“The grass is always greener on the other side,” is what people usually say when someone makes a life decision that is predominantly based on perceived notions. I think we all have an innate ability to try and convince each other to make certain choices. It could be as simple as deciding what to eat with your family or colleagues. In my case, the grass really is greener — my neighbor is a landscaper. I look out the front window of my house and see a perfectly manicured yard year around. On one occasion, I remember he plucked a blade of grass and held it up for me to observe as he explained his methods. One of the most important factors for having the greenest lawn, in my neighbor’s opinion, was irrigation.
Plumbing engineers frequently work with landscape designers to provide water connections for irrigation. Anytime we are asked to provide a water connection, especially for an application other than human consumption, the first question we need to ask ourselves is: “What type of backflow prevention should we provide?” Irrigation is no exception. Backflow prevention is at the core of plumbing engineering — it is one of the first lines of defense when it comes to public safety.
Many of us might think the answer is to provide an “RPZ” or a “Double Check.” The truth of the matter is that there is a wide variety of backflow devices. The “ASPE Plumbing Engineers Handbook, Volume 2,” lists 13 different types of backflow preventers and references 15 different ASSE standards. ASSE stands for the American Society of Sanitary Engineers, which was created in 1906 to “push for the standardization of plumbing practices” and evolved into the organization is it today which creates quality standards, certification and testing programs.
A reduced pressure zone backflow preventer is a common backflow device used to protect the upstream water supply from any hazards. The “ASPE Plumbing Engineer Design Handbook” abbreviates these devices as an RPZ. I have also seen them abbreviated as an RPBP. As one mentor explained to me, the important thing is to make sure it is on your drawing legend correctly, however you chose to designate it.
Another important thing to remember is that an RPZ, or RPBP, is a specific type of device that is listed per ASSE standards 1013 or ASSE 1047 for fire protection. The term RPZ is so commonplace that sometimes people use it to refer to any backflow preventer. A designation like “BFP” for backflow preventer is more general. In order to spark some imagination, I like to compare RPZs to an airlock on a submarine or spaceship. They are designed with an intermediate chamber that isolates one pressure zone from another.
Specifying the right product
How do we determine which type of backflow prevention is appropriate for keeping the grass green on the other side of the street? A reduced pressure principle device would surely provide the level of safety, but our responsibility as designers and engineers is to understand the application and apply best engineering practices. I typically hesitate when it comes to installing RPZs if I do not need to because they require additional testing by the owner. Short of an air gap, RPZs also have the highest-pressure loss, even at the lowest flow rates. Before you add that RPZ to your design, make sure you have an extra 10 to 15 psi to spare. For any device you are going to use, refer to the cut sheet which will have pressure loss curves.
One code I am familiar with lists irrigation systems as a type of hazard with pressure vacuum breakers (PVB) as an acceptable type of backflow preventer. The caveat for specifying a PVB is there is no health hazard and no back pressure possible. When providing a water connection to an irrigation system, I consider any type of chemical injection a health hazard. Backpressure is defined in the “ASPE Engineers Design Handbook, Volume 4,” as something that “occurs when a higher pressure is applied on the outlet, or connection side of the water distribution side than on the inlet.” I have seen my neighbor diligently maintain his lawn and irrigation system, and I know that he does blow out the lines every year to prevent freezing in the winter. I would not necessarily consider this backpressure since the irrigation network would be open to atmospheric pressure for maintenance. A couple of benefits to specifying a PVB for irrigation is there may be less testing requirements, and the pressure losses are on the order of 2 to 5 psi.
PVBs are similar in purpose to atmospheric vacuum breakers (AVB) in that they protect against back-siphonage. The device “breaks” the vacuum if there is a loss of pressure on the inlet side. While PVBs are manufactured and tested to meet the performance requirements of ASSE standard 1020, AVBs are made to meet the performance standards of ASSE 1001. The main difference between the two is pressure type vacuum breakers have a spring loaded poppet and may be acceptable for use with valves installed downstream.
As engineers and designers, we are tasked with making decisions as we specify products for plumbing systems. It is a valuable skill to be able to make those decisions after collecting as much information as we can about an application as well as the piping, valves and equipment that we plan to incorporate. I have worked on many a job where the choice was made to install an RPZ at almost every instance where backflow is required.
Sometimes, there is a valid reason to do so. On one job, the fire department explained to me how their pumper truck could pull water out of a nearby pond that had a history of pollution.
All things being considered, we should be careful not to be swayed by anyone who is trying to convince us that the “grass is greener” if we go with their product or system.