Backflow preventers work by letting water flow through them in one direction, but prevent water from flowing back through them in a reverse direction. This is how the water supply is protected. A commercial building or industrial plant can use all the water it wants, but once the water has passed through the meter and the backflow preventer, it will not be allowed back into the water supply.
An RPZ provides the best level of protection because it has a built-in relief valve, which will open up and actually dump the backflowing water out of the valve, in order to prevent any chance of contaminated water re-entering the water supply. To see how the RPZ works in various situations, watch this less than 6-minute video.
The video includes illustrations and scenarios of when and why the RPZ dumps water. The RPZ indicates whether the valve is working properly or if service is needed. If no water is dumping out of the relief valve, the backflow preventer is working properly. If the relief valve is dumping out water or spitting out small or large amounts of water, then something is not right and maintenance on the valve is required.
This adds a level of safety to protect the public drinking water and is why more and more municipalities across the country are requiring the RPZ for both domestic and fire lines. Now that the RPZ is being required in many jurisdictions, thinking about where to install it becomes important.
There are two options: inside in a mechanical room or in an RPZ backflow cover. Because an RPZ is designed to dump water, the surrounding area is going to get wet. Unfortunately, this is not a good situation if the RPZ is installed inside a mechanical room or in a utility closet as the surrounding area will be damaged by the water pouring out of the backflow preventer. In this picture, the RPZ was installed in a mechanical room. An RPZ valve can often be one of the safest backflow prevention devices available, providing the correct upkeep is undertaken.
These valves need to be installed by competent and qualified plumbers, tested at least annually, and fully maintained to ensure that they perform correctly and continuously safeguard the water supply. Incorrect installation or use can risk contamination and incur costs to put things right, and testing is vital to proper performance. However, these routine tests come at a cost, making the valves expensive to maintain when compared to alternatives such as air gap options.
It is important that the installation is completed by a fully qualified contractor. After the initial installation, an RPZ valve will require annual formal commissioning and testing.
This requires a test kit to be attached to the test cocks to make sure the valve is operating correctly, and can only be completed by an accredited RPZ tester. The 1st check of an RP can have a loading of anywhere between PSI depending on the make, model and size. In our generic RP, we will assume our relief valve spring generates a 2.
Once the 1st check opens, water will travel past and pressurize the area between the 1st and 2nd checks. When this area is pressurized, it will also pressurize the low pressure side of our relief valve. The higher inlet pressure PSI is placed on the high pressure side of the elastic element in the relief valve, and the lower pressure past the 1st check 90 PSI is placed against the low pressure side of the elastic element.
Once this area between the two checks and the low pressure side of the relief valve is pressurized, the pressure will now cause the 2nd check to open.
The loading of the 2nd check spring will be anywhere between PSI depending on the make, model, and size. The pressure after the 2nd check will be reduced by the amount of pressure it takes to open the 2nd check.
For our illustration, we will give the 2nd check a 5. Don't forget that the relief valve spring is continually trying to open the relief valve, which the inlet pressure is keeping closed, and that a properly working relief valve can only open when the pressure downstream of the 1st check plus the relief valve spring load is greater than the upstream pressure to our RP. In our illustration, the pressure past the 1st check 90 PSI must increase to During the normal flow of water through a properly working RP backflow prevention assembly, the relief valve will be pressured closed, and the check valves will modulate between an opened and closed position to fill the water demand of the plumbing system.
The check valves in an RP will open when something downstream of the RP in the piping system opens a water-using fixture, and as the water begins to flow to that fixture, the pressure drops.
The pressure upstream of the fixture begins to drop as the water flows through the fixture. Because of this flow of water, the pressure upstream of the RP is now great enough to cause the check valve to open and flow water to the fixture demanding water. Check valves only open enough to fill the demand for water; they do not open fully, but usually modulate to fill the demand for water.
Backflow is the hydraulic condition that can cause an RP to stop working in the described normal flow pattern. Backflow can happen by either backpressure and or backsiphonage. Backpressure is a condition where a greater pressure is generated on the downstream than the upstream side of the assembly. This condition can happen for many reasons, pumps, thermal expansion, etc.
Let us assume we have a proper working RP, and we apply backpressure to the outlet side of our RP. If the starting downstream pressure 85 PSI increases toPSI for example, and the second check is working properly, the 2nd check closes and keeps the PSI pressure from migrating into the area 90 PSI between the 2 check valves. Even if we have a working 2nd check, and backpressure is applied, we can get a discharge from our relief valve.
