Burkert Type 8791 - Manual (Page 36)

36
ParameterizationofBasicFunctions
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).
The 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. 25: 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
Software Tool - Positioner