Pressure relief discharge system design

Conventional PR valves and discharge systems should be designed such that built-up back pressure does not exceed 10% of set pressure (both measured in psig), to avoid chattering problems. In the case where a pressure relief valve system is sized for fire conditions, with 21 % overpressure, built-up back pressure up to 21 % of set pressure is permissible. However, the lower rates resulting from other contingencies still must meet the 10% limitation. 

Since discharges of vapors from highly hazardous toxic materials cannot simply be released to the atmosphere, the use of a weak seam roof is not normally acceptable. It is best that tanks be designed and stamped for 15 psig to provide maximum safety, and pressure relief systems must be provided to vent to equipment that can collect, contain, and treat the effluent. 

Conventional Flare System - The majority of pressure relief valve discharges which must be routed to a closed system are manifolded into a conventional blowdown drum and flare system. The blowdown drum serves to separate liquid and vapor so that the vapor portion can be safely flared, and the separated liquid is pumped to appropriate disposal facilities. The blowdown drum may be of the condensible or noncondensible type, according to the characteristics of the streams entering the system. Selection criteria, as well as the design basis for each type of blowdown drum, are detailed later in this volume. The design of flares, including seal drums and other means of flashback protection, is described later. 

Not all possible deficiencies are included in the above statistical analysis. There are also concerns about items such as excessive flare radiation levels, inadequate knockout drums, poorly designed quench systems, discharge of toxic fluids to atmosphere, discharge of combustible or toxic liquids and gases to atmosphere and a general lack of process safety information upon which to base a safe pressure relief system design. Therefore, it could be stated that the actual total deficiency rates reported may even be understated. 

If the plant safety shutdown is not rapid enough and an overpressure situation develops, then the pressure relief system is activated. Pressure vessel design codes such as the ASME Boiler and Pressure Vessel Code require relief devices to be fitted on all pressure vessels (see Section 13.17). If the relief system has been properly designed and maintained, then in the event of an overpressure incident, the plant contents will be vented via relief valves or bursting disks into the relief system, where liquids are recovered for treatment and vapors are sent to flare stacks or discharged to the atmosphere if it is safe to do so. The pressure relief system should allow the plant to be relieved of any source of overpressure before damage to process equipment (leaks, bursting, or explosion) can occur. 

The first step in designing a pressure-relief system is to evaluate the possible causes of overpressure so as to determine the rate of pressure accumulation associated with each and hence estimate the relief load (the flow rate that must be discharged through the relief device). The API Recommended Practice (RP) 521 suggests the following causes ... 

Pressure relief devices must be properly sized (capacity), and discharge location is critical. These devices are generally sized for the most likely pressure increasing event, e.g., external fire or internal process upset such as an uncontrolled exothermic reaction. The engineer must calculate the temperature and pressure increases associated with the event as well as the expected release volume. He or she must also account for pressure drops across the relief device as well as for friction losses in the lines. A decision must also be made about whether to discharge to the atmosphere or to a closed system that includes a scrubber, a flare, or even as simple as a water tank, such as is sometimes used for venting anhydrous ammonia. Some of these design decision issues are addressed in more detail by Crowl and Louvar. ... 

Design of Pressure-relief Systems. Discharge rates are calculated by charts, tables, or equations supplied by manufacturers. In general, the method is to establish the discharge-pressure rates by code requirements and use fluid-flow calculation. for the pressure-relief devices, which behave as nozzles or orifice.s, and the associated piping. Discharge coefficients for pressure-relief devices can he approximated as follows ... 

However, in many applications, the reduction in vent header and protective relief equipment, together with the lower probability and frequency of discharge, can justify this technique. The author has experienced one case with a high-pressure, close-boiling system where the reduction in vent header and protective equipment cost was by itself sufficient to pay for the required increase in column design pressure. 

The design of a safe pressure relief system together with an appropriate handling of the discharged substances may be a time-consuming iterative process. An alternative to pressure relief can be pressure containment, i.e. the load hmit is chosen in such a way that no event sequence in the plant leads to pressures exceeding it (passive safety, vid. Sect. 4.2.2). 

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