Pressure Relief Valves
Transcript of Pressure Relief Valves
Ritchie Type Pressure Relief Valve:
An Englishman, Charles Ritchie, achieved the first significant improvement in valve lift in 1848 with a valve that exploited the expansive property of the gas for raising the disc. This was achieved by providing the disc with a peripheral flow deflector, which, together with a lip around the seal, formed an annular chamber with a secondary orifice around the seat, as shown in figure.
As the escaping gas deflects on the disc only through around 90◦, only a portion of its kinetic energy is converted into lifting force so that the valve can open only partially within the normally permissible overpressure. When the overpressure recedes, the static pressure in the annular chamber builds up again, causing the disc initially to huddle above the seat until the operating pressure has dropped to below the set pressure. The resulting difference between the set pressure and reseating pressure is referred to as the blowdown.
Naylor Type Pressure Relief Valve:
William Naylor introduced in 1863 an improved lift pressure relief valve in which a lip around the disc turned the discharging fluid through 180◦, as shown in figure. By this construction, the flowing fluid was able to impart the maximum lifting force on the disc from its momentum. But as the valve can open only in proportion to the rising overpressure, flow rate and the corresponding lifting force were initially too small to raise the disc significantly within the permissible overpressure.
Safety Valve Safety Valve with Cooling
Safety valves, such as those shown in figures, are intended primarily for the relief of steam in industrial boiler plants and other steam systems. Their design is directed towards protecting the spring from excessive temperature rises that might cause drift of the spring setting and possibly spring relaxation over time. To provide this protection, safety valves are commonly provided with an open bonnet that allows steam leaking into the bonnet to escape directly into the atmosphere around the valve.
Safety Valve with Eductor Lift Assistance:
Conventional Safety Valves:
Safety Relief Valve with Open Bonnet
Bellows Balanced Safety Relief Valve
Balanced Relief Valve with Friction Ring to Prevent Valve Chatter
Direct-Loaded Vacuum Relief Valve:
The valve shown in figure relies for loading solely on the light weight of the disc. To achieve a high degree of seat tightness under these loading conditions, the seat seal is made of sponge rubber. To also ensure easy travel of the disc in its guide, the guide rod is PTFE coated. The valve body is designed to be combined with an overpressure relief valve.
Direct-Loaded Vacuum/Pressure Relief Valve:
The pressure relief valve shown in figure combines a vacuum relief valve with a positive pressure relief. The valve has been designed for sanitary application in the beverage, food processing, and pharmaceutical industries.
Figure shows a breather valve that carries separately a direct-loaded vacuum relief valve and an overpressure relief valve. In deviation from other vacuum relief valve designs, the disc swings open in an arc on a point contact hinge. Valve seals consist of soft diaphragms that allow the valves to reseat close to the set pressure. The valve is intended for service on low-pressure storage tanks.
Assisted Safety Relief Valve:First figure shows the design of an
assisted pressure relief valve and Second one the associated control piping diagram. Under normal operating conditions, the assist device is deactivated. When the valve is called upon to open, the assist device is activated and introduces an unrestricted supplementary opening force. When the safe pressure is restored, the assist device is deactivated and the valve closes normally. If the external power supply should fail, the valve opens and closes with no interference from the assist device.