Main Drain Test Question

[Pfairview117]

Hi everyone,

I am new to the fire protection community and had a general question. If a facility's fire protection system is supplemented with a fire pump, is a main drain test expected to kick the pump on? Or is there typically not enough pressure drop.

Any thoughts/advice would be greatly appreciated, thank you!

 

[SprinklerDesigner2]

It will always turn the fire pump on.

Why?

By design jockey pumps simply do not have the capacity to discharge enough water to keep pressures up.

[URL unfurl="true"]https://www.mthpumps.com/applications/jockey.html[/url]

Let's look at a 500 gpm fire pump at 140 psi.

All we need is a 3/4 HP pump which would deliver just 5 gpm @ 140 psi. An open 2" drain would easily discharge 100 gpm and almost immediately cause the main fire pump to start.

 

[skdesigner]

It would definitely turn the fire pump on. NFPA 20 recommends that jockey pumps be sized to provide a flow less than that of a single sprinkler. A test and drain with a restricted orifice is required to allow a flow rate of at least a single sprinkler. When the handle is in the drain position vs. the test position, it would probably flow 10x that amount.

For systems with fire pumps, the annual main drain test is typically not performed separately from the fire pump flow test, as the water supply data can be read from the suction gauge on the pump during the flow test and compared to previous years, which is the purpose of the main drain test.

 

[LCREP]

Definitely check the fire and jockey pump start and stop setting in the jockey and fire pump controllers. NFPA 20 offers some examples as noted below if for some reason the main drain is Not starting the fire pump. Sometimes they are not set properly.

Examples of fire pump settings follow (for SI units, 1 psi = 0.0689 bar):
i. Pump: 1000 gpm, 100 psi pump with churn pressure of 115 psi
ii. Suction supply: 50 psi from city — minimum static; 60 psi from city — maximum static
iii. Jockey pump stop = 115 psi + 50 psi = 165 psi
iv. Jockey pump start = 165 psi − 10 psi = 155 psi
v. Firepumpstop=115psi+50psi=165psi
vi. Firepumpstart=155psi−5psi=150psi
vii. Fire pump maximum churn = 115 psi + 60 psi = 175 psi