This low-tech valve may not have changed much in the last 100 years, but the gate valve plays a major role in virtually every refinery, chemical plant and industrial facility in the world.
The most popular style of valve in the world of flow control is the gate valve. They are the on/off switches of the fluid control industry and they are found in every refinery, chemical plant, power plant and industrial facility. Gate valves exist for one primary purpose- to stop flow. Because of this, they are often referred to as “stop” or “block” valves. Gate valves are manufactured in a wide range of sizes- from ¼” through 144”.
It is not recommended to operate gate valves in the partially open position, or to use them in throttling service. When a gate valve is partially open, it closure element (disc or wedge) can vibrate against the seats and become scratched causing them to lose their seating integrity.
The chief advantage of a gate valve is that it offers virtually no resistance to flow in the open position. Only a full port ball valve can equal the gate valve’s flow characteristics. Due to their symmetrical design and equilateral seating, gate valves can be used to stop flow from either direction. They are available in every material from the shiny brass construction of the diminutive ½” water valves on the hardware store shelf, to the exotic high alloy models found in nuclear power installations.
Gate valves have been an important piece of fluid control equipment for over 150 years. In fact, the very first valve patent issued in the United States was for a “gate valve”. Since those humble beginnings in the 19th century, the gate valve has answered the fluid control call with relatively little basic design change.
From the outside, most gate valves look somewhat similar. However, inside there are a host of different design possibilities. Most gate valves consist of a body and bonnet that contains a closure element, called a disc or a gate. The closure element is attached to a stem that passes through the bonnet of the valve, ultimately interfacing with a handwheel or other device to operate the stem. Pressure around the stem is contained through the use of packing material which is compressed into a packing area or chamber.
The word “trim” is often overheard when valve professionals are talking about industrial gate valves. Trim has nothing to do with how slim and fit a valve is, rather it refers to the internal components of a valve that are exposed to great stress or subject to a harsh combination of erosion and corrosion. In a gate valve the trim components are the stem, disc seating area, body seats and backseat, if applicable. Common utility bronze or brass valves usually have trim parts of the same material as the body and bonnet. Cast and ductile iron valves have either all iron trim components or occasionally bronze trim. The term for an iron valve with bronze trim is “iron body, bronze mounted” or IBBM for short.
Because of their weldability, steel valves can be furnished with a number of different trims. Stellite, Hastelloy, 316ss, 347ss, Monel, and Alloy 20 are some of the materials regularly used for gate valve trim.
During most of the 19th century, valves were predominantly supplied with screwed end connections, even in sizes as large as 12” NPT. Since that time the flanged end connection has become the most popular. Other end connection types in use today include screwed, ring-type-joint (RTJ), Victaulic, Greyloc and water works “mechanical joint”.
Gate valves can have one of two different disc designs: parallel or tapered type. Both operate on the principle of a closure element (disc or gate) sliding into a slot in the pipeline and closing off the fluid path. The tapered disc of the “wedge gate” valve is machined to match a pair of body seats set at the same angle, usually about 10o. If machined correctly, as the tapered disc engages the seats, it locks firmly into place, stopping the flow.
Three types of wedge gates are available: solid disc, one piece flexible type, and two piece split design.
- The solid wedge has been around the longest and at one time virtually all wedge gates were the solid type. The drawback to a solid design is that it does not have any flexibility and if there is any valve body/seat distortion due to extreme temperature fluctuations or pipe stresses, the solid disc can become jammed in the seats. The solid disc is still standard on bronze, cast iron, water service and compact carbon steel valves (API 602 type). Today, solid discs are usually only available as special order items on large diameter gate valves.
- The flexible wedge type is just that- flexible. By the addition of a groove or slot around its periphery, the flexible disc can adapt to temperature changes and adverse piping stresses without binding. The flexible design also is a little easier to manufacture, in that minor imperfections in the seating surface angles can be compensated for by the disc’s flexibility. The “flex-wedge” design is by far the most common type seen on commodity gate valves used in industrial applications.
- The split wedge type consists of a two-piece design with mating surfaces on the back side of each disc half. These mating surfaces allow the downward stem thrust to be uniformly transferred to the disc faces and onto the seats. This flexible design also provides protection against jamming due to thermal expansion. A disadvantage to the split design is that in “dirty” services, residue or debris can cake in between the disc halves, causing the valve to improperly seat or even jam. Split wedge designs are commonly found on stainless steel and high alloy valves, as well as many small bronze valves.
Wedge gates are guided by grooves or ribs cast or welded into the body of the valve. These wedge guides keep the disc in alignment as it opens or closes and also keeps the disc from sliding against the downstream seat during opening and closing.
