Corrosion relief valve protection

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Rupture discs are also used below relief valves to protect them from corrosion due to ves.sel fluids. The rupture disc bursts first and the relief valve immediately opens. The relief valve reseals, limiting flow when the pressure declines. When this configuration is used, it is necessary to monitor the pressure in the space between the rupture disk and the relief valve, either with a pressure indicator or a high pressure switch. Othei-wise, if a pinhole leak develops in the rupture disk, the pressure would equalize on both sides, and the rupture disk would not rupture at its set pressure because it works on differential pressure. 

Vessels should be provided with overpressure protection as required. Vents and relief valve vent piping should be so arranged Uiat Uie vented vapors will not constitute a liazard. Relief valves must be kept free from corrosion or fouling and should be operable at all Umes. 

Figure 7-11. Safety valve and rupture disk installation using pressure rupturing disk on Inlet to safety relief valve, and low pressure disk on valve discharge to protect against back flow/corrosion of fluid on valve discharge side, possibly discharge manifold. By permission, Fike Metal Products Div., Fike Corporation, Inc.

Often, the RD is mounted under the PSV so that it is sealed tight and protects the relief valve from being contacted by corrosive, plugging, hazardous, freezing, or regulated processes. This way the best characteristics of both devices are utilized. The RD can also be installed after the PSV. This installation can be used when the valve discharges into a vent header that might contain corrosive vapors. Table 3.153 provides data on RD materials, sizes, and minimum rupture pressures. 

Rupture disks are often used upstream of relief valves to protect the relief valve from corrosion or to reduce losses due to relief valve leakage. Large rupture disks are also used in situations that require very fast response time or high relieving load (for example, reactor runaway and external fire cases). They are also used in situations in which pressure is intentionally reduced below the operating pressure for safety reasons. 

Another method is also employed to prevent loss of the contents of the reactor. A safety valve, placed after the rupture disc, is set to open about 50 psi below the bursting point of the frangible disc. Thus, a rupture of the diaphragm is.sure to be followed by the op ng of the relief valve which has been protected from the corrosive vapors of the autoclave. It is usually feasible to complete the run and then replace the disc and overhaul the safety valve. 

All pressure regulators should be equipped with spring-loaded pressure-relief valves (see section 6.D.2.2.1 for further information on pressure-relief devices) to protect the low-pressure side. When used on cylinders of flammable, toxic, or otherwise hazardous gases, the relief valve should be vented to a hood or other safe location. The use of internal-bleed-type regulators should be avoided. Regulators should be removed from corrosive gases immediately after use and flushed with dry air or nitrogen. Mercury bubblers should not be used. 

The diaphragm is not a pressure-protective device the breaking pin serves that function. The process-side diaphragm and another one mounted at the top of the valve are there to prevent corrosion by exposure to process gas or the atmosphere. When the set pressure is exceeded, the pin breaks and the valve opens. The upper diaphragm then breaks, and chlorine escapes. When the pressure returns to a safe value, the valve closes and then functions as a standard spring-loaded relief valve until the breaking pin and diaphragms can be replaced. 

The increased tower boil-up overpressured the tower. Since the high-pressure alarm depended on the same tap as the pressure controller, no alarm was sounded. The relief valve, which was set to pop open at 300 psig, did not open even at 450 psig. Corrosion products had completely plugged the relief valve. (A rupture disk is one common method used to protect relief valves from plugging.)... 

A depropanizer was designed to operate at 270 psig on a corrosive stream with a high fouling tendency. It was protected from overpressuring by a relief valve set at 300 psig. On one occasion the operators found... 

Rupture Disk Device in Combination with Pressure Relief Valve. Combination rupture disk/pressure relief devices, as illustrated in Fig. 7-17, consist of a conventional reclosing pressure relief valve in series with a rupture disk. The rupture disk is located between the pressure relief valve and the container. The relief valve is thus sealed from toxic and corrosive ladings and the rupture disk protected against corrosion from the environment. 

Protecting Relief Valves from Fouling and Corrosion... 

There are two methods that 1 have used to protect process relief valves from exposure to corrosive or fouling environments ... 

Install flame arresters on atmospheric vents to prevent fire on the outside of the tank from propagating back into the vapor space inside the tank. Provide fire resistant insulation for critical vessels, piping, outlet valves on tanks, valve actuators, instruments lines, and key electrical facilities. Provide remote controlled, automatic, and fire-actuated valves to stop loss of tank contents during an emergency provide fire protection to these valves. Valves should be close-coupled to the tank, and must be resistant to corrosion or other deleterious effects of spilled fluids. Vessels should be provided with overpressure relief protection. 

A bursting or rupture disk is a pressure relief device that protects a vessel or system from excess pressures. They have been commonly used in aerospace, aviation, defense, nuclear, and oilfield applications often as a backup device for a conventional safety valve. In this instance, if the pressure increases and the fitted safety valve fails to operate, the rupture disk will burst as required. The discs are usually made from thin metal foil, and gold has been used in some instances because of its ductility and resistance to corrosion. Gold discs fitted to liquid ammonia tanks, for example, have shown good durability in this application compared to other metals. The use of gold in this application was reviewed in the 1970s when this industrial application for gold was more common [10]. 

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