Maintenance Fundamentals Episode 2 part 5 docx

Operating Methods Operating methods for control valves, which are designed to control or direct gas and liquid flow through process systems or fluid-power circuits, range from manual to

Repeatability Repeatability is perhaps the most important performance criterion

of a process-control valve This is especially true in applications in which precise flow or pressure control is needed for optimum performance of the process system

New process-control valves generally provide the repeatability required How-ever, proper maintenance and periodic calibration of the valves and their actu-ators are required to ensure long-term performance This is especially true for valves that use mechanical linkages as part of the actuator assembly

Installation

Process-control valves cannot tolerate solids, especially abrasives, in the gas or liquid stream In applications in which high concentrations of particulates are present, valves tend to experience chronic leakage or seal problems because the

Figure 13.8 High-torque electric motors can be used as actuators

particulate matter prevents the ball, disk, or gate from completely closing against the stationary surface

Simply installing a valve with the same inlet and discharge size as the piping used

in the process is not acceptable In most cases, the valve must be larger than the piping to compensate for flow restrictions within the valve

Operating Methods

Operating methods for control valves, which are designed to control or direct gas and liquid flow through process systems or fluid-power circuits, range from manual to remote, automatic operation The key parameters that govern the operation of valves are the speed of the control movement and the impact of speed on the system This is especially important in process systems

Hydraulic hammer, or the shock wave generated by the rapid change in the flow rate of liquids within a pipe or vessel, has a serious and negative impact on all components of the process system For example, instantaneously closing a large flow-control valve may generate in excess of 3 million foot-pounds of force on the entire system upstream of the valve This shock wave can cause catastrophic failure of upstream valves, pumps, welds, and other system components Changes in flow rate, pressure, direction, and other controllable variables must

be gradual enough to permit a smooth transition Abrupt changes in valve position should be avoided Neither the valve installation nor the control mech-anism should permit complete shut off, referred to as deadheading, of any circuit

in a process system

Restricted flow forces system components, such as pumps, to operate outside of their performance envelope This reduces equipment reliability and sets the stage for catastrophic failure or abnormal system performance In applications in which radical changes in flow are required for normal system operation, control valves should be configured to provide an adequate bypass for surplus flow to protect the system

For example, systems that must have close control of flow should use two propor-tioning valves that act in tandem to maintain a balanced hydraulic or aerodynamic system The primary or master valve should control flow to the downstream process The second valve, slaved to the master, should divert excess flow to a bypass loop This master-slave approach ensures that the pumps and other up-stream system components are permitted to operate well within their operating envelopes

Fluid power control valves are used on pneumatic and hydraulic systems or circuits

Configuration

The configuration of fluid power control valves varies with their intended appli-cation The more common configurations include one-way, two-way, three-way, and four-way

One-Way

One-way valves are typically used for flow and pressure control in fluid-power circuits (Figure 13.9) Flow-control valves regulate the flow of hydraulic fluid or gases in these systems Pressure-control valves, in the form of regulators or relief valves, control the amount of pressure transmitted downstream from the valve

In most cases, the types of valves used for flow control are smaller versions of the types of valves used in process control These include ball, gate, globe, and butterfly valves

Pressure-control valves have a third port to vent excess pressure and prevent

it from affecting the downstream piping The bypass or exhaust port has

an internal flow-control device, such as a diaphragm or piston, that opens

at predetermined setpoints to permit the excess pressure to bypass the valve’s primary discharge In pneumatic circuits, the bypass port vents to the

Figure 13.9 One-way fluid-power valve

atmosphere In hydraulic circuits, it must be connected to a piping system that returns to the hydraulic reservoir

Two-Way

A two-way valve has two functional flow-control ports A two-way, sliding-spool directional control valve is shown in Figure 13.10 As the spool moves back and forth, it either allows fluid to flow through the valve or prevents it from flowing

In the open position, the fluid enters the inlet port, flows around the shaft of the spool, and flows through the outlet port Because the forces in the cylinder are equal when the valve is open, the spool cannot move back and forth In the closed position, one of the spool’s pistons simply blocks the inlet port, which prevents flow through the valve

