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.
11/1/2018 12:40:49 pm
4/6/2022 04:43:56 pm
you are overestimating the available pressure you really only have 15 PSI at the bottom of the tank plus gravity. there is a calculation in NFPA 13 that will show that
11/2/2018 10:01:01 am
I agree with EDWH above.
11/2/2018 07:46:49 pm
Agreed..the calcs have to be taken back to the tank discharge.
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.
1/4/2019 11:27:47 am
What is that "NYC quirk" that allows to calculate back to the Riser only?
4/6/2022 04:41:19 pm
The "NYC Quirk" assumes that a correct source curve has been developed to account for source friction losses, fire pump curve and hydrant flow curve
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.
Leave a Reply.
Subscribe and learn something new each day:
Top Feb 2023 Contributors
Get 100 Days of Free Sample Questions right to you!
PE PREP SERIES