Application Report - Rapid control dynamics reduce steam consumption by up to 30 %0 pages

Technical article: Sliding gate valve reins in costs for steam systems

Dr. Rainer Lange, Peter Stein - Ingolstadt, April 2013

The sliding gate valve reins in costs for steam systems
Rapid control dynamics reduce steam consumption by up to 30 %
The level of quality attainable in controlling a
steam plant is heavily dependent on the dynamic
characteristics of the total system comprising
the control element, actuator and controller. The
control parameters for sliding gate valves relating
to dynamic behaviour are clearly superior to
those for conventional control valves. As a result,
the highly responsive dynamics not only improve
the control quality but also form the basis for
control circuits with very short reaction times.
This has proved to be the key factor in steam
savings of up to 30 %. Current comparisons of
steam plant operators show that savings of this
magnitude are attainable only by installing a
sliding gate valve in place of a traditional control valve.
The most important prerequisites for short reaction
times in a control element are short strokes, small
actuators and low actuation forces resulting from these.
All of these characteristics come together in the sliding
gate valve. As it operates, two slotted plates slide
against each other transversely to the direction of flow.
Thus, the sliding gate valve seals itself without the need
for a metallic seat. The typical stroke between opened
and closed is a mere 6 to 9 mm. One of the most
noteworthy advantages stemming from this operating
principle is the low actuation force required for the
positioning movements.
This actuation force F can be determined for the sliding
gate valve by the coefficient of friction of the sliding
pair, the slot area ASlot facing the flow and the differential
pressure p with F1 = ASlot p. The force requirement
for an equivalent globe style valve is obtained from
F2 = ASlot p. Comparing these two forces mathematically results in F1 / F2 = ASlot/ASeat = 0.1 for valves
with the same Kvs value and the same differential
pressure. Therefore, the sliding gate valve needs only

one tenth of the actuation force required by a globe
style valve under the same process conditions. Also
as a direct result of this, the valve actuator for sliding
gate valves is naturally correspondingly lighter and
smaller.
Control dynamics at the highest performance level
By analysing its frequency response, the dynamics of
a control valve can be evaluated in terms of its control
performance. This aspect was studied experimentally,
for example, in [1] for different control valve systems.
The general conclusion which can be gathered from
this is that the use of sliding gate valves in a control
circuit results in higher critical gains. On the one hand,
therefore, the process controller can be set "more
aggressively", while, on the other hand, overshooting
is reduced when approaching a changed set point
value, which is also reached more quickly.
Depending on the plant and process, it is therefore
possible to tap into potential additional savings
simply by exchanging a globe style valve by a sliding
gate valve. By doing so, the quantities of steam
fed unnecessarily into the system by an overshooting action are reduced. This is supported
by the figures recorded by operators of different
steam systems.
Chinese Tobacco manufacturer, Hongta
Tobacco, reduces steam consumption by 30 %
In the three lines at the Chinese tobacco manufacturer,
Hongta Tobacco, the tobacco is conditioned at different
temperatures. The process variable defining the quality
is the temperature setting at 60, 65 or 70 oC. Depending
on the temperature setting required, previously Hongta
Tobacco had required up to 990 kg of steam per hour

Technical article: Sliding gate valve reins in costs for steam systems

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