Relief valves pilot operated

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

A disadvantage of pilot operated valves is that, if there is no pressure in the vessel, back-pressure could cause the disc to lift. This could occur if t was shut-in and depressured for maintenance, the relief val ve wa led in a header, and another valve in the header was opened, 

---

Pilot-operated relief valves use the pressure in the vessel rather than a spring to seal the valve and a pilot to activate the mechanism. Figure 13-6 is a schematic of a typical pilot-operated valve. A piece of tubing communicates pressure between the relief valve inlet and pilot. When this pressure is below the set pressure of the pilot, the pilot valve is in the position... 

All relief valves are affected by reaching critical flow, which corre-spond.s to a back-pressure of about 50% of the set pressure. Pilot-operated relief valves can handle up to 50% back-pressure without any significant effect on valve capacity. Back-pressure correction factors can be obtained from the relief valve manufacturers for back-pre.ssures above 50%. API RP 520 gives a generic method for sizing a pilot-operated relief valve for sub-critical flow. 

Calculate orifice size for pilot-operated relief valve. 

Radioactive decay contributed to further heat buildup. This caused water in the primary circuit to boil. The pilot-operated relief valve on the... 

Pressure-relief devices include direct spring-loaded and pilot-operated relief valves that close or reseat if the vessel pressure is brought back into a safe condition, as well as non-reclosing devices such as rupture disks and breaking-pin devices. 

The pressure-flow response of a pilot-operated relief valve is illustrated schematically in Figure 13.45. Leakage is eliminated and there is no blowdown. 

Figure 13.44. Pop-action pilot-operated relief valve. Reproduced with permission from API Recommended Practice 520.

The TMI accident sequence started when a pilot-operated relief valve stuck open. In the nine years before the TMI incident, eleven of those valves had stuck open at other plants, and only a year before, a sequence of events similar to those at TMI had occurred at another U.S. plant. 

Evaporation of secondary coolant through a pilot operated relief valve (PORV) on the secondary circuit steam pressurizer. 

If the pressure in the RCS exceeds a value which is given by the function of cold leg temperature versus RCS pressure, the protection system will limit the pressure in the RCS by opening the pilot operated relief valve at the pressurizer. This valve is capable of relieving steam as well as water, but its relief capacity is limited. 

In some countries, pilot operated relief valves, (PORVs) and block valves were originally designed as non-safety components intended for operation under normal conditions. The PORVs were provided for pressure control of the RCS during normal operation and transients. The block valves were installed because of expected leakage from the PORVs. The valves were not required for safe shutdown or to mitigate the consequences of accidents. 

The disk in a normal relief valve is kept in the closed position during normal operations by the force of a spring. A pilot-operated relief valve (PORV), however, uses system pressure to seal the valve as shown in Figure 13.2. 

Pilot Operated Relief Valve, solenoid operated valve meant to prevent safety relief valves from opening. This valve stuck-open at TMI producing small break LOCA. 

Shown in Figure 2.1 is a vessel known as a pressurizer (it is the half-fuU vessel to the left of the right-hand steam generator). The pressurizer is a part of the primary water circuit, and plays a critical role in controlling the pressure and temperature in that circuit. If the reactor temperature rises, then the volume of water increases and the level in the presstuizer increases (and vice versa). If the pressure becomes too high, the Pilot Operated Relief Valve (PORV) opens and steam is vented to the drain tank. 

In case of a major problem in meeting the 3% line loss criterion, a pilot-operated relief valve can be installed with a pressure sensing tapping taken from the protected equipment. With such an installahon, it is still required to reduce the inlet pressure drop as... 

For conventional relief valves venting to a closed system, the line is sized to keep the back pressure within 10% of the relief valve set pressure. This is subject to a maximum velocity of 0.75 Mach. API 521 recommends a velocity up to 1 Mach. However, proper analysis of piping integrity is required for a velocity close to Mach 1 such high velocity should not be used for the design of a new system. Outlet line sizing for balanced-bellows and pilot-operated relief valves is based on a maximum velocity of 0.75 Mach. 

---