Published and Presented Content
Safety risks and economic costs make shutting down a process when a pressure relief device needs to be isolated or removed less than ideal. Several alternative methods are available that will enable you to isolate these devices while continuing to run your process.
Explore different approaches to reducing risk during the online isolation and removal of pressure relief devices for maintenance.
Collection, disposal, and treatment systems are an integral part of pressure relief systems for minimizing potential hazards posed by the effluent from pressure relief devices. There are several strategies for effluent handling, as well as common unit operations for executing those strategies in design, including knockout drums and flares.
The design and analysis of effluent handling systems should address the primary goals of ensuring safe operation of relief devices and equipment tied to the system as well as appropriate design of the collection system piping and effluent handling equipment, and a heuristic for the analysis of effluent handling systems is provided to address these goals.
A systematic approach for identifying the pressure relief requirements for typical gas conditioning systems found in the field shows that relief system analysis does not have to be burdensome when similar or standard design features pervade and common sense is applied.
In high velocity gas flows, such as those that may occur within the discharge piping of a pressure relief or depressuring valve, the temperature experienced at the wall of the pipe through which the gas is flowing can be much higher than the flowing stream ‘static’ temperature. In fact, the wall temperature approaches the stagnation temperature, that temperature which would be obtained if the fluid is brought adiabatically and reversibly (i.e. isentropically) to rest. Experimental work in aeronautical engineering has established this effective adiabatic wall temperature, and correlations have been proposed to determine the ‘recovery factor’ as a function of the Prandtl Number of the fluid; the adiabatic wall temperature is about 90% of the difference between the stagnation and static temperatures for a turbulent gas having a Prandtl Number of 0.7, which is typical for many gases. When performing heat transfer calculations between the pipe and the gas, or when specifying temperatures for piping material selection, this recovery factor should be accounted for.
A review of vendor packaged well site and gathering facility gas conditioning equipment demonstrates the need for owner/operator review or oversight of overpressure protection design.
As illustrated by recent process safety related incidents and near-misses, designs of gas conditioning systems must consider robust process safety features, including overpressure protection.
In designing and sizing relief-device and effluent-handling systems, one commonly overlooked aspect of the performance is examining the potential for low temperatures that can cause the components of the system to reach temperatures below their respective minimum design metal temperatures (MDMT), which may result in brittle fracture with subsequent loss of containment. This article points out limitations of the typical overpressure-protection analysis philosophy, discusses common sources of low temperatures for further investigation, and addresses possible design remedies for MDMT concerns.
For sizing pressure relief valves, many turn to the guidelines and practices of ASME and API that present specific sizing equations given a required relief rate and basic fluid properties. Application of these sizing equations is fairly straightforward for idealized systems (nonflashing liquids or ideal gases), but increase in complexity for non-idealized systems. In some cases, there are no sizing equations presented, leaving the engineer to wonder if some simplifying assumptions can be made to allow the use of idealized equations or if further analysis needs to be done. A fundamental understanding of the basis for the relief valve sizing equations, which is founded on isentropic nozzle flow, will allow one to view the sizing equations as part of a unified mathematical approach, to determine the applicability and limits of the idealized assumptions, and to have a general alternative for use in any fluid regime…
In API Recommended Practice 520, the basis for evaluating the ideal gas specific heat ratio has been modified from standard conditions (in the sixth edition) to relieving conditions (in the seventh edition). This provides the impetus for evaluating the use of the ideal gas specific heat ratio in the vapor-sizing equations as well as the validity of the ideal gas assumption to provide a good estimate of the mass flux through a nozzle. Presented here are the results of an evaluation for a few pure components, which indicates that the ideal gas specific heat ratio at the inlet temperature provides a very good estimate of the isentropic expansion coefficient under ideal gas conditions, although the temperature choice does not appear to have a significant effect on the mass flux calculations…
The key to creating and maintaining comprehensive pressure relief system information is the relational database predicated on an equipment-based approach for individual overpressure scenario analysis and relief system design basis documentation. This method is based on the evaluation of all potential overpressure scenarios for a piece of equipment, regardless of the presence or absence of a pressure relief device. With the information for individual overpressure scenarios managed in a relational database, a logical extension can be applied to the common, or ‘global,’ overpressure scenarios with multiple releases to an effluent handling system…