Reducing Variability for Optimal
Plant Performance
Experience the Cernay PlantWeb Dynamic Performance Loop
It is well known and documented that
control loop performance has an
impact on plant performance and
hence profitability. If the variability
within the process is too high it can
lead to scrap, downgrading of product
or rework. Often the solution is to
move the operating point outside of
the most profitable area, even to run
the process in manual. This can lead to
higher than necessary energy or raw
material costs and you may be produc-
ing a product that exceeds specifica-
tion by a wide margin.
Control loop performance naturally
degrades over time and a great deal of
money is spent maintaining equip-
ment and tuning loops to ensure vari-
ability is kept to a minimum. Many
companies have invested heavily in
advanced process control and auto
tune functions to improve perform-
ance. The problem is that these solu-
tions focus on DCS controller func-
tions and do not take into considera-
tion the deterioration in field equip-
ment where the underlying problem
exists.
Control loop performance
An independent process control consul-
tancy has accumulated evidence, from
years of research that suggests control
valves are the biggest contributor to
poor loop performance and the desta-
bilisation of product uniformity. They
advocate that more attention should be
paid to control valve choice, perform-
ance monitoring and maintenance.
Variability will always exist in a process;
it is the function of the process control
equipment to reduce the variability to a
level that is acceptable. It is easy to
understand how a badly maintained
valve may increase variability but what
is less easy to understand is how a brand
new, high quality valve can also
increase the variability.
There are many factors that influence
the ability of a control valve assembly to
minimise variability, these include valve
style, design and size. Reference specifi-
cations, including ‘The control valve
dynamic specification, version 3’, from
EnTech, an independent process con-
trol consultancy, and the ISA standard
SP75.25 both provide performance cri-
teria that can be applied when selecting
a new control valve to ensure it has the
ability to minimise process variability.
Emerson Process Management have sig-
nificant experience of monitoring and
maintaining control valves on process
plants. The experience they have gained
has led to new valve designs and exten-
sive performance testing in their
research and test facilities around the
globe. Tools and techniques have been
developed that enable optimum per-
formance to be established by selecting
the best control valve for the duty,
maintain that performance by routine,
on-line monitoring and retrieve the per-
formance to previous levels should it
deteriorate.
Variability will always
exist in a process; it is the
function of the process
control equipment to
minimise variability,
even new equipment is
not always effective at
doing so.
Establish
Maintain
Retrieve
A three phase
approach to optimis-
ing control loop per-
formance.
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