VALVE
1.1 The Valve
1.1.1 Definition of a Valve
By definition, valves are mechanical devices specifically designed to direct, start, stop, mix, or regulate the flow, pressure, or temperature of a process fluid. Valves can be designed to handle either liquid or gas applications.
By nature of their design, function, and application, valves come in a wide variety of styles, sizes, and pressure classes. The smallest industrial valves can weigh as little as 1 lb (0.45 kg) and fit comfortably in the human hand, while the largest can weigh up to 10 tons (9070 kg) and extend in height to over 24 ft (6.1 m). Industrial process valves can be used in pipeline sizes from 0.5 in [nominal diameter (DN) 15] to beyond 48 in (DN 1200), although over 90 percent of the valves used in process systems are installed in piping that is 4 in (DN 100) and smaller in size. Valves can be used in pressures from vacuum to over 13,000 psi (897 bar). An example of how process valves can vary in size is shown in Fig. 1.1.
Today’s spectrum of available valves extends from simple water faucets to control valves equipped with microprocessors, which provide single-loop control of the process. The most common types in use today are gate, plug, ball, butterfly, check, pressure-relief, and globe
valves.
Valves can be manufactured from a number of materials, with most valves made from steel, iron, plastic, brass, bronze, or a number of special alloys.
1.2 Valve Classification According to Function
1.2.1 Introduction to Function Classifications
By the nature of their design and function in handling process fluids, valves can be categorized into three areas: on–off valves, which handle the function of blocking the flow or allowing it to pass; nonreturn valves, which only allow flow to travel in one direction; and throttling valves, which allow for regulation of the flow at any point between fully open to fully closed. One confusing aspect of defining valves by function is that specific valve-body designs—such as globe, gate, plug, ball, butterfly, and pinch styles—may fit into one, two, or all three classifications.
For example, a plug valve may be used for on–off service, or with the addition of actuation, may be used as a throttling control valve. Another example is the globe-style body, which, depending on its internal design, may be an on–off, nonreturn, or throttling valve. Therefore, the user should be careful when equating a particular valve-body style with a particular classification.
1.2.2 On–Off Valves
Sometimes referred to as block valves, on–off valves are used to start or stop the flow of the medium through the process. Common on–off valves include gate, plug, ball, pressure-relief, and tank-bottom valves (Fig. 1.2). A majority of on–off valves are hand-operated, although they can be automated with the addition of an actuator (Fig. 1.3). On–off valves are commonly used in applications where the flow must be diverted around an area in which maintenance is being performed or where workers must be protected from potential safety hazards.
1.2.2 On–Off Valves
Sometimes referred to as block valves, on–off valves are used to start or stop the flow of the medium through the process. Common on–off valves include gate, plug, ball, pressure-relief, and tank-bottom valves (Fig. 1.2). A majority of on–off valves are hand-operated, although they can be automated with the addition of an actuator (Fig. 1.3). On–off valves are commonly used in applications where the flow must be diverted around an area in which maintenance is being performed or where workers must be protected from potential safety hazards.
They are also helpful in mixing applications where a number of fluids are combined for a predetermined amount of time and when exact measurements are not required. Safety management systems also require automated on–off valves to immediately shut off the system when an emergency situation occurs. Pressure-relief valves are self-actuated on–off valves that open only when a preset pressure is surpassed (Fig. 1.4). Such valves are divided into two families: relief valves and safety valves. Relief valves are used to guard against overpressurization of a liquid service. On the other hand, safety valves are applied in gas applications where overpressurization of the system presents a safety or process hazard and must be vented.
1.2.3 Nonreturn Valves
Nonreturn valves allow the fluid to flow only in the desired direction. The design is such that any flow or pressure in the opposite direction is mechanically restricted from occurring. All check valves are non return valves (Fig. 1.5).Nonreturn valves are used to prevent backflow of fluid, which could damage equipment or upset the process. Such valves are especially useful in protecting a pump in liquid applications or a compressor in gas applications from backflow when the pump or compressor is shut down. Nonreturn valves are also applied in process systems that have varying pressures, which must be kept separate.
1.2.3 Nonreturn Valves
Nonreturn valves allow the fluid to flow only in the desired direction. The design is such that any flow or pressure in the opposite direction is mechanically restricted from occurring. All check valves are non return valves (Fig. 1.5).Nonreturn valves are used to prevent backflow of fluid, which could damage equipment or upset the process. Such valves are especially useful in protecting a pump in liquid applications or a compressor in gas applications from backflow when the pump or compressor is shut down. Nonreturn valves are also applied in process systems that have varying pressures, which must be kept separate.
1.2.4 Throttling Valves
Throttling valves are used to regulate the flow, temperature, or pressure of the service. These valves can move to any position within the stroke of the valve and hold that position, including the full-open or fullclosed positions. Therefore, they can act as on–off valves also. Although many throttling valve designs are provided with a hand-operated
Manual handwheel or lever, some are equipped with actuators or actuation systems, which provide greater thrust and positioning capability, as well as automatic control (Fig. 1.6). Pressure regulators are throttling valves that vary the valve’s position to maintain constant pressure downstream (Fig. 1.7). If the pressure builds downstream, the regulator closes slightly to decrease the pressure. If the pressure decreases downstream, the regulator opens to build pressure. As part of the family of throttling valves, automatic control valves, sometimes referred to simply as control valves, is a term commonly used to describe valves that are capable of varying flow conditions to match the process requirements. To achieve automatic control, these valves are always equipped with actuators. Actuators are designed to receive a command signal and convert it into a specific valve position.
Figure 1.6 Globe control valve with extended bonnet (left) with quarter- turn blocking ball valves (right and bottom) in refining service. (Courtesy of Valtek International)
Figure 1.7 Preassure Regulator(Courtesy of Valtek International)
Using an outside power source (air, electric, or hydraulic), which matches the performance needed for that specific moment.
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