Backflow Talk

Regularly the technical staff at Fluidflo receive calls regarding opening of the relief valve of a reduced pressure zone device (RPZD).

The call usually comes from the building owner, supplier or the person at the job who has just installed the RPZD. The call more often than not sounds something like “we just installed this valve and it doesn’t work” or “we just installed this valve and its leaking”.

Firstly we ask to identify what is occurring, and then we can establish if there actually is a problem and what the possible solutions may be.

Many people may have never heard of a reduced pressure zone device (RPZD), or even understand its operation until they are asked to install one. So the person complaining about the leaking valve may have no concept of what the valve actually does.

They see water coming out and think there is a problem. It is not always possible to determine the problem over the phone, but some problems can be eliminated this way.

The activation of the relief valve must be clarified. Is the relief valve stuck in the open position and releasing large quantities of water? Does the relief valve have a constant steady drip or slight stream discharging from the relief port? Does the relief valve activate periodically spitting water?

If it is determined that the relief valve is discharging large amounts of water, there may be a problem that requires immediate attention. The relief valve could be stuck in the open position due to debris, or the first check of the (RPZD) could be stuck in the open position for the same reason.
This could be caused by a ruptured diaphragm in the relief valve. Another possibility is the second check could be stuck open with a backpressure condition.

No matter what the cause, the assembly will need to be tested by a licensed backflow professional to determine the condition and operation of the check valves and the relief valve.

If the relief valve is discharging a steady drip or stream of water, all of the same conditions may exist, but not to the same degree. The most common problem is the first check. If the first check is not seating properly this will cause the relief valve to discharge under no flow conditions.

The amount of water discharging from the relief valve may be directly related to the size of the fouling of the check. A fine drip may indicate a grain of sand on the sealing surface of the check valve.
A stream of water may indicate something larger. If there is a constant flow of water this problem would not occur until the flow of water is stopped, such as to test the backflow device.

When water flows through the assembly, the pressure drop across the first check valve increases and keeps the relief valve from leaking. When the water flow stops, the check valve leaks causing the relief valve to open or leak. Another possible cause is a fouled relief valve.

Some debris may be caught on the relief valve disc, not allowing the relief valve to seat properly. In this situation, the discharge from the relief valve would not be affected by the flow of water through the valve. Other possibilities are the diaphragm has a small hole in it or the number two check is leaking with backpressure.

In many cases the relief valve may be reported as discharging intermittently or spitting. When this occurs the (RPZ) is most likely doing exactly what it is designed to do. The differential pressure relief valve is designed in such a way that it is hydraulically dependent on the differential across the first check valve.

When the difference in pressure across the first check drops far enough the relief valve opens. This relieves the pressure in the zone between the check valves and keeps the pressure in the zone reduced lower than inlet pressure, hence the name reduced pressure zone device.

Backflow Talk

DateTuesday 24 Oct 2006