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]

psi set at 63
What psi?

Why hold 48 in the tanks if the vfd is trying to keep them at 63?
That's how I (misunderstood) what you said.

Best thing to do is scrap the vfd. Put your pump on a pressure switch to cut off at a couple of psi below the setting of the tank.

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

 

[TD2K]

I'm assuming based on your write-up the pump is intended to start at 48 psig and then stop at 63 psig?

Sounds like the demand on the system is changing (that's why the discharge pressure rises and falls) and is greater than your pump can supply short-term, that's why the pressure is rising and falling but doesn't get high enough to stop the pump for a while (when the demand drops off enough for the pressure to build to the stop pressure).

You mentioned you have a variable speed drive, do you simply adjust the speed as needed and then leave it?

What % of BEP does the flow corresponding to 48 psig and 63 psig correspond to?

 

[noideahow]

sorry for not being specific. the pressure switch for vfd starts at 55 and stops at 62. from what i was told the pressure in the tanks should be at 15 psi under so i put the tanks at 4round 48. i can adjust the pressure of the vfd easily. before this system i had like 6 tanks and no vfd. with that system it would just turn on once and stop..wouldnt cycle this fast. i wonder if this is fine or if i need more tanks so the cycle takes longer to ho through when the demand is high. sorry if im all over the place, i dont know much about this. thanks.

 

[noideahow]

tried giving u a star td2k but its not working?

 

[TD2K]

I'm more than slightly confused trying to understand your numbers in your various posts and how they relate to each other.

However, the tanks supply water to the system, at the expense of the pressure falling, until the pump kicks on and repressurizes the system. So, adding more tanks will lengthen the time before the pump has to kick on.

http://www.cyclestopvalves.com/tanksizing_12.html

I don't know how well this relates to your system but it's interesting they also refer to 119 gal tank. A 40 psig start/60 psig stop setting allows a 119 gal tank to supply 31 gal. You have only a 7 psi margin with a pump that puts out 230 gpm.

You'll need to look I think at your system and see if you have the necessary surge capacity.

 

[noideahow]

thanks i guess i need more tanks. i was told that 1 119 gallon tank should be good enough but i wanted to be sure so i got 2. seems when there is a medium demand of water my pump will start when it goes to 55 psi and will go all the way to 61 psi but not stop and go back to 55 a couple of times then go to 62 then finally stop. this is worrying me because i think this would effect my pump.

 

[TD2K]

If the water pressure isn't getting up to 63 psig which would shut down the pump, it sounds like the water demand is such that the pump just can't get there. As long as you have verified that the pump can put out 63 psig in terms of minimum flow I don't see a problem.

 

[BigInch]

I have no idea why you want a vfd here. You're wasting good money. ON/OFF is the most efficient means of pumping, provided your off time is not too long, which would indicate you could have an even smaller system. There is apparently no need to drive match rpm exactly to a particular demand here and the vfd is probably slowing down when you reach the higher set pressure, even if you have a high demand at that time and actually need more water and pressure!

Remove the vfd. Run this pump to turn on using a (low) pressure switch. Adjust the pressure setting such that the pump turns on at whatever the lowest pressure is that does not deliver sufficient flow for whatever it is that you want to do at the time. If your tasks and water demand vary with task, keep an eye on the pressure and keep in mind what the lowest pressure you see has been when you start needing more water. That's the set pressure.

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

 

[bimr]

I agree with TD2K. The water demand is greater than the pump output at times and that allows the pump to run longer.

If you have a quality pump, the pump is made to operate continuously, so there is no problem. Operating off the BEP and frequent start/stops of a pump would be more of a concern for the life of the pump.

I don't see a need for more tanks with the scheme that you are operating. However, you may need a slightly larger pump if you can not hold water pressure when you are pumping at the flow rate of maximum water demand.

It is true that there is no efficency gain by using a VFD as Biginch says. However, without the VFD, the pump would cycle on-off more frequently and more tanks would be needed.

 

[noideahow]

I rather not get rid of the vfd since I spent some good money on it already. I had 4/6 busted tanks with the old system an was told to get the vfd instead because it would "save on electric, would be better for the pump, and I would need just one tank. Wonder if I got screwed

 

[BigInch]

You got screwed big time. A cycle stop valve would have been a much better deal to protect against any high pressure spikes.

