Pressure-relief valve function

For a flare stack to function properly and to handle the capacity that may be required, the flows under emergency conditions from each of the potential sources must be carefully evaluated. These include, but may not be limited to, pressure relief valves and rupture disks, process blowdown for startup, shutdown, upset conditions, and plant... 

New systems or processes may also need to be qualified from an operational safety perspective. This is particularly relevant in the case of chemical synthesis involving exothermic reactions. Critical safety aspects are usually identified using hazard operability or HAZOP assessments and studies. For example, a HAZOP analysis of an exothermic reaction vessel would involve consideration of the consequence of failure of the motors for mixers or circulation pumps for cooling water. Thus, the qualification of such a system would involve checks and assessment to ensure that the system/process can be operated safely and that pressure relief valves or other emergency measures are adequate and functional. 

Protective system—Systems such as pressure relief valves that function to prevent or mitigate the occurrence of an incident. 

In most materials selection processes, it is virtually impossible to make materials choices independent of the product shape. This includes not only the macroscopic, or bulk, shape of the object such as hammer or pressure relief valve, but also the internal or microscopic shape, such as a honeycomb structure or a continuous-fiber-reinforced composite. Shape is so important because in order to achieve it, the material must be subjected to a specific processing step. In Chapter 7, we saw how even simple objects made from a single-phase metal alloy could be formed by multiple processes such as casting or forging, and how these processing steps can affect the ultimate properties of the material. As illustrated in Figure 8.6, function dictates the choice of... 

It is common on chemical plant to install safety devices such as trips and relief valves which protect the plant in the event of a malfunction of control systems or human error. Unfortunately, these devices can (and do) fail occasionally. The problem is that the failures cannot be seen until they are tested or until they are called upon to act (a plant may operate perfectly normally even though, say, a pressure relief valve is faulty, because under normal conditions the valve is never activated). It is thus necessary to test safety devices periodically to ensure they are functioning. 

If a flammable liquid is in a closed container, the vapor pressure will increase inside the container as the temperature of the liquid increases. This increase in temperature can come from many different sources. Increases in ambient temperature, radiant heat from the sun, or a nearby fire can increase the vapor pressure in a container. As the pressure increases in a container, it will reach the setting on the pressure-relief valve and the relief valve will function. If this pressure increase occurs in a container that does not have a relief valve, the container may rupture. Rupture may also occur in a container with a relief valve if the pressure rises too fast for the relief valve to vent the material into the air, or if the relief valve is not working properly. In either case, the rupture may be violent, with a fireball and flying pieces of tank that can travel over a mile from the blast site. This phenomenon is referred to as a boiling liquid expanding vapor explosion (BLEVE). 

Valves. Valves control the way the gas is used, stopped, or directed. They must function under a variety and range of temperatures. Control or proportional valves in a process system are power-operated mechanisms able to alter fluid flow. A pneumatic valve actuator adjusts valve position by making the air pressure either linear or rotary motion. Ball valves provide the shutoff capability. Gas valves are specialized to control the flow of another medium, such as natural gas. A pressure relief valve is a self-actuated safety valve that relieves pressure. A butterfly valve controls the flow of air or a gas through a circular disk or vane by turning the valve s pivot axis at right angles to the directing flow in the pipe. 

Diversion of flow safely All the above systems discussed are part of electronic system requiring power. As a next layer of protection there are mechanical devices (requiring no power to meet power failure situations) to divert the flow safely. Safety and relief valves are used to in case to depressurize when SIS fails to take care (e.g., say due to control power failure). Pressure relief valve diverts the fluid for safe passage. These relief valves are spring force to close so that when pressure is below setting it is closed. Many cases rupture discs are used but in that case system needs to be closed to attain the disc. To a certain extent quartz bulb in sprinkler system does the same function. 

Breaking pin device in combination with pressure relief valve. This style of pressure relief device, as illustrated in Fig. 8-19, is very similar to the rupture disk and relief valve combination, except that a breaking pin is used in place of the rupture disk. The breaking pin device is a nonreclosing pressure relief device actuated by inlet static pressure and is designed to function by the breaking of the load carrying section of the pin. 

