Minimum Design Water Pressures

[cuels]

The state rules have the following minimum pressure requirements on a public water system:

20 psi - Fire Flow + Peak Day Demand
30 psi - Peak Day Demand - 800 gpd/ERC
40 psi - Peak Instantaneous Demand

If I am looking at a commercial development, I am assuming to use the worst case scenario for my development's water system. So, I would use the 20 psi as my minimum pressure at the connection to the PWS. It doesn't make sense to me that I would be able to maintain that 20 psi to the rest of the development (if it is not all downhill/buildings above grade, etc.). What should I use as a minimum pressure to that development and buildings, if measured data is not available?

 

[cvg]

you need to calculate the friction loss for the on-site piping to the most remote location and also add the elevation change to determine the additional pressure required. However, according to your rules the 20 psi is measured at the public system, not your private system. So at the fire hydrants on the nearest public streets, you must maintain 20 psi and at the meter connection you must maintain 30 or 40

 

[CPENG78]

Cuels,
I am assuming that when you say that "...data is not available" that this refers to no record information available from the city and/or agencies involved. The 20 psi requirement by itself would not only be measured at the connection point but also at any lines that are part of the fire suppression system, whether private or public (Refer to NFPA 24 and NFPA 13 for this requirement). I don't know at what stage of development your project is currently at but discussions with water agency should be started at least to get a general idea of any pressure issues in the area. When applying the fire requirements and/or conbinations of any others, you may ended up finding necessary to upsizing pipes (proposed and/or existing) and worst case scenario potentially having to add a pump. If at the early stages of your development, these are issues that the developer will need to know in order to proceed and understand the potential costs involved.

If no data available, a fire flow test at one of the nearest hydrants would be necessary ir order to investigate current system performance. As a general rule of thumb, depending on the configuration and size of your proposed developement the number of connection points to the public system will equal how many points you will collect your fire flow data from.