Pump education

[FIMCO-MEISTER]

The weakest part of my irrigation resume is pumps. Don't run into them that often so I refer them tho a bud who deals with a lot. Anyway I've decided to learn something. These are the pics. The pump works now but is really noisy. The customer said they have had 5 in the last seven years. Apparently the creek gets really high and floods in the pump area. The inlet pipes are in water 3-4 feet deep. Take a look and advise, educate puhleeeze.

[FIMCO-MEISTER]

More pics.

[FIMCO-MEISTER]

looks like a slow night. I guess I'll crash and see what the morning brings.

[drmiller100]

5 HORSEPOWER?????????

GOOD GRIEF!!!!!!!!! so what are you irrigating, 20 acres??????

how much pressure are you seeing???

[FIMCO-MEISTER]

5 HORSEPOWER?????????

GOOD GRIEF!!!!!!!!! so what are you irrigating, 20 acres??????

how much pressure are you seeing???

This was an existing system so I can't give any history. Just started working on it yesterday. They have 2 acres spread among 20 zones of spray and rotors.(Not mixed) Plenty of pressure.The rotors do not have overpowering pressure. Appear to be 5-6 pgps per zone with no9 nozzles. Now comes dumb question time. Is there a pump that can be installed where this one is and handle occasional flooding? I have another customer with a submersible pump. I'm wondering if there are above ground pumps that can handle being submerged. If not we will be moving and I think replacing this one because it is so noisy they can only run it in the daytime to avoid waking neighbors. Once again this is the fifth pump in 7 years according to the lady so who knows the history of the pump sizes.

[drmiller100]

hmmmmm.
someone check my math please.
a 5 horsepower pump, assume you have 20 feet of lift, and produce 50 psi. I get something like 130 feet of total head.

according to http://www.ajdesigner.com/phppump/pump_equations_discharge.php

that should produce 150 gallons per minute.

Now we take a pgp with 9.5 nozzles. 5 of them at 10 gpm is 50 gallons per minute.
so, in other words, the pump is sitting there cavitating its entire life. Which destroys them pretty quickly.

Figure out each zone, adn how much water each zone takes. for grins, try turning on 3 or 5 zones manually so they all come on at one time, then run the pump.
My bet is the zones are waaaaaaaay undersized for the size of your pump.

[FIMCO-MEISTER]

Thanks I will have more questions after I review the tutorial and understand cavitating. What about an above ground submersible? Such a thing?

[FIMCO-MEISTER]

Also the zones are running closer to 20 to 25 gpm. pgps no 9 nozzle isn't 9gpm which means the situation is even worse as far as cavitation goes I'm guessing.

[drmiller100]

i found exact specs on that exact pump.
http://www.font.co.cr/CDCorporativo/bombas/jacuzzi/112-133.pdf

130 feet of total head, 5 horse, you are going to be something near 70 gallons per minute.
again, try running a couple of zones at once.

[drmiller100]

Thanks I will have more questions after I review the tutorial and understand cavitating. What about an above ground submersible? Such a thing?

above ground submersible is a sump pump. not what you are after.

why not just move the pump further up the hill???? if you do, upsize the suction line, and don't make it more then about 20 feet of vertical from the creek to the pump.

[drmiller100]

again, run 3 or 4 zones manually at once to see what happens.
i've been known to actually design well systems with a bunch of small zones, then do the math on what i built, then wire multiple valves to specific zones on the controller so the controller fires multiple valves to get the gpm i am after.

so, figure out what zones are going to be happy running at once, then rewire teh controller.

[FIMCO-MEISTER]

again, run 3 or 4 zones manually at once to see what happens.
i've been known to actually design well systems with a bunch of small zones, then do the math on what i built, then wire multiple valves to specific zones on the controller so the controller fires multiple valves to get the gpm i am after.

so, figure out what zones are going to be happy running at once, then rewire teh controller.

That was exactly what was going through my mind. Thanks I'm already a 100% smarter than I was. I'm going to pick your brain for some more knowledge soon. my next job today deals with a submersible pump so i'm going to get info on it.

[FIMCO-MEISTER]

above ground submersible is a sump pump. not what you are after.

why not just move the pump further up the hill???? if you do, upsize the suction line, and don't make it more then about 20 feet of vertical from the creek to the pump.

What about replacing this pump with a submersible? Could that be done in 3-4 feet of water? Heading to work so I'll follow up this evening.

[Wet_Boots]

Placing a submersible in a body of water might be prohibited in your area, due to the electrocution hazards. An above-ground pump does need to be located above the flood level. And once the suction height is more than a few feet, you need to have a foot valve on the suction inlet.

As has been mentioned, you aren't using anything near the flow capacity of the pump. Straight centrifugals can almost push 40 gpm per horsepower. Jet pumps are better for your average sprinkler system. I like Goulds' jet pumps, for their ability to hold prime.

[koster_irrigation]

throw a submersible pump in there! be done with it, build a pump sled out of 2" and a 4" main tube to hold the pump. get a good strainer on the end of the 4"


the current pump you say is running LOUD because the creek and ruined the bearings in the pump not to mention the motor its self wont last much longer.

[Dr.Pressure]

I have been lurking here to learn about irrigation system but I frequently do pump sizing in industry and I wanted to mention a few things.

First, if cavitation really is occurring in this system, moving the pump up the hill or increasing the volumetric flowrate will only make the cavitation worse. Cavitation results from insufficient available net positive suction head (NPSHA) and as shown in bottom of the chart on page 126 of the linked specification, this pump requires about 13 ft of NPSHR at 50 GPM. Increasing the flow rate to 70 GPM pushes the NPSHR up to almost 23 ft.

