Valves For Process Piping: An Introduction
This article is intended to help an engineer evaluate generic factors when selecting appropriate valves for process piping. It is not intended to aid engineers in the actual design and selection of valves for specific services.
Also, for purposes of this article the definition of a valve is a mechanical device designed to precisely control flow of any liquid being piped. Not covered are check valves, pressure-relief valves and other special-duty valves. Valves discussed in this article can be classified as on/off service, throttling service (where it would be used for flow control and preventing reverse flow) and pressure control. These valves can be operated manually, remotely or automatically. Related variables include system pressure, temperature, and chemical variations of the fluid in the systems in which they are installed.
Codes that apply to these systems (whether operating separately or together) exist from several bodies, including the American Petroleum Institute, American Water Works Association, Manufacturers Standardization Society, American Society of Mechanical Engineers and American Standards Association. Each of these codes represent general industry, manufacturers, consumers and engineering societies. There are several other codes that may also apply.
Valve Body MaterialsTo begin, let’s discuss some of the materials more commonly used in the manufacturing of valves.
The bronzes, which are a copper base alloy family of metals, offer a fair to excellent corrosion resistance in most general service applications such as steam, water, refined oil and natural gas. They are easily shaped, cast or forged, and machined.
Within their maximum temperature limitations they are widely used for valves from 1/8 through 3 inches. Above the 3-inch size, except where required for specific corrosion resistance, all-bronze valves are seldom used because of their higher cost as compared to iron body valves. There are two main alloys used: ASTM B61 and ASTM B62.
ASTM B61. Commonly known as cast steam or valve bronze, this material is composed of 86-90% copper, 5.5-6.5% tin, 3-5% zinc and 1-2% lead, as well as other miscellaneous materials. Its physical properties are as follows:
- Tensile strength, psi..............34,000
Yield strength, psi...................16,000
ASTM B62. Called “composition brass” or “ounce metal,” this material is also known as 85-5-5-5 because it is composed of 85% copper, 5% lead, 5% tin and 5% zinc. Its physical properties are as follows:
- Tensile strength, psi..............30,000
Yield strength, psi...................14,000
Cast iron is used for the pressure-containing parts, generally manufactured in sizes two inches (DN 50) and larger. An exception is the clip-type valve where cast iron is used in all sizes from 1/4-inch (DN 6) to 4-inch (DN 100) and larger.
Generally speaking, cast iron is not noted for high resistance to corrosion. It usually depends on the formation of a protective coating for its serviceability that forms after initial corrosion occurs; as, for example, surface rust on cast iron that comes from contact with water. This accounts for the fact that cast iron valves so often require more-resistant materials for seating elements and other valve trim.
Two classes of cast iron are used extensively in the valve and fitting industry. They are regular gray cast iron (ASTM A126 class A) and the higher-strength gray iron (ASTM A126 class B). Its physical properties are as follows:
- Class A...............................21,000 psi
Class B...............................31,000 psi
Steel, forged or cast, is regularly used for those valves designed for services in which the pressure, temperature and/or shock conditions exceed the limitations of bronze, iron and ductile (nodular) iron. The high-tensile strength and toughness of these materials - combined with resistance to strain, shock, vibration, freezing, and damage by fire - afford maximum protection when safety and utility are sought.
Except in alloyed varieties, steel (like iron) is not marked by or usually selected for its high resistance to corrosion. As is the case of cast iron, steel depends upon the formation of a protective coating that is formed after the initial corrosion occurs for its serviceability.
Forged carbon steel. Forged carbon steel, when it meets the requirements of ASTM A105 grade II, is regularly employed in small valves in sizes from 1/4- (DN 6) to 2-inch (DN 50). This material is composed of 35% carbon, 0.90% manganese, 0.05% phosphorous and 0.05% sulphur. Its physical properties are as follows:
- Tensile strength, psi..............70,000
Yield strength, psi..................36,000
Cast carbon steel. Cast carbon steel (ASTM A216, grade WCB) is regularly used for larger valves, 1-1/2 to 24 inches. This material is composed of 30% carbon, 1% manganese, 0.05% phosphorous, 0.06% sulphur and 0.06% silicon. Its physical properties are as follows:
- Tensile strength, psi..............70,000
Yield strength, psi..................36,000
Alloy steel. When temperatures and corrosion conditions exceed that of the steels mentioned above, alloys are utilized; a large variety of which are manufactured. Manufacturers should be consulted for additional discussions.
