2. VALVES

VALVES

Control valves are the most commonly used actuators or final control elements in process industries. They manipulate the flowing fluids to keep the variables being controlled in the desired positions. A control valve is known as the final control element because it is the element that ultimately manipulates the value of the variable in the control process. It is defined as a mechanism that alters the value of the variable being manipulated in response to the output signal from a controller, whether automatic, manual, or by direct human action. It is the element that implements the decision of the controllers. Controllers can be set in either automatic or manual mode control. A crosssectional diagram of a typical pneumatic control valve is shown in Fig. 2-1. The purpose of the valve is to restrict the flow of process fluid through the pipe that can be seen at the very bottom of the figure. The valve plug is attached rigidly to a stem that is attached to a diaphragm in an air pressure chamber in the actuator section at the top of the valve. When compressed air is applied, the diaphragm moves up and the valve opens. The spring is compressed at the same time. The valve illustrated in Fig. 2-1 is a fail-closed type of valve because when the air pressure is reduced, the spring forces the valve to close.

A control valve has three basic components:

1.      Actuator. Most actuators are pneumatic. Usually, an actuator works with the help of a diaphragm and instrument air. This is the device that positions the throttling elemen (i.e., the valve plug inside the valve body).

2.      Valve body subassembly. This is the part where the valve plug, valve seats, and valve casing are located. The valve body and the valve plug differ in geometry and material construction. The combined body and plug geometry determines the flow properties of the valve. There are through-flow, blending, and stream-splitting types of configurations. Similarly, valve seats also differ in construction. There are conventional and contoured valve seat types with parabolic and quick-opening plugs whose internals can be inspected only during servicing.

3.      Accessories. These include positioners, I/P (currentto- pressure) transducers, and position sensors.

In the process industries, hundreds or even thousands of control loops are in use  produce marketable end products. Many of these valves are housed in an attractive fashion, as shown in Fig. 2-2. Typically, a control loop consists of three major elements: a sensor and transmitter, a controller, and a control valve. A feedback control loop is shown in Fig. 2-3. The control loop is a closed system consisting of selected instruments that work together as a unit with the single objective of controlling an identified variable. A loop consists of a sensor that can be an orifice, a thermocouple.

Figure 2-1 Cross-sectional diagram of a pneumatic control valve.

Figure 2-2 Assembly of control valves.

or a venture meter; a transmitter, which can be either a differential pressure electropneumatic or pneumatic transmitter; an indicator, which can be a pressure gauge, a level gauge, or a temperature gauge; and a transducer, which converts the signal reported from the form manipulated to a form understandable to the controller. The controller makes the decision and sends it to an I/P converter that converts the electric signal to a pneumatic signal and sends it to the final control instrument, or a positioner that gives proportional positional action to the valve stem so as to position the plug correctly in the valve body and, finally, regulates the flow (Fig 2-3).

2.1 TYPES OF CONTROL VALVES

A variety of types of control valves are used in all sectors of the process industries, depending on the suitability of a valve for a process. Two general types of control valves are based on their motion: linear-motion valves and rotarymotion valves.

2.1.1 Linear-Motion Control Valves Linear-motion valves have a tortuous flow and low recovery. They can be offered in a variety of special trim designs and can throttle small flow rates. Most linearmotion valves are suitable for high-pressure applications. They are usually flanged or threaded and have separable bonnets. Examples of linear-motion valves are gate valves, diaphragm valves and globe valves.

2.1.1.1 Gate Valves Gate valves are generally used when a straight-line flow of fluid and minimum restriction

Figure 2-3 Components of a typical control loop arranged in a feedback configuration.

are desired. They are so named because the part that either stops or allows flow through the valve acts somewhat like the opening and closing of a gate. When the valve is wide open, it is fully drawn up into the valve, leaving an opening for flow through the valve of the same size as the pipe in which the valve is installed. Therefore, there is little pressure drop or flow restriction through the valve. Gate valves are not usually suitable for throttling purposes because flow control would be difficult, due to the valve design, and the flow of fluid slapping against a partially open gate can cause serious damage to the valve. Gate valves used in steam systems always have flexible gates [26]. The reason is to prevent binding of the gate within the valve when the valve is in the closed position. When steam lines are heated, they will expand, causing some distortion of valve bodies. If a solid gate fits snugly between the seat of the valve in a cold steam system, when the system is heated and pipes elongate, the seats will compress against the gate, wedging the gate between them and clamping the valve shut. This problem is overcome by the use of a flexible gate. This allows the gate to flex as the valve seat compresses it, thus preventing clamping [27].

