These valves may also be described by the number of positions provided. Typically their description gives the number of ports and number of positions as figures separated by a stroke-e.g., a 4/2 valve is a four-port, two-position valve. A five-way, two-position valve (5/2) is similar to a four-way valve except that an additional exhaust port is provided. Thus when either of the output ports is switched to exhaust, it does so through a separate exhaust port. This is of particular advantage when the stroke speed of a double-acting actuator must be controlled in both directions. Independent flow control valves may be connected to each exhaust port.

Another useful valve option is the four-way, three-position valve. The function is similar in porting to a four-way, two-position valve except that an additional position is available with all ports blocked. No flow is possible through the valve in either direction in the normal position. A typical application is the control of a double-acting pneumatic actuator that must be held in a given position upon loss of electric or air power. Another version of the 4/3 valve is to have the normal position provide air pressure to both sides of the actuator and the exhaust closed. This holds the actuator in position by pressurizing both sides of the cylinder.

Standard Valve Symbols

Symbols used for designating pilot valves are conventionally in the form of adjacent squares with each square representing one position of the valve. This means that a two-position valve would have two squares and a three-position valve would have three. Flow paths within the valve are indicated by lines and arrows (see Fig. 6.22).

Standard Pilot Vale Symbols

The action of a pilot valve in an electric control circuit is initiated by a solenoid operator. The solenoid operator is composed of a coil plunger, and spring. The coil is a doughnut-shaped electromagnet. When voltage is applied to the coil power leads, the resulting current generates a flux field. This flux field acts like a magnet. A spring and plunger assembly is fitted through the core of the coil. The magnetic field of the coil pulls the plunger inward when the coil is energized, and the spring forces the plunger back to its normal, or original, position when the coil is deenergized. Other means of actuating a pilot valve are manually, by a push button or lever; electrically, by single or dual coils; and pneumatically, by a piston or diaphragm assembly.

Many pneumatic actuators are manufactured with an integral air manifold and internal porting that allow the pilot valve to be mounted in modular fashion directly onto the actuator (see Fig. 6.23). This eliminates the cumbersome adaptation of traditional solenoid valves with nipples and tubing, making a neater, more dependable package.

When selecting a solenoid-operated pilot valve for an actuator, it is important to specify that it be energized to open or energized to close. This means that the pilot valve should be ported in such a way that when voltage is applied to it (energized), it will pressurize the actuator, which in turn will operate the process valve to the open or closed position. With some integrally mounted solenoid valves, this section is satisfied by mounting the valve in one position or 18O^o out of position.

Fig. 6.23 Direct mount valve accepts electric single to pilot pneumatic actuator. Courtesy of El-o-matic, Inc.

To increase the speed of actuation a pilot valve with a higher flow coefficient may be used. In some cases the actuator may require modification to match the port size of a larger valve. For slower actuation a variable restrictor (needle valve) may be connected to the exhaust port. Undesirable speed and operation may result if the restriction is connected to the supply pressure port.

Limit Switches

For a pneumatic actuator, the term limit switch may be a misnomer. The term more properly applies to electric rotary actuators that are fitted with limit switches to interrupt the power to the motor when the actuator has reached its desired limit of rotation. As a functional term, position-indicating switch is more properly applied to limit switches when they are used with pneumatic actuators (see Fig. 6.24).

Fig. 6.24 Top-mounted rotary switch on a pneumatic actuator. Courtesy of El-o-matic, Inc.

Indeed, a switch fitted to a pneumatic actuator does not limit its travel but instead indicates (through switches) when the actuator has reached, or has not reached, a specified point of rotation.

Also referred to as a switch box, the position-indicating switch encloses the switch elements, cams, and terminal strip and has a rotating input shaft that is fitted to the auxiliary shaft of the actuator to pick up rotary motion. The switch housing is composed of an input shaft that externally couples to the actuator's auxiliary drive shaft and is fitted internally with adjustable cams, snap-acting switches that are mounted to align with the cams, and a terminal strip for incoming wiring (see Fig. 6.25). As the actuator cycles, the input shaft of the switch box rotates and the cams actuate the switches. when the switches are used to indicate the limits of the cycle, the cams are adjusted to operate the switch when the desired position is reached.

Figure 6.25 Inside view of switch enclosure shows switches, cams, and terminal strip. Courtesy of El-o-matic, Inc.

Position-indicating switches are used for a variety of applications: light indication (powering indicator lamps on a control panel), system sequence cycling, alarms, electrical interlocking, etc. Some switch enclosures may be fitted with other devices, such as a potentiometer or position transmitter for continuous feedback of the valve's position.

When the switches are connected to signal lights, they should be arranged so that both lights are on in midtravel, with one or the other being extinguished at the ends of travel. This helps the operator avoid being misled by a burned-out lamp.

Switch boxes for pneumatic actuators are often specified by the type and quantity of switches required. Examples of the types of switches available are snap acting, proximity, and pneumatic switches.

Electric limit switch configurations

Electric switches are usually expressed in terms of the number of poles and throws they contain. A pole is a component of the switch that is moved by the switch action to make or break electric contact. The possible electric connections that can be made by a given pole are called throws.

For example, the most elementary switch configuration is shown in Fig. 6.26.

single pole single throw single break

Figure 6.26 Single-pole, single-throw switch (SPST)

There are four configurations of electric limit switches: single-pole, single-throw; single-pole, double-throw; double-pole, single-throw; and double-pole, double-throw.

