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variable frequency drive
5

variable frequency drive

variable frequency drive

(OP)
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?

RE: variable frequency drive

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?

RE: variable frequency drive

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?

RE: variable frequency drive

(OP)
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.   

RE: variable frequency drive

(OP)
tried giving u a star td2k but its not working?
 

RE: variable frequency drive

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.

RE: variable frequency drive

(OP)
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.
 

RE: variable frequency drive

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.

RE: variable frequency drive

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?

RE: variable frequency drive

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.

RE: variable frequency drive

(OP)
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  

RE: variable frequency drive

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?

RE: variable frequency drive

(OP)
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.  

RE: variable frequency drive

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?

RE: variable frequency drive

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

RE: variable frequency drive

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?

RE: variable frequency drive

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?

RE: variable frequency drive

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.

RE: variable frequency drive

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?

RE: variable frequency drive

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.

RE: variable frequency drive

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?

RE: variable frequency drive

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.....

RE: variable frequency drive

(OP)
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

RE: variable frequency drive

(OP)
Or the range of 40-60

RE: variable frequency drive

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?
 

RE: variable frequency drive

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

RE: variable frequency drive

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.

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