If you had to pump to a precisely set and often variable flowrate, or to a set constant pressure all the time accepting a variable flowrate, it might have been a good buy, but IMO you can rest assured that it wasn't from what I knwo about this situation so far. Cut your losses; remove it now.

Pump start and stops. There are BILLIONS of pumps out there that run on ON/OFF level controls, ON/OFF pressure controls, etc. If they are off too much, or cycling on/off too much, NEXT TIME SAVE MORE MONEY AND BUY A SMALLER PUMP. Or buy a big one and only run 1 minute a day.

Pressure tanks are a way to equalize any mismatch between pump capacity, supposedly sized for the average flowrate and running nearly constantly, and the maximum demand, which you would draw from the pressure tanks as the pump continued to pump into those tanks at average flowrate. Thus the pump can be sized for average flow and does not have to be oversized to meet maximum demands itself. You have an oversized, and probably overpressure capacity as well, which is why you busted the previous tanks.

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

 

[noideahow]

So you're suggesting to get rid of vfd and then purchase 7 tanks? I believe the tanks busted because it was more than yrs old. 13 yrs to be precise.

 

[BigInch]

Whatever.

Why 7 tanks?

Start with an assumption of a pump optimum flowrate = Qo = should be as close to the the average flowrate of the system as possible.

Find your maximum demand flowrate, Qmax

Max Demand Volume = (Qmax-Qo) * Duration of Max Demand

That Max Demand Volume is what you have to put in the tanks. Buy enough tanks to do that, or raise the pump optimum flowrate, recalculate and buy fewer tanks.

If you keep raising the pump optimum flowrate, it eventually gets to equal the max demand flowrate and you don't need any tanks. If 7 is the right number and the cost looks OK to you, buy 7. If that's more expensive than a bigger pump and running a vfd at low rates and high rates and paying for pump power plus the 5% vfd surcharge and it's replacement cost when it burns, then do that.

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

 

[TD2K]

Since you purchased the pump with a VFD drive, did you consider controlling the discharge pressure with the VFD?

 

[BigInch]

Isn't that what he's doing now? I don't know what he's doing now.

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

 

[thewellguy]

Assume that you are talking about a submersible well pump, the manufactures require a minimum of 1 minute of run time once the pump has started and they dont like them to be started more than 20 times a day if you are using standard across the line starters.

Standard 119 gallon captive air pressure tanks have a 32 gallon working capacity on a 40/60 psi cycle so you only have 64 gallons of capacity which is no where near enough.

Is sound to me like the set points on your vfd are incorrect. What brand is if? Who programed it?

The pump has a minimum speed of usually half the name plate rpms but other than that it should ramp up and down to follow the water use. What is the delay time between hitting system pressure, and shut off of the pump? What actually shuts the pump off and how long is that delay between hitting that point and shut off?

 

[bimr]

Actually, the maximum number of well pump starts is around 50 for a single phase motor if you are using a franklin motor.

http://www.franklin-electric.com/aim-manual/page-3.aspx

Motors should run a minimum of one minute to dissipate heat build up from starting current. 6-inch and larger motors should have a minimum of 15 minutes between starts or starting attempts.

Your pump is probably less than 6-inch size. Therefore, your pressure tank should be sized to minimize the number of pump starts to approximately 12 per hour or 50 per day. If your number of starts is below that, you don't have to add tanks.

One of the advantages to a VFD system is that the pressure tank volume can be smaller.

I know that Biginch is adamantly opposed to VFD's. But if you already have a VFD, there is no point in spending money to change it. You will not have enough change in efficiency to make it worthwhile.

 

[BigInch]

That's 50 per day.
I am only opposed to vfds where they are not needed.
If you pull the vfd you gain at least 5% efficiency and life-time energy costs, plus possibly replacing them in 5 to 8 years. Might not sound like much, but for some projects, especially those involving renewable energy, and engineers, those can be a make or break numbers.

Hey, if you have a pet VFD... feed it. What else can I say.

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

 

[bimr]

But in this business case, the justification for replacing the VFD most likely involves a return on investment.

It is very difficult to justify a ROI, (especially a ROI after taxes) unless the payback occurs in less than a year.