Friday, January 14, 2022

Jockey Pump Sizing and Options

Pressure maintenance pumps (PMP), or jockey pumps (JP) as they are more commonly referred to, are an often incorrectly sized part of a fire pump system. Lets walk through the fundamentals.

Why/When Required:

To start with, technically you are not required to provide a jockey pump on all systems.  NFPA 20 section A.4.25 states "Pressure maintenance (jockey or make-up) pumps should be used where it is desirable to maintain a uniform or relatively high pressure on the fire protection system."  The goal is to not shock the system with water hammer when the main fire pump starts. You don't want to be boosting the pressure from 10 psi to 150 psi. You want to be boosting the pressure in the 5-15 psi range.

It is our recommendation that in almost all situations, a jockey pump should be provided to minimize pressure surges when the main fire pump starts.

Jockey Pump Types:

Jockey pumps are not required to be listed per NFPA 20-2010 section 4.25.1.  The failure of a jockey pump results in the main fire pump which would be considered a success.

As such any type of commercial booster pump could be utilized.  However, as a practical matter there are three common types of pumps used.

Regenerative Turbine

Pro: Cheap Cost / Low HP

Con: High churn pressure - Even the smallest sizes have churn pressures in excess of 400 psi / Must use a pressure relief valve always / Low flow rates - do note expect to fill a large system quickly with these pumps

(Anvil Fire does not recommend this type due to ease of accidentally over pressurizing your system)

Vertical Multi-Stage

Pro: Very reliable / Can be selected to prevent exceeding system pressure ratings (i.e. 175 psi) / Ease of maintenance

Con: Slightly more expensive

(Anvil Fire includes this type in all our quotes)

Submersible Multi-Stage

If your water supply isn't pressurized (i.e. above-ground tank or city water supply) this is your only option. These are basically "well" pumps and will be submersed down into the water on the end of a 1.25 or 1.5 galvanized pipe.

Sizing of Jockey Pumps

Once you know what type of jockey pump you are using, you need to specify the size/rating. We don't recommend following the commonly referenced rule-of-thumb: a jockey pump at 1% of the rated capacity of the main fire pump.  It does not take into account the volume of your system and anticipated leakages. The sizing of the jockey pump, has very little to do with the size of your fire pump. Lets look at a couple system types and how we would size the jockey pumps.

GPM Rating - Aboveground Only Systems

For situations where the pressure maintenance pump serves only aboveground piping for fire sprinkler and standpipe systems, the pressure maintenance pump should be sized to provide a flow less than a single fire sprinkler. The main fire pump should start during any waterflow situation where a sprinkler has opened, which will not happen if the pressure maintenance pump is too large. 

NFPA 20-2016 section 4.26 states the pump "shall be sized to replenish the fire protection system pressure due to allowable leakage and normal drops in pressure". The appendix language goes on to state "... The main fire pump should start and run (providing a pump running signal) for any waterflow situation where a sprinkler has opened, which will not happen if the pressure maintenance pump is too large. "

One guideline that has been successfully used to size pressure maintenance pumps is to select a pump that will make up the allowable leakage rate in 10 minutes or 1 gpm (3.8 L/min), whichever is larger. From a practical standpoint, jockey pumps are "approximately" rated for 2.5 gpm or 5 gpm. We typically quote our pumps with a 5 gpm rated jockey pump.

GPM Rating - Large Underground Systems

Underground mains are permitted by NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, to have some leakage.  

L = testing allowance (makeup water), in gallons per hour
S = length of pipe tested, feet
D = nominal diameter of the pipe, in inches
P = average test pressure during the hydrostatic test, in pounds per square inch (gauge)
Assuming that you have a system that will not exceed 150 PSI in static pressure, the underground will need to be tested to 200 PSI.

So lets run the following example:
  • 500-feet of 6-inch Class 52 Ductile Underground (D = 6.280")
  • Hydrostatic test pressure of 200 psi
  • Allowable Leakage L = 0.30 gpm
Assuming you have a 2,000 gpm pump, do you really need a jockey pump rated for 20 gpm when the actual leakage should not exceed 0.30 gpm? We are back to our standard practice of a 5 gpm rated jockey pump being adequate unless you have a lot of underground.

PSI Rating - All Systems

Assuming we now know we are going to use a 5 gpm rated pump, what pressure (psi) rating should we select? There are two different design conditions we need to consider:
  1. Flow rate to keep up with expected system leakage
  2. Maximum expected churn (no-flow) pressure so as to not exceed system rated pressure
Since we know the expected system leakage is generally close to zero, we are mainly concerned with just making sure the jockey pump won't over pressurize the system. There are three different approaches we can take:
  1. Adjust controller setpoints so the jockey pump turns off (bad idea for a variety of reasons)
  2. Provide a pressure relief valve to accommodate over pressurization (subject to human error and potential mechanical failure)
  3. Select a pump with a churn pressure (+ suction pressure) that won't exceed you system rating
Guess what option Anvil Fire recommends? 

Jockey Pump Selection Table Summary

Anvil Fire represents AC Fire / Xylem products and generally follow the sizing chart below for jockey pumps. Reach out to your local regional rep and let us help you narrow down your options.