Trim MaterialsTrim is the internal component of a valve that is exposed to great stress, or subject to a combination of erosion and/or corrosion. These include:
A. Stainless steel with 13% chromium (types 4190, 416 and 420), which have high hardness and makes them resistant to wear and erosion. Valves trimmed with this material, nickel-copper and stellite are highly recommended for water, steam and gas up to a temperature of 850°F.
B. Stellite is a cobalt-chromium-tungsten alloy deposited on facings by welding. It is highly recommended for severe service, most commonly steam.
C. A wide variety of additional materials are available. Consult manufacturers for severe conditions.
Body End ConnectionsA. Butt weld end connections are available for only cast steel valves. Appropriate ASA standards should be specified for their use.
B. Flanged ends allow valves to be bolted to a piping system. Flanged ends are available for bronze, iron, ductile iron, cast and forged steels all in accordance with various ASA and ASME specifications.
C. Hub ends allow attachment to a piping system by caulking a joint with lead or a gasket material. Hub ends are available on iron valves, gates and checks.
D. Mechanical ends allow valves to be bolted to a piping system without leakage and are considered more positive than hub ends. This joint incorporates a stuffing box principal using a triangular gasket, held by a gland and bolted to the pipe ends. Mechanical ends use joints that allow some deflection as well as longitudinal expansion and contraction, and are best for underground water and gas.
E. Screwed end connections are furnished with female threaded ends matching the male end of the pipe to which it is to be joined. It is available in bronze, iron, ductile iron, cast and forged steel, and limited to smaller valves up to four inches (DN 200) because of the difficulty in making up the joint.
F. Socket weld ends are available on small forged-steel valves, those up to two inches (DN100). The pipe to be welded slips into a socket on the valve. The socket weld has some advantages over the butt weld, such as not having to tack weld. It is supported and aligned by the socket, and there is no danger of leaving any welding obstruction inside the pipe.
G. Solder end is available for brass or bronze valves used for water, oil and gas having copper tubing as the supply pipe.
H. Union end for bronze valves consists of a union nut and a companion nipple. It is particularly useful where there is a reasonable expectation that the valve has to be replaced.
I. Grooved ends depend on grooves machined into the body of the valve and the pipe to which it is to be connected. A split ring with bolts in the perimeter is placed in the grooves and used as the closure with a gasket added to prevent leakage.
Performance Factors Related to Valve MaterialsSome of the general considerations that go into the selection of materials for a particular application are as follows:
Pressure. Because valves are pressure-containing vessels, they must be designed to confine the fluid being piped under pressure without leakage and without undue distortion of the pressure-containing parts. In addition to the “normal flowing pressure,” shock must be considered. Shock is the sudden increase in pressure caused when the liquid flow is suddenly stopped. The maximum shock pressure must be considered in addition to the working pressure to obtain the pressure rating unless a device to reduce that pressure is used.
Temperature. The operating temperature is an exceedingly important factor in the selection of any material. As the temperature of the fluid being piped becomes elevated, problems become more complex. This is because the tensile strength of all metals and alloys is reduced as the temperature increases. For each material, there is a critical elevated temperature beyond which its tensile strength and resistance to shock is so sharply reduced as to render that particular material unsuitable for use as a pressure-containing vessel.
Corrosion. The ability of a material to resist internal and external corrosion caused by the fluid being piped or location in a backfill is also a major consideration.
Cost. The cost of a finished product must also be considered as a vital factor in the selection of materials as opposed to others for a given application. This would include raw material, casting, and forging into the finished product.
Common Valve TypesGate Valve
The gate valve is the most-popular style in the world for flow control. Gate valves exist for a single primary purpose: to stop flow. They are sometimes known as “stop” or “block” valves. The advantage of using gate valves is that, in the fully open position, they offer almost no resistance to flow. This valve should not be operated in a partially open position because of potential damage to the seating surfaces.