Single-pole, single-throw (SPST). This switch has one movable component, or a single pole. It also has only one possible connection, or a single throw, for that pole. The configuration of single-pole, single-throw is often abbreviated SPST. It is usually used as an on-off switch.

Single-pole, double-throw (SPDT). Figure 6.27 shows a single-pole, double-throw configuration. This switch still has only one movable component, but it has two possible connections, or throws. It is often found in a switch box where the customer may select between normally open or normally closed contacts. Normally open contacts provide an open circuit when the switch is in a free position, while normally closed contacts provide a closed circuit when the switch is in a free position.

single pole double throw single break

Figure 6.27 Single-pole, double-throw switch (SPDT).

Double-pole, single-throw (DPST). This switch (Fig. 6.28) has two movable components, so it is a double pole. There is only one connection for each pole and, therefore, is a single throw. Note that the key phrase in determining the number of throws is "for each pole." The contacts of this switch are normally closed.

double pole single throw single break

Figure 6.28 Double-pole, single-throw switch (DPST).

Double-pole, double-throw (DPDT). This switch (Fig. 6.29) has two movable components, and, therefore, is a double pole. It also has two connections for each pole, so it is a double throw. This type of switch is often used in a switch box to give the customer an extra set of contacts to wire indicator lights or relays. Almost any switch configuration can be identified by applying the principles outlined here.

double pole double throw single break

Figure 6.29 Double-pole, double-throw switch (DPDT)

Proximity switches

These switches operate when a metallic or magnetic object is brought into close proximity to the switch sensing area. These switches are inherently protected against dust and moisture and some require a power circuit. Two types of proximity switches are the proximity sensor and reed switch.

Proximity sensors. Proximity sensors are switches that operate when a metallic object is brought into close proximity to the sensing face. Most proximity sensors comply with several NEMA ratings. The sensors are protected against dust, moisture, and oil. Internal solid-state circuitry prevents shock and vibration from affecting sensor operation.

Reed switches. Another low-current proximity switch (250 to 500 mA) is the reed switch. Action is initiated when a magnet is placed in the proximity of the sensing area. Reed switches do not require a power supply.

Pneumatic switch

In some instances a pneumatic valve may be used to indicate actuator position. This method is sometimes used in hazardous areas to eliminate electrical equipment. A roller and cam are used to initiate valve action. At the control panel a pressure switch or pneumatic or mechanical sensor will indicate the presence of air pressure and indicate the valve position.

Other methods of position indication

If continuous monitoring of an actuator's position is required, as in modulating or "jogging" applications, a switch box may be fitted with a potentiometer. As the shaft of the switch box rotates, it likewise rotates the input shaft of the pot. The continuously decreasing or increasing resistive signal may then be converted into a valve position at the control panel. When the actuator is located far from the control system, the result may be an unreliable resistive signal due to the inherent resistance of the long wire. In this case a resistance-to-current transducer circuit may be preferred. The circuit board is usually installed in the switch box with the potentiometer and provides a 4- to 20- mA signal to indicate valve position.

Switch boxes designed for use in explosive environments must be able to withstand an internal explosion without igniting the explosive mixture surrounding the switch enclosure, The enclosure is thus designed to withstand the maximum expected internal explosion pressure without damage or excessive distortion and to provide venting for the pressure through channels of such dimensions that gases will be cooled below the ignition temperature before reaching the surrounding atmosphere. Thus the design of an explosion-proof switch enclosure involves careful consideration of housing thickness, cover fit, and tolerances.

Many switch enclosures incorporate multiple construction standards (NEMA I, IV, VII, IX, etc.) to satisfy a wide range of applications.

Transducers. A transducer device that converts one signal type to another. In the case of control instrumentation, a current to pneumatic transducer accepts an analog milliamp control signal from a field instrument and converts it to a proportional pneumatic signal for the positioner. The most common conversions used with control valves are shown below.

For systems being controlled and monitored with electronic instrumentation but with pneumatically actuated control valves, transducing is the most practical method for interfacing the two types of equipment. As an electromechanical device, a transducer must be carefully selected for environmental compatibility, hazardous areas, sensitivity, vibrations, etc.

One drawback of transducers is that it is sometimes difficult to locate them near the positioner, which may then require long runs of wire or pneumatic tubing. To satisfy this, some manufacturers have integrated the transducer into the positioner. These hybrids are known as electro pneumatic positioners.

Standard Instrument signals. Instrument signals are used to interface between various elements in the control process. Information may be transmitted from a sensor to a controller, or a controller to an actuator, etc. Standard instrument signals allow a wide variety of products made by different manufacturers to work together. Common standard instrument signal ranges are shown below. The high end of a standard instrument signal range is usually 5 times the value of the low end. For instance, 20 mA is 5 x 4 mA, 15 lb/in² is 5 x 3 lb/in², etc.

The low end usually does not have a value of zero. This provides a positive method of determining the difference between a device that is indicating the low end of a range and a device that is not functioning. This is known as live zero.

The main exceptions to these conventions are resistance-type inputs which usually have a low end of zero and various values of high ends. Split ranges are usually fractions of standard instrument signals. For example, 3 to 15 lb/in² is often split into 3 to 9 lb/in² and 9 to 15 lb/in², each of which is half of the standard range.

In pneumatic devices, pressure [lb/in² (bar)] is the usual variable for instrument signals. In electric devices, the variable may be current (mA), dc voltage (V dc), or resistance [ohms]. The following table gives instrument signal ranges for pneumatic and electric devices.

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