A number of features common to most sliding-spool valves are shown in Figure 13.10 The small ports at either end of the valve housing provide a path for fluid that leaks past the spool to flow to the reservoir This prevents pressure from building up against the ends of the pistons, which would hinder the movement of the spool When these valves become worn, they may lose balance because of greater leakage on one side of the spool than on the other This can cause the spool to stick

as it attempts to move back and forth Therefore, small grooves are machined around the sliding surface of the piston In hydraulic valves, leaking liquid encircles the piston, keeping the contacting surfaces lubricated and centered

Three-way

Three-way valves contain a pressure port, cylinder port, and return or exhaust port (Figure 13.11) The three-way directional control valve is designed to operate an actuating unit in one direction It is returned to its original position either by a spring or the load on the actuating unit

Four-way

Most actuating devices require system pressure to operate in two directions The four-way directional control valve, which contains four ports, is used to control

IN

OPEN

CLOSED IN

Figure 13.10 Two-way, fluid-power valve

the operation of such devices (Figure 13.12) The four-way valve also is used in some systems to control the operation of other valves It is one of the most widely used directional-control valves in fluid-power systems

The typical four-way directional control valve has four ports: pressure port, return port, and two cylinder or work (output) ports The pressure port is connected to the main system-pressure line, and the return port is connected to the reservoir return line The two outputs are connected to the actuating unit

Performance

The criteria that determine performance of fluid-power valves are similar to those for process-control valves As with process-control valves, fluid-power valves also must be selected based on their intended application and function

Installation

When installing fluid power control valves, piping connections are made either directly to the valve body or to a manifold attached to the valve’s base Care

Figure 13.11 Three-way, fluid-power valve

should be taken to ensure that piping is connected to the proper valve port The schematic diagram that is affixed to the valve body will indicate the proper piping arrangement as well as the designed operation of the valve In addition, the ports on most fluid-power valves are generally clearly marked to indicate their intended function

In hydraulic circuits, the return or common ports should be connected to a return line that directly connects the valve to the reservoir tank This return line should not need a pressure-control device but should have a check valve to prevent reverse flow of the hydraulic fluid

Pneumatic circuits may be vented directly to atmosphere A return line can be used to reduce noise or any adverse effect that locally vented compressed air might have on the area

Operating Methods

The function and proper operation of a fluid-power valve are relatively simple Most of these valves have a schematic diagram affixed to the body that clearly explains how to operate the valve

Figure 13.12 Four-way, fluid-power valves

Figure 13.13 is a schematic of a two-position, cam-operated valve The primary actuator, or cam, is positioned on the left of the schematic and any secondary actuators are on the right In this example, the secondary actuator consists of a spring return and a spring-compensated limit switch The schematic indicates that when the valve is in the neutral position (right box), flow is directed from the inlet (P) to work port A When the cam is depressed, the flow momentarily shifts

to work port B The secondary actuator, or spring, automatically returns the valve to its neutral position when the cam returns to its extended position In these schematics, T indicates the return connection to the reservoir

Figure 13.14 illustrates a typical schematic of a two-position and three-position directional control valve The boxes contain flow direction arrows that indicate

Figure 13.13 Schematic for a cam-operated, two-position valve

the flow path in each of the positions The schematics do not include the actuators used to activate or shift the valves between positions

In a two-position valve, the flow path is always directed to one of the work ports (A or B) In a three-position valve, a third or neutral position is added In this figure, a type 2 center position is used In the neutral position, all ports are blocked and no flow through the valve is possible

Figure 13.15 is the schematic for the center or neutral position of three-position directional control valves Special attention should be given to the type of center position that is used in a hydraulic control valve When type 2, 3, and 6 (see Figure 13.15) are used, the upstream side of the valve must have a relief or bypass valve installed Since the pressure port is blocked, the valve cannot relieve pressure on the upstream side of the valve The type 4 center position, called a motor spool, permits the full pressure and volume on the upstream side of the valve to flow directly to the return line and storage reservoir This is the recommended center position for most hydraulic circuits

The schematic affixed to the valve includes the primary and secondary actuators used to control the valve Figure 13.16 provides the schematics for three actuator-controlled valves, as follows: (1) double-solenoid, spring-centered, three-position valve; solenoid-operated, spring-return, two-position valve; double-solenoid, detented, two-position valve

The top schematic represents a double-solenoid, spring-centered, three-position valve When neither of the two solenoids is energized, the double springs ensure

