I was asked to write an article for Working Pressure magazine, so I thought I would share my favorite reduced pressure backflow assembly (RP) troubleshooting method without the need to use a pressure differential gauge and put hoses to test ports.
At one time or another, all of us backflow testers have encountered our easiest backflow assembly test, which is when you come up to an RP and the differential pressure relief valve is already leaking. This test is an automatic failure, which means a repair is in order; but where is it leaking from, what is causing the failure? Let’s find out without having to put gauges on the assembly.
There are three possibilities for the source of an RP leak: 1) the first check, which is 90% of the time, 2) the second check, and 3) the differential pressure relief valve. If you know of a fourth, please reach out to me (wink). After getting permission to turn off the water and repair the backflow assembly, I want you to first turn off the downstream No. 2 shut-off valve.
This eliminates the possibility of any backpressure affecting the relief should the No. 2 check valve not want to close tightly. This does not confirm that the No. 2 check valve is working, but does confirm that it is not causing the relief valve discharge.
POSSIBILITY #1: RELIEF LEAKING
The next step is to take one of your hoses and place it on the fourth test port (TC#4). Open TC#4 to establish flow across the assembly and observe what is happening to the relief valve. If the relief valve keeps dumping, then the problem could only be found in the differential pressure relief valve.
For this trick to work, it is important to know that you must compensate for more water to run out of TC#4 than out of the relief valve. If you have 50 gpm coming out of the relief, you must have more flowing out of TC#4; otherwise, you will not be able to notice if the relief valve has stopped leaking.
Remember while you are running water out of TC#4, the first and second check valves are forced open because water must pass through them for water to flow out of TC#4.
POSSIBILITY #2: CHECK VALVE ONE IS LEAKING
If the relief is not dumping and you’re still running water out of TC#4 but the relief stopped dripping, then the problem could only be in the No. 1 check valve because the relief stopped dumping. Keep in mind that while running water out of TC#4, with check valves No. 1 and 2 flowing water, the problem can’t be in the first check valve because the relief valve can’t leak with water flowing past the first check valve.
POSSIBILITY #3: CHECK VALVE TWO IS LEAKING
While this is a rare occurrence, it is still a possibility; therefore, we need to address it. Three scenarios need to occur at the same time for check valve No. 2 to fail: two failures must happen at the same time and a backpressure condition must be occurring.
The first is obviously that there is dirt or debris in check valve No. 2 because it can’t be closed tightly. The second failure is for the downstream — or No. 2 — shut-off valve to fail. Lastly, a backpressure condition must occur downstream of the No. 2 shut-off valve.
If the second check valve is holding tightly but the No. 2 shut-off valve is not — no matter how much backpressure you have going on in the building — the check will seat even more tightly, thereby not allowing water to enter the reduced pressure chamber to overcome the pressure on the upstream side of the diaphragm and allow the relief valve spring to open and let water run out of the relief valve port.
If, however, the second check can’t close tightly, the No. 2 shut-off valve does not hold tightly and there is a backpressure issue happening in the building. Only then will the differential pressure relief leak from the second check issue.
“But Nick — you just told us if the check is leaking, the problem is in the relief valve.” You would be correct, but in order to notice this, you must be a very patient tester. Remember that slow testers are the best testers because they wait for the problem to expose itself.
What do I mean by this? During your first step, when you turn off the No. 2 shut-off valve, open TC#4 and observe that the relief valve has stopped dripping. Once you see that the relief valve has stopped dripping, you are sure that the problem is in the first check. However, after backpressure has gone through the No. 2 shut-off valve and the second check valve has debris preventing the No. 2 check valve from closing tightly, backpressure will overcome the upstream pressure on the diaphragm, thereby causing the relief valve to open and leak. It will take some extra time for backpressure to get past the second shut-off valve and the No. 2 check valve.
Remember that for the relief valve to leak, you must get above the buffer. The buffer is defined as the difference between the value of check valve No. 1 and the differential pressure relief valve. With a 5-pound check valve, one spring and a 2-pound relief valve spring, the buffer will be 3 pounds. With the exception of a pressure fluctuation issue, you must always overtake the buffer differential for the relief valve to open in this situation.
As you see the relief stop leaking in possibility No. 1, continue to wait and see if the relief valve goes back into a leak situation. When the relief stops — and then resumes — leaking, it is an indication that check valve No. 2 has failed because the downstream No. 2 shut-off valve did not hold tightly, check valve No. 2 is not holding tightly, and the buffer has been overtaken by backpressure condition from the building; all of this took some time to develop.
How can a building develop a backpressure condition when the water utility’s water supply pressure cannot be higher than that of downstream supply pressure across an assembly?
Remember the equation force equals pressure times area (F = P x A). In this example, we will take a building that is 100 feet tall and multiply by 12 inches, which mathematically is 0.433 psi, and we will get an extra 43.3 psi of water. This means that if the water utility is supplying 43.3 psi to a building, the top floor of this 100-foot building will have 0 psi while the base where the water comes in has 43.3 psi.
To make the top floors function properly, the engineer has added a booster pump system and raised the water utility’s pressure by 36.7 psi so that every fixture on the top floor can flush toilets, take showers, do laundry, etc. This building now has an extra 80 psi of pressure at the base of the building and only 36.7 on the top floor. This is how a building archives backpressure.
I hope you have enjoyed my article. For most of the seasoned testers and repairers, this was a good review. For those of you who are newbies to the backflow world, I hope you gained some insight into troubleshooting an RP without using a differential gauge. And to all of you who have read this article — if you have any questions or comments, my contact information is listed below.
#4 sensing tube everytime, the most overlooked part of a rp.
Terminology is confusing. You talk about the relief valve leaking, but is this the correct terminology? Is the relief valve leaking, or is it discharging because the first check valve is not dripping tight, and there is no downstream demand? Is water discharging out of the relief because the second check valve is not drip-tight, or is the second check valve O-ring issue, and backpressure is present with a not drip-tight downstream shut-off valve? In this case the relief is constantly discharging. The relief valve leaks due to a relief valve malfunction; the relief valve is designed to trigger (not leak) when the pressure between the check valve (the zone) equals the inlet pressure with the assistance of a spring. The relief valve will sporadically discharge when there is a drop in inlet pressure. The relief valve triggers when a pressure drop exceeds the difference between the first check valve spring psi and the relief valve spring psi.