hydrant flow tests 1

[mlhardesty]

i'm working on water model calibration for a small municipal water system. because the largest demands are typically fire related, calibration requires that we use hydrant flow tests to stress the system. my first question is just about fire hydrant flow tests in general.

hydrant flow tests are generally conducted with a pitot gauge held in the water stream. this pressure (or velocity head) is converted to a flow by means of a chart or equation which is bascially a form of the small orifice equation. the smoothness of the hydrant outlet is a factor in this calculation.

my question is, for a multiple outlet fire hydrant, wouldn't using a pressure gauge on the other outlet give you a pressure without the use of a pitot gauge. furthermore, wouldn't this negate the need to worry about the friction losses related to outlet smoothness in calculating the hydrant flow?

thanks.

 

[LHA]

A pressure gauge would give you a Static Pressure. The flow gauge gives you a pressure drop due to the flow. That is what you are converting to velocity head, then to velocity, and finally to flow (Q=vA).

The friction losses are a necessary variable in the velocity head portion of the flow equation. During Static Pressure, friction losses are, of course, zero.

Engineering is the practice of the art of science - Steve

 

[mlhardesty]

what i meant was taking a dynamic pressure during the flow test at the other outlet of the fire hydrant. most fire departments use the small 2.5" connections for flowing. mose modern hydrants have two, along with a large steamer outlet. what i meant was, why not take a pressure reading at the other outlet on the same hydrant while flowing? if the pressure head is all converted to velocity head except for the outlet losses, shouldn't the dynamic pressure reading at the same hydrant give you a pressure that could be used in the same pitot equation or chart without worrying about the hydrant coefficient of 0.6-0.09.

 

[TravisMack]

The appendix of NFPA 13 - Standard for Automatic Fire Sprinkler Installation shows a diagram for conducting flow tests. The standard procedure is to use a pitot tube for measuring flow. However, the diagram indicating how to conduct a flow test does offer an option for what you are describing.

I have always been meaning to do the test in that manner and test with a pitot to compare.

You could also reference NFPA 291 for more standards on fire hydrant flow testing.

 

[CarlB]

Lots of good hydrant info at

http://firehydrant.org

In particular flow testing:

http://firehydrant.org/info/ftest1.html