Resilient seat gate valves (available in sizes up to DN 600) have a gate which is encapsulated in rubber (Plate 28(d)) and seals against a clear full bore typically without grooves in which dirt can collect and prevent full closure.
Gate valves should be transported in a closed position. Gate valves do not usually have lifting lugs. Gate and globe valves are lifted by using a cloth wrapped around the body and bonnet connection under the yoke, as shown in Fig. 6.56.
Gate valves should not be installed with their stem below the horizontal line. If they are, complete drainage is not possible and solids will accumulate in the bonnet that can affect the operation and service life of the valve (increasing the possibility of packing damage). Both positions of installation for gate valves shown in Fig. 6.57 are acceptable since the stems are not below the horizontal line.
Gate valves have more height than ball valves due to their high body and upward stem and gate movement for rising stem design. The height of a gate valve could be considered a disadvantage, since it can impose a manifold structure with higher height. Usage of a linear actuator on top of a TCG valve could make the height differential even greater compared to an actuated ball valve. One should bear in mind that rack and pinion or scotch and yoke actuators, which are used for ball valve automation, stand horizontally when the valve is connected to a horizontal pipe, whereas linear actuators stand vertically when the TCG valve is connected to horizontally installed piping. Fig. 4.33 illustrates the height of an actuated slab gate valve.
Gate valves work by inserting a rectangular gate or wedge into the path of a flowing fluid. They are operated by a threaded stem which connects the actuator (generally a hand wheel or motor) to the stem of the gate. If the valve has a rising stem its position can be seen just by looking at the position of the stem. Fig. 5.2 shows the internals of a Gate Valve that is half open.
When fully open, a gate valve has no obstruction in the flow path and so has a very low pressure drop. If the fluid is very viscous a special type of gate valve known as a knife gate valve can be used.
One reason that gate valves are not normally used to regulate flow is that the flow rate of the fluid is not proportional to the amount that the valve is open. Moreover, a partially open gate valve may suffer from vibration in which the valve may move from its assigned position. Also the gate and seat may be subject to excessive wear if the valve is partially open.
A gate valve is generally used to completely shut off fluid flow or, in the fully open position, provide full flow in a pipeline. Thus it is used either in the fully closed or fully open positions. A gate valve consists of a valve body, seat and disc, a spindle, gland, and a wheel for operating the valve. The seat and the gate together perform the function of shutting off the flow of fluid. A typical gate valve is shown in Figure 12.9.
Gate valves are generally not suitable for regulating flow or pressure or operating in a partially open condition. For this service, a plug valve or a control valve should be used. It must be noted that because of the type of construction a gate valve requires many turns of the hand wheel to completely open or close the valve. When fully opened, gate valves offer little resistance to flow and its equivalent length to diameter ratio (L/D) is approximately 8. The equivalent L/D for commonly used valves and fittings is listed in Table 12.3.
Table 12.3. Equivalent lengths of valves and fittings
Plug valve straightway
Plug valve 3-way thru-flo
Plug valve branch flo
Swing check valve
Lift check valve
Standard elbow: 90°
Standard elbow: 45°
Standard elbow long radius 90°
Standard tee thru-flo
Standard tee thru-branch
Mitre bends: α = 0
Mitre bends: α = 30
Mitre bends: α = 60
Mitre bends: α = 90
The gate valves used in the mainlines carrying oil or gas must be of full bore or through conduit design to enable smooth passage of scrapers or pigs used for cleaning or monitoring pipelines. Such gate valves are referred to as full bore or through conduit gate valves.
Gate valves are characterized by a “gate” (Figures 4.49 and 4.50) that closes in a plane perpendicular to the flow of fluid. They are used primarily for on/off, nonthrottling service. Shearing of high-velocity flow will cause a partially open disk to vibrate and chatter, which will damage the seating surfaces and prevent a tight seal. They are suitable for most fluids including steam, water, oil, air, and gas. Gate valves may have either a solid or flexible wedge disk. In addition to on/off service, gate valves can be used for regulating flow, usually in sizes 6 in. and larger, but will chatter unless the disk is fully guided throughout travel.
Gate valves respond slowly, requiring numerous turns of the handwheel, to go from fully open to fully closed. The disks are made in either a solid or flexible wedge disk. Flexible disks were developed to overcome sticking on cooling in high-temperature service and minimize operating torque. High-pressure service of large sizes is usually cheaper than plug or ball.
