Industrial Machinery Repair Part Episode 1 Part 9 pot

184 Control Valves Figure 9.4 Butterfly valves provide almost unrestricted flow Straight-flow Angle-flow Cross-flow Figure 9.5 Three globe valve configurations: straight-flow, angle-flow, and cross-flow Control Valves 185 View A View B Figure 9.6 Globe valve When the valve is open, as illustrated in View B of Figure 9.6, the fluid flows through the space between the edge of the disk and the seat. Since the fluid flow is equal on all sides of the center of support when the valve is open, there is no unbalanced pressure on the disk to cause uneven wear. The rate at which fluid flows through the valve is regulated by the position of the disk in relation to the valve seat. The globe valve should never be jammed in the open position. After a valve is fully opened, the handwheel or actuating handle should be closed approx- imately one-half turn. If this is not done, the valve may seize in the open position making it difficult, if not impossible, to close the valve. Many valves are damaged in the manner. Another reason to partially close a globe valve is because it can be difficult to tell if the valve is open or closed. If jammed in the open position, the stem can be damaged or broken by someone who thinks the valve is closed. Performance Process-control valves have few measurable criteria that can be used to deter- mine their performance. Obviously, the valve must provide a positive seal when closed. In addition, it must provide a relatively laminar flow with min- imum pressure drop in the fully open position. When evaluating valves, the following criteria should be considered: capacity rating, flow characteristics, pressure drop, and response characteristics. Capacity Rating The primary selection criterion of a control valve is its capacity rating. Each type of valve is available in a variety of sizes to handle most typical 186 Control Valves process-flow rates. However, proper size selection is critical to the per- formance characteristics of the valve and the system where it is installed. A valve’s capacity must accommodate variations in viscosity, temperature, flow rates, and upstream pressure. Flow Characteristics The internal design of process-control valves has a direct impact on the flow characteristics of the gas or liquid flowing through the valve. A fully open butterfly or gate valve provides a relatively straight, obstruction-free flow path. As a result, the product should not be affected. Pressure Drop Control-valve configuration impacts the resistance to flow through the valve. The amount of resistance, or pressure drop, will vary greatly, depending on type, size, and position of the valve’s flow-control device (i.e. ball, gate, disk). Pressure-drop formulas can be obtained for all common valve types from several sources (e.g., Crane, Technical Paper No. 410). Response Characteristics With the exception of simple, manually controlled shutoff valves, process- control valves are generally used to control the volume and pressure of gases or liquids within a process system. In most applications, valves are controlled from a remote location through the use of pneumatic, hydraulic, or electronic actuators. Actuators are used to position the gate, ball, or disk that starts, stops, directs, or proportions the flow of gas or liquid through the valve. Therefore, the response characteristics of a valve are determined, in part, by the actuator. Three factors critical to proper valve operation are: response time, length of travel, and repeatability. Response Time Response time is the total time required for a valve to open or close to a specific set-point position. These positions are fully open, fully closed, and any position in between. The selection and maintenance of the actuator used to control process-control valves have a major impact on response time. Length of Travel The valve’s flow-control device (i.e., gate, ball, or disk) must travel some distance when going from one set point to another. With a manually operated valve, this is a relatively simple operation. The operator moves Control Valves 187 the stem lever or handwheel until the desired position is reached. The only reasons a manually controlled valve will not position properly are mechani- cal wear or looseness between the lever or handwheel and the disk, ball, or gate. For remotely controlled valves, however, there are other variables that directly impact valve travel. These variables depend on the type of actuator that is used. There are three major types of actuators: pneumatic, hydraulic, and electronic. Pneumatic Actuators Pneumatic actuators, including diaphragms, air motors, and cylinders, are suitable for simple on-off valve applications. As long as there is enough air volume and pressure to activate the actuator, the valve can be repositioned over its full length of travel. However, when the air supply required to power the actuator is inadequate or the process-system pressure is too great, the actuator’s ability to operate the valve properly is severely reduced. A pneumatic (i.e., compressed air-driven) actuator is shown in Figure 9.7. This type is not suited for precision flow-control applications, because the compressibility of air prevents it from providing smooth, accurate valve positioning. Hydraulic Actuators Hydraulic (i.e., fluid-driven) actuators, also illustrated in Figure 9.7, can provide a positive means of controlling process valves in most applica- tions. Properly installed and maintained, this type of actuator can provide Pneumatic or hydraulic cylinder actuator Figure 9.7 Pneumatic or hydraulic cylinders are used as actuators 188 Control Valves Motor actuato r Figure 9.8 High-torque electric motors can be used as actuators accurate, repeatable positioning of the control valve over its full range of travel. Electronic Actuators Some control valves use high-torque electric motors as their actuator (see Figure 9.8). If the motors are properly sized and their control circuits are maintained, this type of actuator can provide reliable, positive control over the full range of travel. Repeatability Repeatability is, perhaps, the most important performance criterion of a process-control valve. This is especially true in applications where precise flow or pressure control is needed for optimum performance of the process system. New process-control valves generally provide the repeatability required. However, proper maintenance and periodic calibration of the valves and their actuators are required to ensure long-term performance. This is Control Valves 189 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 where high concentrations of par- ticulates are present, valves tend to experience chronic leakage or seal problems because the 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 three 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 con- trol mechanism should permit complete shutoff, referred to as deadheading, of any circuit in a process system. Restricted flow forces system components, such as pumps, to operate out- side of their performance envelope. This reduces equipment reliability and sets the stage for catastrophic failure or abnormal system performance. In applications where radical changes in flow are required for normal system operation, control valves should be configured to provide an adequate bypass for surplus flow in order to protect the system. 