Process plants are a network of complex systems and processes. Just as arteries, veins, and the heart are vital to human life, pipes, valves, and pumps are indispensable in a process plant. The primary purpose of a valve is to direct and control the flow of fluids by starting, stopping, and throttling (restricting) flow to make processing possible. Valves are designed to withstand pressure, temperature, and flow and can be found in homes and industry across the world.
Process operators classify valves by (1) flow-control elements, (2) function, and (3) operating conditions such as pressure, flow, or temperature. The most common way to classify valves is by the valve’s flow-control element design. This part of the valve controls or regulates the flow of fluid through the device. Some valves have movable metal gates, balls, plugs, diaphragms, discs, needles, or even butterfly-shaped elements. Most valves are named for the type or design of the flow-control element. Valves that are used for isolation are classified as block valves. While gate valves are the most common valve used for isolation, any valve can be used for this type of service. Another term associated with valve operation is valve capacity. Valve capacity is a term used to describe the total amount of fluid a valve will pass with a given pressure difference when it is fully open.
- Gate Valves
One of the more common valves found in industry is a gate valve. A gate valve places a movable metal gate in the path of a process flow in a pipeline. The gates are sized to fit the inside diameter of a pipe, so very little restriction occurs when it is in the open position. Valves vary in size from 0.125 inches to several feet. Gate valves typically are operated in the “wide open” or “completely shut” position. This type of valve is used where flow rates are not restricted. Gate valves should not be used to throttle flow for extended periods. Turbulent flow rates across the valve body will cause metal erosion, seat damage, and damage to the flow-control element, which can prevent the valve from blocking the flow completely.
The seats in a gate valve fall into two categories: replaceable and fixed. Smaller valves typically have fixed or cast seats because it is easier and more cost efficient to replace the valve than to replace the seats. In most cases, a gate valve has two vertical seating surfaces. The edges of the seats match up with the parallel discs or wedge.
The typical gate valve consists of a gate, body, seating area, stem, bonnet, packing, stuffing box, packing gland, and handwheel. The gate can be wedge shaped or may consist of parallel discs. It can be composed of a variety of materials. The gate is placed directly in the path of a process flow when it is shut and is lifted completely out of the way when open.
The body is the largest part of the valve. The body can be connected to the process piping in three ways: flanges, threaded connections, or welding. The rest of the valve is attached to the body.
- Globe Valves
Globe valves are the second most common valves used in industry. A globe valve places a movable metal disc in the path of a process flow. This type of valve is most commonly used for throttling service. The disc is designed to fit snugly into the seat and stop flow. Process fluid enters the globe valve and is directed through a 90° turn to the bottom of the seat and disc. As the fluid passes by the disc, it is evenly dispersed. Globe valves must be installed properly to work efficiently. If the valve is installed backward, it will tend to wear unevenly and push the flow-control element down.
The typical globe valve consists of a disc, body, seating area, stem, bonnet, packing, stuffing box, packing gland, handwheel, and back seat. The disc is attached to the stem in three ways: slip joint, threading, or one piece manufacturing. The disc can be classified as plug, ball, composition, or needle shaped. It is composed of a variety of materials. The disc, or flow-control element, rests in the seat, directly in the path of a process flow when it is shut. Unlike the gate valve, the globe valve is designed to be used for throttling. Flow can be regulated by the percentage of opening of the flow-control element.
The body is the largest part of the valve. The body can connect to the process piping in three ways: flanges, threaded connections, or welding. The rest of the valve is attached to the body.
- Ball Valves
Ball valves take their name from the ball-shaped, movable element in the center of the valve. Unlike the gate and globe valves, a ball valve does not lift the flow-control device out of the process stream. Instead, the hollow ball rotates into the open or closed position. Ball valves provide very little restriction to flow and can be fully opened with a quarter turn on the valve handle. In the closed position, the port is turned away from the process flow. In the open position, the port lines up perfectly with the inner diameter of the pipe. Ball valves come in a variety of shapes and sizes. Larger valves require handwheels and gearboxes to be opened but only require one-quarter turn on a handle. Ball valves should not be used for throttling service. During prolonged exposure, the ball seats and ball can be damaged, stopping the valve from sealing properly. Typically, ball valves are not designed for excessive temperatures. The seats are made of a plasticcoated material that tends to break down under high temperatures. Process technicians should be familiar with the temperature specifications of the valves.
Ball valves do not generally seal as well as globe valves in high-pressure service. Some ball valves (multiport valves) are designed with multiple ports, so an operator can switch fluid sources without stopping flow.
- Check Valves
A check valve is designed to prevent reverse flow and to avoid possible contamination or damage to equipment. The check valve limits backflow but is not considered a tight shutoff. Check valves come in a variety of designs and applications. Check valves have flow direction stamping on the valve body. A typical check valve design is the swing check, which has a hinged disc that slams shut when flow reverses. Flow lifts the disc and keeps it lifted until flow stops or reverses. The body of the check valve has a cap for easy access to the flow-control element. Another design is the lift check, which has a disc that rests on the seat when flow is idle and lifts when flow is active. Special guides keep the disc in place. Like the swing check, it is designed to close when flow reverses. Lift checks are ideal for systems in which flow rates fluctuate. The lift check is more durable than the swing check. In the horizontal or vertical lift check design, a piston or ball is lifted up and out of the seat by process flow. A third design is the ball check design, which has a ball-shaped disc that rests on a beveled, round seat. The ball is down when flow is idle and up when flow is active. Special guides keep the ball disc in place. Like the swing check, it is designed to close when flow reverses. Ball checks are ideal for systems in which flow rates fluctuate or the fluid contains some solids. The ball check is as durable as a lift check and more durable than a swing check. A fourth design is the stop check design, which has characteristics of a lift check and a globe valve (Figure 2.13). In the closed position, the stop check disc is firmly seated. In the open position, the stem rises out of the body of the flow-control element and acts as a guide for the disc. In the open position, the stop check functions like a lift check with one exception. The degree of lift can be controlled.
