VFD to maintain constant irrigation pressure 4

[gare]

I have searched the forums and I know I have missed something, but:

We have a submersible 5hp Franklin motor/pump, 3 ph 208 VAC, 18.3 FLA. The pump resides about 350 feet from the controller location (I need to measure it). There is a pneumatic tank next to the controller set at 60 psi.

The motor was being started and stopped across a nema starter via a pressure switch set at 75psi on and 95psi off. When irrigation starts it cycles every couple of minutes, less on larger zones. Motors were not lasting long and it is tough to set sprinkler patterns with 20psi swings.

We replaced the controller with a Gould SPD-2-0050 VFD (max current 17.8amps). We left dips switches at 100% output. We set the jumper for the system to run at 75 PSI. With no irrigation on and system pressure at zero, the motor draws FLA at startup then drops into the 11-13 amp range after about 10-15 seconds (down to 11 up to 13 non-stop) and never reaches pressure, it just keeps running at about 70 psi.

When irrigation is turned on the system gradually drops to 40 psi and does not recover. We reversed two leads with no apparent change in current draw or pressure.

I am merely a somewhat ignorant electrician working with an irrigation company, we are wits-end, the client is angry (no irrigation 100 degrees...). The Gould rep said, take a guess, "this will take care of everything". I would appreciate any input that may help to resolve this issue.

gare

 

[gare]

I called ITT/GOULD technical support in NY.at 5:15 PST. I can't go to the site, so it is great to call everyone on the east coast.

They are having us check input and output voltage LL and LG, I had only confirmed input LL. Not knowing VFD's I did not think I could really check output voltages and know what I should read. I live and learn.

We will confirm that we are getting full output voltage or not. They also want us to remove the jumper setting the unit to a factory default 75 PSI. Removal sets the unit at 50 PSI. We will confirm the system gets to 50 psi and then use the INCREASE button on the unit to bring it up to the 75 PSI we would like to maintain.

They were nice enough to give me a direct phone number so I will not go into a waiting queue when I call them with the results.

 

[jgailla]

gare,
I'm ignorant on irrigation issues, but a VFD pump for an irrigation system seems like overkill.
I'm used to seeing a bladder tank to handle the pressure surges and so that there is a small buffer of water storage so the pump doesn't cycle.
You may want to check to see if the other components of the system are working before you start messing with the pump.

 

[bimr]

jgailla

That is a good point that you made. There is no advantage to using a VFD. The pump output should be matched to the sprinkler system design.

 

[cvg]

all components including the pump should be reviewed:
is the tank large enough
is the pump curve acceptable
is 5 horsepower sufficient
is the pipe sizing acceptable
is a pressure / flow regulator necessary

 

[gare]

We did not install the system. It seemed the pump/motor might be oversized, as it reached max pressure very soon, even with the largest zone active. To get accurate pump information we would have to pull it out of the tank to see what it is and get the information on it. Plus, the piping lies under at least 350' of very finished landscape (between pump and controller), so redoing everything is just not feasible.

We thought of making larger irrigation zones so that the pump would tend to run longer, but again, this would be quite a task (there are thousands of feet of irrigation pipe in the ground). All things said, the owner does not want to have to spend a lot of money to make something work that should already be ok.

It trips out about once a month, we assume it is on overload. We assume because the LEDS on the monitor are not visible unless the controller is open and the manager resets it by throwing the disconnect off and on.

We ran the system and found that even on large zones it got up to 95 psi very quickly, then dropped to 75 very quickly (30 seconds). We thought of a bypass valve to just dump excess back in the sump to keep the pump from cycling but ultimately decided on the VFD.

We neglected to take proper notice a Cycle Stop Valve in line after the check valve. It seems to cause odd pressure differentials on either side (upstream or pump discharge side) of the check valve versus downstream where the transducer resides. We think that because pressure on the downstream side starts approaching our pressure setting, the VFD slows down, then the CSV sees a pressure differential and begins to close causing the pressure to drop upstream and remain constant at the transducer, then the CSV starts opening again and the pump speeds up. Over and over again.

 

[cvg]

even though you didn't design it, you are re-designing it.

You will take the blame from this point onwards if it doesn't work properly. Without really knowing what is wrong, it seems that you are shooting in the dark, hoping that this VFD will solve your problems. Hope so...

 

[gare]

yes cvg, very true. And, though I do not like it, we needed the work. I attached a picture to show the CSV in place. I have no idea how to link to the picture though.

 

[gare]

Oh, I see how...sheesh:

http://files.engineering.com/getfil...2-7821-4db1-bbf9-eb79afa10ccd&file=csv001.pdf

 

[gare]

Good thing feet grow back cuz I have shoved plenty down my throat. The CSV is designed to do allow a motor to run at full speed, throttling the output of water into the system. When there is no more demand it is supposed to divert water to the singular hydro tank and the pressure increase shuts the pump motor off. It stops hammering, stops cycling of the motor, and is supposed to provide a constant pressure regardless of demand... Well I do not mind learning.

So either the one there failed, is not adjusted correctly, or is in the wrong location.

gare

 

[atook]

I've specified VFD pumps for larger irrigation systems, in order to deal with the problem of variable flow requirements, but a constant pressure.

If the VFD is sized right, you shouldn't need to use a CSV to throttle the output of the water, let the VFD do that.



Tim Grote - The Irrigation Engineers.

http://www.irrigationengineers.com

 

[gare]

I had another thread going on this, with a slightly different theme. We ended up using the VFD. It is along story and I will not burden this forum with it:

http://www.eng-tips.com/viewthread.cfm?qid=254975

gare

 

[gare]

Sorry, Thank you for all of the input.
gare

 

[thewellguy]

it sounds to me like the cycle stop on the vfd are fighting each other.

 

[BigInch]

The CSV is a pulse pressure control valve, not an operational pressure control valve, so probably not, unless the vfd pressure signal is coming from a very fast acting transducer and the pump responds equally as fast to the vfd control signal, but it is a possibility.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/