Check Pump Selection

[ckeller]

I need to pump from a lake over a levee to another impoundment. The pump will be placed on top of the levee. Static head on suction side of pump is 23.5 ft. Length of suction line is about 250 ft. Discharge is about 300 ft from pump and elevation drops 10 ft from pump to point of discharge.

Looking at a 4" gas powered trash pump with the following specs: suction lift of 25 ft, total head of 85 ft, max discharge of 450 gpm.

Assuming that the suction/discharge piping is designed so that frictional losses are less than about 60 ft, will this pump work?

Thanks.

 

[BigInch]

Don't know. The most important dimension is the height from the lowest water level of the impoundment on the pump's suction side to the pump centerline. That can't be greater than 25 ft -Suction Piping Head Loss + Static head at the inlet of the suction - a bit of safety elevation.

So, how high is the centerline of the pump over the lowest water elevation of the impoundment you're drawing from?



BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[RossABQ]

How can the "static head" at the inlet be positive, if the levee is retaining the water in the lake?

 

[MikeHalloran]

I think it'll take forever to prime. How about mounting the pump on a float in the lake?



Mike Halloran
Pembroke Pines, FL, USA

 

[BigInch]

Ross,

Yup. I thought it was a bit confusing too. From the description, one would normally think the suction impoundment level would be higher than the pump, but that would bring up the question you have already asked. It might also be "possible" that the OP is referring to the static head over the inlet to the suction pipe, which could be as much as the depth of the water, if the inlet to that pipe was on the bottom of the impoundment. That's why I asked for the dimension.

Mike, depending on the dimensional clarification, U R so right.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

The confusion arises because static head can be implied as either positive or negative and unfortunately it is not always quantified as being positive of negative as in this case. Although from reading the post I guess you have to assume it is negative ie. suction lift.

A good "self-priming" pump should be able to handle this duty without to many problems but the inlet side losses need to be carefully calculated and I doubt if the 4" pump being considered is the right pump for this application.

 

[TenPenny]

I will start by assuming you mean you have a suction LIFT of 23.5 ft from the lake level up to the pump. If your pump can handle a lift of 25 ft, you will be pushing it to the limit in this application. I would venture to say that it will take 'forever' to prime, and also that as soon as the pump wears in any appreciable amount, you will not be able to pump at all. For a 6 inch line, your friction will be in the order of 1.5 ft / 100 ft for 450 gpm, so you're looking at an 8in or larger suction line...

Also, assuming that all works, this pump will only be able to lower your lake water level by 1.5 ft before you've hit the limit of your pump, and that's with no friction allowance in the suction, as noted above.

You're probably far better off to build a small raft to float the pump in the lake.

 

[ckeller]

Thanks for the input so far. I'll attempt to clarify the question.

Consider the lake water (the water source) surface elevation as 0 feet (all other elevations will be expressed as + or - relative to the lake at 0). Top of levee elevation that I need to pump over is +21 feet (I can't trench through it). The elevation of the discharge end of the pipe is about +10 feet relative to the lake. So, I basically have to pump over a wall.

The pump can be located anywhere, including on the lake shore at an elevation of about +3 feet. The total distance from the lake to the discharge is 500-600 feet.

Rather than sit the pump on top of the levee and fight the suction lift limit of 25 ft, can I put the pump at the lake edge, to minimize suction side losses, and push the water over the levee? In that scenario, the discharge pipe will be about 500 ft in length, but I suppose I can increase the diameter to reduce frictional losses.

 

[TenPenny]

Okay, stick the pump down by the lake, say a 3 ft lift from the lake to the pump. Use a 4 or 6" suction line.

So you'll have about 500 ft of discharge line...at a flow rate of 450 gpm, that'll be approx 7.5 ft friction if you use 6" pipe...call it 10 ft to allow for fittings, etc.

On start up, your static discharge head will be about 20 ft...once the system is full, assuming no leaks, your static head will be only 10 ft or so (since your discharge is only 11 ft above the lake)...allowing 10, even 15 ft of friction, the answer is that the 4" pump will be just fine. If you use 4" discharge line, it's going to be a much higher friction loss, about 50 ft or so, I'd stick with the 6" if possible.

That said, one thing to note is that pump manufacturers often give "maximum flow" and "maximum head" numbers, but the thing to note is that the max flow is at little or no head, the max head is at no flow, so you'll never get 450 gpm at 85 ft, for example.

You'll probably be able to get 300-400 gpm out of your pump in this application; if that's sufficient, go for it and be happy.

 

[ckeller]

Thanks TenPenny. Would you recommend an air release valve at the high point?

 

[BigInch]

With your permission I'll answer. No, you should have enough velocity whenever you start the pump to clear and then keep it clear of air.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com