Mankenberg M XTRA RP 840, Operating Manual

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Know How Pressure Reducing Valves
Pressure reducing valves reduce a high and frequently fluctuating pressure to an adjustable constant pressure downstream of
the valve. A spring keeps the valve open and this closes as the outlet pressure rises.
Page No. DM/13.1.152.1 - Standing
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Selecting valve type and nominal diameter
Using your maximum operating data and the smallest differential
pressure Δp, you should calculate the characteristic performance figure
K
v
(see leaflet Calculation of Pressure Regulators). Select a valve whose K
vs
value is 30% greater than the calculated K
v
figure. Additional
allowances must be made for high-viscosity liquids or liquids which
vaporise when depressurised.
You should also note the reduction ratio i.e. inlet pressure p
1
divided by
outlet pressure p
2
. The inlet pressure acting on the cone causes the valve
to open whereas the outlet pressure acting on the diaphragm/spring
system causes it to close. If the reduction ratio calculated from the
operating data is greater than the quoted ratio, the valve will not close.
Pressure reducing valves should not be overdimensioned. Their optimum
working range is within 10% to 70% of their K
vs
value.
Selecting rated pressure and valve material
The rated pressure must exceed the maximum system pressure,
irrespective of safety allowances. Please note also the effect of the
temperature (see DIN 2401).
Selecting the setting range
For good control accuracy you should select a setting range which
places the required outlet pressure near its upper limit. If, for example,
the controlled outlet pressure is to be 2.3 bar, you should select the 0.8
to 2.5 bar setting range, not 2 to 5 bar. If the available setting range is
not wide enough you may go below the bottom limit of the setting
range provided that the valve loading is kept low and a high control
accuracy not required.
Selecting elastomer materials
You should select elastomers according to the operating temperature
and the requirements of the medium. High-pressure gases, for example,
can diffuse into the elastomer and cause damage when being
depressurised.
Flow velocity
Depending on pressure drop and permitted maximum noise level, we
recommend the following flow velocities:
Liquids 1 - 5 m/s
Saturated steam 10 - 40 m/s
Superheated steam 15 - 60 m/s
Gases below 2 bar 2 - 10 m/s
Gases above 2 bar 5 - 40 m/s
Sense line (control line)
You should install a sense line if the selected pressure reducer is designed
for sense line operation. The sense line should be connected at a distance
of not less than 10 times nominal diameter downstream of the pressure
reducing valve. No isolating valves should be installed in the sense line to
avoid an excessive pressure differential between valve body and
diaphragm.
To attenuate any oscillations occurring in the pipeline system, the sense
line may be fitted with a restrictor which must never be fully closed
during operation.
In the case of steam and liquids the sense line must be installed so as to
fall towards the valve. Under special operating conditions, for example
intermittent operation with dry steam, an compensation vessel must be
installed. The sense line must be rigid as elastic hoses can induce
oscillations.
Protecting your system
To protect your system you should install a safety valve downstream of
the pressure reducer to prevent the maximum permitted operating
pressure (normally 1.5 x maximum set pressure) being exceeded. The
safety valve operating pressure should be set approximately 40% above
the maximum set pressure of the pressure reducer to avoid blow-off
during slight pressure fluctuations. For example: if the pressure reducer
setting range is 2 - 5 bar the safety valve operating pressure must be 1.4
x 5 bar = 7 bar.
Protecting the pressure reducing valve
To protect the pressure reducer against damage from solid particles
carried in the pipeline, a strainer or filter should be fitted and serviced at
regular intervals.
With steam as medium, the pressure reducer should be preceded by a
water trap, which is also called steam dryer, to protect it from cavitation
(see below chapter "Steam Operation").
Valve seat leakage
These valves are no shut-off elements ensuring a tight closing of the
valve. In accordance with DIN EN 60534-4 and/or ANSI FCI 70-2 they
may feature a leakage rate in closed position in compliance with the
leakage classes II – V:
Leakage class II (metal sealing double seat cone) = 0.5% Kvs value
Leakage class III (metal sealing cone) = 0.1 % Kvs value
Leakage class IV (PTFE seal cone) = 0.01 % Kvs value
Leakage class V (soft seal cone) = 1.8 x 10-5 x Δp x D* [l/h]
*D=seat diameter
Any low leakage requirement must be expressly specified when ordering.
Valve leakage can be considerably reduced by special measures such as
lapping the valve seat, using special cone seals and increasing the
control (diaphragm) surfaces.
Cut-off
For the purpose of installation, servicing and isolation of the valve,
shut-off valves should be installed upstream and downstream of the
pressure reducer. When closing the shut-off valves the upstream valve
must always be closed first. A bypass line may be necessary to maintain
emergency operation.
Stellited seat and cone
In the case of abrasive media or liquids with pressure drops (inlet
pressure minus outlet pressure) of more than 25 bar the valve cone must
be stellited; for pressure drops above 150 bar the seat must be stellited
as well.
Leakage line
If toxic or hazardous media are used the valve must feature a sealed
spring cap (including setting spindle seal) fitted with a leakage line
connection. When the pressure reducer is installed on site a leakage line
must be fitted capable of safely and pressureless draining the escaping
medium in case the control valve should become defective.
Mounting position
For gases a pressure reducing valve can normally be fitted in horizontal
pipelines with the spring cap at the bottom or at the top. Installation in
vertical pipe runs is possible but can result in increased wear and loss of
control accuracy owing to increased friction.
In the case of liquids a pressure reducer should be installed with the
spring cover at the bottom. Thus gas traps upstream of the valve are
avoided which would cause the valve to oscillate.
For steam a pressure reducer should likewise be installed with its spring
cover at the bottom to protect the diaphragm against overheating by
means of a layer of condensate. In case the valve must be emptied
completely during operation (angle valves), it must be installed with the
spring cap pointing upwards.
Start-up
Pressure reducers should be started up and operated without pressure
surges, if possible. A sudden operation of upstream or downstream
valves should be avoided.
Steam operation
If a pressure reducer is installed in a steam plant the diaphragm water
reservoir must be filled before the plant is started up. There must be no
danger of overheating at the installation site caused by excessive
ambient temperatures or insufficient heat dissipation. Pressure regulators
must not be insolated. In some cases an insulating of the body is
permitted, but only with cast bodies. Never insulate diaphragm housing,
mid section and spring cap (or open springs). Overheating caused by
insulating destroys the elastomere of the control unit.
Many steam generators send a lot of water through the piping together
with the steam. Even an initial overheating can get lost through piping
heat losses, so that the steam gets "wet". A piping speed of up to 25
m/s is normal for "dry steam", whilst wet steam already has the effect of
a sandblasting machine at this speed, and the condensate and/or the
water droplets eat holes into pipings and valve seats. In addition, water
obstructs heat transition especially in heat exchangers. To avoid it, the
water should be removed by a water trap, also called steam dryer, as
quickly as possible and without steam losses.
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