Making process control valve choices

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

control valves 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.

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

pressure relief valve 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

common rail control valve F00vc01334. 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.

Advanced control schemes can't produce optimum results unless the

diesel common rail fuel valve F00rj01428 operate properly.

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.

The flow of these fluids can be effectively controlled by installing control

valves. They are available for various fluids, such as steam, water, and chemicals.

Some can also withstand harsh environments, such as high temperatures and toxic

chemicals.

Manual control valves are simple—humans manually control the opening of the valve

through a manual mechanism such as a handle. The handle’s motion is directly

connected with the internal valve mechanism, which controls the fluid flow.

 

Control Valves with Feedback Signals

Another control valve style uses a feedback signal to control the fluid flow.

Before controlling the valve working, let’s briefly introduce feedback signals,

electrical to pneumatic converters, and actuators.

Feedback Signal

The feedback signal monitors the target system for flow requirements. If the

feedback signal detects any need to increase or decrease the fluid flow, it alerts the

valve. The valve then responds and adjusts its opening to accommodate the fluid flow.

The feedback signal can be electrical or pneumatic, and is often from a programmable

logic controller (PLC) or electrical controller, which monitors the change in fluid

flow requirement.

Electrical to Pneumatic Signal Converter

The control valve accepts feedback signals in the form of air. In the case of the

electrical feedback signal, the signal is converted to equivalent pneumatic pressure.

The electrical to pneumatic converter converts the incoming electrical feedback signal

to equal pneumatic pressure, fed to the control valve as a feedback signal.

Examples of pneumatic converters include a current-to-pressure transducer that

converts the incoming current to a pneumatic signal.

Actuator

The actuator is a component attached to the

injector control valve F00rj02130, which controls the valve

movement. Without the actuator, the internal mechanism of the valve cannot be moved.

The feedback pneumatic signal comes to the actuator, directly connected to the

main valve body. The main control valve moves relative to the actuator. When the

feedback signal becomes high—indicating a more increased flow—the actuator closes,

closing the main valve. 

When the feedback signal decreases—indicating low flow—the actuator opens,

simultaneously opening the valve to increase the flow.