Is Pitot the Same As Residual on Flow Test?

12/27/2019

Fire hydrant flow test question - is the pitot the same as the residual pressure on a water flow test from a fire hydrant?

Is it possible to know the residual pressure and 'chart out' the flow of the fire hydrant, or is there a way to determine the flow with only the static and residual pressures?

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5 Comments

Mike L

12/27/2019 08:09:36 am

Pitot Pressure and Residual Pressure are two separate measurements. Pitot pressure is used to calculate the flow rate from the hydrant that correlates to the residual pressure being read.

The values of Static Pressure (no flow) and residual pressure (at the calculated flow rate) are then plotted on a graph to produce the water supply curve.

Please read NFPA-291 for Hydrant Flow Test procedures and resultant calculations.

Wayne Ammons

12/27/2019 09:17:05 am

To add to what Mike said, you can conduct a flow test if you do not have a pitot tube:

NFPA 291, 2016 Edition:

4.9 Determination of Discharge Without a Pitot.

4.9.1 If a pitot tube is not available for use to measure the hydrant discharge, a 50 or 60 psi (3.4 or 4.1 bar) gauge tapped
into a hydrant cap can be used.

4.9.2 The hydrant cap with gauge attached is placed on one outlet, and the flow is allowed to take place through the other outlet at the same elevation.

4.9.3 The readings obtained from a gauge so located, and the readings obtained from a gauge on a pitot tube held in the stream, are approximately the same.

joe

12/28/2019 02:26:17 pm

What do you do if the city fire don't have a residual for a preliminary calculation on a bid? some cities i deal with only have static and pitot it is not smart to drive 2 hours one-way for to make flow test on a job you may not get. i just trying to figure out a way to get a residual what the city doesn't have a residual pressure.

Joe Meyer

12/30/2019 07:41:29 am

We don't have flow test information for many bids we go after. It's extremely annoying and not helpful to the project.

If we are calculating a system we will generally have some idea of what the area can handle from a water-supply perspective.

If an area is very hilly, I will check the elevation of the project compared to nearby projects I've completed or the elevation compared to the nearest water tower. I've been able to estimate water supplies using this method too.

Ultimately, you may consider protecting your bid by qualifying it with an assumed water supply - if the job actually requires a fire pump then provide an alternate price for it. Most projects for us are generally pretty straight-forward as to whether it would need a pump or not based on the building's height and hazard.

Mike L

12/30/2019 02:43:49 pm

1 more thought...I believe that in order for a water supply to be considered a municipal supply, it must be able to deliver a minimum of 20 psi residual. So you could use that as a basis to develop your bid/pump requirement. I would qualify the bid stating that absent of verified residual pressure an assumption of 20 psi was assumed.

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