Valves handle a wide variety of functions related to liquids and gases — turning flow on or off, controlling flow rate, and preventing reverse or back flow, as well as regulating and relieving pressure. Valves fall into two general classifications — linear (such as a gate valve) or rotary (such as a ball valve) — based on the action of the closure member. They also are categorized by their closure member’s shape (e.g., gate, globe, butterfly, ball, plug and diaphragm). Selection primarily depends upon the application and the pressure and temperature conditions. In addition, certain services, such as those handling flammable materials, may require valves to be fire-safe or approved for fire protection use.
Some valves are actuated manually. Valves that are located remotely in a machinery package, require frequent operation or must automatically respond to equipment needs (such as to prevent surge in turbo-compressors), or control system demands (such as to adjust flow rate) use a powered actuator.
The valve body houses all the internal working components of a valve and includes flanges or other hardware for joining the valve to the piping system. Inside the body, the closure element moves to adjust passage of fluid through the valve. To shut off flow, the element closes so that its mating surface presses against a seat that provides a surface capable of sealing against the flow. The seat usually is attached to the valve body. Most valves have a metal closure element sealing on a soft elastomeric seat; metal-seated valves may require as much as 50% more seat material. The distance the closure element travels from fully open to fully closed position is called its stroke.
A movable stem connects the valve’s actuator to the closure element. The stem may go through a bonnet, which provides a leak-proof entrance to the body. However, many valves (such as plug and some ball designs) don’t have bonnets.
The bonnet often is semi-permanently screwed into the valve body or bolted onto it (which is preferable for high performance valves); special valves feature other bonnet designs. Accessing internal parts of a valve, e.g., for maintenance, usually requires removing the bonnet.
A stuffing box (the interior area of the valve between the stem and the bonnet) contains packing to seal the stem to prevent leakage to the outside of the valve; a packing nut on the bonnet often keeps the packing in place.
In many valves, the bonnet has a backseat to seal the stem to avoid leakage into the packing when the valve is in its fully open position. A bushing on the stem provides the mating surface. Backseating is useful if the packing begins to leak and also prevents the stem from being ejected from the valve.
The particular fluid being handled impacts the operation of a valve. For instance, the operating torque necessary will depend upon the lubricity of the fluid; gases usually don’t provide any lubrication while liquids may. Liquids carrying solids can clog clearances between the stem and other components. The fluid also may corrode internal parts over time, considerably increasing the torque needed — up to twice or more of that when new.
For fire safety, some valves may require secondary metal-to-metal seating. Another important consideration in some situations is fail-safe operation; this typically calls for a larger size actuator.
The power source for a valve actuator should be capable of exceeding by an adequate margin the torque needed. In the case of throttling, determining the necessary torque may require a detailed analysis. The worst case involves providing the breakaway torque (usually the highest value). Valve-operating torque varies with closure member position. As a rough indication, the peak torque is required at breakaway, decreases to about 30% of breakaway in the half-open position and increases to about 90% of breakaway at closure.
The valve-operating torque depends upon the size and characteristics of the valve itself and the type of seat. A valve operating at full-rated pressure usually requires more operating torque than one operating at a lower pressure. The pressure differential typically varies throughout the valve’s entire stroke.
Breakaway torque plus a proper margin generally is used to select actuators for on-off service. For quarter-turn valves requiring throttling, calculating the torque often is more complicated because additional torque is necessary to counterbalance the momentum of the flowing fluid; unbalanced forces generate ‘‘hydrodynamic torque.’’ For smooth operation, the actuator torque output should significantly exceed the operating torque.
Torque requirements usually are lowest at ambient temperature. High and cryogenic temperatures necessitate greater operating torque. Fluid temperatures above 140°C may mandate a special operating and mounting assembly, often a stem extension. Even at ambient conditions, actuators located outdoors may require special considerations.
Another important factor is the cycling rate. Pneumatic and hydraulic actuators that cycle more than 30 times per hour are considered to have high operating rates. The same is true for electric actuators cycling more frequently than 10% of their duty cycle (say, operating for one cycle and resting for a time equivalent to nine cycles); such a situation calls for an extended-duty motor. Cycle speeds faster than one-half the standard cycle time demand particular care. A sudden physical shock associated with fast operating speed combined with rapid cycling rates can damage valve and actuator parts. Pneumatic actuators may need quick exhaust valves, special solenoids and larger actuators. Higher speeds are accomplished using different gearing devices, which may increase torque output, or via an electronic speed control, which will not affect torque output.
An often overlooked, but important consideration when designing piping systems and packages is stem orientation. A valve stem not aligned vertically may cause stem seal leakage or galling due to side thrusts induced by an overhung load on the actuator. The use of heavy-duty couplings and mounting brackets will minimize these problems.
Electric actuators frequently are chosen for valve operation. Either solenoid- or motor-operated, they usually are (relatively) large and heavy but often have the lowest total installed cost because electricity generally is readily available and the wiring and control instrumentation are relatively simple. Solenoid operation typically is limited to smaller lines. Motor-operated actuators tend to be bulky and slow, particularly when large gear reduction is used to increase torque — but their torque output is constant throughout their stroke and their response is more-or-less linear. Critical systems should have an emergency power supply because a power cut or a power system failure is a possibility.
