Important installation advice, including handling, plant conditions, pipework configuration, markings and noise considerations.
Safety Valve Installation
Reaction forces when discharging
In open systems, careful consideration must be given to the effects of the reaction forces generated in the discharge system when the valve lifts. In these systems, there will be significant resultant force acting in the opposite direction to that of discharge. It is important to prevent excessive loads being imposed on the valve or the inlet connection by these reaction forces, as they can cause damage to the inlet pipework. The magnitude of the reaction forces can be calculated using the formula in Equation 9.5.2:
The reaction forces are typically small for safety valves with a nominal diameter of less than 75 mm, but safety valves larger than this usually have mounting flanges for a reaction bar on the body to allow the valve to be secured.
These reaction forces are typically negligible in closed systems, and they can therefore be ignored.
Regardless of the magnitude of the reaction forces, the safety valve itself should never be relied upon to support the discharge pipework itself and a support should be provided to resist the weight of the discharge pipework. This support should be located as close as possible to the centreline of the vent pipe (see Figure 9.5.7).
Figures 9.5.8 and 9.5.9 show typical safety valve installations for both open and closed systems.
Changeover valves (see Figure 9.5.10) permit two valves to be mounted side by side, with one in service and one isolated. This means regular maintenance can be carried out without interruption of service or the vessel being protected. Changeover valves are designed in such a way that when they are operated, the pass area is never restricted.
Changeover valves can also be used to connect safety valve outlets so that the discharge pipework does not have to be duplicated. The action of both inlet and outlet changeover valves has to be limited and synchronised for safety reasons. This is usually by means of a chain drive system linking both handwheels.
Consideration must be made to pressure loss caused by the changeover valve when establishing the safety valve inlet pressure drop, which should be limited to 3% of the set pressure.
Although discharge from a safety valve should not occur frequently, should it occur, the noise generated can often be significant. It is therefore necessary to determine the sound level of safety valves to ensure that relevant health and safety regulation levels are not exceeded.
Assuming a sonic flow nozzle discharge, an approximate value of the sound level, LP, in decibels at a flange outlet can be calculated using the formula given in Equation 9.5.3 (Source API 521).
There are several ways to reduce noise level, the simplest being to use larger diameter discharge pipes, or to lag the discharge pipe (however, the valve must not be lagged). It is also permissible for a silencer to be used in extreme cases, in which case any backpressure generated must then be taken into account.