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About:
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Making process control valve choices
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Control valves are used to manage the flow rate of a liquid or a gas and in-turn
control the temperature, pressure or liquid level within a process. As such, they are
defined by the way in which they operate to control flow and include globe valves,
angle seat, diaphragm, quarter-turn, knife and needle valves, to name a few. In most
cases the valve bodies are made from metal; either brass, forged steel or in hygienic
applications 316 stainless steel.
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Actuators will use an on-board system to measure the position of the valve with
varying degrees of accuracy, depending on the application. A contactless, digital
encoder can place the valve in any of a thousand positions, making it very accurate,
while more rudimentary measurements can be applied to less sensitive designs.
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One of the main areas of debate when specifying <a
href="http://www.dingchengmachinery.com/control-valve/" target="_self">control
valves</a> is determining the size of the valve required. Often process engineers will
know the pipe diameter used in an application and it is tempting to take that as the
control valve’s defining characteristic. Of greater importance are the flow
conditions within the system as these will dictate the size of the orifice within the
control valve. The pressure either side of the valve and the expected flow rate are
essential pieces of information when deciding on the valve design.
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Inside the valve body, the actuator design is often either a piston or a diaphragm
design. The piston design typically offers a smaller, more compact valve which is also
lighter and easier to handle than the diaphragm designs. Actuators are usually made
from stainless steel or polyphenolsulpide (PPS), which is a chemically-resistant
plastic. The actuator is topped off by the control head or positioner.
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Older, pneumatically operated positioners had a flapper/nozzle arrangement and
operated on 3-15psi, so no matter what the state of the valve, open closed or
somewhere in between, the system was always expelling some compressed air to the
atmosphere.
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Compressed air is an expensive commodity, requiring considerable energy to
generate and when a manufacturing line is equipped with multiple <a
href="http://www.dingchengmachinery.com/pressure-relief-valve/pressure-relief-
valves.html" target="_self">pressure relief valve</a> all venting to the atmosphere,
this can equate to a considerable waste of energy. It is important to not only
establish the most appropriate valve design, but also a cost-effective solution that
takes account of annual running costs.
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Modern, digital, electro-pneumatic valves that use micro-solenoid valves to
control the air in and out of the actuator have introduced significant improvements
for operators. This design means that while the valve is fully open, fully closed or
in a steady state, it is not consuming any air. This, and many other engineering
improvements, have made substantial advances in both economy and precision.
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Valve seats can be interchangeable within a standard valve body, which allows the
valve to fit existing pipework and the valve seat to the sized to the application more
accurately. In some cases, this can be achieved after the valve has been installed,
which would enable a process change to be accommodated without replacing the complete
valve assembly.
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Selecting the most appropriate seal materials is also an important step to ensure
reliable operation; Steam processes would normally use metal-to-metal seals, whereas a
process that included a sterilization stage may require chemically resistant seals.
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Setting up and installing a new valve is now comparatively easy and much less
time-consuming. In-built calibration procedures should be able perform the initial
setup procedures automatically, measuring the air required to open and close the
valve, the resistance of the piston seals on the valve stem and the response time of
the valve itself.
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