Static and residual pressure in water calcs

[Spacemouse]

When designing domestic water pipe systems, plumbing code indicates to use available static pressure as starting pressure in calculations. I have read elsewhere to start off with residual pressure (which is lower than static) as starting pressure. Which is correct?

 

[PEDARRIN2]

Both are correct

The International Plumbing Code indicates the static pressure must be less than 80 psig, otherwise a pressure reducing valve or regulator is required. I think the Uniform Plumbing Code is similar. This is mainly to reduce wear and tear on plumbing fixtures.

For operation of the plumbing system, you would use the residual pressure to determine pressure loss at the design flow in the system piping to determine if adequate water pressure is available for fixture use.

Flush valves, for instance have a pressure requirement at a certain flow (25 gpm at 35 psig in the IPC). If the residual is not enough, you would need to design a boosted system.

 

[Spacemouse]

So if I have results of a hydrant flow test which list static and residual pressure at a specific GPM (usually at a much higher flow rate than my design GPM), I can graph these points, dramas a line through them, and find the residual pressure available at the design GPM?

 

[IRstuff]

Wouldn't a hydrant be supplied by a much bigger line than a residence would have?

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[PEDARRIN2]

Spacemouse,

The static and residuals are points on the graph, but it is not a linear reduction between them (unless you use special graphing paper formatted for this application).

From Hazen-Williams, psi loss varies with flow to the 1.85 power. Generally, a straight line between them on normal graph paper would give a lower pressure than is actually available.

Look at chapter 3 of this document (

http://www.usfa.fema.gov/downloads/pdf/publications/Water_Supply_Systems_Volume_II.pdf)

Based upon the system flow curve, the pressure at the design flow (which is typically the peak flow, based on fixtures and other equipment)of your building would be the pressure available. You would have to calculate all the piping friction losses, including static and equipment to determine if you need a booster pump or not.

IRstuff,

The hydrant is on a much larger pipe. They test the hydrants to determine what pressure is available for the residence/business.

 

[Spacemouse]

Thanks for your help.