Relief devices

Relief systems are expensive and introduce considerable environmental problems. Sometimes it is possibly to dispense with relief valves and all that comes after them by using stronger vessels, strong enough to withstand the highest pressures that can be reached. For example, if the vessel can withstand the pump delivery pressure, then a relief valve for overpressurization by the pump may not be needed. However, there may still be a need for a small relief device to guard against overpressurization in the event of a fire. It may be possible to avoid the need for a relief valve on a distillation column... 

For most processes, the optimum operating point is determined by a constraint. The constraint might be a product specification (a product stream can contain no more than 2 percent ethane) violation of this constraint causes off-specification product. The constraint might be an equipment hmit (vessel pressure rating is 300 psig) violation of this constraint causes the equipment protection mechanism (pressure relief device) to activate. As the penalties are serious, violation of such constraints must be very infrequent. 

Pressure-relief-device requirements are defined in Subsec. A. Set point and maximum pressure during relief are defined according to the service, the cause of overpressure, and the number of relief devices. Safety, safety relief, relief valves, rupture disk, breaking pin, and rules on tolerances for the reheving point are given. 

Testing, certification, and installation rules for reheving devices are extensive. Eveiy chemical engineer responsible for the design or operation of process units should become veiy familiar with these rules. The pressure-relief-device paragraphs are the only parts of Sec. TII, Division I, that are concerned with the installation and ongoing operation of the facility all other rules apply only to the design and manufacture of the vessel. 

Care of Pressure Vessels Protection against excessive pressure is largely taken care of by code requirements for relief devices. Exposure to fire is also covered by the code. The code, however, does not provide for the possibility of local overheating and weakening of a vessel in a fire. Insulation reduces the required relieving capacity and also reduces the possibihty of local overheating. 

Stanley Grossel/ President, Proce.ss Safety h- Design, Inc. Fellow, American Institute of Chemical Engineers Member, American Chemical Society Member, The Combustion Institute Member, Explosion Protection Sy.stems Committee ofNFPA. (Emergency Relief Device Effluent Collection and Handling)... 

Emergency Relief Device Effluent Collection and Handling. 26-31... 

Safety Devices Pressure relief devices, flame arresters, and methods for handhng effluent from controlled releases provide control of accidental undesirable events. Special equipment should be considered for highly toxic chemical service. The following matters are considered ... 

In some instances, plant-specific information relating to frequencies of subevents (e.g., a release from a relief device) can be compared against results derived from the quantitative fault tree analysis, starting with basic component failure rate data. 

Accumulation The rise of pressure above the MAWP of the pro-tec ted system, usually expressed as a percentage of the gauge MAWP. Note The MAWP and accumulation terms are not included in the ANSI definitions since they relate to the protec ted system instead of the relief device. 

Pressure Relief Devices The most common method of overpressure protection is through the use of safety rehef valves and/or rupture disks which discharge into a containment vessel, a disposal system, or directly to the atmosphere (Fig. 26-13). Table 26-8 summarizes some of the device characteristics and the advantages. 

EMERGENCY RELIEF DEVICE EFFLUENT COLLECTION AND HANDLING... 

Introduction In determining the disposal of an effluent vent stream from an emergency relief device (safety valve or rupture disk), a number of factors must be considered, such as ... 

Types of Equipment The three most commonly used types of equipment for handling emergency relief device effluents are blowdown drums (also called knockout drums or catch tanks), cyclone vapor-liquid separators, and quench tanks (also called passive scruh-hers). These are described as follows. 

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

Equipment Selection Criteria and Guidelines A number of factors should be considered in order to determine when to select a blowdown drum, cyclone separator, or quench tank to handle a multiphase stream from a relief device. Among these are the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

When the pressure relief device is set to open at greater than 15 psig (critical flow will result), it is normally not uecessaiy to be concerned about the pressure drop in the separator. If the hquid is to be drained from the separator during the emergency blowdown, a vortex breaker and false bottom should be used (Fig. 26-18, view BB). 

Maintenance and testing. It is not a good idea to apply vacuum on a vessel during maintenance or testing without full knowledge of the external pressure rating, unless a suitable vacuum relief device is in place and operable. 

Location of Vacuum Relief Device (Carl Schiappa, Michigan Engineering, The Dow Chemical Company, Midland, Mich., personal communication, March 20, 1992.) If a vacuum relief device is used, locate the device at the highest point on the top of the tank. If the vacuum relief device is not installed in this location and the tank is overfilled with liquid, the relief device will be sealed in liquid and will be ineffec tive in protecting the tank. This is especially true for the part of the tank above the vacuum relief device if it is sealed in liquid, tne liquid level is lowered, and the tank goes into a partial vacuum. 

Examples of Vacuum-Related Accidents Figure 26-47 shows a jacketed tank, where the jacket was designed for low-pressure steam. When the steam was turned off and the drain valve and trap were closed, the steam condensed, causing the jacket to collapse. The jacket should have been designed for full vacuum, or a suit le vacuum relief device should have been installed on the jacket. 

Design for pressure containment Provide adequately designed relief device Use less severe operating conditions... 

Provide adequately designed emergency relief device... 

Insufficient diluent due Provide automatic control of diluent to under feed or exces- addition sive evaporation result-, Select diluent less susceptible to evaporation ing in insufficient heat sink. Possibility of run- automatic/manual isolation based on away reaction due to detection of unexpected reaction rate high temperature excur- Provide emergency cooling Sion or high concentra-. adequately designed relief device tion of reacting species Monitor liquid level CCPS G-11 CCPS G-23... 

Provide vacuum relief device/system (can be a source of oxygen in vapor space resulting in flammable atmosphere)... 

Provide deflagration pressure relief device/system... 

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