank, not an open tank. You can hook up a float switch, but you disconnect the pressure transducer. It's either or, but the unit comes with the transducer pre-wired to the drive, so you pay for it whether you use it or not.

Also, it protects against the pump running dry by a combination of the pressure feedback and by monitoring the motor power for the given pressure, which makes sense. If you use the float switch control you have to turn that off, so you lose that protection.

 

[sparkyrick]

Filling a simple cistern type water tank, definitely not required. Maintaining a constant pressure on a closed water system, like in a home, a nice thing to have.

 

[GoldDigger]

Filling a simple cistern type water tank, definitely not required. Maintaining a constant pressure on a closed water system, like in a home, a nice thing to have.

A pump with a constant pressure valve (requiring a minimum flow to work) is one of the alternatives to a cyclic pressure tank system for residential. That type of system has both advantages and disadvantages. The pump runs longer but at a reduced load because of the throttling. It is not particularly suitable for the average submersible deep well pump which requires a minimum water flow for proper cooling.
It sounds like the OP's system may have been designed to hold a constant tank pressure by matching the outflow via the VFD working with an analog pressure sensor.
At some point (low flow rate) it would have to just stop the pump when the speed got too low and cycle it back on with hysteresis.

 

[ptonsparky]

...
That Goulds drive is not really doing anything that cannot be done with any other VFD, except they tailor the user interface to make it very simple for people using it to understand, mainly by hiding all of the internal complexity from them. Plus they extend the warranty on the entire system when used as a package, and there is no finger pointing when something goes wrong, you make one phone call to Goulds. When people use their own VFDs on Goulds pumps, they get the basic warranty on the pump and as a rule, any issues are immediately blamed on the VFD first (even if it isn't the problem). It's just good marketing really, and several of the major pump companies are following suit now.

I sold quite a few VFDs for wells until Gould and others stepped in and marketed them that way. Now the pump installers sell almost all of them.

 

[sparkyrick]

We did a trucking firm with one of the fancy schmancy constant pressure well pumps. During construction, they tried bring the pump online with just a simple hose bid attached inside where the water entered, so the rockers had water to do their work. The pump refused to work. The pump guys came out and said it was because there wasn't enough of the buildings plumbing system attached. I guess their high dollar pump and controller couldn't supply a single hose bib without malfunctioning, but once the entire building went online, everything was fine. Dumb.

 

[GoldDigger]

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.

 

[Open Neutral]

I just looked at a manual. This is intended to be used with a pressurized diaphragm tank, not an open tank.

A 5000 gallon pressurized tank? What would ASME have to say about that?

Yes, I'm thinking about an open tank, with an air-break between the well output and the stored water. I'm not worried about the pump load as much as the piping stress.

There is a 2nd 2HP pump and Gould VFD to pump to the orchid; it's rated 27 GPM @ 75 PSI. I might see that use of constant pressure regulation but at a steep price..