17(19)
The time axis of the trend is from left to right, with oldest
values on the left and newest values on the right. The trend
adjusts periodically along the time axis as follows:
When 24 hours have passed, a new
Day
value is
added to the trend and the device begins calculating a
new
Now
value.
When 30 days have passed, a new
Month
value is
added to the trend.
When 12 months have passed and 12 new
Month
val-
ues have been calculated, a new
Year
value is added
to the trend.
3.3.3.1.2 Reference Value
Reference value represents valve performance at its best.
During the first three months of operation, the device cal-
culates reference values for all trends. Note that the device
does not calculate reference values if supply pressure is
not connected.
You can use reference values later for analyzing changes
in the trend.
3.3.3.1.3 Steady State Deviation
Steady State Deviation is used to determine the basic con-
trol accuracy of the valve. It is updated whenever the set-
point is considered to have reached the desired position as
precisely as possible. Figure 22 illustrates this concept.
Steady State Deviation trend is updated during the periods
marked
steady
.
Figure 22. This graph illustrates steady states and dynamic
states.
3.3.3.1.4 Dynamic State Deviation
Dynamic State Deviation can be used to estimate valve
dynamics such as response times. It is updated whenever
the setpoint changes and the valve is expected to move
accordingly. Updating continues throughout the valve
movement and is switched back to
Steady State
when
both setpoint and valve position have reached a steady
state.
Figure 22 illustrates this concept. Dynamic State Deviation
trend is updated during the periods marked
dynamic
.
3.3.3.1.5 Stiction
Stiction
,
Stable State Load
, and
Load for Opening
are
the three pneumatic load measurements. You can use the
Stiction value to estimate internal frictions in the control
valve package:
Increased
stiction suggests an increase in internal fric-
tion, which can cause accuracy problems and ulti-
mately prevent the valve from moving.
Decreased
stiction suggests a decrease in internal
friction. This may indicate problems such as extensive
wear or valve shaft break.
Whenever the valve moves, required pneumatic pressure
in the actuator is registered. The stiction value can be cal-
culated when measurements in both upscale and down-
scale directions have been registered.
3.3.3.1.6 Stable State Load
This trend shows Stable State Load measurement, which is
especially significant for single-acting actuators since you
can use it estimate the actuator spring force and spring
state. The trend is based on the pneumatic pressure
measurement that is carried out whenever the valve is in a
stable state.
If the spring is broken, Stable State Load decreases no-
ticeably.
3.3.3.1.7 Load for Opening
You can use Load for
Opening to estimate the
following:
Seat wear, especially in
butterfly valves
Medium
crystallization
in
e.g. ball valves
The Load for Opening trend
is updated whenever the
valve is opened from a fully
closed position. Like stiction,
this trend is based on the
pneumatics measurement.
3.3.3.1.8 Spool Valve Position
This trend shows Spool Valve Position, which is measured
as a percentage of the movement range. Spool Valve Posi-
tion is typically in the middle of the range, but changes in
the trend may suggest actuator (pneumatic) leak, espe-
cially with single-acting actuators.
3.3.3.1.9 Supply Pressure
You can use Supply Pressure trend as follows:
To analyze possible problems with supply pressure
