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priming an irrigation pump

priming an irrigation pump

priming an irrigation pump

(OP)
Our vegetable garden is irrigated from creek water. The creek runs about 3m below the pump house, and the garden is 20m above the pump house. The pump is a 1.1 kW centrifugal unit, designed for the purpose.

Up until now it has worked very well, but the level of the creek has now dropped lower than we've ever seen it and while messing about with the position of the intake I managed to drain the intake side so nothing works at the moment.

Fear not, there is a priming plug, and the previous owner fitted a valve slightly upstream of the pump, and a tap upstream of that, so I have a lot of high pressure water available. So, I could close the valve,connect a hose to the tap, stick the hose into the priming hole, turn the tap on and away we go, is the theory.

Hang on, should the motor be running or not? Should the valve between pump and the main outlet pipe be open or closed?

Spoilsport answer, but not definitive, is that I refilled the pump casing via the hole, switched the motor back on, and nothing happened. So I refilled the stationary pump again, wandered off for a fantastic lunch(pea and ham soup cooked by my wife, rye bread baked by the local baker, and ham and prosciutto and shrubbery from our favorite deli, since you ask) , switched the pump on for no particular reason, and it all worked.

So what should I have done (other than had lunch earlier)?



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: priming an irrigation pump

Umm. Use the high pressure priming water from upstream to water the garden and skip the pump?

Keith Cress
kcress - http://www.flaminsystems.com

RE: priming an irrigation pump

Ahhh, isn't patience a virtue!

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

RE: priming an irrigation pump

You must have a check valve near the pipe inlet to prevent the whole line from draining back to the creek every time the pump is off. Your pump was air locked when you first primed it. With time, and with the pump off, water trickled down the suction pipe from uphill of the pump and displaced the air in the suction line, which would have bubbled up through the pump to the discharge pipe.
Priming requires the suction line to be filled with water. This can be a slow process when filling a small line from the top while air has to escape upward against the flow of water downward. If you were running the pump while priming you filled only the discharge pipe. Any high point in the suction line can cause an air pocket that can make priming difficult or impossible.
Self-priming pumps just have a water reservoir at the suction which allows the impeller to churn the water in the casing. Air in the churned water rises in the discharge pipe. In this way air is slowly pumped from the suction-side to the discharge side of the pump, but with little pressure being developed.

RE: priming an irrigation pump

If your suction check valve is not at the very botom of the suction line, there may be air trapped in the line between the inlet screen and the check valve. When this air hits the pump it may cavitate and pumping will stop. However, the air pocket may have moved up to the pump. The second time you primed the pump, there was no air pocket in the lower suction line, it had moved up into the pump body.

Self priming pumps often have an internal venturi in addition to the reservoir.
The pump pumps the water through the venturi and the ventui sucks the water up the suction line. Any air in the suction goes to the top of the reservoir, and out the discharge.
This is typical of self priming shallow well pumps.
Some well pumps have a removable venturi which may be connected with twin pipes and lowered down a deep well.
For shallow wells the venturi is bolted to the face of the pump.
For deep wells the venturi is removed from the face of the pump. Both the pump housing and the venturi will have two NPT tapped holes (supply and delivery) to allow the venturi to be remote mounted down the deep well.
The largest self priming pump that I have seen was a bilge pump. The pump drew in sea water from the sea chest. The water was pumped through a venturi which sucked from the bilges. In rough water, the bilge suction may at times be uncovered and suck air. This was no problem for the venturi which did not care if it was sucking air or water.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: priming an irrigation pump

I see nothing flawed in the early lunch theory. If I were able to convince my employer, I'd be more than willing to test it on a wide array of interesting challenges, for science, of course!

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

RE: priming an irrigation pump

Probably just airlocked, is the priming plug in the pump case at the highest part of the volute? On such a small pump it doesn't take much to put it off-prime.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: priming an irrigation pump

(OP)
Yes its at the top.

The questions i'm interested in are, that as i run the water from the tap into the priming port:

should the motor be running or not?

Should the valve between pump and the main outlet pipe be open or closed?

