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Pressure Tanks - Hydraulic Calculation Method

11/1/2018

6 Comments

 
I have a question regarding performing or checking hydraulic calcs for a pressure tank supplied system in a retrofit application.

As an example, suppose our system is located 5 floors below the existing pressure tank for a high rise building in New York City (NYC). The tank was designed and installed under prior codes and utilized the pipe schedule methods. We may have 2 tanks connected that are 9,000 total gallons capacity each with 1/3 air and 2/3 water pressurized at 75 psi (the initial pressure) at the tank.

Our connection to the sprinkler riser being 5 floors down now has 75 psi plus the pressure gained by gravity (5 psi per floor typical NYC arrangement) equaling 100 psi static available.

The jurisdiction now requires all systems to be hydraulically calculated. The calculation are done to the point of connection to the riser and not all the way back to the water source (NYC quirk) and no hose demand.

The contractor provides a calculation stating that the system demand is below the 100 psi (95 psi residual) and less than 400 gpm (light hazard occupancy 30 minute supply), and claims it works. We’ve argued that as the tank drains, the air-to-water ratio changes, therefore the pressure available at the end of the 30 minutes is not the same as initial tank pressure.

Using the formula given in the Annex of NFPA 13, we can solve for what a system demand should be given the existing initial conditions of the tank plus the pressure gained by gravity and say that as long as the calculated system is less than that pressure, the system is acceptable. But is that a correct assumption?

I want to give them all the correct procedure, methodology and theories behind this subject.

Posted anonymously by a member for discussion. Discuss this | Subscribe.
6 Comments
EdwH
11/1/2018 12:40:49 pm

Two things:

The tank capacity can only meet a system demand of ~310 gpm for 30 minutes ...

NFPA 13 -2016 24.2.4.1.2 requires the tank pressure to be maintained until the tank is completely empty ...

either of these could be an issue

Reply
JOE MEYER
11/2/2018 10:01:01 am

I agree with EDWH above.

I think the hydraulic calculations must be performed back to the pressure tank as the source in order to prove that there's sufficient pressure and capacity in the tanks. NFPA 13 only gives the annex guidance which describes calculating back to the pressure tank for systems that use a pressure tank as a water supply.

If a system riser is sufficiently tested to understand the residual pressure at the end of the required flow duration is known, then perhaps a calculation could be done to that point. Without that data, though, I suspect that only calculating back to the pressure tank would be appropriate.

Reply
Mike
11/2/2018 07:46:49 pm

Agreed..the calcs have to be taken back to the tank discharge.

NFPA-13 24.2.4.1.2 requires an automatic means to maintain pressure. It does not state that constant pressure must be maintained during tank draining. The appendix formulas are calculated to provide 15 psi as the last of the water leaves the tank for pipe schedule systems.

For hydraulically calculated systems, the formulas work backwards to what the tank pressure should be set to. But in a retrofit situation, the tank pressure is given. As the tank empties, and pressure changes, does that require a differential equation to solve for the final pressure?

Or assume 15 psi at the end of the 30 minutes plus whatever is gained from the elevation difference and that's the "residual" pressure you have available???

Reply
Nimal Tissa Wijetunga
11/3/2018 01:58:44 am

I think the project of this nature requires the services of a Fire Safety Consultant or consultancy as this is not only NFA 13 as you may have to consider NFPA 14 as well and possibly NFPA 14 requirement may be overriding the NFPA 13 requirements.

If it is connected to External yard hydrants, then you need to consider NFPA 24 requirements also.

.

Reply
John
1/4/2019 11:27:47 am

What is that "NYC quirk" that allows to calculate back to the Riser only?

Reply
Gillian Babcock link
2/7/2019 05:15:25 pm

My dad wants to make sure that the water will reach all of the parts of our home, so, he's planning to have pressure tanks. It was mentioned here that the position of the pressure tank should be calculated for efficiency. Furthermore, it's highly recommended to consult professional contractors when considering having pressure tanks.

Reply



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  • Blog
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