Irrigation pump

Anonymous Poster · Mar 31, 2003

Howdy. I figure you guys might be my best bet here.

I have an orchard served by a 3-HP/80GPM centrifugal pump, but I am allowed only 42-gpm from the ditch (3-1/2-miner's inches). Since it makes no sense to run this pump for half it's capacity, I'm looking for a 1-HP/45 GPM pump that will cost alot less.

I'm also looking at rigging the pump in 110v mode rather than 220v which it is now. Maybe i might get 45-gpm at 1-HP?

The pump is on the source. Amount of lift-- about 1-foot. Amount of head needed to pump-- about 20-feet (apx. 10-psi). Mains are 2-inch sched. 40.

If there is a 1-HP pump that will pump more than 45-GPM, all the better, as reserve would be nice. Some outfits are telling me I'd be best off with a 2-HP pump with the preceding in mind.

Pump is a Sta-Right, bronze.

OK experts! What do you think? Can I do it with 1 horse?

Anonymous Poster · Apr 1, 2003

Tracy.....you didn't say if the source was pressure or not? Do you have any pressure coming INTO the pump, or is this a suction lift? You can figure HP by this formula.....GPM X HD (in feet) divided by 3960. Then divide that number by the efficiency. Usually in those little suckers the efficiency is about 70% so use .70 as your divisor. Forget going to 120 volts, bad idea! There IS a way you can use the pump you have.......you can TRIM the impeller and get a lower GPM/Head. The efficiency will suffer a bit, but it will still be cheaper than replacing the pump. To determine the trim, you can either get a curve from the MFG and look on the curve for the new trim, or you can calculate it yourself. Just remember that the head varies as the square of the difference, the GPM varies directly, and the HP varies as the CUBE of the difference. /forums/images/graemlins/smile.gif

Egon · Apr 1, 2003

You could also put a valve or limiting orfice on the discharge to cut you back to the reqired gpm.
A bypass back to the suction would also do the same job.
As said trim the impellers.

Whichever is cheaper.

Stay at 220 volts as you already have it.

Egon

Anonymous Poster · Apr 1, 2003

Taking into consideration the sky high prices that they have to pay on the jest coast for electricity /forums/images/graemlins/mad.gif, the impeller trim approach is definitely out in front from an economy standpoint. /forums/images/graemlins/tongue.gif When he downloads that 3HP motor to less than 2 HP by trimming, the efficiency of the motor AND the pump will most likely suffer, but it WOULD draw less running current. Yes, throttling could lower the input HP; how much you could drop it would depend on the characteristics of that particular impeller; whereas by-passing to the suction wouldn't necessarily reduce input HP, and does have the potential to entrain enough air to initiate some cavitation, even at a reduced flow through the impeller. He would be OK with by-passing as long as the re-entrance of the by-passed water was a LONG way upstream from the pump suction itself. /forums/images/graemlins/tongue.gif Tracy could do a "polish" job on that impeller after trimming by hand filing the vanes to about 1/8", and slicking up the insides of those ports. A few evenings in front of the TV with a cloth in his lap and a good file to work over that wheel would pay him back in increased efficiency. /forums/images/graemlins/crazy.gif Yes, it IS kind of "sweeping out the corners" to do that, but in todays energy market it might make good sense. /forums/images/graemlins/blush.gif I've done several impellers that way when I was looking for something that the curve didn't quite show we had. /forums/images/graemlins/crazy.gif

Egon · Apr 1, 2003

In situations where volume is fluxuating and the pump is of fixed volume discharge the bypass and reduced disharge are very common practices with no cavitation problems. The amperage draw goes down as the discharge pressure increases and volume decreases.

The original pump also does not require a new impeller if it is placed into another service demanding it's full output.

Egon

Anonymous Poster · Apr 1, 2003

Throttling may not actually decrease HP signigicantly as many of the small, single-suction centrifugals have a rather flat HP curve. Throttling is kind of a last resort measure and was used a lot more when electricity was cheap. Because of component wear and resultant changes in overall losses, and therefore Total Dynamic Head, throttling may not be a completely dependable method of holding a constant net discharge from the plant, and he could exceed his lawful allotment from the water district turnout. By-passes, while inexpensive, are more often used on axial flow units in a pit application where discharge head must be kept constant as flow decreases, and not so much on centrifugals any more since they, well, waste horsepower and HP is money. Of course if Tracy really wanted to do this right, he could apply an AC Adjustable Frequency Variable Speed Drive. /forums/images/graemlins/wink.gif He can get a nice one for $611.00 USD from WW Grainger. While it seems kind of spendy up front, the savings in HP will return the investment in the VFD, and as you pointed out, the untrimmed impeller would be there for him later on. /forums/images/graemlins/smile.gif

Anonymous Poster · Apr 2, 2003

I'm with Egon on this one, adjustable bypass back to the suction. Cavitation isn't a consideration, deep well pumps do this very same thing in a jet housing. I'd even try coming up with a deep well jet setup for this purpose, on the hunch the venturi effect would increase efficiency some. If I couldn't get my hands on a jet housing I'd plumb the return to suction in a manner to minimize suction line turbulance before the pump.

Egon · Apr 2, 2003

In my experience bypass systems are usually used to maintain a level. The high pressure discharge is a first line secondary feature to keep within the design limits of the discharge system. The next is a high pressure/high temp. pump shutdown followed by a preset PSV as the last resort.

Egon

Anonymous Poster · Apr 4, 2003

Using a jet would increase the discharge pressure somewhat, while definitely reducing the flow, but it doesn't sound like Tracy is especially low on discharge head; and isn't one of the goals here to conserve on power (electricity useage)? Not to put too fine a point on it, but a jet isn't quite the same as a by-pass-to-suction would be, and it doesn't have the cavitation-inducing effect that returning flow to the suction line in a closed system usually has. Sorry, but I just have to stick to my guns here, and say that by-passing in the age of high energy costs is a non-solution /forums/images/graemlins/crazy.gif. Thirty years ago when I was CHIEF OF APPLICATIONS for a major US pump manufacturer, we could, and did sometimes, trade up-front equipment costs for a slight increase in input HP. But that was thirty years ago. Nowadays you have to sweep out ALL of the efficiency corners to find that last little bit of cost-avoidance. /forums/images/graemlins/tongue.gif I'm going to full circle back to my original idea of trimming the impeller; because it's the right "equipment" thing to do, and because it has the potential to recover the cost of the mods by the savings in energy. /forums/images/graemlins/wink.gif

Egon · Apr 4, 2003

Tracy:
Forgot to ask but how old is the three HP. electric motor?

Reason I' m asking is if it is real old a new motor [ much more effient ] and pump may be the answere.

Egon