190 Control Valves Spring Poppet Body No flow Free flow In Out Figure 9.9 One-way, fluid-power valve For example, systems that must have close control of flow should use two proportioning 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 upstream system components are permitted to operate well within their operating envelope. Fluid Power 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 application. 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 (see Figure 9.9). Flow-control valves regulate the flow of Control Valves 191 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 set-points to permit the excess pressure to bypass the valve’s primary discharge. In pneumatic circuits, the bypass port vents to the 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 9.10. 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 through the outlet port. Because the forces in the cylinder are equal when 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 9.10. 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 In In OutOut Open Closed Figure 9.10 Two-way, fluid-power valve 192 Control Valves #1 #1 #1 #2 #3A #2 #3B #2 #3C Figure 9.11 Three-way, fluid-power valve 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 (see Figure 9.11). 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 in order to operate in two directions. The four-way directional control valve, which contains four ports, is used to control the operation of such devices (see Figure 9.12). 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. Control Valves 193 Air introduced through this passage pushes against the piston which shifts the spool to the right Centering washers Springs push against centering washers to center the spool when no air is applied Pistons seal the air chamber from the h y draulic chamber Figure 9.12 Four-way, fluid-power valve 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 con- nected 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 must also 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 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 indi- cate the proper piping arrangement, as well as the designed operation of [...]... the product Table 10 .1 Approximate capacities of chain conveyors Flight width and depth (inches) Quantity of material (Ft3 /Ft) Approximate capacity (short tons/ hour) Lump size single strand (inches) Lump size dual strand (inches) 12 × 6 15 × 6 18 × 6 24 × 8 30 × 10 36 × 12 0.40 0. 49 0.56 1. 16 1. 60 2.40 60 73 84 17 4 240 360 31. 5 41. 5 5.0 4.0 5.0 6.0 10 .0 14 .0 16 .0 Conveyors 207 Table 10 .2 Capacity correction... three-position valves A B A B A B P T Type 0 P T Type 1 P T Type 2 A A A B P T Type 3 B P T Type 4 B P T Type 6 Figure 9 .15 Schematic for center or neutral configurations of three-position valves Control Valves 19 7 A B A B A B P T P T (1) P T A B A B P T P T (2) A B A B P T P T (3) Figure 9 .16 Actuator-controlled valve schematics The top schematic, in Figure 9 .16 , represents a double-solenoid, springcentered,... actuators used to control the valve Figure 9 .16 provides the schematics for three actuator-controlled valves: 1 Double-solenoid, spring-centered, three-position valve 2 Solenoid-operated, spring-return, two-position valve 3 Double-solenoid, detented, two-position valve 19 6 Control Valves A B P A B T P 2-Position valve B P A T T A B A B P T P T 3-Position valve Figure 9 .14 Schematic of two-position and three-position... Figure 9 .16 , 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  19 8 Control Valves The bottom schematic, in Figure 9 .16 , represents... devices that are an integral part of the chain to drag the conveyed material through the ductwork Performance Data used to determine a chain conveyor’s capacity and the size of material that can be conveyed are presented in Table 10 .1 Note that these data are for level conveyors When inclined, capacity data obtained from Table 10 .1 must be multiplied by the factors provided in Table 10 .2 Installation The... and no flow through the valve is possible Figure 9 .15 is the schematic for the center or neutral position of threeposition 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 9 .15 ) are used, the upstream side of the valve must Control Valves 19 5 “P” “T” Roller (Cam follower) A Push rod trips... will handle material on inclines up to 35 degrees Capacity is reduced in inclined applications and Table 10 .3 provides the approximate reduction in capacity for various inclines Table 10 .3 Screw conveyor capacity reductions for inclined applications Inclination, degrees Reduction in capacity, % 10 10 15 26 20 45 25 58 30 70 35 78 ... Troubleshooting Although there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure Table 9 .1 lists the more common causes of these failures Table 9 .1 Common failure modes of control valves • • • Line pressure too high Manually actuated Opens/closes too slow Galling • • • • Mechanical damage • Opens/closes too fast • •... the next detent is reached Pilot Although there are a variety of pilot actuators used to control fluid-power valves, they all work on the same basic principle A secondary source of fluid Control Valves 19 9 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 preselected limits, the sealing device prevents the control... to open Valve fails to close THE PROBLEM • Not packed properly • Packed box too loose • Packing too tight • • Threads/lever damaged • • Valve stem bound • • Valve undersized • • Control Valves 2 01 Table 9 .1 continued • Galling • • Mechanical damage (seals, seat) • • • Pilot port blocked/plugged • • • Pilot actuated • Pilot pressure too high • Pilot pressure too low • Corrosion • • • Dirt/debris trapped . the spool than on In In OutOut Open Closed Figure 9 .10 Two-way, fluid-power valve 19 2 Control Valves #1 #1 #1 #2 #3A #2 #3B #2 #3C Figure 9 .11 Three-way, fluid-power valve the other. This can cause. 4 Type 6 TT Figure 9 .15 Schematic for center or neutral configurations of three-position valves Control Valves 19 7 A (1) (2) (3) PPPT PPTT TT AAB A AABB PPTT ABB BB Figure 9 .16 Actuator-controlled. two-position valve 19 6 Control Valves AB AAABBB AB PT PPPTTT P 2-Position valve 3-Position valve T Figure 9 .14 Schematic of two-position and three-position valves AA ABB B PPPT Type 0 Type 1 Type 2 TT AA