Minimum Water Main Pressure 1

[eea]

I know the code is a minimum operating pressure of 20psi at service connections, but what are the restrictions against low pressure along the course of the water main? In our case we have a private 4" main that is functioning more like a transmission main feeding a camp with demand of 30gpm. It is tapped into the public system with 35psi at POC. Our review shows the existing line to have localized pressure drops along its length of <20psi but due to a lower elevation difference at the point of delivery the pressure is nearly the same since the elevation difference negates the friction losses. We are planning to add air release valves along the major high points, but we are trying to prepare for potential client pushback against a 6" replacement recommendation.

 

[coloeng]

Without more information it is hard to determine if the pressure loss is due to the friction losses or elevation gains in the pipeline. Based on your 30 gpm you have an approx. velocity in the 4-inch of 0.77 fps, which shouldn't generate significant friction losses. The concern I would have as the client with changing to a 6-inch is not only the cost of construction, but the additional residence time of the water due to the larger capacity of the pipeline. If this is a campground, is the system a transient non-community water system? If it is, would they have to flush additional water in order to maintain their residual disinfectant requirements? Is the 30 gpm a peak flow or an average daily flow?

 

[LittleInch]

What are the reasons for the "localised pressure drops"?

You normally don't want to fall below atmospheric pressure at any point and risk infiltration of non potable water.

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[bimr]

What is the purpose of the air release valves? You are just going to make things worse.

 

[swazimatt]

it sounds like there are some high points on your delivery line which, hence the air valves. the low pressure is not really an issue, however you will need to make sure your air valves are not above your hydraulic grade line or you will create a pressure break, and if you have to siphon over the crest without an air valve you run the risk of getting an air lock

 

[hydrae]

Friction loss is not your problem. 30 gpm in a 4 inch has minimal friction loss.
increasing to 6 inch will not fix that.

Is the pressure from the supply consistent?

Do you have power? Can you get power?
if no power, a small bank of solar panels could power
a set of 1/6 or 1/3 hp circulating pumps which can add 10 to 20 psi when the sun is shinning
add a few pressure tanks and your installation cost will be way less than replacing the
main.

Hydrae

 

[LittleInch]

Reading this again I think the issue is low internal pressure at high points along the line??

If your total loss is only 15 psi from one end to the other a larger line isn't going to make much difference.

Along a line as long as you are positive pressure at all times I can't see the issue.

What are your predicted pressures at the high points?

A bit of response would help.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.