Pressurized thermal expansion tanks differ from plain steel expansion tanks in that a flexible bladder or diaphragm separates the air cushion from the system fluid. The air side pre-charge of the tank must be field adjusted to equal the system supply pressure. As the system water expands, the bladder expands open, pushing against the air cushion to accept the expanded water.

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Your plumber or licensed backflow assembly testing company will likely know which type of assembly to use. Backflow assemblies most commonly installed on residential irrigation systems are the double check valve assembly (DCVA) and the pressure vacuum breaker assembly (PVBA). The DCVA may be installed in a vault below finished grade. The PVBA must be installed at least 12 inches above finished grade.

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The reduced pressure backflow assembly (RPBA) is designed to prevent backflow caused by backpressure or backsiphonage and may be installed on either low- or high-hazard connections. Backsiphonage is when water moves in the direction opposite its regular flow.

During normal operation, the pressure between the two check valves, referred to as the zone of reduced pressure, is maintained at a lower pressure than the supply pressure. If either check valve should leak, the relief valve is designed to open and discharge water to the outside.

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The TX Series tanks are designed for use in low pressure residential systems and in light commercial systems where ASME construction is not required. This is a replaceable bladder tank, ranging in capacity from 2.1 gallons to 79.2 gallons, full acceptance volume. TX tanks are factory pre-charged to 40 psig (field adjustable) and are rated for 200°F operation.

This assembly uses a mainline reduced pressure backflow assembly equipped with two resilient seated indicating shutoff valves, a bypass arrangement containing a smaller reduced pressure backflow assembly, and water meter. The RPDA must be installed at least 12 inches above finished grade or the 100-year flood plain, whichever is greater.

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The T Series thermal expansion tanks are designed using a heavy-duty fixed butyl diaphragm, and are offered in sizes ranging from 2 to 132 gallons.

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The assembly consists of two spring-loaded check valves, a bypass assembly with a water meter and double check valve assembly, and two resilient seated indicating shutoff valves.

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A properly functioning RPBA will periodically release small amounts of water due to changes in line pressure up or downstream of the assembly.  For this reason, this type of assembly should be installed in areas that are not subject to water damage and have proper drainage. When backflow happens, a properly functioning RBPA may release some water. If the RPBA becomes clogged by debris or is not functioning properly and needs repair, it may discharge a large volume of water.

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Thermal expansion tanks are designed for pressure control in a potable water system. The building's water heater or water heating system typically creates the thermally expanded water. A properly sized thermal expansion tank will accommodate this additional volume of water created through expansion during the heating cycle, and control the system pressure increases, keeping pressures from reaching critical limits. The tank uses compressed air to maintain system pressures by accepting and expelling the changing volume of water as it heats and cools. These tanks are designed utilizing a flexing butyl barrier that separates the stored water from the captured pressure control air cushion. This barrier, a replaceable bladder or fixed diaphragm, allows the water to be contained within the bladder preventing corrosion and potential water logging. The thermal expansion tanks are engineered to meet ASME standards, and are also available in non-code designs. Wessels' thermal expansion tanks are offered in industry's broadest range from 2 to 4,000 gallons and up to 250 psi.

The TXA Series bladder style tanks boast many advantages over other style thermal expansion tanks. The bladder can be easily replaced in the field. The free standing models have a system connection at the bottom of the tank. This is subtle but important difference from the top connection type of tank offered by some manufactures. The TXA tank is designed to prevent any harmful build-up of dirt, grit or system contaminants in addition to providing for easy draining to assist in establishing proper air charge.

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The double check valve assembly (DCVA) is commonly used to protect against backflow hazards that do not pose a health risk. They may be used under continuous pressure and protect against both backsiphonage and backpressure conditions. They can be installed above or below ground, but no more than 24 inches deep or five feet above the floor or grade.

The AVB is among the simplest and least expensive mechanical types of backflow prevention device that the end user can buy. The PVBA is an outgrowth of the AVB and evolved out of a need to have an atmospheric vacuum breaker that was testable and could be used under constant pressure.

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The double check detector assembly (DCDA) protects the potable water supply line from possible contamination or pollution from:

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There are two types of vacuum breakers: the atmospheric vacuum breaker (AVB) and the pressure vacuum breaker assembly (PVBA). The Water Bureau doesn't allow either the AVB or PVBA to be used for backflow protection.

The location of the assembly, service size, and hazard level of the connection determine which backflow assembly you should use. If you have questions about which type of assembly you need, contact the Water Bureau Water Quality Inspection staff at 503-823-7480. Below are some assemblies commonly used in commercial applications. More information about the all the following assemblies can be found on our backflow assembly installation requirements page.

TTA Series tanks are actually a tank within a tank. A non-ASME bladder tank is placed in an ASME tank. The bladder cannot expand any larger than the inner tank that contains it. This is why the acceptance volume of the TTA tanks is less than the total tank volume. The bladder is fixed and cannot be replaced. TTA tanks are generally less costly than the comparable TXA tank.

Some residents choose to install a backflow device on their hose bib. These are called vacuum breakers and they do not need to be tested. You can purchase them at most plumbing and hardware stores.

One step you can take to protect our drinking water is to keep the ends of hoses out of pools or other containers while you are filling them. For example, if a hose is submerged in the pool, the chlorinated pool water may enter the drinking water system during a backflow condition. When filling pools, keep the end of the hose out of the pool and have a gap between the end of the hose and the surface of the water. This gap must be twice the diameter of the hose and never less than one inch.