RPs & RPDAs

My circuitous career path that has resulted in an ongoing 30-year stint in the plumbing industry wasn’t necessarily per my initial plans. I graduated from North Carolina State University in 1986 with a degree in aerospace engineering and set my sights on securing a job designing airplanes or spacecraft; however, the Challenger disaster in January of that year had a chilling effect on my job prospects. Eventually I did land a job with an aerospace component manufacturer, and later with a company that made automotive tire and air conditioning valves. Suddenly a light went off in my head — valves were definitely in my wheelhouse! I could apply what I had learned in college about fluid flow to valve design.

Anxious to expand my horizons, I was hired by “Apollo”® Valves in the mid-1990s. It was with some confidence that I strolled into famed Apollo engineer Bob Funderburk’s office to review one of my first projects, helping to design a new reduced pressure principle (RP) backflow assembly. “So how does one of these RPs work?” I asked. “Do you know what backflow is?” Bob responded, stroking his Tom Selleckworthy mustache. “Sure, a reversal of fluid flow.” Bob went on to explain there were two types of backflow — backpressure and backsiphonage. Backpressure backflow occurs when an increase in pressure exists on the downstream side of a connection, causing a reversal of flow from its intended direction. Backsiphonage backflow occurs when the supply pressure falls below atmospheric pressure and can be caused by water main breaks or excessive water demand. When atmospheric pressure becomes greater than the supply pressure, a reversal of flow can take place.

Bob pointed to a prototype on his desk and further explained that an RP was a testable assembly consisting of two independently acting check valves separated by an intermediate zone in which there is a hydraulically operated relief valve. The assembly is completed with the inclusion of inlet and outlet resilient seated shut-off valves, and four properly located test cocks. The RP’s purpose is to make sure that once water is delivered, it can’t return to the potable supply. The check valves are internally force loaded to a normally closed position, and the relief valve to a normally open position. When the inlet shut-off valve is opened, water enters a relief valve sensing passage and pressurizes the high-pressure side of the relief valve. The first check will next open with a minimum required opening pressure of 5 psi. After the first check opens, the zone between the two checks is pressurized. This pressure is lower than the inlet condition due to the pressure expended to open the first check valve. This lower pressure impedes on the low-pressure side of the relief valve. The high versus low pressure condition on the relief valve causes it to close. The residual pressure next causes the second check valve to open. The second check valve is required to hold at least 1 psi prior to opening. Under normal operating conditions, the RP’s relief valve is closed, and the check valves move between positions of open and closed to satisfy water demand. If the pressure between the first and second check valve (also called the “zone of reduced pressure” or “zone”) increases to the point where it is within 2 psi of the inlet pressure, the relief valve opens.

Bob then explained that in the event of a backpressure event, the second check valve prevents water from reversing its flow direction past the RP. In the case of backsiphonage (say the supply pressure suddenly goes subatmospheric due to several fire hydrants being opened), the first check closes, the pressure at the high side of the relief valve becomes negative and the relief valve again opens.

“You want to know the wonderful thing about RPs?” Bob inquired, barely able to mask his enthusiasm. “Sure,” I said, still trying to digest what he had just told me about how an RP works. He then went on to explain an RP would prevent backflow even if:

• the first check failed in the direction of flow — zone pressure would equal supply pressure and the relief valve would open
• the second check failed with backpressure — zone pressure would increase and surpass the minimum 2 psi differential from the inlet pressure, and the relief valve would open
• the relief valve failed — continuous dump, no backflow

After a few days of testing RPs in “Apollo”® Valves’ engineering laboratory, I was eager to prove my worth by suggesting some design parameters. I said, “This seems like a simple force balancing exercise, Bob. If we make the components this size and use these spring forces, it should work like a charm.” Bob glanced at my sketches and calculations and then replied by mumbling through his signature chortle, “I wish it was that easy.” Bob was right. RP designs must consider component materials of construction, their smoothness and friction characteristics, wear patterns, fluid flow dynamics, flow path geometry, water chemistry, disc compression, elastomeric durometers and chloramine resistance, to name a few factors. My hat is off to anyone who has designed an RP. I know from experience there is much trial and error required before a functional design can be realized.

ASSE 1013 PERFORMANCE REQUIREMENTS FOR REDUCED PRESSURE PRINCIPLE BACKFLOW PREVENTION ASSEMBLIES (FIRST PUBLISHED IN 1971)

This standard provides the minimum requirements that an RP must meet in order to be certified. Besides marking and component requirements, ASSE 1013 includes 15 functional tests. These include an independence of components test, a hydrostatic test of the entire assembly, a seat leakage test for the shut-off valves, a hydrostatic test of the check valves, an allowable pressure loss test, a relief valve opening test, a sensitivity of differential pressure relief valve test, drip tightness of the first and second checks tests, relief valve discharge tests with atmospheric and positive supply pressures, an air gap device backsiphonage test, a deterioration at extremes of temperature and pressure ranges test, a cycle test, and a field evaluation test for assemblies when required by the authority having jurisdiction (AHJ). It is this gauntlet of tests that gives consumers confidence that an ASSE-certified RP will perform as expected once installed. ASSE 1013 provides a minimum threshold that all manufacturers must meet in order to proudly display the ASSE certification mark. It is a tribute to the relatively small group of manufacturers of RPs that new designs continue to be brought to market that are smaller, lighter and easier to maintain while still providing the protection of the potable water supply ASSE 1013 requires.

ASSE 1047 PERFORMANCE REQUIREMENTS FOR REDUCED PRESSURE DETECTOR BACKFLOW PREVENTION ASSEMBLIES (FIRST PUBLISHED IN 1990)

This standard specifies compliance testing requirements for a product very similar to an RP but made specifically for fire protection systems. A reduced pressure detector assembly (RPDA) is usually installed on a fire sprinkler line. Like an RP, the RPDA prevents the reverse flow of contaminated water back into the potable water supply; however, the “detector” part of the name points to an important secondary function. Most fire sprinkler lines are not metered, making them prime targets for water theft. The RPDA’s mainline is an RP. RPDAs are also fitted with a bypass line that provides a visual or audible indication of system leakage or unauthorized water use. The first flow up to 2 gpm (0.13 l/s) is directed through the bypass line, which is also fitted with an RP and a water meter and/or alarm signaling device. The bypass line of an RPDA bypasses the first and second check valve of the mainline assembly. An innovative variation of the RPDA, the RPDA-II uses the mainline’s first check valve for both the mainline and the bypass line, affording the same protection as the RPDA with a more compact footprint.