The hood pressure control system functions as follows When the pressure in the hood rises, which may occur for example as a result of a decrease in the exit gas flow or an increase in the cooling air flow, the controller increases the volume of air delivered by the exhaust air fan. It does this by adjustment of a damper or an inlet vane control unit or by varying the fan drive motor speed. Conversely, when the pressure in the hood goes down, the air delivery rate of this fan is reduced by the control system. Thus, with the aid of the hood pressure controller, the exhaust air fan performs the function of a pressure relief valve. 

The function of the control system is to regulate the operation of the pump. The latter pumps fluid from an infinitely large reservoir into the tank. We shall assume that it takes 60 s to pressurize the tank. The pressure switch has contacts, which are closed when the tank is empty. When the threshold pressure has been reached, the pressure switch contacts open, de-energizing the coil of relay K2 so that relay K2 contacts open, ranoving power from the pump, causing the pump motor to cease operation. The tank is fitted with an outlet valve that drains the entire tank in an essentially negligible time the outlet valve, however, is not a pressure relief valve. When the tank is empty, the pressure switch contacts close, and the cycle is repeated. 

One reason for this type of degradation in the pressure relief function is deficiencies in the quality assurance programme. The quality assurance programme should ensure that pressure relief valves installed are correctly calibrated, set to the right trip point and equipped with correct springs. 

Water in the steam lines in a BWR may degrade the pressure relief function. The probability for getting water in the steam lines are relatively high for some occurrences. There are two possibilities to overcome this problem to verify the capability to reduce the reactor pressure with existing pressure relief valves even if there is water in the steam lines or to install qualified valves designed with this capability. 

A new gasoline terminal was installed some miles downstream and commissioned on December 17, 1997. The pressure relief valve was not set up properly so it did not function, and it was not tested so the improper setup was not revealed. 

The primary advantage of using the pressure relief valve is that functioning of this type of device will not release all of the contents of the cylinder, but is designed to reseal after reseating pressure has been achieved. This characteristic, in fire conditions, will minimize feeding the fire in the case of a flammable lading. 

High Pressure is a cause of many serious events. Some of the causes of High Pressure are listed in Table 17.1. For High Pressure to create an incident, it is likely that a protective interlock system failed and/or that the system s pressure relief valves did not function properly. The worst case is selected for the design of the relief device. 

We mean any function that specifically provides safety in any situation. E.g. a seat belt in a car, an air bag, a pressure relief valve on a boiler or an instrumented shutdown system. Thus an air bag has a safety function to prevent injury in the event of collision. The safety system of an air bag comprises the sensor, the release mechanism, the inflator and the bag itself. 

In the analysis of CR ejection at hot standby , the inserted reactivity is set as 2.8%dk/k. The peak fuel enthalpy is below 150 cal/g. The increase in the pressure is more extensive compared to the full power case. The active initiation of the SRVs (in their relief valve function) is not credited. The SRVs are assumed to open passively as the safety valve function. The peak pressure is 27.2 MPa. 

Rupture Disks A rupture disk is a device designed to function by the bursting of a pressure-retaining disk (Fig. 26-15). This assembly consists of a thin, circular membrane usually made of metal, plastic, or graphite that is firmly clamped in a disk holder. When the process reaches the bursting pressure of the disk, the disk ruptures and releases the pressure. Rupture disks can be installed alone or in combination with other types of devices. Once blown, rupture disks do not reseat thus, the entire contents of the upstream process equipment will be vented. Rupture disks are commonly used in series (upstream) with a relief valve to prevent corrosive fluids from contacting the metal parts of the valve. In addition, this combination is a reclosing system. 

Pilot-operated valves have the advantage of allowing operations n the set point v/ith no leakage, and the set position is not affected by ba pressure. However, they will not function if the pilot fails. If the sens line fills with hydrates or solids, the valve will open at 25% over pressure trapped above the disc (usually the normal operating pre the vessel). For this reason they should be used with care in dirty vice and liquid service. They are used extensively offshore where all platform relief valves are tied into a single header because up to 5 back-pressure will not affect the valve capacity. 

The main function of the relief valve is to act as a back up to a pressure governor to prevent the pre-set downstream pressure being exceeded. In particular, it will relieve the small amount of creep that will occur by slippage past the valve seating in a governor. 

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