Safety valve discharge area

Credit for additional height of the flame center for multiple valve installations may be taken by clustering the safety valve discharge pipes to the atmosphere. The following procedure should be used for determining equivalent diameter and exit velocity to be used in the flame center calculation. Diameter and velocity are based on the total acmal area of the clustered vents. 

The ALMS is composed of sensors (accelerometers) which are installed on the pressurizer safety valve discharge lines (one per safety valve). Signals from the sensor area are routed to the in-containment amplifiers. The amplifier output is subsequently directed to the control room. Within the alarm instrumentation, the signal is compared to a threshold value obtained during startup testing. 

The flow rate used for this calculation should be the best estimate flow rate for the safety valve. BS 6759 19] requires that any safety factors used in determining G for relief system sizing, including the 10% de-rating of the safety valye discharge coefficient, should be removed. The actual flow area through the valve should be used., ... 

As the safety valve has a de-rated discharge coefficient of 0.87, then A=0.0383/0.87 = 0.044 m2. In this case, Fauske s method gives a slightly higher relief area than the 0.0378 m2 given by Leung s method. 

Sizing formulae based on equations (A6.4) to (A6.6) are given in many safety valve manufacturers catalogues. Where possible, the method given by the manufacturer of the particular safety valve should be used to find its capacity because this will contain the correct values of flow area and discharge coefficient for the valve. (This is not usually true in the USA where National Board certification information should be used.) The relief lines upstream and downstream of the safety valve also need sizing. Further information is given in 9.7 and references 2 and 4. 

The pressure drop allowed through the inlet and discharge lines is unlimited as long as the capacity of the line is adequate for the relief requirements. That is, at the required flow rate the vessel pressure must not exceed the maximum allowable accumulated pressure. In sizing a safety disc, it is usually assumed that the entrance loss at the nozzle is the governing restriction insofar as capacity is concerned. Thus the effective orifice area is considerably larger than the effective orifice area of a safety valve of the same pipe size. Consequently,... 

Ethyl acetate is stored at 450 K and 1,200,000 Pa. The maximum required discharge rate for pressure relief is 10 kg/s, the backpressure amounts to P2 = Patm = 100,000 Pa. What is the required minimum cross sectional area of the safety valve, which has a set pressure of 1,300,000 Pa ... 

Pressure relief equipment includes relief valves, safety valves, rupture discs, piping, drums, vent stacks, pressure indicators, pressure alarms, pressure control loops, and flare systems. Pressure relief devices can be placed on pumps, compressors, tanks, piping, reactors, distillation columns, refrigeration systems, and many other kinds of equipment. Materials that cannot be released to the atmosphere are recycled back to the system, or sent to a scrubber or flare system. The discharge from pressure relief equipment is collected in a closed piping system and sent to a flare stack. Harmless gases are discharged at a safe distance from plant operations areas. 

Gm,nozzie/A eat = 3.98 X 10 kg/(m s). It is, therefore, 63% larger than the value in accordance with EN-ISO 4126-7. If the same discharge coefficient of the valve is used, the noinimum required area of the safety valve would be 63% lower. 

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. 

A safety valve on a liquid oxygen storage container has a free-flow area of 15 cm and a discharge coefficient of 0.7. Calculate the capacity of the valve during discharge if the oxygen enters the valve at 245 K and 600 kPa absolute. 

Safety Showers. Safety showers and eyewash fountains or hoses should be installed where corrosive or toxic materials are handled. A large-volume, low velocity discharge from directly overhead should effect continuous drenching, ie, a minimum flow of 20 L/min (50 gal /min). Water to outside showers may be heated to a maximum temperature of 27°C by an electric heating cable. The valves for all safety showers should be at the same height and relative position to the shower head, and they should operate in the same way and direction. The shower station should be identified by paint of a bright, contrasting color. In areas where chemicals harmful to the eyes may be encountered, an eyewash fountain or spray should be available in case of splash accidents. 

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