A condition called disc compression can cause discharge from a properly working RP. When backpressure occurs, this increase in pressure placed on the downstream side of the 2nd check causes the 2nd check disc to embed farther into the 2nd check seat. The volume of water in the body between the 2 checks is being squeezed as the 2nd check disc embeds farther into the seat. Water is a not a compressible fluid in these pressure ranges, so this squeezing of this water causes an increase in pressure in the area between the 1st and 2nd check valves.
If this increase in the pressure between the 2 checks, which started at 90 PSI in our illustration, increases to the point where it is greater than the inlet pressure minus the relief valve spring loading - 2. In our field test procedures, when we perform the 2nd check test of an RP, we are simulating a backpressure condition by bringing the higher inlet pressure PSI around to test cock 4 85 PSI.
If you remember from your field test procedures, when an apparent 2nd check failure is observed, you are required to open your low side bleed valve on your test kit. This will draw the elevated pressure from the area between the 2 check valves, while the second check disc stays embedded into the 2nd check seat from the applied backpressure.
When the low bleed is opened, you are reestablishing the pressure in the area between the 2 checks back to its normal pressure of 90 PSI while the elevated PSI is maintained after the second check. Disc compression is one of the most common errors made by a backflow prevention assembly tester when performing a field test. Once the relief valve discharges when testing the second check, you must open the low bleed one more time to determine if the 2nd check is actually working or not.
A disc compression scenario may occur, and the tester may incorrectly assume the 2nd check is not working. Let us see what happens when we apply backpressure to a non-working 2nd check.
Once the pressure begins to increase on the outlet of our assembly, the 2nd check cannot maintain the separation of pressures between the inlet and outlet of the 2nd check, and the pressure will equalize on both sides.
As the pressure increase begins from 85 PSI in our illustration the area between the 2 checks will also increase. Remember that the area before the 2nd check is where our low pressure is applied to the low pressure side of our relief valve elastic element. As the pressure increases above our starting pressure of 85 PSI, and goes to the point equal to the inlet pressure minus the relief valve opening PSI - 2.
Backsiphonage is a condition that causes a sub-atmospheric pressure to be applied to the upstream side of the assembly. Backsiphonage can happen for several reasons; one of the more common is excessive water demand in the distribution system. Let's examine the effect of backsiphonage on our RP and see how it functions. When the inlet pressure to our assembly PSI in our illustration goes down to sub-atmospheric, or negative, the PSI is reduced to a negative pressure. The pressure at the inlet of the RP is what keeps the relief valve closed.
When the pressure at the high pressure side of the elastic element in the relief valve is reduced to a negative, the relief valve will open because of the relief valve spring load, and any pressure remaining in the area between the 2 check valves is now applied to the low pressure side of the relief valve elastic element.
The relief valve of an RP can only open for the two backflow conditions of backpressure or backsiphonage or the simulation of these two conditions. One of the more common simulations of backpressure happens when there is a pressure fluctuation at the inlet of the RP.
If there is no flow going through the assembly, and then the upstream pressure drops quickly from PSI down to 80 PSI, this can cause the relief valve to open. This would happen because there would be higher pressure on the low pressure side of the relief valve elastic element versus the high pressure side high pressure side would be 80 PSI and the low pressure side would be 90 PSI plus the relief valve spring loading.
If a water hammer condition happens on the downside of a properly working RP because of a quick closing solenoid, this increase in pressure would create a backpressure condition, which could cause the relief valve to open by disc compression. Just because a relief valve discharges water, does not always mean the RP is not working.
Pressure fluctuations can simulate conditions that can lead a person to assume the assembly is not working properly. To be sure whether the assembly is working or not, a test kit must be attached and proper test procedures applied to determine the working condition of the assembly. Now let us look at how an RP reacts when components of the assembly are not working properly and how we can diagnose the condition.
Let's assume we perform a field test on an RP and generate a 1. Does this mean the assembly is "leaking" or will not prevent backflow? The answer is probably no. We know from our tester training that 2. If the relief valve opens at 1. An RP with a 1. For the assembly to perform optimally, it must operate at or above this minimum standard, in this case 2.
The cause of a relief valve opening below the 2. Usually, the incorrect assumption is made that a spring has worn out and that is why the relief valve will not open.
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