18.4.17
Modulating Valves Type: VSR
3. Functional description
9
3 .
F u n c t i o n a l d e s c r i p t i o n
The GUTH modulating valve VSR with multi-spring diaphragm actuator has a travel (stroke) of 20,
30 or 60 mm as standard for the nominal diameters DN 10 to DN 150. The linear movement of the
valve spindle is induced by means of an actuator shaft connected by a coupling piece. The dia-
phragm plate moves axially on account of the overpressure in the pressure chamber of the dia-
phragm actuator. It moves until the pressure of the compressed air on the one side is in equilibrium
with the spring load on the other side. The travel of the diaphragm plate is approximately propor-
tional to the air pressure present, assuming that the spring has a linear characteristic curve. The air
pressure is dependent on the amount of air introduced into the pressure chamber. The principle of
operation of the valve positioner involves controlling the air supply or extraction of air by means of
a control piston. It is always possible to work with the maximum air flow rate (flow volume).
There are one or more control cones on the valve spindle that are supported by a metal seat ring.
Depending on the direction of operation of the diaphragm actuator, the spindle is either completely
raised or completely lowered when the pressure chamber is completely void. In this state, the dia-
phragm plate lies flush against the housing, or another limit point has been reached in the valve
housing, i.e. cone seat. However, when a cone is flush on its seat ring, it is held against it by the
pre-tensioning force.
The control cones (parabolic cone) have an exactly computed form that determines the free cross-
section in the ring gap in dependency on the length of travel. Depending on the flow rate reference
for the valve, the so-called Kvs value, and the desired characteristic curve as a function of the length
of travel and flow volume, different cone forms are needed. Normally, so-called linear or equal-per-
centage characteristic curves are used, see Figure 8. For the purpose of throttling media where
there is a high differential pressure between the inlet and outlet sides, special cone and housing
shapes are used, so-called multi-stage modulating valves.
For optimal regulation characteristics, the direction of flow must always be towards the pointed end
of the control cone.
The standard modulating valves are fitted with metal seat seals.
In this connection it should be borne in mind that leakage according to VDI/VDE 2173, and EN
1349/ IEC 60534-4 of leakage class 4 are permissible.
This means that, in view of the less stringent leakage class, modulating valves with metal seat seals
may not also be used as shut-off valves.
If more stringent leak flow restrictions are required, GUTH modulating valves with elastomer seat
seals can be used. In this case, elastomer seat rings are chambered without gaps by means of
dividing the cone in two.
As standard, the spindle seal is provided by an O-ring placed close to the product and open to flush-
ing via a generously dimensioned gap. This represents a sensible compromise in respect of
hygiene aspects and the service life of this functional element. A second O-ring towards the spindle
seal prevents the ingress of foreign substances from outside.
Linear characteristic curve
Equal-percentage characteristic curve
Diagram. 1
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