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: priming an irrigation pump

Not necessary to run the pump.
No reason to shut the delivery valve as it is somewhere for any trapped air to go to, If the outlet is lower than the pump - which in your case it's not - you might need to close the discharge valve.

There is always a likely chance that some air is trapped in the inlet pipe and you might need to preserve in the priming, ie, go for another enjoyable lunch , but leave the priming water supply slightly on as high pressure might negate any air in the inlet pipe rising and discharging into the case and out thru' the outlet

Do you have a tapping in the bottom of the casing?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: priming an irrigation pump

You have to fill the port with the pump off and then re-install the plug before starting the pump. But then sometimes leaving it loose or open can help the pump purge any leftover air. Just be ready for the surge of water and to shut the pump off when the pump starting pumping.

From your description, I would remove the plug and then just open the upstream valve a bit and let water back-flow into the pump and down the intake pipe. Or is there another check valve on the outlet of the pump?

A neat priming trick I learned years ago is to take the inlet pipe and check valve and move it somewhat vigorously back and forth in the water. Each time it's moved forward, water is pushed into the pipe. This fills the pipe from the bottom and avoids air bubbles in the pipes. Keep doing it until water comes out of the pump priming hole and the pipe and pump will be full of water with almost no air left in it.

RE: priming an irrigation pump

At his cottage, my brother took a gallon just (windshield washer fluid), bottom cut off, upside down, with a pipe/hose kludged onto it; he attached this to a valve piped into the priming port.

At the start of the season, he can fill the jug with a gallon of water, open the valve to let it start filling the pump, and turn the pump on. We've always found that filling while the pump runs works better, but you want a valve to shut off the priming port, because believe me, trying to thread a plug in when there's water gushing out is a royal pain.

RE: priming an irrigation pump

You can't get all the water in unless you let the air out. Connect the priming water anywhere but the priming port. Install a stop cock on the priming port that you can open to let the air out.
You may be unable to run the pump. Some pumps may pump but many will cavitate. Once cavitation starts it may continue until the pump is stopped.
In the tropics our main water supply tended to run dry and lose prime.
I had a 5 gallon bucket with a 1/4" plastic hose attached. The hose ran to a stop cock at the bottom of the pump housing.
There was another stopcock on the priming port. When the pump lost its prime I would open both stop cocks. When water ran out of the hi stopcock, I would close both the hi cock and the lo cock (grin) and start the pump.
This was a daily ritual.
The system described by TenPenny should have piping large enough to let the air counter-flow past the water entering. You have to let the air out. It has an added advantage that the back-flow when the pumping starts may be used to refill the jug before the valve is turned off, ready for the next priming cycle.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: priming an irrigation pump

waross : a point worth raising for clarification is air entrapped in a pump casing, this is not cavitation, this is simply - entrapped air, air binding, unprimed pump, or similar description.
Cavitation on the other hand is a lack of NSPHa as compared to the NPSHr required at that particular flowrate.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: priming an irrigation pump

Maybe I have used the word cavitate inappropriately. When the pump starts pumping with air in the casing, it may be able to flush the air out with the water and continue to pump. However, depending on the design and the level of water in the housing below the air, the pump may not start to pump. I have seen it both ways. Open face impellers are much worse than closed impellers.
I had issues daily with a pump losing its prime due to low supply water level. I tried a lot of methods toprime the pump. Some metods didn'e really work. Some methods worked some of the time or worked on the seccond or third try.
Filling from the bottom and venting the air out of the priming port worked every time. By the way, this was our primary supply. We did not have the option of using a hose from another supply.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: priming an irrigation pump

waross : if the supply is critical and causing major concerns and the varying level is the problem, investigate a vacuum pump priming system, either as a complete pumpset or a stand alone priming system - -, will overcome all your problems.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

RE: priming an irrigation pump

My pump issues were during my sojourn in Central America a few years back. The municipal water system never developed more than about two feet of head. Not a misprint, 24" of water column. That was when there was any water at all. I put in a pressure tank and pump with the pump connected directly to the incoming water service. During part of the day, there was no water at all in the water main. Even a selfe priming pump would lose its prime.
I tried a lot of things before I hit on a simple cheap system that worked dependably.

Bill
--------------------
"Why not the best?"
Jimmy Carter

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