The second disc design is the parallel type. Unlike the wedge type gate valve, which relies on the stem thrust to “wedge” the disc into the seats to seal, the parallel seat valve needs some assistance to seal properly. The sealing assistance is usually in the form of a spring loaded or mechanically activated spreading action between the two disc halves as the valve closes fully. On most parallel seat designs the friction and sealing force is relieved as the gate disengages from the seats.
The most common use for parallel disc valves today is in the pipeline industry, where elastomer seat seals and ambient operating temperatures make valve virtually leak proof. Parallel gates are also used in some high pressure, high temperature steam applications, to help reduce the possibility of locking the disc in the closed position due to a radical change in temperature.
Regardless of disc design or type, the gate valve closure element must come in perfect contact with seats in the valve body. The body seats may be welded, screwed, pressed or swaged in, or be integral with the valve body. Most industrial steel gate valves utilize seat rings that are welded into the valve body. For most of the 20th century the norm was screwed in seat rings in steel valves. However, advances in welding and valve repair techniques made the screwed-in rings obsolete. Seat rings and valve discs are also often overlaid with corrosion or abrasion resistant alloys to increase their service life.
Gate valves are normally available in five different body/bonnet joint designs. They are: screwed, union, bolted-bonnet, welded-bonnet & pressure-seal.
- The screwed joint is the simplest design. However it is only used for inexpensive bronze valves that rarely if ever require disassembled.
- The union joint is also primarily used on bronze valves, but the union design allows for easier disassembly for repair and maintenance.
- The bolted-bonnet joint is the most popular joint and it is used on the vast majority of gate valves in industrial use today. Unlike threaded and union bonnet valves, the bolted-bonnet connection requires a gasket to seal the joint between the body and bonnet. On lower pressure valves, sheet gasket materials are used. ANSI Class 150 steel valves usually employ a corrugated soft iron or graphite/corrugated soft iron gasket. Valves of class 300 and higher employ either a spiral-wound or ring joint type gasket.
- The pressure-seal joint is energized by the fluid pressure in the valve body acting upon a wedge shaped, soft iron or graphite gasket wedged between the body and bonnet. On a pressure-seal valve, the higher the body cavity pressure, the greater the force on the gasket. Pressure-seal bonnets are used extensively for high-pressure high-temperature applications, such as the power industry. Pressure-seal valves are much lighter than their corresponding bolted bonnet designs. Due to the pressure energization of the seal ring, they are normally not used in pressure classes below ANSI class 600.
- Welded bonnets are a very popular body-bonnet joint for compact steel valves in sizes ½” through 2” and pressure classes 800 through 2500, where disassembly is not required. The higher pressure welded-bonnet type valves rely on threads to handle the force generated by the body cavity pressure, while a small peripheral weld bevel actually contains the pressure. Like pressure-seal valves, welded-bonnet valves are much lighter than their bolted-bonnet counterparts.
Three different bonnet/stem designs are predominant in gate valve construction. They are: inside screw, rising stem (ISRS), non-rising stem (NRS), and outside screw and yoke (OS&Y).
- The ISRS bonnet/stem design is the most popular design in use today on bronze valves. Due to the fact that it exposes the stem threads to the process fluid and potential corrosion damage which could cause a stem to disc failure, the ISRS design is not used for critical service industrial applications.
- The NRS type is another special purpose type that is used in applications where there is limited vertical clearance above the handwheel, because on an NRS valve, the stem does not rise up as the valve is opened. Most NRS valves today are manufactured of either iron or bronze. Some applications such as marine use, where clearances are tight, often use NRS steel gate valves.
- The most common stem/bonnet design in use on industrial valves is the OS&Y. The OS&Y design is preferred for corrosive environments because the threads are outside the fluid containment area. It also differs from other designs in that the handwheel is attached to a bushing at the top of the valve yoke, and not to the stem itself, thus the handwheel does not rise as the valve is opened.
Also in the gate valve family are knife and sluice gates. The bonnetless knife gate is especially suited for use in slurries such as in pulp and paper mills. Knife gates are very thin, only slightly wider than there closure element (disc). Because of their unique geometry and thin cross-section, knife gates are limited to low pressure applications.
In appearance, the sluice gate doesn’t look like it even belongs in the gate valve family, however based upon its sliding disc design; it is characterized as a gate valve. Sluice gates are limited to very low pressures, in most cases, simple head pressure. They are used primarily in waste water and irrigation systems.
Gate valves standards are produced by several standards making organizations, for a multitude of industries. Here are some of the better known gate valve specifications:
American Petroleum Institute
*API 600 “Steel Gate Valves, Flanged & Buttwelding Ends”, it is a companion document to ISO 10434.
*API 602 “Compact Steel Gate Valves”
*API 603 “Corrosion Resistant Bolted Bonnet Gate Valves”
*API 6D “Specification for Pipeline Valves”, it is a companion document to ISO 14314.
