Dry Standpipe System - Jockey Pump Requirement

[SherifH]

thread184-444593

I was reading this thread and its quite confusing and has conflicting points. I wanted to revive it and give a more detailed explanation as to the question so that its understood correctly, to hopefully attract more relevant discussion and be able to obtain a more assuring conclusion.

The Facts
1) A Dry Standpipe system exists.
2) The water supply pressure (city water pressure) is too low to be self-sufficient and thus needs a NFPA-20 Fire Pump.
3) An air compressor ensures adequate air pressure downstream of the air valve to keep it closed.
4) The Fire Pump & Alarms are triggered only when the Air Valve is opened, i.e the air pressure downstream falls enough that the water pressure upstream pushes the seat. (I do not know if this one is correct, its my understanding)

Is a jockey pump still required UPSTREAM of the air valve with the fire pump, or can the fire pump function alone?

I want to know your opinions on the following logic:

A) If there is a drop in city water pressure, the air valve will not be opened. If the fire pump is only triggered on an opening of the air valve, this tells me that a jockey pump is not required. (This goes back to assumption stated in #4 above, because if the fire pump is also activated by a loss of water pressure this would not apply). As such, a jockey pump is not required.

On the other hand, if the water pressure upstream of the air valve drops, then consequently that would mean that the air pressure drop would have to be bigger downstream to open the valve which could cause problems since this would mean a delayed response to a fire.

B) A drop in city water pressure as an issue can be indicated by a pressure gauge that would alert operators there is something wrong (leaking) without having to trigger the alarm system. The jockey pump here would keep pressure through the leak, but if they are alerted and manage to fix the issue, the pipe shall go back to city water pressure and thus a jockey pump can still be avoided.

Unless it is mandated by the NFPA or Building Code for reasons im unaware of atm.



Looking for some insight or some previous experience Please & Thanks!

 

[UFT12]

NFPA 20 mandates a pressure maintenance device (see 4.27.1) but it does not mean necessarily a jockey pump. However, practically that ends up being a jockey pump almost always. I have seen installations (non-US) with a bladder tank instead but I wouldn't recommend it. Observe however that 4.27.7 prohibits the use of main fire pumps for pressure maintenance. If pressure fluctuations are expected in your system, a jockey pump makes sense. Also, if a system is supplied by an underground serving main, leakages from this mains are expected, aka pressure loss. Therefore (per 4.27.7) a jockey pump is again necessary. But regardless of underground mains, according to my experience, when a water supply system is supplying an extended system (multiple buildings) which requires system expansions every now and then of which portions need to be emptied and refilled, you are always better off to let the jockey pump pressurize and refill the system. There are cases where the main pumps where activated causing surges due to high residual pressures. This is also stated at the end of the link you posted and may be particularly important in your case where the static (city) system pressure may be substantially lower than the main pump churn pressure.

To your writings, by city pressure drop the main pump without a jockey will start and most likely create a surge. Do keep in mind that dry pipe valves have clappers which after activation cannot rest back to the original closed position. Typically they have a latch which prevent them to rest back unless you manually reset the valve. That can be problematic in case the system is not tripped as the valve will remain open. Remember, air is compressible.

 

[SherifH]

1) Pressure fluctuations due to city water pressure I think are within a reasonable rate that this might not be an issue? In your experience would a city water main connection to a transit station heavily fluctuate?

I do not know enough about pressure setting requirements but a city pressure of 50 psi should be able to refill the rather physically small portion of the pipe & sprinkler system that lay upstream of the dry air valve if needed, but is the fire pump pressure signal to activate usually much higher than the intake pressure? The pump is designed to lift 40 psi to approx 90 psi. How low is a pump pressure switch usually from the pressure output (when would the fire pump need to activate according to regulations)?

Also as there is a back flow preventer upstream of the pump, would that not decrease the risk of pressure leakage?