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Dry Fire Systems & Backflow Prevention Testing

November 28, 2017

When testing a backflow prevention assembly with OSY or NRS valves on a dry fire sprinkler system the simple act of closing shut off valve #2 can build up enough pressure on the upstream side of the Clapper valve to trip the system. It happens to many backflow testers every single year, it has never happened to me because I follow these simple steps.

Dry Systems.jpg

  1. In the photo there are 2 gauges, the lower gauge is the water pressure on the upstream side of the Clapper valve. The upper gauge is the air pressure on the downstream side of the Clapper valve. The air pressure is what keeps the Clapper valve closed. There are many different dry systems on the market and some systems require more air pressure than others. If the readings are equal to the values below you are in little to no danger of tripping the clapper valve when testing the backflow prevention assembly.
  2. Water PSI = Air PSI

    60-80 PSI = 30-35 PSI

    80-100 PSI = 35-40 PSI

    100-120 PSI = 40-45 PSI

    120-140 PSI = 45-50 PSI

    If your air pressure is lower than what is indicated on the above chart for a given water pressure that “may” indicate a problem and you should inform your customer and make it clear you are not certified in dry systems and it would be wise to have a certified technician inspect it.

  3. As you’re slowly closing shut off valve #2 on the backflow preventer keep an eye on the lower water pressure gauge on the dry system. If the needle begins to rise STOP what you’re doing and slightly open test cock #4. The water pressure gauge on your dry system should drop back down to the original reading. Continue to slowly close shut off valve #2 while continuing to watch the gauge; the water pressure should remain stable. Once you have the shut off valve closed tight you can close test cock #4 and begin your test.
  4. If the air pressure is much lower than the chart above for a given water pressure your best option is not to touch the assembly until the dry system has been checked out by a certified technician.
  5. Remember it is extremely important to know what the downstream process is before you shut the water off. It’s up to you to educate yourself on the various types of systems to avoid costly damages to the equipment downstream of the assembly.


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