loss in a dist. system

[stanford94]

suppose i have flow losses totaling 500gpm at about 80psi in a dist. system. if i reduce the pressure to say 50psi, what kind of reduction in flow losses would you expect? i would use flow through an orifice i suppose as a first start to approximate this but suggestions are welcome. thanks.

 

[bimr]

You can expect about a 25% reduction in flow rate.

 

[cvg]

start by calibrating or verifying your flow meter(s). 500 gpm loss is a lot of leakage and I would want to make sure that is the correct number.

 

[stanford94]

ok.. this statement is part of a work scope i was reading by ch2mhill.....

"........ since leakage rates are directly related to system pressure, lowering the pressure will reduce water loss from any existing leaks. For example if a portion of the distribution system has leaks totaling 500gpm with a pressure of 80psi, the leakage will be reduced to 105gpm (nearly an 80% reduction) by lowering the pressure to 50psi........"

i know the pressure drop varies with the square of the velocity but maybe i missed something here...

 

[bimr]

One would think that the leakage would occur from flow through small cracks and pinholes for the most part. So the most likely simulation of the flow of fluid would probably be with a formula such as the flow through an orifice formula:

Q = 19.65 (d) (d) C (sq root h)

If you assume all of the pressure is headloss, then;

Square root of 80/square root of 50 = 1.25. So that would be a decrease in flow of 25% going from 80 psi to 50 psi.

I would prefer to state the proposed leakage reduction in a range of values rather than a specific number. The proposed leakage reduction is nothing more than an educated guess.

 

[stanford94]

yes, i would model this like flow thru an orifice..

so... something's wrong with the statement? i.e. we would expect the flow to reduce the same percentage as the pressure.

 

[cvg]

q=Ca (2gh)^.5
C = orifice coefficient (0.6)
a = area of orifice sq ft
g = 32.2
h = head in feet

using the orifice equation, the result is approximately 21% less flow going from 80 to 50 psi. CH2M Hill was wrong - go figure...

 

[stanford94]

ok. yes that is what i got. 21%. thanks. that's way too many opinions for one small relatively easy eqn.