What is a Valve?
A valve is a mechanical element used in piping systems to start, stop, mix, control, or regulate the flow or pressure of the fluid by modifying the passage through the pipe. Valves are an essential part of any piping system conveying liquids, vapors, slurries, gases, and mixtures of liquid, and gaseous phases of various flow media. Valves are the most costly rigid piping components in a plant. They can account for up to 20% to 30% of overall piping item costs for a process plant. The size of a valve is decided by the size of its ends connected to the piping system.
Common types of valves can be self-actuated or manually operated or have actuators. Actuators of valves can be electric, pneumatic or hydraulic powered, or a combination to operate the valve. Valves are made of metals and non-metals depending on application.
Types of Valves / Valve types
Common types of valves used in piping systems can be classifies based on various parameters like
- valve types based on functions
- type of valves based on end connections
- valves based on construction material
- valve types based on actuator operation
- valve types based on the mechanical motion of the closing member
- valves based on pressure-temperature ratings
- types of valves as per port size.
Valve Types based on Functions
The valve type classification based on functions are shown in Fig. 2.
The main functions of a valve is
- To start or stop a flow.
- To increase or reduce a flow.
- To control the flow.
- To prevent backflow.
- To relieve a pipe system at a certain pressure to safeguard the system.
- To throttle.
Types of valves based on End Connection
Based on end connection to piping or equipment nozzles, the valves can be classified as follows:
- Flanged ends (Normally 2″ and larger valves)
- Butt-welded ends (Class 900 and higher)
- Screwed ends (1.5 inch and smaller sizes)
- Socket welded ends (2″ and smaller sizes)
- Wafer type
Valve types based on the material of construction
Based on the construction material, the available valve types are
- Carbon Steel
- Alloy Steel
- Cast Iron
- Stainless Steel
- Ductile Iron
- Nickel Plated Ductile Iron
- Silicon Bronze
- Aluminum Bronze
- Lined valves
- Special steels
- Non-metals like PP, PVC
Valve types based on actuator operation
Based on operator types, industrial valves are classified as shown in Fig. 3
The operator is a device that aids in opening or closing a valve. Various operators available in industrial valves are
Hand Wheel: For the majority of popular smaller valves, Handwheel (Fig. 5) is the most common means of rotating the stem. Normal handwheels can be substituted using a hammer blow or impact handwheels when an easier operation is needed.
Chain: It is used where a handwheel would be out of reach. The stem is fitted with a chain wheel or wrench (for lever operated valves) and the loop of the chain is brought within one meter of the working floor level.
Gear operated valves: Gear operators are used to reduce the operating torque. It consists of a hand wheel operated gear train actuating the valve stem in case of a manual operator. A thumb rule for gear operators consideration is
- valves of 350 mm NB and larger up to 300#,
- valves of 200 mm NB, and larger up to 600#,
- valves of 150 mm NB, and larger up to 1500# and
- valves of 100 mm NB and larger for higher ratings.
Pneumatic and Hydraulic: Situations where the possibility of flammable vapor is likely, pneumatic or hydraulic operator is used. They are of following forms:
- Cylinder with double-acting piston driven by air, water, oil, or other liquid, which usually actuates the stem directly.
- Air motor, which actuates the stem through gearing. These motors are commonly piston and cylinder radial type.
- A double-acting vane with limited rotary movement in a sector casing, actuating the stem directly.
Electric Geared Motor: For large valves in remote locations, Electric Geared Motor is used to move the valve stem.
Solenoid: These can be used for fast-acting check valves, and with on/off valves in light-duty instrumentation applications.
Types of Valves based on the mechanical motion of the closing member
The closure members of the valve exhibit various kinds of mechanical motion during operation. Accordingly, valves are grouped as follows:
Linear Motion Valves:
The valves in which the closure member moves in a straight line to allow, control, regulate, stop, or throttle the flow are known as linear motion valves. Gate, globe, diaphragm, pinch, and lift check valves are linear motion valves. Linear valves are of two types: rising stem (multi-turn) and axial. In both valve types, the flow obstructer moves linearly but there are wide differences in construction and operation.
In case of multi-turn rising stem valves, the obstructer is moved by the rotation of a threaded rod (stem) attached to the obstructer. Typical examples of multi-turn valves are globe valve, gate valves, pinch valves, needle valves, and diaphragm valves which are commonly used for flow control applications.
On the other hand, Axial valves use electro-magnetic or pneumatic force to move the flow obstructer along an axis. Coaxial valves, angle seat valves, etc are examples of these type of valves which are typically fast acting and only used for on/off process applications.
Rotary Motion Valves:
In butterfly, ball, plug, eccentric, and swing check valves, the valve-closure member travels along an angular or circular path. These types of valves are known as rotary motion valves as these valves rely on the rotary motion of the flow obstructer. The rotation is usually limited to one quarter turn or 90 degrees.
Quarter Turn Valves:
Some rotary motion valves on the other hand require approximately a quarter turn i.e, 00 through 900 of motion of the stem to travel from fully open to a fully closed position or vice versa. Such valves are called quarter-turn valves.
Fig. 4 below shows the common types of valves possessing various kinds of mechanical motion.