2.1.1.2 Diaphragm Valves In a diaphragm control valve, operating air from the pilot acts on the valve diaphragm. The substructure that contains the diaphragm is direct acting in some valves and reverse acting in others. If the substructure is direct acting, the operating air pressure from the control pilot is applied to the top of the valve diaphragm. If the substructure is reverse acting, the operating air pressure from the pilot is applied to the underside of the valve diaphragm [26]. Diaphragm valves are lined to pressures of approximately 50 psi. They are used for fluids containing suspended solids and can be installed in any position. In this valve, the pressure drop is reduced to a negligible quantity. The only maintenance required in this valve is the replacement of the diaphragm, which can be done without removing the valve from the line.

2.1.1.3 Globe Valves These are probably the most common valves in existence. The globe valve derives its name from the globular shape of the valve body. However, positive identification of a globe valve must be made internally because other valve types may also have globular bodies [26]. Globe valve inlet and outlet openings are used extensively throughout the engineering plant and other parts of the ship in a variety of systems. In this type of valve, fluid passes through a restricted opening and changes direction several times. It is used extensively for the regulation of flow.

2.1.2 Rotary-Motion Control Valves

Rotary-motion control valves have a streamlined flow path and high recovery in nature. They have more capacity than that of linear-motion valves. This type of valve has an advantage in handling slurries and abrasives. They are easy to handle because they are flangeless and have an integral bonnet. Rotary-motion valves are designed to have high rangeability. Examples of this type of valve are butterfly valves, ball valves, and plug valves.

2.1.2.1 Butterfly Valves The butterfly valve is used in a variety of systems aboard vessels. These valves can be used effectively in saltwater, lube oil, and freshwater systems [25]. Butterfly valves are light in weight, relatively small, quick acting, provide positive shutoff, and can be used in throttling. This valve has a body, a resilient seat, a butterfly disk, a stem, packing, a notched positioning plate, and a handle. The resilient seat is under compression when it is mounted in the valve body, thus making a seal around the periphery of the disk and both upper and lower points where the stem passes through the seat. Packing is provided to form a positive seal around the stem for added protection in case the seal formed by the seat should become damaged. Butterfly valves are easy to maintain [26]. The resilient seat is held in place by mechanical means, and neither bonding nor cementing is necessary. Because the seat is replaceable, the valve seat does not require lapping, grinding, or machine work.

2.1.2.2 Ball Valves These are stop valves that use a ball to stop or start the flow of fluid [25]. When the valve handle is operated to open the valve, the ball rotates to a point where the hole through the ball is in line with the valve body inlet and outlet. When the valve is shut, which requires only a 90◦ rotation of the hand wheel for most valves, the ball is rotated so that the hole is perpendicular to the flow openings of the valve body, and flow is stopped. Most ball valves are of the quick-acting type, but many are planetary gear operated [26]. This type of gearing allows the use of a relatively small hand wheel and operating force to operate a fairly large valve but increases the valve operating time. Ball valves are normally found in the following systems: desalination, trim and drain, air, hydraulic, and oil transfer. They are used for general service, high-temperature conditions, and slurries.

2.1.2.3 Plug Valves These are quarter-turn valves that controls flow by means of a cylindrical or tapered plug with a hole through the center which can be positioned from open to close by a 90◦ turn. They are used for general services slurries, liquids, vapors, gases, and corrosives [26].

Other types of control valves are used either to control the flow of fluids or to control the pressure of fluids: nonreturn valves and relief valves.

2.1.3 Nonreturn Valves

Also known as reflux valves or check valves, these valves possess automatic devices that allow water to flow in one direction only (Fig. 2-4). They are made of brass or gun metal. Usually, a valve is pivoted at one end and can rest on a projection on the other end. This valve is provided in the pipeline that draws fluid from the pump [27]. When the pump is operated, the valve is open and the fluid flows through the pipe. But when the pump is suddenly stopped or fails due to a power failure, the valve is closed automatically and the fluid is prevented from returning to the pump [28].

2.1.4 Relief Valves

Relief valves are also known as pressure relief valves, cutoff valves, or safety valves [25]. These are automatic

Figure 2-4 Nonreturn valve.

valves used on system lines and equipment to prevent overpressurization. Relief valves normally have a spring, and the power of the spring is adjusted such that a valve always remains in the closed position up to some permissible fluid pressure in the pipeline. When the pressure of the fluid suddenly exceeds the permissible pressure, the valve opens (lifts) automatically and the excess pressure is released instantaneously and then resets (shuts). Thus, the pipeline is protected from bursting. These valves areprovided along the pipeline at points where the pressureis likely to increase. Other types of relief valves are highpressure air safety relief valves (PRVs) and bleed air surge relief valves. Both are designed to open completely at a specified lift pressure and to remain open until a specific reset pressure is reached, at which time they shut [25]. However, the PRV is also the one piece of equipment that we hope never needs to operate. Because the PRV is the last line of defense against the catastrophic failure of a pressurized system, it must be maintained in “like new” condition if it is to provide the confidence necessary to operate a pressurized system.