Leser 411 Instructions De Service page 20

walled valve bodies should not be stamped.
Safety valves have a name plate showing the
following data:
• Date of order (Serial-No.)
• Technical data
• Test pressure
• VdTÜV type test approval number
• CE-marking and identification number of the
notified body.
For safety valves without type test approval, only
E
order data and technical data are included.
Further marks required are either moulded into
the casting, or, for safety valves with threaded
connections, punched in. Safety valves with a
heating jacket have a separate name plate for the
heating jacket.
If technical changes are made, always check
whether the identification has to be adjusted.
Modifications on valves or identifications may
only be carried out by trained personnel (refer to
section 12.14).
4 Pressure
Definitions:
a) Test pressure: the pressure that the safety
valve is set to at LESER's. The outlet side of the
safety valve is open to atmospheric pressure.
b) Set pressure: the predetermined pressure at
which a safety valve under operating
conditions commences to open.
c) Overpressure: a pressure increase over the
set pressure, at which the safety valve attains
the lift specified by the manufacturer, usually
expressed as a percentage of the set
pressure.
d) Reseating pressure: the value of the inlet
static pressure at which the disc re-
establishes contact with the seat or at which
the lift becomes zero.
e) Operating pressure: the pressure at which
the plant operates.
f) Built-up back pressure: the pressure existing
at the outlet of a safety valve caused by flow
through the valve and the discharge system.
g) Superimposed back pressure: the pressure
existing at the outlet of a safety valve at the
time when the device is required to operate.
h) Back pressure: the total of built up and
superimposed back pressure.
Pressure shall be stated as overpressure [bar g or
psig] above atmospheric pressure.
Unless stated otherwise, the pressure specified
by the customer is set with atmospheric pressure
acting on the outlet side (test pressure = set
pressure).
If there is pressure on the outlet side
(superimposed back pressure), a force is
20
produced, which acts on the rear side of the disc.
This increases the set pressure by exactly the
value of this pressure. If the superimposed back
pressure is constant, it is possible to adjust the
differential pressure by reducing the value of the
test pressure by the value of the back pressure
(test pressure set pressure).
If there is no superimposed back pressure, the
set pressure will drop. The back pressure
intended must not be exceeded because then it
would also exceed the set pressure.
The maximum pressure that a safety valve may
be operated at regardless of the test pressure
depends on a number of factors.
Among these are:
• Materials
• Medium temperature
• Design pressure
• Flange classes
• Others.
These should be taken into consideration when
selecting a safety valve.
The value of the operating pressure must
permanently be lower than the reseating pressure
difference plus 5%. Otherwise LESER cannot
guarantee that the valve will close securely after
opening (exception: if the valve is fitted with a
supplementary loading system, refer to section 15).
5 Function of a safety valve
A performance certificate is required to ensure
that the required mass flow is discharged by the
safety valve if necessary.
Pipes leading to the safety valve have to be fitted
so that large hydrodynamic losses are prevented.
The edges at the pipe inlet should be at least
chamfered, but preferably rounded. The notes on
dimensioning given in the regulations, standards
and manufacturer's information sheets must be
adhered to.
Safety valves may only be disabled by means of
shut-off valves provided that the pressurised
equipment is protected against excessive
pressure by other safety devices or that the plant
or equipment is shut down altogether.
Safety valves are guaranteed to work perfectly, if
the built-up back pressure on the outlet side does
not exceed 15% of the test pressure minus the
superimposed back pressure (if available).
Built-up and superimposed back pressures may
be compensated by an amount not exceeding
35% of the set pressure by using stainless steel
bellows specially designed for this, which
compensates the force acting on the rear side of
the disc. Function and set pressure remain