O.T. - Water pump suction rate

Calling all plumbers (and others)!

I've posted a few times recently concerning a waterline issue I had. Well, turns out the pipe went bad (full of holes) and needed to be replaced. That was accomplished a few days ago. I had to hire a horizontal driller to run the pipe under the road to the tune of big $$$.

Anyway, I am now trying to calculate the flow rate through this pipe. What I have is a concrete cistern from which I am sucking water to the house with a shallow-well pump. The specs are as follows:

1/2 HP pump putting out a max of 8.5 GPM (6.5 at 40 PSI)

240' of 1" plastic pipe

Water pressure tank - 29.5 gallons with a drawdown of 8.9 gallons

Waterline runs between 4 and 10 feet deep underground

It takes my pump about 4.5 minutes to fill the tank going from 30 PSI to cutting out at 50 PSI. I have heard that a pump should run no more than 1.5 - 2 minutes at a time. I am just trying to calculate flow rate through the pipe and see if my pump is too small for the setup I have. Yes, I know that tees and ells and such create drag and effectively make a longer pipe, but the websites I've visited don't seem to give me a formula for what I want. Any and all help would be appreciated.

If the pump motor is thermally protected, it can run for a long long time. If it is getting very warm, I would look to get a pump with a higher duty cycle rating. Look at the manufacturer's site to get duty cycle info, or call them.
Pumping for a long time is going to happen every timne you use water to fill something (like the bath tub) so it will be running no matter what.
Do not listen to everything you hear about things "they heard". Jim

Sounds like you are exceeding the capabilities of your pump. What is the lift that you are having to raise the water. A shallow well pump will only lift water about 34 feet at sea level. you lose 1 foot lift per 1,000 feet above sea level. If you were lifting water about 20 feet and had a short suction line you would be ok. But with the resistence in 240 feet of pipe and the lift you have is more than the pump can do. If you could move the pump to the cistern and push the water to the house you would not have a problem. I don't know your flow rate but it must be almost nothing. A bigger pump may not help you. DH

You are confused about pumps. The pump should not run less than 1.5 to 2 min. Starting is what is bad for a pump, any pump that is set up right should be able to run continuous. Sounds to me like you should of used larger suction pipe if you are sucking water through 240' of 1"!

Did you ever see a fire truck? Those 10 foot lengths of big hose on the side are suction line. They are typically 8 inch or larger and discharge lines are 1 and a half to six inch even though the discharge lines go twenty times further. Friction loss is a much bigger problem on the suction side than discharge, but friction loss is a function of distance. Any part of that suction that you can replace with significantly larger will help a whole lot.

Just a thought , dad had the same problem, had to put the pump at the beginning of the line and push the water, not suck it. Worked for years this way.

You're sucking it too far. The pump should be at the cistern so it pushes the water. This creates a whole bunch of new problems with power wire/freezing, and on and on it goes. Doesn't look like an easy or cheap fix to me if you want more flow and less run-time for the pump. Jim

As previously stated, there is only 3-5 psi to push the water from the cistern to he pump.
That is why most pump systems run a much large suction line on the pump vs the pressure line.

I'd be more concerned about the effictive lift you're asking of the pump rather than the simple length of the line. Friction loss does come into play... but when you're moving 6 gpm in a 1" line it's not a big deal. When you're trying to shove 100 gpm through the same line... friction loss IS a big deal. Are you lifting water 5'? 10'? 20'? 5' is fairly reasonable. 20' on that run, with a jet pump... is probably asking for trouble.

Rod

You are losing less than 2.5 psi, more like 2 psi in the run of pipe without fittings. Fittings add a lot of equivalent length especially the cheap plastic ones that have small id's and sharp bends. So say 5 psi in fittings.

You don't want a bigger pump, if anything a smaller pump and bigger tank is better for what you're trying to do. How high are you lifting with the pump? You've got a backwards setup, you'll always have more problems with leaks and priming that if you had a pump at the water source.

I think you are starving the suction of the pump, typically the suction should be larger than the discharge. You can only suck about 30" of mercury, so any pressure drops due to velocity have to be thought of that way.

How big was your suction line before?

An off the shelf single stage centrifugal water pump would do well to develop 25" of mercury at zero flow . Even at 20" of mercury " lift" , the flow is a mere fraction of the pump"s capacity.

Thanks to all who responded. Yes, maybe I should have considered a 1.25 inch instead of a 1 inch line, but that's definitely not changing now. The 240 feet are basically horizontal. The lift is no more than about 5 feet, since the line dips to about 10 feet underground and the pump is 5 feet below grade in the cellar.

The old line was a 1 inch iron pipe.

Sounds to me like I should just get used to the fill rate is being "normal" for my setup.