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6.4.4 Back Pressure

6.4.4.1 Definitions

Back pressure is the pressure that exists at the outlet of a pressure relief device as a result of the

pressure in the discharge system. It is the sum of the superimposed and built-up back pressures andhas an influence on the function of the safety valve.17)

The type of back pressure that occurs depends on the type of installation. The simplest version ofinstallation is a vessel with a safety valve but no connected outlet line (see Figure 6.4.4.1-1). Thisconfiguration is used for uncritical mediums like water or air and small safety valve sizes. With thisconfiguration no additional back pressure arises.

Safety valve

Vessel

Figure 6.4.4.1-1:Without Back Pressure

Figure 6.4.4.1-2:Built-up Back Pressure

Figure 6.4.4.1-3:Superimposed Back pressure

Built-up back pressureThe safety valve can also be connected to an outlet line which blows off into the open air (see Figure6.4.4.1-2). Pressure that arises at the outlet of a safety valve and is caused by flow through the valveand the discharge system is called built-up back pressure. The diameter, the length of the dischargepipes, elbows, silencers, etc. determine the level of built-up back pressure. Excessive built-up backpressure leads to chattering of the safety valve.

Superimposed back pressureThe medium can also be discharged into a closed blowdown or discharge system (see Figure6.4.4.1-3). This is necessary when discharge in the open air is not wanted or not allowed e.g. for toxicor highly corrosive media. In this case pressure exists at the outlet of a safety valve at the time thesafety valve is required to operate. This pressure is called superimposed back pressure. It is theresult of pressure in the discharge system coming from other sources and may be constant orvariable. Superimposed back pressure cause a change of the set pressure of a conventional safetyvalve.

17)API 520 Part I, 8

thEdition 2008, Sect. 3.3

Back Pressure = Built-up + Superimposed

Safety valve

Vessel

Outlet line

Safety valve

Vessel

Outlet line

BlowdownSystem

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Compensation by CDTP-correction (CDTP: Cold Differential Test Pressure)lThe inlet static pressure at which a pressure relief valve is adjusted to open on the test stand. Thistest pressure includes corrections for service conditions of superimposed back pressure and/ ortemperature.18)

This means:

The CDTP-correction is the correction of set pressure at test bench conditions to achieve the correctset pressure at service conditions. Example:

Pressure form Pressure

Set pressure 10 bar

Superimposed back pressure 2 bar

Differential pressure (CDTP)Setting of safety valve

8 bar

When the superimposed back pressure is taken into account LESER will deliver the safety valve witha spring which is designed for the differential pressure (From the example: 8 bar instead of 10 bar).

Variable Superimposed Back Pressure:To compensate for variable superimposed back pressure a balanced bellows has to be installed. Thiscompensation reaches up to 50 % for safety valves of the API product group and up to 35% for allother LESER safety valves with balanced bellows.

18)ASME PTC 25-2001, chapter 2.7