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newSchoolWorks.com › VoTechWorks › Building & Construction › Plumbing & Pipefitting › Backflow Prevention: Principles, Devices, and Field Testing › Lesson 10
BLD-PLUM06-08 · Lesson 10 · 12 clock hours · lab

Repair and Maintenance of Backflow Prevention Assemblies

Objective. The learner disassembles a fouled DCVA or RPZ assembly, replaces worn check discs, O-rings, and springs using manufacturer repair kits, reassembles the device, and performs a post-repair field test to verify acceptable operation.

Why Backflow Assemblies Need Repair

Backflow prevention assemblies are mechanical devices exposed to water pressure, sediment, chlorine, and biological growth every day of their service life. Over time, rubber check discs harden and crack, O-rings compress and extrude into gaps, springs lose tension, and debris lodges on seats. A fouled assembly may fail in either direction: it may allow backflow that contaminates the potable supply, or it may hold a check valve open and dump water continuously through the relief valve port. Neither outcome is acceptable. Routine maintenance, performed to manufacturer specifications and verified by a field test, restores the assembly to like-new performance and keeps the installation in compliance with local cross-connection control program requirements.

Personal Safety Before You Touch the Assembly

Before disassembly, depressurize and isolate the assembly using the two shutoff valves that are part of every listed assembly. Open the test cocks to vent trapped pressure. If the assembly is on a system that carries non-potable fluid, consult the site's chemical inventory and apply appropriate PPE—at minimum, safety glasses and nitrile gloves for any system. Never assume a test cock is fully depressurized until you crack it slowly and confirm no flow. Surfaces and internal components of large-bore assemblies can be heavy; have a helper or use a bench vise to support the body during disassembly so it does not roll or drop.

If the assembly is located in a vault or pit, evaluate confined-space entry requirements before descending. Apply lockout/tagout to any electrically actuated isolation valve upstream so it cannot be remotely energized while your hands are inside the body.

Understanding the Internal Parts

Reduced Pressure Zone (RPZ) assembly contains two independently acting spring-loaded check valves (first check, second check) and a differential pressure relief valve between them. The relief valve is held closed by the pressure differential between the inlet zone and the zone between the two checks. If either check leaks, that differential drops, and the relief valve opens to drain—protecting the supply.

Double Check Valve Assembly (DCVA) contains two independently acting spring-loaded check valves in series without a relief valve. It provides a lower level of protection and is listed for non-health-hazard applications.

In both assemblies, the wearing parts are:

Disassembly Procedure

1. Close the No. 2 (downstream) shutoff valve, then the No. 1 (upstream) shutoff valve.

2. Open all four test cocks to vent pressure. On an RPZ, confirm the relief valve port is draining freely before you open any cover.

3. Remove the access cover bolts in an alternating pattern to release pressure evenly. Use a rubber mallet if the cover is stuck—never a steel hammer on a bronze body.

4. Lift or slide out the check module or cartridge, keeping it oriented so you can note which end faces upstream. Photograph or sketch the assembly if you are new to the model.

5. Remove the check disc from its retainer using a flat-blade screwdriver or the tool specified by the manufacturer. Note the disc orientation: the rubber face seats against the metal seat ring.

6. Remove springs and O-rings. Lay all parts in order on a clean rag so none are lost.

Diagnosing Worn Components

Reassembly and Torque

Use only the manufacturer's repair kit for that model and size. Mixing parts from different manufacturers voids the listing and may cause immediate failure. Lightly lubricate new O-rings with the food-grade silicone grease supplied in the kit—never petroleum-based grease, which attacks rubber. Install each part in reverse order of removal. Seat covers must be torqued to the manufacturer's specification using a calibrated torque wrench; under-torqued covers leak, over-torqued covers crack.

Post-Repair Field Test

After reassembly, restore pressure slowly by cracking the No. 1 shutoff and listening for leaks at all covers and test cocks before opening it fully. Then perform a complete field test following ASSE 5013 procedures using a calibrated differential pressure test kit. Record readings on the ASSE field test report form. Compare your post-repair readings to the pre-repair readings you took or were given. The assembly must meet the acceptance criteria listed for its type:

If the assembly still fails after repair, do not return it to service. Tag it out of service and notify the owner that replacement is required.

Documentation

Complete the field test report form in full: assembly type, size, serial number, location, pre-repair readings, parts replaced, post-repair readings, and pass/fail result. File the report as required by the local water authority. A repaired assembly with no documentation is treated as untested.

Key terms9

Check discThe rubber-faced poppet inside a check valve module that seats against the metal seat ring to stop reverse flow; the primary wear item in a backflow assembly.
O-ringAn elastomeric loop seal used on covers, plugs, and internal cartridges; must be replaced at every service interval regardless of visible condition.
Differential pressure relief valveThe internally operated valve in an RPZ assembly that opens to drain when the pressure differential between the inlet zone and the zone between the two checks drops below a set threshold, protecting the potable supply.
Free lengthThe uncompressed, at-rest length of a coil spring; compared to manufacturer specification to determine whether the spring retains adequate closing force.
Repair kitA model-specific and size-specific set of replacement parts (discs, O-rings, springs, grease) provided by the assembly manufacturer; the only approved source of replacement components for a listed assembly.
SeatThe machined metal surface inside the assembly body against which the check disc seals; must be cleaned at every service and inspected for scoring that would prevent a watertight seal.
ASSE 5013The ASSE International standard that establishes field test procedures for backflow prevention assemblies; defines the acceptance criteria and documentation requirements used during post-repair verification.
Lockout/tagout (LOTO)An energy-control procedure that isolates and de-energizes hazardous energy sources (electrical, hydraulic, pressure) and physically prevents re-energization while workers are performing service, as required by OSHA standards.
Post-repair field testA complete differential pressure test performed on a reassembled backflow assembly using a calibrated test kit and ASSE 5013 procedures to verify the repaired assembly meets acceptance criteria before it is returned to service.

