Draining the suction side of a pump intake

[rangercap]

Question - I am in the process of developing/designing a water withdrawal and intake system for a natural gas drilling company. Here is the basic system setup: I currently have the system set up with an intake placed in a stream, below the water surface (elev = 988') and the pump is placed inside a building approximately 900 LF away. Pump intake elevation is 1001'. The pump will withdrawal water from the stream through a 12" HDPE DR11 waterline and into a 2MG above ground storage tank (top elev 1010').

My question is: we typically use an airburst system to clean the inlet screen, but in this case, I believe we have enough elevation difference between the intake side of the pump and the water surface elevation to utilize the water contained in the intake piping (12" DR11 HDPE) to back flush the pump. I had thought that using a manual valve (ball valve or plug valve) to introduce air and break the vacuum on the suction side would be a solution for a simple back flush. Environmental regulations aside (I used to work for a the state regulatory agency so what I proposed is acceptable) is there any thing that I should be concerned about with this set up? This would only be a periodic backflush, and the site will be manned 24/7 during withdrawal operations. I have in my design a suction side vacuum gage to display vacuum pressure, and back flushing would only occur if the vacuum was above a certain value, say greater than 34 psi. In order to back flush, the manual valve would be opened flushing the intake screen. We are using a prime assist pump so I am not to concerned about reestablishing the pump prime.

Thanks for any input you can provide.

Brian

 

[bimr]

You may have a lot of energy in a 12-Inch pipe. If you have a quick closing valve such as a ball valve, you may encounter a water hammer situation.

I don't think there is enough pressure for backflush of an intake screen if you just open the pump suction with a valve.

Why don't you backflush the intake screen with the water in the storage tank?

Your post is a little confusing when you are referring to "back flush the pump". I assumed that you mean "backflush the intake screen".

Also, is your storage tank only 10-feet tall? Sounds odd for a 2 mg tank.

 

[racookpe1978]

But, if he is breaking the vacuum by the high point vent valve in the pump discharge being already open when the 12" drain ball valve is opened, then where would water hammer be coming from? Would he not have a short 4 - 12 foot drain pipe on the suction side of the pump, the 12" drain ball valve, and a short drain point discharge pipe.

When he closes (or opens) the12 inch drain ball valve, then only that little bit of drain pipe between the ball valve and the end of the drain pipe woulld see a pressure cycle. Above teh drain ball valve, the water would be flowing, then stop as the ball valve closes, but the pipe would stay full.

 

[bimr]

racookpe1978, reread and notice the word "may". In addition, I said the post was confusing. However, my preference is to use slow closing valves on larger pipe.

You might note that the practical maximum suction height is approximately 23 feet at ambient temperature or about -10 psig. The pump will cavitate prior to -34 psig.

Don't think you have enough pressure (5 .6 psig) to backwash the screen with just the static pressure in the inlet pipe.

 

[cvg]

you have 13 feet of head and should see up to 7 fps velocity to backflush at the beginning. however the flow will quickly empty the pipe so you will only have a minute or so of good backflush

 

[LittleInch]

The amount of flow and velocity will depend on the pipe profile. If it is flat then rises at the pump end you won't get much flow, the opposite then you will.

Clearly a typo on the vacuum - -14/5 psig is outer space so -34 is not possible....

Even if you have a priming pump, still seems a poor idea to drain the line simply to have to re-fill it. Would make more sense to me (and bimr) to have a line bypassing the pump from the tank and you then control the flow with a suitable valve. This gives you much bigger volumes to flush your filter at a constant pressure. You don't loose all the fluid in the pipe and can start pumping again as soon as you close the bypass valve. Much simpler.

Seems quite a long way to go with a decent vertical lift so your NPSH must be quite low?


My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rangercap]

Back from vacation... I'll clear up a few things.

Also, is your storage tank only 10-feet tall? Sounds odd for a 2 mg tank.

We typically use porta-basin type tanks for the oil/gas industry. It's an above ground storage tank, 10 feet high. There are two tanks which will hold 25,000 bbl, or just over 1MG each.

Your post is a little confusing when you are referring to "back flush the pump". I assumed that you mean "backflush the intake screen".

Yes - back flush the screen - my error. It was late when I had posted this originally.

Why don't you backflush the intake screen with the water in the storage tank?

We can't backflush from the tank as we would need an NPDES permit to do so.

Thanks for the discussion. I'm looking for ideas and this has definitely helped clear some things up.

Brian