Gate valves are more commonly used in refineries and petrochemical plants where pressure remains relatively low, but temperature may be very high. Gate valves are used less in upstream oil and gas production facilities due to high operating pressures, long opening/closing times, and severe environmental conditions when operating in marine atmospheres.
The gate valve is a block valve. Due to its design, it cannot control flow; even at the beginning of opening, erosion of the seat and disk occurs, resulting in the destruction of the tight faces.
18.104.22.168.2 Parallel gate valves
Parallel gate valves utilize a parallel-faced, gatelike seating element. A double-disk parallel gate valve has two parallel disks that are forced, on closure, against parallel seats by a “spreader.” They are used for liquids and gases at normal temperatures. On the other hand, the seating force in a single-disk parallel gate valve is provided by the fluid pressure acting on either a floating disk or a floating seat. This configuration allows closure with flow in either direction. They are used for liquid hydrocarbons and gases. If the fluid pressure is low, the seating force provided by the fluid pressure may be insufficient to produce a satisfactory seal in metal-sealed valves. If the fluid pressure is high, frequent valve operation may lead to excessive wear of the seating forces; thus, parallel gate valves are normally used for on/off duties that require infrequent operation.
22.214.171.124.3 Full-bore through conduit gate valves
Full-bore through conduit gate valves prevent solids from entering the body cavity. The valve body extends equally on both sides of the valve centerline to form a cavity long enough to contain a disk or gate having a circular port of the same dimension as the pipe internal diameter (full port valve) (Figure 4.51). These valves may be a solid plate or two-piece plate design, which isolates the valve body cavity against the fluid in both the open and closed position. They are used in pipelines that must be scraped or where a full-bore valve is required.
126.96.36.199.4 Wedge gate valves
Wedge gate valves differ from parallel gate valves in that the seating element is wedge-shaped, instead of parallel (Figure 4.52). The disk or wedge can be a single-piece or a two-piece design. The purpose of the wedge shape is to introduce a high supplementary seating load that enables metal-sealed wedge gate valves to seal against not only high but also low fluid pressure. The wedge shape also results in a seal on both sides of the gate. Since the disk is in contact with the seats only when the valve is closed, the wedge gate valve offers a maximum resistance to wear, where turbulent flow is present.
188.8.131.52.5 Plug disk gate valves
The plug disk gate valve differs from other gate valves in that it can be used in a throttling service (Figure 4.53). This configuration offers minimum resistance to flow when fully open, a feature common to most gate valves. They are used where minimal pressure drop (unrestricted flow) is required.
Gate valve: An on-off valve that works by inserting a rectangular gate or wedge into the flow of the fluid. The fugitive emission standard for gate valves is covered by API 624 and ISO 15848-1. It should be noted that the stem motion in a gate valve is typically linear, which creates a lot of friction between the valve stem and packing. This friction can cause packing wear and tear as well as leakage. Gate valves are available in different types, such as slab, expanding and wedge. Wedge gate valves have a sealing element in the shape of a wedge. A wedge gate valve is a torque seated valve, meaning that the wedge is expanded from both sides due to the stem force and provides sealing. The expansion of the valve closure member due to the stem axial force is called “wedging effect.” Expanding gate valves are also torque seated valves with a closure member in two sections, one male and the other female. Slab gate valves have a flat disk or closure member that provides sealing due to the fluid pressure. Slab gate valves, unlike expanding and wedge gate valves, are not torque seated.
Gear box: Gear boxes are typically used with handwheels to facilitate valve operation. This is a simple and cheap method of valve operation in which gears are used to increase the force and efficiency produced by the operator moving the handwheel. The gears inside the gear box are wheels with teeth that slot together. Let us suppose that a gear box contains two gears, as illustrated in Fig. 1.10; the first one on the left, the "driving" gear, is smaller and has 20 teeth. The second, “driven” gear is larger with 40 teeth. The gear box in this case increases the input force applied by the operator through the handwheel, since the driving gear has lower numbers of teeth. The amount of increase depends on the gear ratio, which is calculated through Formula (1.1).