Worked examples4

1. Before repair, your RPZ first check reads a differential pressure of 3.2 psid. After replacing the check disc and O-ring and reassembling, you test again and read 8.6 psid. The manufacturer's minimum acceptable differential for the first check is 5.0 psid. Did the repair succeed, and by how much does the post-repair reading exceed the minimum?
  1. Identify the minimum acceptable differential: 5.0 psid.
  2. Compare pre-repair reading to minimum: 3.2 psid is below 5.0 psid, so the assembly failed before repair.
  3. Compare post-repair reading to minimum: 8.6 psid vs. 5.0 psid.
  4. Calculate margin above minimum: 8.6 − 5.0 = 3.6 psid.
Answer: The repair succeeded. The post-repair first-check differential of 8.6 psid exceeds the 5.0 psid minimum by 3.6 psid, so the assembly passes.
2. A manufacturer's spec sheet states the first-check spring free length is 3.25 inches. You measure the spring removed from the assembly and it measures 2.87 inches. Should you replace the spring, and what is the difference from specification?
  1. Record specified free length: 3.25 in.
  2. Record measured free length: 2.87 in.
  3. Calculate difference: 3.25 − 2.87 = 0.38 in.
  4. Determine action: the spring is 0.38 in shorter than specification, meaning it has lost significant closing force and does not meet manufacturer criteria.
Answer: Yes, replace the spring. It is 0.38 inches shorter than the specified free length of 3.25 inches, indicating loss of closing force that could allow the check to leak under low back-pressure conditions.
3. You are tightening a test cock plug after reassembly. The manufacturer specifies a torque of 25 ft-lb. Your torque wrench is set in in-lb. What is 25 ft-lb expressed in in-lb, and at what in-lb reading should you stop?
  1. Recall the unit conversion: 1 ft-lb = 12 in-lb.
  2. Multiply: 25 ft-lb × 12 in-lb/ft-lb = 300 in-lb.
  3. Set the torque wrench to 300 in-lb and tighten until the wrench clicks or deflects at that value.
Answer: 25 ft-lb equals 300 in-lb. Stop tightening when the torque wrench signals 300 in-lb.
4. Your field test report must record the change in the second-check differential before and after repair. Pre-repair reading: 4.1 psid. Post-repair reading: 7.8 psid. What is the numerical improvement, and expressed as a percentage of the pre-repair value (round to one decimal place)?
  1. Calculate absolute improvement: 7.8 − 4.1 = 3.7 psid.
  2. Calculate percentage improvement: (3.7 ÷ 4.1) × 100.
  3. Divide: 3.7 ÷ 4.1 ≈ 0.9024.
  4. Multiply by 100 and round: 0.9024 × 100 ≈ 90.2%.
Answer: The second-check differential improved by 3.7 psid, which represents approximately a 90.2% increase over the pre-repair reading of 4.1 psid.
In class: Instructor teardown demo: disassemble a fouled RPZ, identify worn components, demonstrate proper cleaning of seat and disc, and reassemble with new O-rings using the manufacturer repair kit · Hands-on repair: each learner receives a pre-fouled assembly (instructor-introduced debris or worn disc), disassembles, diagnoses, replaces components from a repair kit, and reassembles · Post-repair verification: learner performs a full field test on the repaired assembly, records results on the ASSE form, and compares pre- and post-repair differential pressure readings
Materials: Pre-fouled DCVA and RPZ assemblies; manufacturer repair kits (O-rings, discs, springs); flat-blade screwdrivers, internal snap-ring pliers, torque wrench; calibrated test kits; ASSE 5013; field test report forms; clean rags, wire brush
Check: Post-repair field test: repaired assembly passes ASSE acceptance criteria; learner correctly names all replaced components and explains why each required replacement; lab log entry completed with pre- and post-repair readings

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASSE Backflow — safe field procedures
1. Before removing the access cover from a pressurized RPZ assembly, which action must be completed first?
NCCER Plumbing — backflow device maintenance
2. Which lubricant is correct for installing new O-rings in a potable-water backflow prevention assembly?
ASSE Backflow — component diagnosis
3. A technician measures a check spring and finds its free length is shorter than the manufacturer's minimum specification. What is the correct action?
ASSE Backflow — post-repair verification
4. After reassembling an RPZ, a technician performs a post-repair field test. The relief valve opens and does not reseat when the test is completed and the No. 1 shutoff is slowly opened. What is the most appropriate next step?
ASSE Backflow — device listing and compliance
5. Why must replacement parts come exclusively from the manufacturer's repair kit for that specific model and size?
NCCER Plumbing — backflow device inspection
6. During inspection of an assembly seat, you can feel a distinct groove across the seating surface with your fingernail. What does this finding indicate?
ASSE Backflow — component diagnosis
7. Which condition on a check disc requires immediate replacement rather than reinstallation?
NCCER Plumbing — safety; State Journeyman — workplace safety
8. A backflow assembly is located in a below-grade vault. Before descending to perform maintenance, the technician must first evaluate which additional hazard beyond standard assembly lockout/tagout?
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