What is an RP Assembly ?

Reduced Pressure Principle Backflow Prevention Assembly

The RP is the most protective type of assembly available for backflow prevention, rated for use in high hazard situations that can have a backpressure condition. They are regulated by two main specifications, ASSE 1013 and AWWA C511, depending on the codes and systems used ( Plumbing for ASSE, Water Distribution for AWWA ) they will have to adhere to one of these specifications or sometimes both, if you read both specifications, they largely say the same things with very small differences in the wording. Typically they are installed either for domestic water containment protection which protects the water purveyor's lines from an entire facility, or the they are used to isolate hazardous systems from the potable water supply inside of a plumbing system in a building, for example an HVAC make up water connection or a connection to irrigation. They will always have some type of outlet to atmosphere on the bottom or side, called the Relief Valve, which differentiates the RP from a Double Check which may look the same but without the relief valve.

Components of an RP Assembly

The RP Assembly incudes:

No. 1 Test Cock - This test cock is not normally used during testing, but can be used to flush components of the valve during repair, or used to bypass a leaking No. 2 Shutoff during testing.

No. 1 Shutoff Valve - This valve is normally open during testing, but would be closed if repair is required for the valves. During some single hose testing methods, you would also use the No. 1 Shutoff.

No. 2 Test Cock - This test cock shows the pressure in the zone upstream of No. 1 Check Valve and the Relief Valve, which is also equal to the static line pressure for the system.

No. 1 Check Valve - This Check Valve is extra heavy ( Minimum 5.0 psi Differential ) and, although would stop backflow in an extreme situation, under normal operation should never be in a Backsiphonage or Backpressure Condition, and serves as a component of the Relief Valve System by reducing the pressure between the upstream and downstream sides of the relief valve. The Relief Valve should remove all water or contaminating liquids from the Reduced Pressure Zone in the event of a Backpressure or Backsiphonage event.

No. 3 Test Cock - This test cock shows the pressure in the zone downstream of No. 1 Check Valve and the Relief Valve, and upstream of No. 2 Check Valve, also referred to as the "Reduced Pressure Zone"

Relief Valve - The relief valve is hydraulicly dependent on the No. 1 Check Valve, meaning that if the No. 1 Check Valve leaks, the Relief Valve will discharge continuously. This valve will discharge any contaminants that may leak through No. 2 Check Valve in a Backpressure or Backsiphonage condition, such that even if the components of an RP are broken, as long as the relief valve is not stuck closed, backflow will not occur.

No. 2 Check Valve - This Check Valve is normally the component that stops backflow in a Backpressure condition, holding backpressure continuously in some applications, and holds the system's water inside the building during a Backsiphonage condition, until pressure is restored. This check valve has a normal pressure differential ( Minimum 1.0 psid )

No. 4 Test Cock - This test cock shows the pressure in the zone downstream of No. 2 Check Valve, which is used in testing to determine No. 2 Check Valve tightness against backpressure and Differential Pressure ( if required ).

No. 2 Shutoff Valve - This second shutoff valve is normally used during testing and interrupts the flow to the building or the hazardous service being isolated.

More info on how the RP works on our YouTube Video here

Examples of Reduced Pressure ( RP ) Principle Assemblies

Watts 009

A very common and inexpensive option for smaller applications is the Watts 009. These are more commonly used for smaller sizes ( 1/2" - 2" ). This is referred to as the BMW of Backflow Preventers, because in order to repair or clean any component, you have to remove all of the other components ( similar to getting an oil change with a BMW ) - these are popular in most markets and effectively stop backlow across the USA - Spec Sheets and More Info Here

Wilkins 975

Another common option for smaller sizes ( 1/2" - 2" ), also in the inexpensive price range, is the Zurn / Wilkins 975. There have been many versions of this valve, most common in the field is the 975XL and 975XL2, but recently released the 975XL3 which has a sleeker look and is much more simple to do a full rebuild. More info on the links above. Spec Sheets and More Info Here

Febco 825Y

A less common option for new installations, but often seen in the field, is the Febco 825Y, which is also only available in smaller sizes ( 1/2" - 2" ). Febco as a company was purchased and integrated with Watts Regulator Co in 2005, such that their assemblies are made in the same factories as Watts, and often distribution centers prefer to use the Watts 909 or also the Watts 009 in lieu of this valve. They are very sturdy valves and are built to survive hard water and other issues that often foul other preventers, but typically do cost a premium over the low cost options. Spec Sheets and More Info Here

Apollo RP-4A

Another option for an RP Assembly in smaller sizes is the Apollo RP-4A, which replaced Apollo's 40 series in 2008. Formerly under the brand Conbraco, Apollo is currently owned by AAlberts and although less popular, is a commonly available valve that you will come across in the field. Spec Sheets and More Info Here

Watts 909

Another sturdy option available both in smaller sizes and larger sizes, also one of the first backflow assemblies available on the market when the designs first came out in the years following the passage of the "Safe Drinking Water Act" of 1974, is the Watts 909. These also pre-date many of the regulations and standards such as ASSE 1013. Although they do still make new ones, they are a more expensive and heavier option than others, so you will often see them only as existing installations, and are likely not lead free and may need to be replaced. Spec Sheets and More Info for Small Sizes Here - Large Sizes Here

Ames 4000SS ( Watts 994 )

Another option, typically used on larger sizes and for fire systems that require RP protection, such as ones using glycol or other toxic additives, is the Ames 4000SS. These are only available in larger sizes, and similarly to FEBCO, AMES company was bought and integrated by Watts Regulator Company in 1997. These are also very similar in size and shape to the Watts 994, likely sharing the same or very similar components. The entire body is cast as one large piece of Stainless Steel with a Groove Coupling on top for access, and became popular in the mid 2000s. For fire systems they will have large OSY gate valves or indicating wheeled butterfly valves, for domestic water they will typically have regular non-indicating gate valves. More info for Ames 4000SS here and Watts 994 here

Wilkins 375

The Wilkins 375 RP is actually two separate types of valves. For the smaller diameters ( 1/2" to 2" ), it is a valve where the main mechanisms are contained in a removable module that is completely plastic, which are often used for irrigation and sometimes domestic water. The larger diameters ( 2-1/2" + ), it is a standard valve that is designed similarly to the Ames 4000SS layout, but are made of ductile iron instead of stainless steel. Both are popular and you will see them in the field for domestic and irrigation services. Spec Sheets and More Info for Small Sizes Here - Large Sizes Here

Deringer 40

Deringer is a very new type of valve, with manufacturing starting in 2014, but has become popular in most recent years. They have a very complex internal design, which are vulnerable to water quality issues, but are very light and cost effective, utilizing integral butterfly valves that are available in the wheeled butterfly style with indicators for fire systems. They were also bought by Watts in 2019 and are now called the Ames-Deringer. Spec Sheets and More Info Here

Many More

We tried to compile as many examples as possible, and have included most of the normal ones you will see in the field, but many others exist.