Spring-loaded discharge valve

As soon as the piston reverses its direction of travel, the pressure of the gas inside the cylinder drops. The gas pressure drops below the discharge line pressure, and the spring-loaded discharge valve slams shut. The piston continues its travel toward the crank end of the cylinder. The pressure of the gas inside the cylinder continues to fall, but the suction or intake valve remains closed. 

Both the discharge valve and the intake or suction valve are simply spring-loaded check valves. The discharge valve permits gas only to be forced out of the cylinder. The intake valve permits gas only to enter the cylinder. 

The reactor coolant system pressure is controlled by the pressuriser, where water and steam are maintained in equilibrium by the activation of electrical heaters or a water spray, or both. The pressure is increased through the generation of steam that is formed by the heaters, and deereased by the condensation of steam by the water sprayers. Spring-loaded safety valves are installed at the top of the pressuriser and provide overpressure proteetion for the reactor coolant system. These valves discharge into the containment atmosphere. 

The suction and discharge valves inside a cylinder are spring-loaded check valves, lliey require a small differential pressure m open them. Therefore, at.sactiDn stroke, the pressure inside the cylinder is less than the suction line pressure, and at discharge stroke, the pressure inside the cylinder is higher than the discharge line pressure. 

Example 7.9. Determine the capacity of a spring-loaded safety valve installed on a liquid nitrogen storage vessel if the discharge area of the valve is 40 cm and its discharge coefficient is 0.65. The nitrogen gas may be assumed to enter the valve at a temperature of 200 K and a pressure of 700 kPa absolute. 

If test certifications for spring-loaded valves on gas show the valve coefficient (API coefficient of discharge, Kd) to be less than 0.95, vendors proposals for that particular valve design shall be submitted to the end user for verification and approval of valve sizes. 

The spring pressure produced imder the valve, which is the spring load in pounds, divided by the area of the valve A, may be 1 per cent of the working pressure, for the discharge valve, with a maximum of 5 lb. per square inch. For the suction valve the spring pressure should be limited to 0.5 lb. per square inch. If the suction comes in under a head heavier springs should be used. 

Overpressure protection for the reactor coolant pressure boundary is provided by four spring-loaded ASME Code safety valves connected to the top of the pressurizer. These valves discharge to the in-containment refueling water storage tank, where the steam is released under water to be condensed and cooled. If the steam discharge exceeds the capacity of the in-containment refueling water storage tank, it is vented to the containment atmosphere. 

Unlike spring-loaded PRV, the open or close of pilot-operated PRV is not affected by its backpressure. This means it can be used to protect equipment with higher pressure in relief valve discharge manifolds. 

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