CONTROL VALVES MAINTENANCE
Control valves can be grouped into two major classifications: process and fluid power. Process valves control the flow of gases and liquids through a process system. Fluid-power valves control pneumatic or hydraulic systems.
PROCESS VALVES
Process-control valves are available in a variety of sizes, configurations, and materials of construction. Generally, this type of valve is classified by its internal
The device used to control flow through a valve varies with its intended function. The more common types are ball, gate, butterfly, and globe valves.
Ball valves
Ball valves are simple shut-off devices that use a ball to stop and start the flow of fluid downstream of the valve. As the valve stem turns to the open position, the ball rotates to a point where part or the entire hole machined through the ball is in line with the valve-body inlet and outlet. This allows fluid
to pass through the valve. When the ball rotates so that the hole is perpendicular to the flow path, the flow stops.
Gate valves
Gate valves are used when straight line, laminar fluid flow, and minimum restrictions are needed. These valves use a wedge-shaped sliding plate in the valve body to stop, throttle, or permit full flow of fluids through the valve. When the valve is wide open, the gate is completely inside the valve bonnet. This leaves the flow passage through the valve fully open with no flow restrictions, allowing
little or no pressure drop through the valve.
The butterfly valve has a disk-shaped element that rotates about a central shaft or stem. When the valve is closed, the disk face is across the pipe and blocks the flow. Depending on the type of butterfly valve, the seat may consist of a bonded resilient liner, a mechanically fastened resilient liner, an insert-type reinforced resilient liner, or an integral metal seat with an O-ring inserted around
the edge of the disk.
The globe valve gets its name from the shape of the valve body, although other types of valves also may have globular-shaped bodies. shows three configurations of this type of valve: straight-flow, angle-flow, and cross-flow. A disk attached to the valve stem controls flow in a globe valve. Turning the valve stem until the disk is seated, The edge of the disk and the seat are very accurately machined
to form a tight seal. It is important for globe valves to be installed with the pressure against the disk face to protect the stem packing from system pressure when the valve is shut.
Performance
Process-control valves have few measurable criteria that can be used to determine their performance. Obviously, the valve must provide a positive seal when closed. In addition, it must provide a relatively laminar flow with minimum 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.
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 process-flow rates. However, proper size selection is critical to the performance characteristics of the valve and the system in which it is installed. A valve’s capacity must accommodate variations in viscosity, temperature, flow rates, and upstream pressure.
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 process-flow rates. However, proper size selection is critical to the performance characteristics of the valve and the system in which 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
Response Characteristics
With the exception of simple, manually controlled shut off 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 setpoint to another. With a manually operated valve, this is a relatively simple operation. The operator moves the stem lever or hand wheel until the desired position is reached. The only reasons a manually controlled valve will not position properly are mechanical wear or looseness between the lever or hand wheel and the disk, ball, or gate. For remotely controlled valves, however, there are other variables that directly affect 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.
Hydraulic Actuators
Hydraulic (i.e., fluid-driven) actuators also can provide a positive means of controlling process valves in most applications. Properly installed and maintained, this type of actuator can provide 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 . 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 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. However, proper maintenance and periodic calibration of the valves and their actuators 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 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.
FLUID POWER
Fluid power control valves are used on pneumatic and hydraulic systems or circuits.
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 . 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.
Two-Way
A two-way valve has two functional flow-control ports. 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.
Three-way valves contain a pressure port, cylinder port, and return or exhaust port . 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 the operation of such devices. 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 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 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.
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.
TROUBLESHOOTING
there are limited common control valve failure modes, the dominant problems are usually related to leakage, speed of operation, or complete valve failure.
Special attention should be given to the valve actuator when conducting a Root Cause Failure Analysis. Many of the problems associated with both process and fluid-power control valves are actually actuator problems.
In particular, remotely controlled valves that use pneumatic, hydraulic, or electrical actuators are subject to actuator failure. In many cases, these failures are the reason a valve fails to properly open, close, or seal. Even with manually controlled valves, the true root cause can be traced to an actuator problem. For example, when a manually operated process-control valve is jammed open or closed, it may cause failure of the valve mechanism. This over-torquing of the valve’s sealing device may cause damage or failure of the seal, or it may freeze the valve stem. Either of these failure modes results in total valve failure.
Common Failure Modes of Control Valves
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