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The materials used for internal components must be at least as corrosion-resistant as those used for the body and bonnet , but never in cast iron or carbon steel . Springs in safety and relief valves use specific materials depending on the application :
• For water , oils , steam , compressed air and other less aggressive fluids : alloy carbon steel AISI 5160 or AISI 6150
• For superheated steam : AISI H12 ( tungsten steel ) or Inconel ® X 750
• For moderately corrosive applications : AISI 302 , AISI 316 and AISI 420
• For extremely low temperatures ( -450 ° F to -75 ° F or -268 ° C to -60 ° C ): AISI 316 For highly corrosive fluids , materials such as Monel and Hastelloy C are recommended for internal components . All materials used in the construction of safety and relief valves , including body , bonnet , stud and nuts , and internal components , must comply with ASME Code Section II – Part D and ASME Section XIII – 3.3.1 .
Types of corrosion in springs
The main types of corrosion found in springs are surface corrosion and stress corrosion . Both can significantly impact valve performance and safety . Surface corrosion can weaken the spring , reducing the valve ’ s opening pressure . This weakening can occur when incorrect materials are selected for the application , particularly when exposed to high temperatures and corrosive fluids . This occurs due to a reduction in the wire diameter , which weakens the spring . Stress corrosion cracking is more difficult to detect before spring failure occurs . To mitigate this risk , manufacturers must select springs with :
Corroded 5160 carbon steel spring applied in the boiler water treatment process
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1 . Chemical compatibility with the process fluid 2 . Correct hardness 3 . Appropriate heat treatment These factors affect the spring ’ s resistance to stress corrosion cracking . It is recommended to replace the spring and / or change its construction material when corrosion has consumed more than 1.5 % of the wire diameter . To enhance corrosion resistance , springs can receive a superior surface finish for protection against oxidation .
Resilient seats
Resilient seats are “ O ” ring sealing rings that enhance valve tightness , allowing for effective sealing at 95 % of the valve opening pressure on valves adjusted from 15 psig ( 1.0 barg ). Resilient PTFE seats are recommended for set pressures from 100 psig ( 6.9 barg ). Specifications for resilient seats are recommended for the following applications :
• Fluids that are difficult to confine , such as light hydrocarbon , anhydrous ammonia , helium or hydrogen , and others
• Toxic or flammable gases
• Operating pressure very close to the opening pressure
• Installations subject to vibrations
• Fluids containing solid particulates in suspension
• Applications subject to vacuum
• Fluids that may freeze during relief through the valve , such as certain gases
• Stresses in discharge piping caused by inadequate support
The application of resilient seats is constrained by pressure , fluid temperature , valve nozzle area and chemical compatibility with the process fluid . When the valve opening pressure increases , the nozzle passage area to be selected must be smaller . For processes requiring high opening pressure and high flow capacity , it is advisable to specify , size and install multiple valves with smaller orifices . Common materials for resilient seating applications include Buna-N , Perbunan , Silicone , Viton , Kalrez , Ethylene Propylene , Neoprene , Nitrile and PTFE . These materials are suitable for temperatures around -150 ° F to 450 ° F ( -101 ° C to 232 ° C ), depending on the material , setting pressure and valve nozzle area , while PTFE can be applied at temperatures up to 500 ° F ( 260 ° C ). Maximum application temperature varies depending on resilient seat material , nozzle area and valve setting pressure .
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Conventional safety and relief valve with resilient seat on the disc
For valves with set pressures up to 500 psig ( 34.5 barg ), the elastomer hardness should be 70 Shore . For set pressures between 501 psig and 1500 psig ( 34.54 barg to 103.42 barg ), the elastomer hardness should be 90 Shore . It is important to note that resilient seats may have reduced lifespan under stress due to degradation caused by temperature , aging or expansion . For fluids containing solid particulates , especially when the resilient seat material cannot withstand the process fluid ’ s pressure and temperature , and depending on the valve nozzle area , it is recommended to use disc and nozzle seating surfaces coated with Stellite ® .
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Balanced pressure relief valves with bellows
In applications where the valve discharges into a header , it is recommended to specify a balanced valve with bellows , in addition to nullifying the effects of back pressure on the opening pressure value and flow capacity of the valve , also when the fluid from the header discharge system ( discharge closed ) or the system itself protected by the valve is corrosive or has a tendency to crystallize , polymerize or corrode the valve body , bonnet and internal components . For fluids with these characteristics , the bellows performs others two main functions : 1 . Isolating the clearance between the disc holder shaft and the valve guide hole
2 . Isolating all components above the bellows ( housed inside the bonnet ) By using a bellows with an isolation function , components housed inside the bonnet ( including the bonnet itself ) can be manufactured from materials that are less resistant to corrosion . This approach can significantly reduce the final cost of the valve and installation .
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