Safety Valve Sizing
Safety valves must always be sized and able to vent any source of steam so
that the pressure within the protected apparatus cannot exceed the maximum
allowable accumulated pressure (MAAP). This not only means that the valve has to
be positioned correctly, but that it is also correctly set. The safety valve
must then also be sized correctly, enabling it to pass the required amount of
steam at the required pressure under all possible fault conditions.
Once
the type of safety Valves has been established, along with its set pressure and
its position in the system, it is necessary to calculate the required discharge
capacity of the valve. Once this is known, the required orifice area and nominal
size can be determined using the manufacturer's specifications.
In order
to establish the maximum capacity required, the potential flow through all the
relevant branches, upstream of the valve, need to be considered.
In
applications where there is more than one possible flow path, the sizing of the
safety valve becomes more complicated, as there may be a number of alternative
methods of determining its size. Where more than one potential flow path exists,
the following alternatives should be considered:
- The safety valve can be sized on the maximum flow experienced in the flow path with the greatest amount of flow.
- The safety valve can be sized to discharge the flow from the combined flow paths.
This choice is determined by the risk of two or more devices failing
simultaneously. If there is the slightest chance that this may occur, the valve
must be sized to allow the combined flows of the failed devices to be
discharged. However, where the risk is negligible, cost advantages may dictate
that the valve should only be sized on the highest fault flow. The choice of
method ultimately lies with the company responsible for insuring the plant.
For example, consider the pressure vessel and automatic pump-trap (APT)
system as shown in Figure 9.4.1. The unlikely situation is that both the APT and
pressure reducing valve (PRV 'A') could fail simultaneously. The discharge
capacity of safety valve 'A' would either be the fault load of the largest PRV,
or alternatively, the combined fault load of both the APT and PRV 'A'.
This document recommends that where multiple flow paths exist, any
relevant safety valve should, at all times, be sized on the possibility that
relevant upstream pressure control valves may fail simultaneously.
Related Products of Safety
