NFPA 25 (1998 edition) section 9-2.6 says main drain tests are to be conducted annually at each sprinkler system riser to determine if there has been a change in the water supply, piping or control valves. The original purpose for main drains on sprinkler risers is to drain water from the overhead piping after the system is shut off.
But the added value of the main drains is to perform the test to determine whether there is a major reduction in water-flow to the system, such as might be caused by the an obstruction from a dropped gate on a valve, a partially closed valve, a check valve stuck on its seat, or a foreign object like a rock or a tool left in the pipe from a recent service.
The Annex section of 9-2.6 does allow standpipe risers to have their main drain tests performed at the low point drain where the water enters the building, but that option is not permitted for the sprinkler system risers.
A large drop in the full pressure of the main drain test when compared to previous tests normally indicates a dangerously reduced water supply. After closing the main drain test valve, a slow return to normal static pressure is confirmation of the suspicion of a major obstruction, and is just cause to investigate why the water supply is reduced. A main drain test is considered satisfactory when the pressures and time to restore to static pressure are nearly the same as previous main drain tests.
Please remember that sprinkler riser main drain tests are performed at the sprinkler riser – not at the location where the main water supply enters the building. It is not unusual that the older hospitals are not outfitted with the main drain test valve and pressure gauge on each riser, but that is what NFPA 25 requires. Since each sprinkler riser is supposed to have a main drain test conducted, that means you need to have the same number of main drain test results. Whatever number of risers you have, that’s how many main drain tests you should have documented each year.
Main drain tests are required annually at each system riser, and downstream of any valve that is shut-off, then re-opened. The main drain tests should be coordinated to be performed just after the annual sprinkler control valve exercise. Here is the procedure to conduct a main drain test:
With the fire pump off, but the jockey pump on:
1. Record the static pressure.
2. Open the main drain valve slowly.
3. After the pressure gauge has stabilized, record the residual pressure.
4. Slowly close the main drain valve.
5. Record the time it takes to return the residual pressure back to static pressure.
This main drain test should provide the following findings on the test sheet:
- Static pressure
- Residual pressure
- Time to restore back to static pressure
These findings should be compared to previous main drain test findings to determine if they were consistent. If they are not consistent with previous main drain tests, then an investigation should be conducted to determine if there is an obstruction in the sprinkler water supply.