58
Operation and function
The flow characteristic k
V
= f(s) indicates the flow-rate of a valve, expressed by the value k
V
as a function
of the stroke s of the actuator spindle. It is determined by the design of the valve seat and the seat seal. In
general two types of flow characteristics are implemented, the linear and the equal percentage.
In the case of linear characteristics, equal k
V
value changes dk
V
are assigned to equal stroke changes ds.
(dk
V
= n
lin
⋅
ds
).
In the case of an equal percentage characteristic, an equal percentage change of the k
V
value corresponds
to a stroke change ds.
(dk
V
/k
V
= n
eqlprct
⋅
ds
).
T
he operating characteristic Q = f(s) specifies the correlation between the volumetric flow Q in the installed
valve and the stroke s. This characteristic has the properties of the pipelines, pumps and consumers. It
therefore exhibits a form which differs from the flow characteristic.
Position set-point value
[%]
Standardized valve stroke [%]
Fig. 34: Characteristic
In the case of control tasks for closed-loop control systems it is usually particular demands which are placed
on the course of the operating characteristic, e.g. linearity. For this reason it is occasionally necessary to correct
the course of the operating characteristic in a suitable way. For this purpose the positioner features a transfer
element which implements different characteristics. These are used to correct the operating characteristic.
Equal percentage characteristics 1:25, 1:33, 1:50, 25:1, 33:1, and 50:1 as well as a linear characteristic can
be set. A characteristic can be freely programmed using nodes.
english
Type 8696, REV.2
Summary of Contents for 8696
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