BAT Material Selection & Properties 2 — Questions and Answers
Question 1: Which elastomer is most commonly used for check valve discs in standard potable water backflow preventers?
- EPDM (ethylene propylene diene monomer)
- Silicone
- Nitrile (NBR) (Correct answer)
- Natural rubber
Correct answer: Nitrile (NBR)
Nitrile (NBR) rubber is widely used for check valve discs due to its good resistance to water, oils, and typical plumbing conditions.
Question 2: Which rubber compound is preferred for backflow preventer seals in systems where chloramines are used as the primary disinfectant?
- Nitrile (NBR)
- EPDM (Correct answer)
- Natural rubber
- Neoprene
Correct answer: EPDM
EPDM has superior resistance to chloramines and chlorinated water compared to nitrile rubber, which degrades faster in these conditions.
Question 3: What is the primary failure mode when nitrile rubber (NBR) seals are exposed to petroleum-based fluids in a backflow assembly?
- NBR becomes brittle and fractures
- NBR discolors and stains the water supply
- NBR swells and loses its sealing properties (Correct answer)
- NBR hardens and increases seating force
Correct answer: NBR swells and loses its sealing properties
NBR swells significantly when exposed to petroleum-based products, causing the seal to deform and lose the ability to create a watertight seat.
Question 4: What spring material is standard in spring-loaded check valves used in backflow prevention assemblies?
- Brass
- Copper
- Aluminum
- Stainless steel (Correct answer)
Correct answer: Stainless steel
Stainless steel is the standard spring material because it provides the corrosion resistance, consistent spring tension, and fatigue strength needed for reliable check valve operation.
Question 5: Galvanic corrosion within a backflow assembly is most likely to occur when:
- Water temperature exceeds 140°F continuously
- Two dissimilar metals are in electrical contact within a conductive solution (Correct answer)
- System pressure exceeds the assembly's rated working pressure
- The assembly is installed in a vertical-down configuration
Correct answer: Two dissimilar metals are in electrical contact within a conductive solution
Galvanic corrosion requires an electrochemical cell: two dissimilar metals acting as anode and cathode connected in a conductive electrolyte such as water.
Question 6: For backflow preventer seals in high-temperature applications exceeding 250°F, which seal material is most appropriate?
- Nitrile (NBR)
- EPDM
- Neoprene
- PTFE (polytetrafluoroethylene) (Correct answer)
Correct answer: PTFE (polytetrafluoroethylene)
PTFE has the highest continuous service temperature of common seal materials, making it suitable for hot water and steam applications where elastomers would degrade.
Question 7: What is the primary function of O-rings in a reduced pressure principle backflow preventer?
- Increase flow velocity through the assembly body
- Provide structural support for check valve discs
- Create watertight seals between body sections and moving components (Correct answer)
- Regulate differential pressure across the relief valve
Correct answer: Create watertight seals between body sections and moving components
O-rings form the static and dynamic seals between body sections, bonnet connections, and moving parts to prevent leakage through the assembly body.
BAT Material Selection & Properties 2
Which elastomer is most commonly used for check valve discs in standard potable water backflow preventers?