Valves based on pressure-temperature ratings
Based on pressure-temperature ratings, following types of valves are available in the market.
- Class 150 (Allowable Pressure=16 bar)
- Class 300 (Allowable Pressure=40 bar)
- Class 400
- Class 600 (Allowable Pressure=100 bar)
- Class 900 (Allowable Pressure=160 bar)
- Class 1500 Allowable Pressure=250 bar) and
- Class 2500 valves (Allowable Pressure=400 bar)
However, the Class 400 valve is used occasionally in the piping industry. So less common.
Types of valves as per Port Size
The port is the maximum internal valve opening for the flow. As per port size valves are of two types
- Full Port Valves: Available area almost equal to the full bore of the pipe
- Reduced Port valves: They have less port area that produces a venturi effect to restore a large percentage of velocity head loss through the valve and produce a resultant total pressure drop of relatively low order.
Major Valve Components /Valve Parts
There are two kinds of components in a valve
- Pressure Retaining Parts (Body, Bonnet, Cover Bolting, Disc, Valve Trim, etc) and
- Non-Pressure Retaining Parts (Valve Seats, Stem, Packing, Bushings, Yoke, Handwheel, Actuators, Gland Bolting, etc)
The valve body is the house of the valve’s internal parts and the path for fluid passage. The valve body is manufactured by casting, forging, fabricating, or a combination of two or more methods. A variety of metals or nonmetals can be used to produce Valve bodies based on size and pressure-temperature rating and service requirements. The valve body (Fig. 5) ends are designed to connect with the pipe or equipment nozzles. Some Valve bodies are lined with corrosion-resistant materials.
Bonnet or Cover of the valve:
The valve body is fastened to the bonnet or cover, another pressure retaining shell. It normally provides an opening for the valve stem to pass through. The bonnet (Fig. 5) contains a stuffing box. Valve internals are accessed through the bonnet or cover. It serves as the base for the valve top works like the yoke, hand-wheel, etc. The Bonnet is classified based on the type of attachment as Bolted, Bellow, Sealed, Screwed-on, Welded, Union, Pressure Sealed, etc.
Bolts, nuts, and washers are combinedly called Bolting. The bolting material is selected based on the application conforming to the applicable codes and standards.
The valve disc is one of the major components of the valve that allows, throttles, or stops the flow depending on its position. So, disk directly affects the flow. In various valves, it is known with different names. For example, the disc is known as a plug in a plug valve, ball in a ball valve, gate in a gate valve. A disc is made from casting, forging, or fabrication. When the valve is in a closed position, the disc is seated against the stationary valve seat. Using the motion of the valve stem, the disc (Fig. 5) can be controlled to open or close the valve.
Valve trims are the removable and replaceable internal parts of the valve. These parts come in contact with the flow medium and are collectively known as valve trim. Examples of such parts are valve seat(s), glands, disc, spacers, guides, bushings, and internal springs.
Note that, The valve body, bonnet, packing, etc even though come in contact with the flow medium are not considered valve trim.
The Non-moving part that the body bears is termed as a valve seat. There may be one or more seats in a valve. Globe or swing-check valves have one seat while a gate valve has two seats; one each on the upstream and downstream side. The valve disc along with the seats form a seal that stops the flow.
The valve stem imparts the required motion to the disc to open or close the valve. One side of the stem (Fig. 5) is connected to the valve handwheel, actuator, or the lever; while the other end with the valve disc. In gate or globe valves, linear motion of the disc is needed to open or close the valve, while in a plug, ball, and butterfly valves, the valve disc is rotated to open or close the valve. However, check valves (exception: stop-check valves) do not have valve stems. There are four categories of the valve stem.
- Rising Stem: Handwheel can either rise with the stem, or stem can rise through the stationary handwheel.
- Non-Rising Stem: The Hand Wheel and the stem are in the same position whether the valve is opened or closed. In such a case, the screw is inside the Bonnet and comes in contact with the fluid.
- Sliding Stem: In this variation, the valve stem slides in and out the valve for opening or closing the valve.
- Rotary Stem: In ball, plug, and Butterfly valves, Rotary type valve stem is frequently used. To open or close such a valve, a quarter-turn motion of the stem is sufficient.
Valve Stem Packing:
Depending on the application; the Stem packing performs one or both of the following two functions,
● Preventing leakage of flow medium to the environment
● Preventing outside air from entering the valve in vacuum applications
Stem packing is contained in the stuffing box. Packing rings are packed and compressed by tightening a packing nut or packing gland bolts. Compression must be adequate to achieve a good seal.
Sealing between Valve Stem and Bonnet
The proper sealing between bonnet and valve stem is achieved by
- Using Gaskets in between a bolted bonnet and valve body.
- Using Metal Bellows where high vacuum or corrosive, flammable fluids are to be handled.
- Using gaskets in Flanged Valves to seal against the line flanges.
- Using a resilient seat serves as line gaskets for Butterfly Valves.
Valve Yoke and Yoke Nut
Using a Valve Yoke the valve body or bonnet is connected with the actuating mechanism. A Yoke must be designed strong enough as it will be subjected to forces, moments, and torque developed by the actuator.
A Yoke nut is an internally threaded nut and is placed in the top of a Yoke by which the stem passes.