Gear ratio calculation
Thus the gear ratio in this case is equal to two. It means that if the operator input force on a handwheel with two hands is 300 newton (N), the gear box in the example given above will increase the force to 600 N.
Global warming: Global warming is the gradual heating of the earth’s surface, oceans and atmosphere caused by air pollutants such as methane and carbon dioxide and the greenhouse effect.
Globe valve: A type of valve used for flow regulation or throttling. As illustrated in Fig. 1.11, the fluid makes two 90-degree turns inside the valve, which creates a significant pressure drop.
Greenhouse effect: The greenhouse effect is a natural process that warms the earth’s surface. When the sun’s energy reaches the earth’s atmosphere, some of the heat is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases.
TCG valves have two common types of design, slab gate (Fig. 4.9), and expanding.
Expanding gate valves can be produced as either single expanding or double expanding (Fig. 4.10). A double expanding valve has two half disks and it is used for isolation of the piping system or a component when it is closed.
What is the difference between single expanding and double expanding gate valves? A double expanding wedge is expanded in both open and closed positions, and the two seats are sealed in both open and closed conditions. A single expanding wedge is expanded only in the closed position and the two seats are sealed. Thus, fluid enters the cavity in the open position in a single expanding gate valve, but the cavity is not filled in either open or closed positions for a double expanding gate valve. Double expanding gate valves can guarantee zero leaks to the cavity in both closing and opening conditions. One advantage of a double expanding valve is that this valve contains two half wedges with more disk flexibility in high temperatures due to thermal expansion. Double expanding is the preferred choice of valve for very dirty services in high temperature ranges above 200°C.
TCG valves are not recommended for fluid control (throttling) since prolonged usage under throttling will result in quick damage to the seats of the valves as well as internal valve components. The main parts of the double expanding and slab gate valves are body, bonnet, wedge (gate) halves, seats, and stem. The body and bonnet are the main pressure-containing parts. The disk(s) are positioned by the stem to block the flow or open the fluid path. The wedge assembly in a double expanding gate valve includes two halves, one male and the other female, which are coupled together, usually with springs.
A slab gate valve, also called a single disk, is designed to provide isolation of the piping system or a component when it is in a closed position. This type of valve is also not suitable for fluid control (throttling). Just as the double expanding gate valves, prolonged usage of a slab gate valve under a throttling operation will result in wearing and damage of the seats and internal components. The main parts of a double expanding and slab gate valve are body, bonnet, wedge (gate), seats, and stem. The body and bonnet are the main pressure-containing parts. The seats are usually floating-type seats with the springs at the back to ensure proper tightness between the gate and seats.
A double expanding valve is two wedge gate valves in series with one bleed gate valve between. A double expanding valve can be as much as 50% more expensive on average than a slab gate valve in small sizes and 25%–30% more expensive in larger sizes. Single expanding valves are usually less expensive than double expanding. Single expansion is a good choice if a more robust option than a slab gate valve is required, and the valve is most often in the closed position. Comparing the metal seat ball valve with a TCG valve, both slab and expanding are more expensive in large sizes such as 30″ and 36″. In some cases, a ball valve in smaller or medium sizes could be more expensive than a TCG valve. For example, a 12″ Class 1500 ball valve with API 6D design could be more expensive than a slab gate valve in the same size and pressure class and API 6D design. One way to estimate the relative cost of the valves is to compare their weights. As an example, a 3″ slab gate valve in class 300 was compared to a ball valve with the same size, pressure class, and material. The slab gate valve was almost twice as heavy as the slab gate valve. Of course, the weight of a valve is different from one supplier to another one.
A double expanding gate valve is not necessarily a better valve than a slab gate type. But double expanding is a more robust valve since less friction and wearing happens between the seat and the disk during disk traveling. In addition, less fluid is accumulated and pressurized into the cavity so double expanding has a longer packing life compared to the slab gate valve. However, the actuation of a double expanding valve is challenging because this valve is a torque-seated type and over-torquing the valve could damage the disk. Therefore, extra attention should be paid to sizing and selecting the actuator for a double expanding gate valve to avoid over-torquing.
