For each common contingency, the hydraulic modeling output data consists of the fluid temperature, pressure, composition, and phase at each node defined in the physical description of the relief collection system being evaluated. Also included are the stream velocity and mass flow rate within each defined piping segment (i.e., between nodes). These modeling results are evaluated against the design criteria outlined below. These criteria can be summarized as follows. The flow out of the relief devices and depressuring valves into the collection system should not be compromised by the pressure built-up at each device’s outlet, and the design conditions of all piping, fittings, and equipment in the collection and disposal system should be appropriate for all conditions predicted by the hydraulic modeling for all common contingencies.
1. Back Pressure on Pressure Relief Valves
The impact of the back pressure on a pressure relief valve is dependent on the type (conventional, balanced, or pilot-operated) of the valve. Section 1230 describes the effects of back pressure on the performance of each type of pressure relief valve. See Item 1, “Pressure Relief Device Discharges” on page 1200-82 for a discussion of the acceptable values of built-up back pressure for the various types of pressure relief valves.
Consideration needs to be given to the mechanical back pressure limit for each balanced-bellows type relief valve connected to the disposal system. The maximum back pressure rating for the bellows can be significantly less than the outlet flange pressure rating.
2. Back Pressure on Control Valves
The impact of the back pressure on control valves should be evaluated using the valve manufacturer’s capacity calculation methods. The total input to the disposal system may be reduced by the developed back pressure. The impact of back pressure on fully open control valves is determined using the guidance presented below for depressuring valves.
3. Back Pressure on Depressuring Valves
Back pressure may cause the capacity of a depressuring valve to be reduced due to sub-critical flow. The reduction in capacity should be included when designing systems to be depressurized in a specified amount of time. The back pressure may serve to reduce the overall initial discharge to the disposal system from the depressuring valve.
4. Mechanical Design of Collection System Components
The pressures and temperatures predicted by the hydraulic model to be developed within the collection and disposal system should be considered when specifying (or evaluating) the design limits of all system components. Particular attention should be given to the materials of construction of components at which temperatures below -20 °F are predicted.