Manufacturers Standardization Society
*SP-70 “Cast Iron Gate Valves”
*SP-80 “Bronze Gate, Globe, Angle and Check Valves”
*SP-81 “Stainless Steel Bonnetless, Flanged, Knife Gate Valves”
American Waterworks Association
*AWWA C500 “Metal-Seated Gate Valves for Water Supply Service”
*AWWA C509 “Resilient-Seated Gate Valves for Water Supply Service”
*AWWA C515 “Resilient-Seated Gate Valves for Water Supply Service”
American Society of Mechanical Engineers
*B16.34 “Valves- Flanged, Threaded and Welding End”
Materials of Construction
Gate valves are manufactured in virtually every metal from Aluminum to Zirconium. They are also manufactured in a variety of engineering plastics. The most common materials however, are steel, iron and bronze.
Bronze offers the greatest machinability and the lowest manufacturing cost. The features that make bronze easy to machine, its lower strength and softness, also make the valve only suitable for lower pressure applications. The predominant service for bronze valves is on water and utility lines where pressures are lower than about 300 psi.
Iron valves are in between bronze and steel as far as strength goes. The iron is slightly harder to machine, but the iron castings are relatively easy to pour. Iron for valves is commonly two types; grey or cast iron and malleable iron. In refinery and petrochemical service iron valves are usually restricted to low pressure water lines. The high carbon content and better rust resistance of iron valves makes them more suitable for buried service than steel valves.
For industrial valves, steel is the material of choice. A broad spectrum of steels are utilized for valve construction, from the lowest grade WCB, to the chrome/moly’s. Unlike the brasses, bronzes and irons, most steels and low alloys are readily weldable, which makes them easier to modify, repair and in some cases even easier to manufacture. Gate valves are also manufactured in a number of exotic alloys from Titanium to Zirconium.
There have been several attempts to make the gate valve obsolete and take away its market share, but they have only met with limited success. The first challenger to the gate valve throne was the ball valve, which came into prominence during the middle part of the 20th century. Ball valves have been substituted for gate valves in many lower pressure and lower temperature applications, but in some cases they are more expensive to manufacture and repair. The elastomer seats of the ball valve also limit them to temperatures below about 500 degrees F.
Butterfly valves have supplanted gate valves in some of the larger (48” and above) low pressure applications, such as water works usage. The metal-seated butterfly valve has also been successful in certain critical service applications that once were solely the realm of the gate valve, but their high initial cost and very high repair costs make them unlikely to ever completely replace the venerable gate valve.
Gate Valve Actuation
The most common method of opening and closing (actuating) a valve is through a handwheel attached to the yoke or bonnet. This works fine on moderate size valves operating at reasonable pressures, but some severe operating situations call for more muscle. For example, an 18”, class 1500, main steam isolating valve in a power plant operating at 1750 psi and 1000 degrees F. requires a huge amount of torque to open under pressure. The only solution is remote actuation- usually in the form of an electric motor or hydraulic actuator.
Additional gate valve actuation can be provided by pneumatic cylinders. In some cases these sit directly on top of the yoke and are attached directly to the stem, to provide a quick-opening form of actuation. For additional leverage a standard gate valve might have a manual gear operator attached to it to decrease the amount of force required to open and close it under pressure. These devices are called bevel gears.
Repair of Gate Valves
Industrial gate valves are often used in harsh environments and sometimes these valves need to be repaired. The decision to repair or replace a valve usually is a result of comparing the replacement cost to the repair cost. When the repair cost exceeds 50-65% of the cost of a new valve, the decision is usually to replace the valve, unless the delivery is unacceptable.
Generally speaking, all bronze valves, except for expensive cryogenic designs, are replaced rather than repaired. Iron valves, except for the largest sizes, are also replaced rather than repaired. Steel and alloy gate valves are the most repaired types. Steel valves smaller than 12”, class 150 are usually not repaired, unless replacements are not readily available. On the other hand, high alloy gate valves as small as ½” size may be repaired because of their high cost and long lead time.
Some gate valves, such as large diameter, buttweld end and pressure-seal types are often repaired in the field. These field repairs are often difficult and pose logistical challenges, but compared to the cost of removing them from the line and shipping them to a repair facility, field repair is more economical option.
Gate valves are still the primary choice for many service applications. Their cost of manufacture to value ratio is still very high. On typical petrochemical and refining projects today, the percentage of gate valves on the requisition is about 60-70%.
Although science and technology has made tremendous leaps during the past 50 years, most gate valves are still being produced to the same basic designs developed a hundred years ago. And until someone invents a Buck Rogers laser valve with no moving parts, tens of thousands of gate valves will still be manufactured each year, in plants from South Carolina to Southeast Asia.