Pressure Relief Valves
cold differential adjustment ( CDTP ) to compensate for the effect of the backpressure on the valve set pressure value , as is mandatory for a conventional valve . In applications for fluids with the characteristics mentioned above , the presence of the bellows allows components above the bellows , such as the bonnet , stem , spring , spring supports , spring adjustment screw , adjustment screw lock nut and cap , to be constructed of materials that are less resistant to corrosion , thus reducing the final cost of the valve and installation . For extremely corrosive applications , such as hydrogen sulfide , the NACE MR 0175 standard requires that the bellows be manufactured in Inconel ® × 625 .
Maximum set pressure limits
Conventional and balanced valves have maximum set pressure limits that are reduced as the nozzle area and set pressure increase . The table below shows the set pressure limit values for balanced valves for fluids such as water , compressed air and saturated steam . The values are in barg and were taken from the catalogs of the main manufacturers :
Minimum set pressure
Due to the presence of the bellows , balanced valves have a minimum set pressure that is always higher than that of conventional design valves . This is because the elastic constant of the bellows prevents a lower set pressure . This minimum set pressure is 1 barg ( 15 psig )
Rupture disc at outlet
If the user wishes to isolate a pressure relief valve from the manifold due to corrosiveness or viscosity of the fluid , he can install a rupture disc on the outlet flange of this valve . However , if this valve is conventional , a leak from the upstream side can increase the pressure between the nozzle outlet and the rupture disc . For these applications , it is also recommended to specify a balanced bellows valve .
Backpressure limits according to API Std . 526
The table below shows the pressure limits recommended by API Std . 526 at a temperature of 100 ° F ( 38 ° C ). Because of the presence of the bellows , the backpressure limits are lower than those allowed for conventional design pressure relief valves . For example , a conventional design valve with a Class 150 outlet flange is 285 psig at a temperature of 100 ° F ( 38 ° C ). For temperatures above 100 ° F , the pressure limits are lower . The valve manufacturer should be consulted for higher temperature and back pressure values .
Facilitating reseating in applications with low set pressures
The bellows can also be applied to larger valves , for example , for Q , R and T orifices , at set pressures below 3.0 barg ( 43.5 psig ) even if there is no variable backpressure and the fluid is clean . This recommendation is to facilitate the reseating of the disc during the pressure relief phase of the valve . The presence of the bellows facilitates this reseating , that is , the reseating becomes smoother when compared to a conventional valve for the same conditions of size , type of fluid and adjustment pressure .
Relief hole in the bonnet
Every balanced valve with bellows must have an open relief hole in the bonnet . This hole has two functions : the main one is to keep the pressure inside the bonnet equalized with the atmospheric pressure ( acting in an area equivalent to or greater than the sealing area of the nozzle , consequently , there is no pressure to push the sealing disc downwards and against the inlet pressure ). This open relief hole also prevents the bellows from breaking during valve operation . This rupture could occur
Balanced safety valve with bellows . Model JBS . Manufacturer : Crosby
due to the reduction in air volume inside the bonnet if there were no relief hole , and consequent increase in pressure caused by the disc elevation stroke . A secondary function for this hole , also known as a “ sentinel hole ”, is to monitor the physical integrity of the bellows . A punctured or cracked bellows should always be immediately replaced or removed from the valve , especially if it is operating with fluids that tend to crystallize , since the accumulation of this fluid inside the bellows can limit or prevent the disc from lifting during valve operation .
Orifices D E F G H J K L M N P Q R T
WATER 455 455 379 276 207 207 165 110 83 76 76 69 58 23
AIR 414 414 345 255 186 186 152 102 76 69 69 69 58 21
STEAM 200 200 200 200 171 168 123 103 76 69 69 69 58 21
34 Valve World December 2024 www . valve-world . net