- Butterfly Valves
Butterfly valves are commonly used for throttling and on/off service. The body of this type of valve is relatively small when compared with other valves and, therefore, occupies much less space in a pipeline. The flowcontrol element resembles a disc. A metal shaft extends through the center of the disc and allows it to rotate one-quarter turn. A one-quarter turn is all it takes to fully open or close the valve.
Technicians should be aware that butterfly valves are 100% open shortly after the valve handle passes one-eighth of a turn. During a throttling operation, a butterfly valve handle should be carefully secured. As flow enters the body of the valve, it contacts the disc and will cause it to open if the handle is not latched into position. Butterfly valves are used for throttling; however, it should be noted that they have nonuniform flow characteristics. Fifty percent open may provide near-maximum flow.
Butterfly valves are designed to be operated at low temperature and low pressure. They are commonly found in cooling water and heat exchanger systems throughout the process industry. The seats in a butterfly valve can be made of natural gum rubber or suitable plastics.
- Plug Valves
Quick-opening, one-quarter turn plug valves are very popular in the process industry. The plug valve takes its name from the plug-shaped flowcontrol element it uses to regulate flow. Plug valves provide very little restriction to flow and can be opened 100% with a one-quarter turn on the valve handle. In the closed position, the port is turned away from the process flow. In the open position, the port lines up with the inner diameter of the pipe.
- Diaphragm Valves
In a chemical plant, a variety of slurries, corrosive or sticky substances, are transferred from place to place. Standard valves would have a difficult time with this type of product, but diaphragm valves are specifically designed for the job. Diaphragm valves use a flexible membrane and seat to regulate flow. The handwheel operates just like the handwheel on a gate or globe valve. The stem is attached to a device called the compressor. The compressor pushes on a flexible diaphragm. The internal parts of the valve never come into contact with the process material. The diaphragm conforms to the setting on the handwheel. Diaphragm valves typically are used in low-pressure applications. The diaphragm valve seats are made of chemical-resistant plastic, rubber, or neoprene. This type of valve does not use packing.
Diaphragm valves come in two designs: weir and nonweir. The weir diaphragm valve has a weir (a dam) located in the body of the valve. Fluid must go over the top of the weir and under the diaphragm in order to exit. There is a large pressure drop across the body of the valve. It uses thick, durable diaphragm materials. The nonweir, or straight-throughdiaphragm valve, has a flexible membrane that extends across the pipe. Pressure drop across the valve depends on the position of
- Relief and Safety Valves
Relief valves have been engineered to respond automatically to sudden increases of pressure in liquid services. A relief valve is designed to open at a predetermined pressure. In a relief valve, a disc is held in place by a spring that does not open until the system pressure exceeds operating limits. Tremendous pressures can be generated in process units.
Relief valves are designed for pressurized liquid service. They do not respond well in gas service. Relief valves are designed to open slowly. This is a poor feature for gas service. Another reason relief valves are not used in gas service is because of the damage high-velocity gas would do to the seats and disc. Steam cut and wire drawn are two terms applied to damaged trim. Relief valves vary in design and style. Relief valves have a specific amount of travel, or lift, which varies from fractions of an inch to several inches. When a relief valve is lifted until it completely compresses the spring, it is said to be in “the fully open position.” The difference between the initial lift-off pressure and the pressure of the fully open position is called accumulation.
The two advantages of relief valves are that they will reseat as soon as the pressure drops below the relief pressure and the spring tension is adjustable.
Safety valves are considered to be a process system’s last line of defense. They are designed to respond quickly to excess vapor, or gas, pressures. When a system overpressures, safety valves respond to allow excess pressure to be vented to the flare header or to the atmosphere. This venting prevents damage to equipment and personnel. This type of valve is very similar in design to a relief valve. The three major differences between a relief and a safety valve are liquid versus gas service, the pressure response time, and a larger exhaust port. Relief valves are designed to lift slowly, whereas safety valves tend to pop off. Because the exhaust port is much larger in a safety valve, it can release more flow at much lower velocities. This feature keeps the trim from being damaged.
- Automatic Valves
The chemical processing industry uses a complex network of automated instrument systems to control its processes. The smallest unit in this network is called a control loop. Control loops usually have a sensing device, a transmitter, a controller, a transducer, and an automatic valve. Because automatic valves can be controlled from remote locations, they are invaluable in processing.
The most common type of automated valve is a globe valve because of its versatile, on/off or throttling capability. Control loops use on/off or throttlingtype valves to regulate the flow of fluid into and out of a system. Automatic valves can be used to control pressure, temperature, flow, or level.
Automatic valves are categorized as either control valves or spring- or weight-operated valves. Control valves can be air operated, electrically operated, or hydraulically operated. Spring- or weight-operated valves hold the flow-control element in place until pressure from under the disc grows strong enough to lift the element from the seat; a check valve would fall into this category.
Any type of valves can be automated by the installation of a device known as an actuator. The actuator controls the position of the flow-control element by moving and controlling the position of the valve stem. Actuators come in three basic designs: pneumatically (air) operated, electrically operated, and hydraulically operated.
1-ENGINEERING DATA BOOK by Gas Processors Suppliers Association
2-Process Technology - Equipment and Systems by Charles E. Thomas