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Back pressure safety valves
1. Introduction
2. Safety valve in the installation
3. Influence of back pressure on functioning behaviour
4. Limits of admissible back pressure (without metal bellow)
5. Back-pressure compensation using stainless steel bellow
6. Limits of admissible back pressure (with metal bellow)
7. Particularities of construction with stainless steel bellow
8. Summary
9. Bibliography
Dipl.-Ing. Erhard Stork, ARI-Armaturen Albert Richter GmbH & Co. KG, D-33756 Schloß Holte-Stukenbrock
Tel.: 05207/994-0, Fax: 05207/994-297, E-Mail: info.vertrieb@ari-armaturen.de
, Internet:: http://www.ari-armaturen.de
t000010301-2.doc 21.12.2005
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1. Introduction
When dimensioning and selecting safety valves for vessels and installations, it is
important that this fitting should not be considered as being separate from its supply
and blow-off pipes. The pressure and flow situation in neighbouring pipes and
installation components may have appreciable adverse influence on the functioning
of safety valves. The resulting reduction of the mass flow leads to an inadmissible
rise of pressure. In extreme cases, unstable functioning behaviour in the form of
flutter and hammering may lead to the valve becoming torn away. An accurate
knowledge of the behaviour of safety valves and of the admissible limits in relation to
back pressure, is a condition of avoiding the problems described. Various regulations
[1,2,3] either specify the admissible back pressure, or refer to the manufacturers of
the safety valves.
2. The safety valve in the installation
The simplest variant of the installation of a safety valve is the direct arrangement on a
container without an associated blow-off pipe, as shown in Fig 1. This is used in the
case of non-critical media such as air and of small valve sizes. By means of free
blow-off into the environment, no additional back pressure, which would have to be
taken into account, is generated.
Druckbehälter
icherheitsventil
A
bblaseleitung
Druckbehälter
Sicherheitsventil
Safety valve
S
Safety valve
Blow-off pipe
Pressure vessel Pressure vessel
Fig 1: Free blow-off without pipe Fig 2: Blow-off pipe leads into
the environment
In most cases, a blow-off pipe, which leads into the environment, is attached to the
safety valve (see Fig 2). The back pressure, which is generated on the output side of
the safety valve during the blow-off process which is called built-up back pressure,
depends on the length of this pipe, the number of bends and of any loss elements
(e.g. silencers).
In the case of critical media which are toxic or strongly corrosive, it is essential to
capture the blown-off quantity in closed vessel systems (see Fig 3). The pressure
generated during this process is present on the output side, even when the safety
valve is once again closed and is called superimposed back-pressure. During the
Dipl.-Ing. Erhard Stork, ARI-Armaturen Albert Richter GmbH & Co. KG, D-33756 Schloß Holte-Stukenbrock
Tel.: 05207/994-0, Fax: 05207/994-297, E-Mail: info.vertrieb@ari-armaturen.de
, Internet:: http://www.ari-armaturen.de
t000010301-2.doc 21.12.2005