Water pump..another question for the experts...Glenn and Don

Okie_Bob · May 11, 2011, 01:52:27 PM

First of all, I'm missed all of you and have no good excuse except, I'm now living on the lake fulltime and retired and find I
have less time than when I had a real job! Go figure. Glad to see Glen, John, Don, Scott and Peg still active on the website!

Anyway, I have a boat dock on the lake in front of my house. I have a new 2 HP centrifical pump located on the deck of the dock and it has a 2" PVC suction line going down into the water, some 8 feet below the pump. I use this water only for my garden and lawn. The suction line has a foot valve (check valve) and a screen and mud anchor at the bottom.
On the outlet of the pump I have a 1 1/2" line running about 150' up to the house feeding my sprinkler valves and a couple of faucets
where I attach a garden hose for watering. Tee'd off of the oulet line is a 3/4" line that goes thru a pressure controller and into a bladder storage tank. This is typical of most water well setups.
Problem is that the max pressure I can get out of the pump is 44 pounds. Question is, why can't I get more pressure? Would reducing the size of the suction line help?
Also, the pump cycles way too much. I've checked the pressure after drawdown and it is 2 psi below as Glen noted in the other post on similar question.
I believe I have way too much pump HP for one thing. But, that doesn't explain the problem with low pressure. Any ideas?
BTW, I had a 5 gallon tank to begin and replaced it with a 36 gallon. Still short cycles way more than it should.
As always, thanks in advance!
Okie Bob

muldoon · May 11, 2011, 09:18:24 PM

I am not one of the experts, but I'll suggest something.

Seems like you have alot of fittings and connections from the point of your pump and the end tank.

QuoteOn the outlet of the pump I have a 1 1/2" line running about 150' up to the house feeding my sprinkler valves and a couple of faucets where I attach a garden hose for watering. Tee'd off of the oulet line is a 3/4" line that goes thru a pressure controller and into a bladder storage tank. This is typical of most water well setups.

So the water path is uphill to a a few hose bibs and sprinklers, and then into a pressure tank? It seems to me that if any of the faucets were even slightly open you would introduce negative pressure or a pressure drawdown if that was the case. That could manifest as the pump cycling frequently and constantly being under pressure. Also, every fitting or connection would need to be airtight or the same thing would happen. Perhaps a walk with a squirt bottle of simple green doing a bubble test might reveal a leak?

May 12, 2011, 12:25:25 AM

Hey, BoB, so nice of you to drop in. We miss you too but I can't blame you if you are out having fun.

A couple of things come to mind. Every pump is different and based on a design dictated by your needs as perceived by your pump man, and actual site conditions. It may be that your pump is not designed to produce more pressure than that and rather is a high volume low head pump. I just may not produce more pressure than that due to the type it is.

But then we come to the cycling problem. If your pump is a high volume pump then even a 36 gallon tank may fill in less than a minute and cycle the pump off even if the bladder is good. Longer pumping time between cycles requires more bladder tanks to be added, or get a lot more things on at the same time, such as design your sprinkler system to use full flow at 40 psi or something.

Possibly you can find info on your pump to help figure out design....and don't be a stranger too much... [ouch]

Okie_Bob · May 13, 2011, 12:33:23 PM

Glenn, thanks for the response. I found the spec on the pump as follows:
2 HP 230V 2"suction 1 1/2" discharge
With 10' suction lift and 30 psi should give 2916 gph and at 0 lift should give 44 psi shut off pressure.

From this I think this would qualify as a high volume, low pressure pump?

? And it that is the case, I'm either going to have to add
additional tank storage and/or add more sprinkler heads?

Can you tell me what the result would be if I used a 1" suction pipe or even 1 1/2"?

MountainDon · May 13, 2011, 01:23:59 PM

If you had more air volume it would likely allow the pump to cycle on and off with longer cycles.

May 13, 2011, 02:05:34 PM

I assume you mean more storage tank, Don?

MountainDon · May 13, 2011, 02:29:02 PM

Read this http://countryplans.com/smf/index.php?topic=10536.msg135000#msg135000 might help

ScottA · May 13, 2011, 03:57:23 PM

2HP is a pretty big pump for what you're trying to do most likely why it's short cycling. The simplest thing to do might be to introduce a leak back into the lake to siphon off some of the excess pressure instead of buying more pressure tanks or sprinker heads. Good to see you again Okie_Bob.

glenn kangiser · May 13, 2011, 09:33:05 PM

So Bob, from the pump spec, then you see why you only get 44 lbs. That is its shut off meaning it will not produce more pressure than that.

You could do as Scott mentioned although you would be paying for power to pump water back into the lake. Best would be to get sprinklers big enough to git er done fast. 2900 gallons per hour so lets round that up to 3000 for easy numbers. That is 50 gallons per minute so a 500 gallon storage tank would take 10 minutes to shut off per cycle.

You could use 10 sprinklers at 5 gallons per minute each or possibly a couple more to keep the pressure down a little... or 50 at one gallon per minute and a few more to drop the pressure a little. When you use all of the water produced by the pump then you are operating efficiently and not wasting money.

May 14, 2011, 07:06:22 AM

Thanks Scott, Don and Glenn. My normal practice of buying 2 hp when all I need is 1 hp bit me on the butt this time. Just goes to prove bigger isn't always better!!! Thanks guys, I knew I could count on you.

glenn kangiser · May 15, 2011, 12:48:58 AM

Our pleasure, Bob.

MountainDon · May 17, 2011, 12:59:54 AM

Note from PEte on this...

You'd need a municipal water tower, not another 36 gal. tank. 50 gals./min. at 30psi is a lot of water, and a fairly good size pump. You shouldn't have taken that pump off the city fire truck. :-) Go to your plumbing supply store or a big box store for that matter, and get a properly sized pump for your application. Someone has to figure the head loss on your system and make a judgement about demand or flow rate in gals./min. Then you pick a pump for continuous service for an hour at a time, or size the tankage to cycle the pump 5 minutes on, out of every 15 or some such.

Head loss is made up of two components: one is physically lifting the water on the suction side and on up to the house, whatever that elevation change is; and the other is friction loss in piping, foot valves, elbows, other fittings and the like. You have answered your own question about the max. pressure, that's a function of the type, size and quality of the pump. The 10' lift limit on the suction side is also a function of the specific pump, so you don't cause it to overwork, run dry, or cause cavitation; and you do not want to use a smaller dia. pipe on the suction side.. But, sprinklers on your typical municipal water system run at 40-50psi, don't they? Watch your city house water meter for an hour with a couple sprinklers running and average this a couple times to get a pretty good idea what your type of sprinkler uses at that pressure.