Pressure-relief valve characteristics

Conventional Safety Relief Valve - A conventional safety relief valve is a closed-bonnet pressure relief valve that has the bonnet vented to the discharge side of the valve and is therefore unbalanced. The performance characteristics, i.e., opening pressure, closing pressure, lift and relieving capacity, are directly affected by changes of the back pressure on the valve. 

Pressure relief valves in liquid service (i.e., relief valves and safety relief valves) have the characteristic of progressively increasing lift with rising inlet pressure until the full open position is reached at about 1 % overpressure. This characteristic may vary between types and between makes. 

If the superimposed back pressure is less than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is unaffected and back pressure is not a factor. However, builtup back pressure on a conventional pressure relief valve will affect its flow capacity and operating characteristics, and should not exceed 100% of its set pressure. If total back pressure (superimposed plus built-up) is greater than the calculated critical flow pressure, the capacity of a conventional PR valve in vapor service is affected, and total back pressure is incorporated into the sizing procedure. Any back pressure reduces the capacity of a conventional PR valve in liquid service, and... 

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. 

Note To emphasize the difference between overpressure (safety relief valve characteristic) and accu-mulation (code limitation on the pressure vessel), safety relief valves installed for fire cases will have an overpressure of 10% like most safety relief valves, even if the allowed accumulation on the pressure vessel is 21% in the case of ASME VIII. ... 

The safety valve is a pressure relief valve that is designed to open fully, or pop, with only a small amount of pressure over the rated limit. Where conventional safety valves are sensitive to downstream pressure and may have unsatisfactory operating characteristics in variable backpressure applications, pressure-balanced safety relief valve designs are available to minimize the effect of downstream pressure on performance. 

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. 

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 it will not release all of the contents of the cylinder because it is designed to reseal after reseating pressure has been achieved. This characteristic in fire conditions will minimize feeding the fire in case of a flammable lading. 

Sizing and selection of relief valves must comply with criteria specified in applicable codes and standards as well as the manufacturer s data concerning the flow characteristics for each design type. Pressure-relief valve manufacturers provide catalogs and computer sizing programs to aid in the sizing and selection of their specific products (Fig. 5.186). 

Balanced Safety Relief Valve. A balanced safety rehef valve is a pressure-relief valve which incorporates means of minimizing the effect of backpressure on the operational characteristics (opening pressure, closing pressure and relieving capacity). 

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. 

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 operation and characteristics of a conventional safety relief valve are illustrated diagrammatically in Figure 3. The action of the valve as pressure rises from the initial normal operating pressure (assuming no back pressure) is described below. The effect of back pressure on PR valve operation is described later. 

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. 

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. 

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. 

Fig. 7 Effect of pressure ratio on force characteristics of a high-lift relief valve.

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