variable frequency drive 5

[noideahow]

some info,

goulds variable frequency drive
12 hp motor
230 gallon a minute
2 119 gallon water pressure tanks
psi of tanks set at 48
psi set at 63
commercial business

Was wondering if i would need more tanks? seems like my pump keeps cycling without turning off for awhile. ( goes to 61 then down to 53 then up to 61 then back down. pump doesnt shut up unless this happens like 3-4 times. does anyone know if i need more thanks or if i should decrease psi? is this bad for the life of my pump?

 

[BigInch]

It will only make that roi 5% slower.

It's simply a pet vfd. There's pet rocks, or there were. There's virtual pets. Why not a pet vfd.

What would you be doing, if you knew that you could not fail?

 

[1gibson]

Did you get screwed? Well, it sounds like you took system design advice from a salesperson for one of the (superfluous) components. So yeah.

If you are running a pump continuously at 1/2 BEP flow, then yes a VFD can save you money. If you have the option to run the pump for 50% less time, at BEP, then operating on the VFD will just cost you more in utilities, not to mention the initial cost.

You use a VFD to tailor the pump operation to a specific, instantaneous demand. In your scenario, the tanks take care of the system demand, the pumps just top the tanks up.

What you are doing now with the VFD is analogous to filling up a tire with an air compressor, but as you get closer to desired tire pressure, you turn down your line pressure. You could fill all 4 tires with "full" line pressure, in the time it takes to fill 1 tire with "variable" line pressure.

But if you are interested, I could sell you a variable line pressure tire filling machine, they're great, and you can use them with a smaller air compressor tank.....

 

[noideahow]

So the pump turns on at 57 and shuts off at 62. Should I increase the range to 50-62. If so what should I put the pressure of the tanks? Doing this will itt take longer for it to cycle? If I got screwed it is what it is. I just want to know how to fix this. Thanks for all the feedback

 

[noideahow]

Or the range of 40-60

 

[hacksaw]

as a late commer, a bit confused about what you are trying to accomplish,


1. are constant pressure tanks being used?
2. are you attempting to deliver constant pressure, if so at what pressure.
3. what is your flow demand? Is it 230 gmp at the desired pressure?

 

[hydrae]

Given
You have a VFD
you have two 119 gallons tanks, do you have room for more?

Do you know the typical diurnal demand curve for the facility?
(how many hours per day do you need 10 gpm, 50 gpm, 230 gpm...

What is your goal?
1) save money I would assume.
2) provide a stable constant pressure.
3) minimize extreme well draw downs to avoid certain water quality issues.

In any case to achieve the goal, you need to run the numbers.

If your goal is to save money:
Get the pump curve, this is more than one curve since you are using a VFD, if you can not get the multi speed curve use the affinity laws to get the other curves.

Use the system demand, system curve, and pump curves to find the BEP of the SYSTEM, this would include suction head, discharge head, friction losses, at each of the flows in your diurnal curve.
I assume you are drawing from a well which may have a high or low specific capacity, this will change how the numbers come out. In your calculations include the parasitic losses (3-5% constant from a VFD), head losses from a Cyclestop valve if you choose to change out to that type of control, losses in pump efficiency from running pump beyond the expected operating range.
Add in some costs for each start cycle. (more starts = shorter pump life, more important in the big motors).
Add in costs for shorter pump life when running out of normal range (that is, normal range for that speed).

My point, there is a set of operation points that will result in lowest operating cost, but you will not know if you are there unless you run the numbers.

As an example, if the well has a low specific capacity, it is more efficient to run the well at a slower pumping rate for longer run times. To do this you have 3 options, use the VFD, install the Cyclestop (or equal), or put in a smaller pump. Each of these solutions have capital and O&M costs. They also have other drawbacks such as, if you put in a smaller pump you may not have enough water when you need the high flows and then you are sacrificing customer service to save a few dollars.


Hydrae

 

[wjmeng]

I've used a number of VFD's in this application with one or two small tanks to smooth out instantaneous demand to allow the VFD to ramp up and down without creating pressure spikes. We install a pressure transducer and connect the VFD, set the pressure at exactly what we want and it's maintained anywhere between zero and maximum pump output. It sounds like you are lacking the transducer and are using the VFD like a pressure switch which doesn't really result in a constant pressure or in electrical savings. Also, when installing the VFD you need to set the ramp up and down times so the pump speed isn't jumping around trying to find the operating speed/pressure. Typically we have around 300 GPM pumps (2) with two 119 gallon bladder tanks and we can maintain 50 PSI constant for 200-300 unit condos and mobile home parks. We do have pretty good lightning protection in front of the VFD because these facilities are in central Florida.