To calculate NPSHA, you:
1. take atmospheric pressure in feet (34 ft)
2. add the static head to the centerline of the inlet which in this case is a negative number because the pump is higher than the water supply (e.g. -10 ft)
3. subtract the head losses in the suction piping at the flowrate of interest (e.g. 10' of 1.5" PVC with 2 90° elbows loses 1.5 ft at 50 GPM). Any other restrictions in the suction line such as check valves, strainers, or a clogged intake pipe will significantly add to this loss.
4. subtract the vapor pressure of the fluid being pumped (water @ 68°F = 0.67 ft).

The number that results from this calculation must be greater than the NPSHR shown on the pump curve.

To reiterate, increasing the output flow also increases NPSHR and increases suction line head losses. This makes it even more likely that the NPSHA will not be greater than NPSHR resulting in cavitation. Similarly, moving the pump up the hill increases the magnitude of #2 above again making it more likely that NSPHA will not be greater than NPSHR.

I couldn't see the suction line all that well in the pictures, but there may be significant enough losses to cause cavitation at the current flowrate. If that is the case, increasing the size of the suction line and minimizing restrictions would definitely help. However, the flooding of the pump may have also trashed the shaft and/or motor bearings causing the noisy operation.

As already mentioned, a centrifugal pump was probably a poor choice for this application.

Regards.

[FIMCO-MEISTER]

I have been lurking here to learn about irrigation system but I frequently do pump sizing in industry and I wanted to mention a few things.

First, if cavitation really is occurring in this system, moving the pump up the hill or increasing the volumetric flowrate will only make the cavitation worse. Cavitation results from insufficient available net positive suction head (NPSHA) and as shown in bottom of the chart on page 126 of the linked specification, this pump requires about 13 ft of NPSHR at 50 GPM. Increasing the flow rate to 70 GPM pushes the NPSHR up to almost 23 ft.

To calculate NPSHA, you:
1. take atmospheric pressure in feet (34 ft)
2. add the static head to the centerline of the inlet which in this case is a negative number because the pump is higher than the water supply (e.g. -10 ft)
3. subtract the head losses in the suction piping at the flowrate of interest (e.g. 10' of 1.5" PVC with 2 90° elbows loses 1.5 ft at 50 GPM). Any other restrictions in the suction line such as check valves, strainers, or a clogged intake pipe will significantly add to this loss.
4. subtract the vapor pressure of the fluid being pumped (water @ 68°F = 0.67 ft).

The number that results from this calculation must be greater than the NPSHR shown on the pump curve.

To reiterate, increasing the output flow also increases NPSHR and increases suction line head losses. This makes it even more likely that the NPSHA will not be greater than NPSHR resulting in cavitation. Similarly, moving the pump up the hill increases the magnitude of #2 above again making it more likely that NSPHA will not be greater than NPSHR.

I couldn't see the suction line all that well in the pictures, but there may be significant enough losses to cause cavitation at the current flowrate. If that is the case, increasing the size of the suction line and minimizing restrictions would definitely help. However, the flooding of the pump may have also trashed the shaft and/or motor bearings causing the noisy operation.

As already mentioned, a centrifugal pump was probably a poor choice for this application.

Regards.

Wow that is a lot of good info. Thanks for the post.:waving:

[drmiller100]

ok. so educate me.

you are saying cavitation is when there is too much restriction in the intake line to where the pump is going totally inefficient and quitting pumping.

what do you call it when a 50 gpm pump is restricted down to 5 gpm??? for instance, take a shop vacuum. turn it on. Then stick your hand over the outlet and the engine revs up and makes a lot of racket and the pressure drops to almost nothing.
Is this not also cavitation????

[Wet_Boots]

Man, that complete line of Jacuzzi pumps has some head-scratchers. A five horse centrifugal with an inch-and-a-half suction input? I bet I could get that 20-to-25-GPM system running fine with a Goulds J15S shallow-well jet pump.

[Dirty Water]

what do you call it when a 50 gpm pump is restricted down to 5 gpm??? for instance, take a shop vacuum. turn it on. Then stick your hand over the outlet and the engine revs up and makes a lot of racket and the pressure drops to almost nothing.
Is this not also cavitation????

I think thats more akin to deadheading a pump.

Cavititation is simply the pump being able to pump more than the suction line can provide, it has little to nothing to do with the pumps actual GPM Outlet.

[Dr.Pressure]

Cavitation is simply the pump being able to pump more than the suction line can provide, it has little to nothing to do with the pumps actual GPM Outlet.

In the simplest terms, this is exactly what cavitation is.

On the other hand, when you reduce a pump's output significantly below its best efficiency point (BEP) which happens to be ~ 53% @ 138 ft of head and 63 GPM as indicated in the annotated pump curve below, the pump begins to experience radial thrust. Excessive radial thrust causes the pump shaft to deflect, leads to noisy vibration, and eventually ruins the pump bearings. This is one reason why dead-heading a centrifugal pump is bad.

Also notice at 63 GPM that the pump requires at least 18 ft of NPSH.

82788


A pump should always be sized and operated as close to its best efficiency point as possible.

[FIMCO-MEISTER]

Hurray I found it. TO THE TOP

[FIMCO-MEISTER]

Please go to:

http://www.lawnsite.com/showthread.php?t=199599

If you wish to further advise on this job.
Thanks
Peter