OVERPRESSURE AND THERMAL RELIEF

The term pressure relief refers to the automatic release of fluids or gases from a system or component to a predetermined level. Pressure relief systems are designed to prevent pressures in equipment or processes reaching levels where rupture or mechanical failure may occur by automatically releasing the material contained within. 

Almost all portions of a hydrocarbon process or system can conceivably be exposed to conditions that would result in internal pressures (either positive or negative) exceeding the normal operating pressures of the system. The most common causes are listed below  

There are numerous types of pressure relieving devices available, which include relief valves, safety valves, rupture or frangible disc and blow out hatches or panels. 

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Thermal relief valves are small, usually liquid relief valves designed for very small flows on incompressible fluids. They open in some proportion of the overpressure. Thermal expansion during the process only produces very small flows, and the array of orifices in thermal relief valves is usually under the API-lettered orifices, with a maximum orifice D or E. It is, however, recommended to use a standard thermal relief orifice (e.g. 0.049in2). Oversizing SRVs is never recommended since they will flow too much too short, which in turn will make them close too fast without evacuating the pressure. This will result in chattering of the oversized valve and possible water hammer in liquid applications. 

Within a chemical plant and over distances of several kilometers, chlorine can be transported by pipelines, either as gas or liquid [24], [240]. Every precaution should be taken to avoid any vaporization of chlorine in a liquid-phase system or any condensation in a gas-phase system. Wherever liquid chlorine could be trapped between two closed valves or wherever the system could be overpressurized by thermal expansion, an expansion chamber, a relief valve, or a rupture disk should be provided [241], [242]. 

In North America, the eductor pipe inside the vessel has an excess-flow valve at the top, immediately below the manhole cover. This valve closes the eductor pipe when the rate of liquid flow exceeds a set rate [2], [24]. North American tank cars have a spring-loaded. safety relief valve, which protects the vessel against overpressure in case of external heat. The tanks have thermal insulation. In Europe thermal insulation and safety relief valves are not used or recommended. 

A reactor has a volume of 2 m3. The worst case runaway reaction has been identified and the data from a suitable adiabatic, low thermal inertia test, with a thermal inertia ( ) of 1.05, is given in Figure 6.4. Under these conditions, the reactor would contain 793 kg of reactants. The reacting system is a vapour pressure system. It is desired to relieve the runaway via a safety valve, if possible, with a set pressure of 0.91 barg (relief pressure of 1.0 barg = 2.0 bara). Evaluate the required relief size for an overpressure of 30% of the absolute relief pressure, which gives a maximum pressure of 2.6 bara = 1.6 barg. 

The cold water feed can be provided by direct coimec-tion to the mains (in which case all components must be able to withstand mains pressure) or from a feeder tank (which is in an elevated position and filled from the mains through a float valve similar to the water closet-cistern float valve). For mains pressure systems most utilities or water supply authorities require the installation of a nonreturn valve. Whereas low pressure (feeder tank) systems are open to the atmosphere through a vent pipe, mains pressure systems must be protected against overpressure (due to thermal expansion of water as heated) by a pressure relief valve, as well as against collapse (implosion), which could be caused by a suction effect (vacuum formation) in case of an aceidental water discharge, by an air intake (or breather) valve. 

Thermal expansion and fire cases are not required to be checked, if the existing equipment is re-used, with the same service and also the same level control setting. Overpressure relief requirements due to each utility failure, fire cases and any other combination scenarios need to be estimated. API 521 (2014) has a comprehensive list of effects for utilities failure. All the PRV manifolds shall be checked to estimate back pressures at the PRVs. PRD overpressure calculations for equipment shall be documented as shown in Table 3.4. Vacuum relief (if the vessel/s is/are not designed to withstand full vacuum) shall also be documented. All the flare scenarios and flare network shall be properly documented. An example of PRV sizing calculations for the system shown in Figure 3.5 is presented in Table 3.4. 

Thermal stability testing showed that the reaction mixture could decompose exothermically with self heating occurring on the plant scale from 145°C (the boiling point of the mixture is about 160°C). Decomposition of the reactant mass would lead to a runaway reaction with the generation of a toxic and irritant gas. This would be vented safely, as far as protecting the reactor from overpressure is concerned, by the emergency relief vent, but would cause a serious toxic and corrosive aerosol emission. 

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