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.
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.
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, firstname.lastname@example.org.
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.
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.
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.