A

P 2−Position Valve

P

A

A

P

3−Position Valve

Figure 13.14 Schematic of two-position and three-position valves

P

Type 0

Type 1 Type 2 T

B

Figure 13.15 Schematic for center or neutral configurations of three-position valves

A (1)

(2)

(3)

Figure 13.16 Actuator-controlled valve schematics

that the valve is in its center or neutral position In this example, a type 0 (see Figure 13.15) configuration is used This neutral-position configuration equalizes the pressure through the valve Since the pressure port is open to both work ports and the return line, pressure is equalized throughout the system When the left or primary solenoid is energized, the valve shifts to the left-hand position and directs pressure to work port B In this position, fluid in the A-side of the circuit returns to the reservoir As soon as the solenoid is de-energized, the valve shifts back to the neutral or center position When the secondary (i.e., right) solenoid is energized, the valve redirects flow to port A, and port B returns fluid to the reservoir

The middle schematic represents a solenoid-operated, spring-return, two-position valve Unless the solenoid is energized, the pressure port (P) is connected to work port A While the solenoid is energized, flow is redirected to work port B The spring return ensures that the valve is in its neutral (i.e., right) position when the solenoid is de-energized

The bottom schematic represents a double-solenoid, detented, two-position valve The solenoids are used to shift the valve between its two positions

A secondary device, called a detent, is used to hold the valve in its last position until the alternate solenoid is energized Detent configuration varies with the valve type and manufacturer However, all configurations prevent the valve’s control device from moving until a strong force, such as that provided by the solenoid, overcomes its locking force

Actuators

As with process-control valves, actuators used to control fluid-power valves have

a fundamental influence on performance The actuators must provide positive, real-time response to control inputs The primary types of actuators used to control fluid-power valves are mechanical, pilot, and solenoid

Mechanical The use of manually controlled mechanical valves is limited in both pneumatic and hydraulic circuits Generally, this type of actuator is used only on isolation valves that are activated when the circuit or fluid-power system is shut down for repair or when direct operator input is required to operate one of the system components

Manual control devices (e.g., levers, cams, or palm buttons) can be used as the primary actuator on most fluid-power control valves Normally, these actuators are used in conjunction with a secondary actuator, such a spring return or detent,

to ensure proper operation of the control valve and its circuit

Spring returns are used in applications in which the valve is designed to stay open

or shut only when the operator holds the manual actuator in a particular position When the operator releases the manual control, the spring returns the valve to the neutral position

Valves with a detented secondary actuator are designed to remain in the last position selected by the operator until manually moved to another position

A detent actuator is simply a notched device that locks the valve in one of several pre-selected positions When the operator applies force to the primary actuator, the valve shifts out of the detent and moves freely until the next detent is reached Pilot Although a variety of pilot actuators are used to control fluid-power valves, they all work on the same basic principle A secondary source of fluid

or gas pressure is applied to one side of a sealing device, such as a piston or diaphragm As long as this secondary pressure remains within pre-selected limits, the sealing device prevents the control valve’s flow-control mechanism (i.e., spool

or poppet) from moving However, if the pressure falls outside of the pre-selected window, the actuator shifts and forces the valve’s primary mechanism to move to another position

This type of actuator can be used to sequence the operation of several control valves or operations performed by the fluid-power circuit For example, a pilot-operated valve is used to sequence the retraction of an airplane’s landing gear The doors that conceal the landing gear when retracted cannot close until the gear is fully retracted A pilot-operated valve senses the hydraulic pressure in the gear-retraction circuit When the hydraulic pressure reaches a pre-selected point that indicates that the gear is fully retracted, the pilot-actuated valve triggers the closure circuit for the wheel-well doors

Solenoid Solenoid valves are widely used as actuators for fluid-power systems This type of actuator consists of a coil that generates an electrical field when energized The magnetic forces generated by this field force a plunger that is attached to the main valve’s control mechanism to move within the coil This movement changes the position of the main valve

In some applications, the mechanical force generated by the solenoid coil is not sufficient to move the main valve’s control mechanism When this occurs, the solenoid actuator is used in conjunction with a pilot actuator The solenoid coil opens the pilot port, which uses system pressure to shift the main valve Solenoid actuators are always used with a secondary actuator to provide positive control of the main valve Because of heat build up, solenoid actuators must be limited to short-duration activation A brief burst of electrical energy is