Here “vessel” is intended to mean any equipment item. Closed Outlets is a potential causes of overpressure when the following two criteria are met:
a. A mechanism, such as a block valve, exists to block the item’s outlet. If a system of vessels in open communication can be blocked by the action of a single outlet block valve (i.e., on the outlet of the last vessel), the potential for a blocked outlet should be considered for each item in the system.
b. A pressure source (pump, compressor, high-pressure reservoir, heat etc.) in excess of the design pressure is present upstream. The maximum pressure of sources can generally be determined as follows:
* The physical limitations of equipment such as the dead head pressure for centrifugal pumps and compressors. That is, the upstream pump or compressor must be able to exceed the design pressure in order to be a potential source of overpressure.
* The maximum temperature achievable due to heat input. For example, if the heat source can not exceed 300 °F, then the internal pressure cannot exceed the vapor pressure of the fluid at 300 °F. As with the pumps and compressors, the heat source must be capable of elevating the vessel pressure above the design pressure to qualify as a potential source of overpressure.
The closed outlet causes of overpressure often applies to the following types of equipment items:
Heat Exchangers Pumps
Absorbers Distillation Systems
The relief flow rate required to prevent overpressurization of the item is equal to the flow rate into the item, with the item at its design pressure plus any allowable accumulated pressure. The throughput of most compressors and of centrifugal pumps decreases with increasing discharge pressure, so the required relief flow rate often can be less than the normal operating flow rate. Similarly, if any outlets on the equipment remain open, credit may be taken for the flow out through them.