The speed at which the valve closure member operates also is important. Relatively high rates are available — but exceeding the maximum specified speed will damage the seat and closure member. Gate and globe valves are torque-seating valves when closed. In the open direction, a limit switch often is provided to protect the seat against backseat over-tightening. Quarter-turn valves (such as ball valves) are position-limited open and closed because seating is based on position, not force. Electric motors don’t stop instantaneously but coast to a stop. So, it may make sense to use a solenoid brake to prevent the motor from over-tightening the closure member; check with the valve manufacturer. Unless an emergency power source is available, don’t use electric motors where cycling to a fully opened or fully closed position is necessary in the event of a power failure. Limit motor operators to moderate cycling functions; avoid them for services where severe cycling is necessary.
Pneumatic actuators handle a variety of valve applications and are well suited for services requiring frequent operation and fast response times. Depending upon the system selected, these air-driven devices usually operate in a range of 2–10 bar, with 4–6 bar most common. The compressed air supply should be a dedicated one and preferably supplied from an instrument air (known as IA) compressor package to ensure the air is clean and dry. Actuators come in two types: piston and diaphragm. The piston actuator usually is used for on-off operation. The piston stroke can be long, making it suitable for large valves. The diaphragm actuator is appropriate for modulating service but its short travel often limits the size of valve on which it can be used. Fail-safe operation typically depends upon either an internal spring or a secondary accumulator tank to provide the necessary power to cycle to an opened or closed position. The internal spring may cause the assembly to flex, which may be a problem for some installations. The accumulator tank is externally mounted, often on a nearby wall or column. Pneumatic actuators are (relatively) large in size and often need frequent maintenance because of air leakage over time (particularly for piston types) that also makes response time longer. There usually is a limitation on maximum valve differential pressure.
Hydraulic actuators are less common but preferred for some special valves. These devices use hydraulic fluid (hydraulic oil) to produce torque and rely on special hydraulic pumps (usually positive displacement pumps, most often gear pumps). They can provide fast actuation; they also are suitable for modulating service. Hydraulic actuators can handle large valves with high pressure differentials and can tolerate frequent cycling. They generally have no fail-safe mode unless emergency electrical power is available. Stroke is easily adjustable in service.
By the nature of their service, some valves require a “fire-safe” designation. Basically, this means the valve shouldn’t melt in a fire or leak after a fire and the seat should close adequately. The standard used most frequently is API-607 (“Fire Test for Soft-Seated Quarter Turn Valves”). Valve makers often provide ratings for their products. Fire-safe valves usually must be tested to show they provide the necessary performance as far as, e.g., “minimum internal leakage,” “minimal external leakage” and “continued operability.”
A valve should offer acceptable seating prior to and after exposure to high temperatures (from fire, etc.) without depending on supplementary pressure from spring-loaded or other devices or a critical seal. The valve body design should minimize external leakage by using fire-resistant stem seals and avoiding large gasketed body joints. The valve should be operable despite fire damage; the body and actuator should resist harm from high temperatures.
A number of designations are used to indicate the pressure ratings of valves; these denote a valve’s ability to withstand pressure within a range of temperatures. Standard pressure ratings have been established to match ASME/ANSI ratings of flanges and fittings and are designated by class, conforming to ASME/ANSI B16.34 ratings. Two types of designation are “WSP,” which stands for working steam pressure, and “WOG” for (cold) water, oil and gases. When both ratings are provided, WSP is called the primary rating. When only one rating is given, the valve generally isn’t used for a service not covered by the rating. A class 300# rating indicates a valve with a working pressure of 300 psig. If a valve primarily is used for water service, a common designation is “WWP” or water working pressure, which rates the ability to handle cold (say, from 0°C to 30°C) water.
High temperatures require de-rating of the valve pressure. Likewise, high pressures mandate de-rating the temperature. The temperature limit of most metallic valves usually is based on the capabilities of the seat and interior trim materials. In general, valves used for utility services (such as in machinery packages) rarely are selected based on pressure drop through the valve but rather for their suitability for the application. Calculations typically aren’t needed because established equivalent lengths of pipe for each type of valve are sufficiently accurate to determine the approximate pressure drop through the valve.
In some situations, such as where pressure drop must be kept to a minimum, precisely determining the pressure drop through a valve is necessary. This usually is done using the standard measure of valve flow, i.e., the coefficient “Cv,” which indicates the flow that will pass through a valve in the wide-open position with a certain pressure drop. The valve manufacturer determines this coefficient, typically via actual flow tests.
Making A Selection
The first factors to consider in choosing a valve are temperature and pressure. The valve body, trim and operating parts should be capable of withstanding the highest temperature expected during sustained normal and transient operating conditions. The valve should be rated for the highest transient pressure anticipated. Then, consider the degree of shutoff necessary. For utility applications (for instance, cooling water for a machinery package), allow for some minor (internal) leakage; completely eliminating such leakage is extremely costly. Bubble-tight valves are those that exhibit no visible leakage through the elastomeric seat of the valve for the duration of a test. Corrosion resistance also is important, and usually is affected by the nature, concentration and temperature of the fluid. Other crucial parameters are velocity of the fluid through the valve, the nature of its operation (e.g., on-off or modulating), whether a fire-safe version is needed, and fluid details (such as whether a gas, liquid or stream with some particulates).
AMIN ALMASI is a rotating equipment consultant based in Sydney, Australia. E-mail him at [email protected].