The wedge is expanded at the top (open) and bottom (closed) points in a double expanding gate valve. Expansion of the wedge toward the seats by the mechanical stem force and sealing pressure between two halves provides tight sealing with two seats in both directions. Tight contact of the wedge with the seats at open and closed positions increases the breakaway torque in both closed and open situations, which increases the actuator sizes. For this reason, an end user may avoid selecting a double expanding valve to prevent having a larger actuator. As mentioned earlier, over-torquing an actuated double expanding gate valve can damage the seat, wedge, and stem. Slab gate valves are not torque-seated so there is no risk of damage to valve components due to over-torquing.
Gate valves or ball valves are two typical valves used in the manifolds. Gate valves have a long history of use in subsea blowout preventer (BOP) stacks, trees, and manifolds and are considered relatively reliable devices because both the valve and the valve actuators have been through extensive development with proven field use and design improvements. Figure 19-6 illustrates two types of subsea gate valves. Figure 19-6A shows a WOM(Worldwide Oilfield Machine, Inc.) subsea gate valve with actuator, compensator, and ROV bucket. The hydraulic actuator is designed with a fail-safe model and spring returns with the ROV. The mechanical ROV is for backup. Figure 19-6B shows a WOM subsea gate valve with only an ROV bucket. Both valves are designed, built, and tested based on API 6A  and 17D , which can be used up to a water depth of 13,000 ft (4000 m).
Ball valves also are proven items and their use in deeper water depths is increasing. In some deepwater applications, ball valves can provide operational and cost advantages over gate valves, and improvements in nonmetallic seals and coatings are raising the reliability of ball valves. Ball valves were initially used downstream by the gas industry in gas pipeline valves. At that time, pipeline gate valves were the standard valves used in liquid pipelines . Even today, gate valves are frequently specified for liquid pipelines, and ball valves are specified for gas pipelines. When gas wells were completed in the Gulf of Mexico in the 1960s, ball valves were installed in pipelines both as isolation valves and as terminal valves to tie in lateral lines from future wells and platforms. In the late 1970s, ball valves were installed in the North Sea and encountered problems due to the more challenging conditions of the sea. Later, ball valves were installed in subsea projects as emergency shutdown (ESD) valves to prevent gas in a pipeline from flowing back to a platform in the event of a major leak.
Figure 19-7 shows a typical two-way subsea ball valve from Autoclave Engineers that is designed to facilitate operation by an ROV. The valve design incorporates additional O-ring seals, which prevent the ingress of seawater into the valve. Seawater would adversely affect the operation of the valve and also contaminate the process fluid. The valve can be used in water depths to 12,500 ft (3800 m) with maximum internal pressures of 20 ksi.
Gate valves are used when a straight-line flow of fluid and minimum flow restriction are needed. Gate valves use a sliding plate within the valve body to stop, limit, or permit full flow of fluids through the valve. The gate is usually wedge-shaped. When the valve is wide open, the gate is fully drawn into the valve bonnet. This leaves the flow passage through the valve fully open with no flow restrictions. Therefore, there is little or no pressure drop or flow restriction through the valve.
Gate valves are not suitable for throttling volume. The control of flow is difficult because of the valve's design and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve. Except as specifically authorized by the manufacturer, gate valves should not be used for throttling.
Gate valves are classified as either rising-stem or non-rising-stem valves. The non-rising-stem valve is shown in Figure 7-2. The stem is threaded into the gate. As the handwheel on the stem is rotated, the gate travels up or down the stem on the threads while the stem remains vertically stationary. This type of valve will almost always have a pointer indicator threaded onto the upper end of the stem to indicate the position of the gate.
Valves with rising stems (Figure 7-3), are used when it is important to know by immediate inspection whether the valve is open or closed or when the threads exposed to the fluid could become damaged by fluid contamination. In this valve, the stem rises out of the valve bonnet when the valve is opened.
Gate and globe valves can have fire test certificates according to API 6FA or ISO 10497 standards. The fire test certificate is not usually required for gate and globe valves with no nonmetallic parts. A fire test guarantees that the valve will function properly during a fire. ATEX is the European regulatory framework for manufacturing, installation, and use of equipment in explosive atmospheres. ATEX certification indicates that the valve does not have any source of ignition, which is applicable for equipment in potentially explosive atmospheres. Valves with actuators are usually in the ATEX scope of work because the ATEX directive does not consider the process source of ignition inside ATEX. Only external sources of ignition such as actuators with electrical parts make the valve fall inside ATEX.