Pressure relief device characteristics

Pressure relief of a runaway reaction is likely to be via a bursting disc or a safety valve, or a combination of both these items. Further information about these is given in Chapter 9. For relief system sizing, it is important to know the pressure at which a relief device will open. 

Within this Workbook, the maximum pressure required to fully open the pressure relief device will be referred to as the relief pressure . (Caution some papers on relief sizing refer to set pressure but mean relief pressure ). For a bursting disc, the relief pressure will be the maximum specified bursting pressure and for a safety valve, it will be the set pressure plus 10% overpressure (or whatever percentage overpressure the valve has been certified at). 

It should be remembered that, for safety valve systems, a vacuum can occur in the reactor when it cools down after a runaway. It is important to take account of this in the reactor design. 

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IIL Characteristics of Pressure Relief Devices Characteristics of difTereitt PRD mentioned in Section II are explained in this section. 

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. 

This section describes the various pressure relief devices that are commonly used, with their characteristics and criteria for selection. Basic calculation procedures for sizing PR valves are covered in subsequent discussions. 

The method used for the safe installation of pressure relief devices is illustrated in Figure 8-1. The first step in the procedure is to specify where relief devices must be installed. Definitive guidelines are available. Second, the appropriate relief device type must be selected. The type depends mostly on the nature of the material relieved and the relief characteristics required. Third, scenarios are developed that describe the various ways in which a relief can occur. The motivation is to determine the material mass flow rate through the relief and the physical state of the material (liquid, vapor, or two phases). Next, data are collected on the relief process, including physical properties of the ejected material, and the relief is sized. Finally, the worst-case scenario is selected and the final relief design is achieved. 

If the reactor has a relief device, the pressure response depends on the relief device characteristics and the properties of the fluid discharged through the relief. This is illustrated by curve A (Figure 8-2) for vapor relief only and by curve B for a two-phase froth (vapor and liquid). The pressure will increase inside the reactor until the relief device activates at the pressure indicated. 

Although many pressure relief devices are called SRVs, not every SRV has the same characteristics or operational precision. Only the choice of the correct pressure safety device for the right application will assure the safety of the system and allow the user to maximize process output and minimize downtime for maintenance purposes. Making the correct choice also means avoiding interference between the process instrumentation set points in the control loop and the pressure relief device limits selected. These SRV operational limits can vary greatly even when all are complying with the codes. 

Pressure relief device is the general term for a device designed to prevent pressure or vacuum from exceeding a predetermined value in a pressure vessel by the transfer of a fluid during emergency or abnormal pressure conditions. There are, however, different definitions for specific devices, their testing and their operating characteristics. 

Bench or test stand testing Testing of a pressure relief device on a test stand using an external pressure source with or without an auxiliary lift device, to determine some or all of its operating characteristics, without necessarily flowing the rated capacity. This is required on a regular basis when the valve is taken into the maintenance cycle (see Chapter 10) at least to see that there is no shift on the set pressure and that the valve would open correctly during a pressure upset. 

Flow capacity testing The usually special testing of a pressure relief device to determine its operating characteristics, including measured relieving capacity. This tests whether the valve flows the capacity as stated in the literature or as per given flow coefficients, or to simply determine the flow coefficient of the valve as such. This is done on a spot-check basis by independent notified bodies in limited locations worldwide especially approved for that purpose. 

API Recommended Practice 520 Part I, Sizing and Selection This API design manual includes basic definitions and information about the operational characteristics and applications of various pressure relief devices. It also includes sizing procedures and methods based on steady state flow of Newtonian fluids. This RP covers equipment that has a maximum allowable pressure of 15 psig (1.03 barg) or greater. 

Alternative energy processes, just like any others, need to be protected from excessively high or low pressures. The methods of protection include pressure regulation, alarm, or safety interlock actuation when preset pressure limits are violated, and providing pressure relief devices, which need to be replaced after each operation (rupture disks) or can automatically reclose (relief valves). The features and characteristics of these devices are discussed in the following subsections. 

This chapter describes the various types of pressure relief devices used in the compressed gas industry and the standards developed to describe their design criteria, performance characteristics, and applications. The sections covered are ... 

In this chapter, following topics will be discussed type of pressure relief devices, their characteristics, ASME code PRD set pressure, maximum operating pressure, contingency analysts, pressure reliefvalve and rupture disk sizing, pressure relief valve inlet/outlet piping sizing, Eind PRD selection. 

II1.2 Characteristics of rupture disk and pin-actuated pressure relief device They are either close or open, depending on the system pressure is below or above its set pressure. 

An important issue to be considered at an early stage is whether there are any common oause failures between redundant parts within each layer (for example, between 2 pressure relief valves on the same vessel), between safety layers or between safety layers and the BPCS. An example of this could be where failure of a basic process control system measurement could oause a demand on the safety instrumented system and a device with the same characteristics is used within the safety instrumented system. In such cases it will be necessary to establish if there are oredible failure modes that could cause failure of both devices at the same time. Where a common cause of failure is identified then the following actions can be taken. 

A substance is supplied to a system, for example by a pump or a compressor. The mass flow rate then depends on the capacity of the pump, respectively the compressor. If its stagnation pressure lies above the failure pressure of the system, its failure must be assumed. The mass flow rate to be relieved corresponds to the flow rate which results from the pressure difference after opening the relief device according to the characteristic of the pump or compressor. 

Transfer hoses should have bursting pressures not less than five times the maximum relief device setting on the connected tank. Minimum design pressures are 2,586 kPa (375 psig) for rail or road tanker service and 2,086 kPa (300 psig) for barge service (P6). Other characteristics of metal hoses should be ... 

Mechanical Relief Devices. The water seals discussed above in Section 9.1.10.1 are effective only at very low differential pressures. After the compression of chlorine, and particularly in liquefaction and storage systems, more conventional relief devices, rupture discs and pressure relief valves, are used. With some fluids, there is a simple choice to be made between discs and valves. While the former are less likely to permit bypassing of small quantities of fluid, they are destroyed when they open. A release will continue even after the pressure on the system drops below the set point of the disc. Relief valves have the opposite characteristics. 

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. 

Often, the RD is mounted under the PSV so that it is sealed tight and protects the relief valve from being contacted by corrosive, plugging, hazardous, freezing, or regulated processes. This way the best characteristics of both devices are utilized. The RD can also be installed after the PSV. This installation can be used when the valve discharges into a vent header that might contain corrosive vapors. Table 3.153 provides data on RD materials, sizes, and minimum rupture pressures. 

Relief valves and inlet check valves are simple devices that suppress the unsteady effects of an over-pressure or under-pressure, respectively. The relief valve is fitted against an aperture in the pipe by means of a spring whose stiffness is chosen so as to allow the valve to open, should the over-pressure exceed a limiting value. Some liquid is then forced out of the pipe, thus decreasing the pressure in the pipe. The inlet check valve operates in the opposite manner, opening inwards of the pipe when the pressure drops below a limiting value. The mass of the valves and the spring constants should be chosen in such a way that the characteristic frequency of the protective system differs from the characteristic frequency I/4 of the hydraulic... 

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