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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.

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.


One of the main areas of debate when specifying

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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.


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.


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.


Compressed air is an expensive commodity, requiring

considerable energy to generate and when a manufacturing line

is equipped with multiple

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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.


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.


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.


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.


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.

Improving safety
Control valves should be specified so they operate in the 40-

85% range so if the valve is commanded to a 10% setting, it can

detect if something has potentially gone wrong with the control

system and the best course of action is to close the valve

completely. If the valve is commanded to a position of 10% or

less this can cause very high fluid or gas velocities, which

have damaging effects on the system and cause considerable

noise and damage to the valve itself.

Modern control functionality can offer a solution that acts as

a safety device to prevent damage to the process pipework and

components. By building in a fail-safe mechanism, any valve

position setting below a pre-set threshold will result in the

valve closing completely, preventing damage to the surrounding

system.


Control inputs can also include safety circuits to ensure safe

operating conditions within the process equipment. For example,

if an access panel on a vessel containing steam is opened, an

interlock switch will open and the valve controlling the steam

supply to the vessel can be automatically closed, helping

mitigate any risks.


Improving reliability
Many process control environments offer less than ideal

conditions for long-term reliability. Moisture-laden

atmospheres, corrosive chemicals and regular wash-downs all

have the capacity to shorten the service life of

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F00vc01334.[/url] One of the potential weaknesses of the

actuator is the spring chamber where atmospheric air is drawn

in each time the valve operates.


One solution is to use clean, instrument air to replenish the

spring chamber, preventing any contamination from entering.

This offers a defense against the ingress of airborne

contaminants by diverting a small amount of clean control air

into the control head, maintaining a slight positive pressure,

thus achieving a simple, innovative solution. This prevents

corrosion of the internal elements and can make a significant

improvement to reliability and longevity in certain operating

conditions.


While choosing the most appropriate process control valve can

be a complex task, it is often best achieved with the

assistance of expert knowledge. Working directly with

manufacturers or knowledgeable distributors enables process

control systems to be optimized for long-term reliability as

well as precision and efficiency.


Damien Moran is field segment manager, Hygienic –

Pharmaceutical at Bürkert. This article originally appeared on

the Control Engineering Europe website. Edited by Chris Vavra,

associate editor, Control Engineering, CFE Media and

technology, cvavra@cfemedia.com.


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Instrument technicians must understand these final control

elements as well as their diagnostic software to ensure the

valves in the plant operate as the system designers intended.


Renewed interest in the performance of control valves is

emerging, partly as a result of numerous plant audits that

indicate roughly one-third of installed control valves are

operating at substandard levels. Even though properly operating

control valves are essential to overall plant efficiency and

product quality, maintenance personnel frequently don't

recognize the signs of poor performance. The basics of control

valve design and operation must be well understood for end-

users to reap the benefits of improved valve operation.


Basic types of control valves
The most common and versatile types of control valves are

sliding-stem globe and angle valves (see Figure 1). Their

popularity derives from rugged construction and the many

options available that make them suitable for a variety of

process applications, including severe service. For example,

sliding stem valves typically are available with options that

satisfy a range of requirements for ANSI Class pressure-

temperature ratings, shutoff capability, size, temperature

compatibility and flow characteristics.


Achieving complete valve shutoff is important in many

applications to prevent leakage that either could contaminate a

process fluid or result in product loss. Tight shutoff also

prevents erosion damage that could occur if a high-velocity

stream leaked across seating surfaces.


Many control valves are oversized as a result of inaccurate

information and safety margins added by each individual or

group that participates in the sizing procedure. Oversized

valves are a problem for three reasons.


First, the valve operation may become unstable because it never

opens very far from the fully closed position. Process gain is

generally high when the valve is throttling near its seat. The

combined valve and process gains may be too high to maintain

stable operation at low lifts. Second, excessive seat wear may

result from high velocity flows between the closure member and

the seating surface. Third, the design flow characteristic may

not be achieved, resulting in controller tuning problems.


Control valves are a common element in the process and

manufacturing industry. They control the fluid flow in the

attached network. The fluid goes into one side of the valve;

its flow is adjusted and comes out the other side. The fluid

flow can be controlled by manual or automatic mechanisms and

comes in various shapes, sizes, and applications.

Controlling the flow rate of process fluids enables personnel

to control many relevant parameters. For example, the

temperature in a closed container is directly proportional to

the steam pressure being applied to it. Similarly, water flow

in a vessel storage system is monitored and controlled to

prevent overflow.

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