INSTRUMENTATION • ELECTRONICS VALVE DESIGN DEVELOPMENTS
Process control valves: making the right choice. By Damien Moran
Choosing valves can be a complex task
M
odern process control valves off er a wide range of features and benefi ts for industries that require precise control over
fl uids, steam and other gases. With so many control valves on the market, it is important to establish the features that will deliver the most cost-eff ective design for a particular application. Here we look at some of the basic diff erentiators as well as some recent design developments. Control valves are used to manage the
fl ow rate of a liquid or a gas and in turn control the temperature, pressure or liquid level within a process. As such, control valves are defi ned by the way in which they operate to control fl ow 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.
ESTABLISHING THE PARAMETERS Actuators use an on-board system that measures the position of the valve with
Working directly with valve manufacturers is key for success
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. One of the main areas of debate when specifying control valves is determining the size of the valve required. Quite often process engineers will know the pipe diameter that is used in an application and
it is tempting to take that as the defi ning characteristic for the control valve. Of greater importance are the fl ow conditions within the system as these will dictate the size of the orifi ce within the control valve. T e pressure either side of the valve and the expected fl ow rate are essential pieces of information when deciding on the valve design.
IMPROVING EFFICIENCY Inside the valve body, the actuator design is predominantly either a piston or a diaphragm design. T e piston design typically off ers a smaller, more compact valve that 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. T e actuator is topped off by the control head or positioner. Older, pneumatically operated positioners
had a fl apper/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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