Since the earliest days of the industrial revolution, industrial processes working under other than atmospheric pressures (both overpressure and vacuum) typically will require (mandatory) measures to assure a safe operation. National and transnational legislations are developed and in place to assure that the required safety levels are not breached and the environment and investments are safe.
As a first line of defense, pressure control systems are typically used. These systems monitor the pressure developments in the process equipment and timely interact with the process control system to limit the pressure to acceptable levels. Control and/or monitoring devices, which are not a necessary part of a safety system, are usually excluded from safety design standards since they are typically active in advance of a safety system. They also may combine other activities related to the process and cannot always be considered as dedicated safety systems. The efficiency of these pressure control systems depends on input received from instrumentation devices and require extensive and validated reliability analysis, based on probability of failure on demand (PFD) or safety integrity level (SIL) assessment.
As in most cases, pressure control systems may not assure the required level of reliability in all service conditions; the use of (last-line of defence) pressure relief systems is often required. In cases where the pressure control systems would fail to achieve the required pressure safety levels, these dedicated protection devices safeguard the installation when the critical pressure threshold is reached. Figure 1 illustrates the correlation between pressure control and monitoring systems, and pressure relief systems.
It is essential to not only consider the pressure relieving device but the complete pressure relief system, so as not to reduce the relieving capacity or adversely affect the proper operation of the pressure relieving devices. Operating problems – where observed – within pressure relief systems, frequently result from incorrect selection of the appropriate device or from improper handling, incorrect installation or lack of maintenance. A risk assessment and determination of all possible
upset condition scenarios which could potentially lead to unacceptable and dangerous pressures needs to be done with sufficient attention and based on experience in running the process. A multi-disciplinary group of experts may be required to collect the necessary information.
To attain the required safety against pressure risks the industry has been using pressure relief devices. Such pressure relief devices are categorized as reclosing and non-reclosing types, and both offer unique characteristics which make them a viable selection for the design engineer.
This document will focus on pressure relief devices only.