Although gate valves are manufactured with virtually every metal, the most common are bronze, steel and iron. Bronze (or brass) is the easiest to machine and has the lowest manufacturing cost. It is mostly used for water and utility service at pressures of 300 psi or lower. Steel is the material choice for industrial valves. Two types of iron are used for valves: gray or cast iron and malleable.
The gate valve’s closure element is a member (disk) that slides into a slot in the body, which closes off the flow. There are three types of disks available: solid, one-piece flexible and two-piece split.
Butterfly valves are regarded as the oldest valve type used in the process-control industry and are available in larger sizes - eight inches (DN 200) and up. They control flow using a one-quarter turn disk that rotates 90 degrees. In the closed position, the disk blocks flow. All butterfly valves are not similar. Quality is important in the manufacturer of these valves because closure depends on a precise fit. High-performance valves can be used for bubbletight shutoff and precise throttling. Primarily used for water and chemical systems, butterfly valves are not considered suitable for slurry or systems containing particulates.
The globe valve is manufactured of the same materials as gate valves. The globe valve’s operation involves a linear rising stem with a multi-turn handwheel. The globe valve’s complex flow path makes it excellent for throttling purposes. It is well suited for automatic and remote operation. Typical applications are for steam, chemical processing and gas service.
The plug valve was developed several thousand years ago and used by the Romans to control water into homes supplied by an aqueduct. The valve is manufactured of the same materials as gate valves. It uses a one-quarter turn valve with an oblong plug containing a hole for liquids to flow through. This valve could be either lubricated (which helps operation) or non-lubricated. It could be either sleeved or fully lined. For general use, the non-lubricated type is preferred. Plug valves are well suited for many corrosive chemical applications and water. Their one negative is the high turning torque necessary for operation.
Ball valves are the most-used valve in the chemical industry. They are a one-quarter-turn type that uses a ball with a drilled-out center, which allows fluids to pass. Turning the ball 90 degrees in either direction cuts off or allows flow. Ball valves range in size from 1/2 to 12 inches, and come in flanged, socket weld, butt weld and threaded ends with a maximum of 600 psi.
The valves are manufactured in ductile iron through high alloys and a variety of seating materials in three configurations: two-piece, three-piece and top entry. The three-piece allows the body of the valve to be removed, keeping the end connections intact. The two-piece is less expensive and must be removed entirely. Top entry valves allow the ball to be easily removed for maintenance. The ball valve is recommended for normal and severe chemical service applications, water and gas. It is not recommended for slurry or liquids containing particulates.
Also known as pinch valves (see Figure 1), diaphragm valves are manufactured from basic ferrous alloys and high-performance thermoplastics. The valve consists of an elastomeric sleevelike material that limits the maximum pressure and temperature. It is available in two configurations: weir and straight-through patterns. It operates by using a closure member outside the sleeve and a multi-turn hand wheel that closes the sleeve to shut off flow. This type of valve is characterized by streamlined flow and chemical resistance of the sleeve. Shut-off is not bubbletight. It is generally used for chemical and high-purity operations.
A regulator valve is a low-cost, standalone, self-actuating, proportional controller whose controlling action is proportional to any deviation from a set point. It does not require any auxiliary piping system. A typical regulator valve is shown in Figure 2. Limitations of regulators include a maximum size of typically six inches and fixed failure modes, plus they are not generally applicable for severe service.
Control valves are not standalone valves that can only control the flow rate. Rather, they are the final control element in a system and need to be evaluated in that context. In principal, any continuously variable system parameter that can be measured and compared to a set point can be controlled. This parameter requires the addition of a proportional and integral control mode that will return the measured variable to the approximate set point following a load change.
When deciding between a control valve and a regulator valve, the following questions should be considered:
- *What accuracy of control is required?
*What is the cost of the installation?
*Is power available for the regulating valve?
*What are the line size and material of the valve?
*Are noise and cavitation considerations?
*Is the environment potentially hazardous or explosive?
*Is a predetermined failure mode required?
Methods of OperationDiscussion of actual valve operation is beyond the scope of this article. What follows are the more commonly employed methods of operation.
- * Chain wheel
* Floor stands
* Indicator posts
* Spur gears
* Bevel gears
* Worm gears
* Hydraulic and pneumatic piston operator
* Electric and air motor operation