Soft seat knife gate valve.

A metal seated knife gate valve (left) and resilient, a soft-seated valve (right side).

The differences with standard design are:

• A standard valve is available with flanged, butt weld, and socket weld connections (knives have lugged or wafer connections mainly)
• A standard valve has a V-ring packing set that seals the shaft that is attached to the gate. Knife valves feature instead of a packing gland area that seals around the gate
• Gate valves are bidirectional, whereas the knife type is generally uni-directional
• A knife valve has a smaller profile than the ANSI gate valve, which is more bulky and refined
• Knife gate valves are lighter and cheaper than API and ASME types

GATE VALVE VS. OTHER TYPES OF VALVES

GATE VALVE VS. BALL VALVE

What is the difference between a gate and a ball valve?

GATE VALVE VS. GLOBE VALVE

What is the difference between a gate and a globe valve?

Gate valves and globe valves are two fundamental types of valves used in piping systems to control the flow of liquids, gases, and slurries. While they share some similarities, they have distinct features, operating principles, and applications that make them suitable for different scenarios.

Understanding the differences between these two valve types is crucial for selecting the right valve for a specific application.

Design And Operation

• Gate Valve: Utilizes a flat gate or wedge that moves vertically to the flow direction to open or close the valve. When open, the gate fully retracts into the valve body, allowing for a full, unobstructed flow path. Gate valves are primarily used for on/off control and are not suitable for throttling due to the potential for gate and seat damage.
• Globe Valve: Features a movable disk-type element and a stationary ring seat in a generally spherical body. The disk moves perpendicularly to the seat, allowing for precise flow control. Globe valves are characterized by their spherical body shape, with the internal baffle that splits the inside space into two chambers. They are used for on/off control as well as for throttling flow, offering better control over flow rates.

Applications

• Gate Valves: Ideal for applications where a straight-line flow of fluid and minimum restriction is desired. Commonly used in water supply, natural gas pipelines, and in applications where the valve will remain either fully open or fully closed for long periods.
• Globe Valves: Suited for applications requiring flow regulation and frequent operation. Their ability to adjust the flow with precision makes them popular in cooling systems, fuel oil systems, marine applications, and where pressure drop is not a significant concern.

Advantages And Disadvantages

• Gate Valves:
  • Advantages: Minimal pressure drop when fully open; suitable for both slurries and viscous fluids; provides a tight seal when closed.
  • Disadvantages: Slow to open and close; not suitable for throttling; can be prone to gate and seat damage from vibration if partially opened.
• Globe Valves:
  • Advantages: Good for throttling and regulating flow; faster to open and close compared to gate valves; provides better sealing.
  • Disadvantages: Higher pressure drop across the valve; not ideal for applications requiring full, unobstructed flow.

Choosing Between Gate And Globe Valves

Choosing between a gate valve and a globe valve often depends on the specific requirements of the system, including:

• Purpose: Gate valves are preferred for on/off control where the flow rate is not adjusted frequently. Globe valves are chosen for applications where flow needs to be regulated or adjusted regularly.
• Flow Characteristics: If minimal pressure drop and full flow are required, gate valves are more suitable. For precise flow control, even at lower flow rates, globe valves are preferred.
• Space and Orientation: Globe valves, due to their design, may require more space in a piping system and are sensitive to flow direction. Gate valves are less restrictive in terms of space and flow direction.

In summary, the choice between gate and globe valves hinges on the need for either unobstructed flow and infrequent operation or the need for flow regulation and frequent adjustments. Both valves serve critical roles in controlling system flow, and their selection should align with the operational needs and constraints of the application.

GATE VALVE VS. CHECK VALVE

Gate valves and check valves are two distinct types of valves used across various industries for controlling fluid flow in piping systems. Each serves a different primary function and operates based on different principles.

Gate Valve

Design and Function: A gate valve features a movable gate or wedge that slides vertically to control the flow of fluid. It is operated manually, typically using a handwheel or an actuator for larger sizes. The primary function of a gate valve is to start or stop the flow, providing a clear and unobstructed path when fully open, and a tight seal when fully closed.

Applications: Gate valves are widely used in applications where a full, unrestricted flow of fluid is necessary. They are ideal for on/off control but are not suitable for throttling purposes, as partial opening can cause vibration and damage to the gate and seats. Common uses include water supply, oil and gas pipelines, and other systems where flow needs to be completely shut off or allowed freely.

Advantages:

• Minimal pressure drop when fully open.
• Suitable for both liquid and gas applications.
• Provides a tight seal when closed.

Disadvantages:

• Slow to open and close.
• Not suitable for throttling.
• Prone to wear and corrosion, which can affect sealing over time.

Check Valve

Design and Function: A check valve, also known as a non-return valve, allows fluid to flow in one direction and automatically prevents backflow when the fluid in the line reverses direction. It operates based on the flow pressure and does not require manual operation. The internal mechanism varies by design, including ball, swing, and lift check valves.

Applications: Check valves are essential in preventing backflow, protecting equipment, and ensuring the safety of the system. They are used in a wide variety of applications, including water and wastewater treatment, chemical processing, and residential plumbing systems. Any system where backflow could cause problems or where fluid needs to be maintained in a single direction benefits from the use of a check valve.

Advantages:

• Prevents backflow automatically.
• Can be used in a wide range of pressures and temperatures.
• Available in various designs to suit specific flow characteristics.

Disadvantages:

• Cannot be used to regulate or stop flow.
• Some designs may cause a significant pressure drop.
• Requires careful selection and installation to function correctly.

Key Differences Between Gate And Check Valves

• Primary Function: Gate valves are used to start or stop the flow, while check valves are designed to prevent backflow and allow flow in only one direction.
• Operation: Gate valves require manual or actuator operation to open or close, whereas check valves operate automatically based on flow conditions.
• Application Use: Gate valves are chosen for system isolation or where full flow is necessary. Check valves are selected to prevent backflow and protect against reverse flow conditions.

Understanding these differences is crucial when designing or maintaining a piping system, ensuring that the right type of valve is used for its intended function, thereby optimizing system performance and safety.

GATE VALVE DIAGRAM

Gate valve diagram showing the key parts of a gate valve for piping

GATE VALVE WEDGE TYPES

In gate valves, the wedge is the movable part that seals against seats to stop the flow or opens to allow flow. The design of the wedge is crucial for the valve’s performance, especially in terms of sealing capability, ease of operation, and durability. There are several types of wedges used in gate valves, each suited to different applications and operating conditions:

1. Solid Wedge

The solid wedge is the simplest and most robust type, made from a single piece of metal. Its simplicity makes it highly reliable and suitable for a wide range of conditions, including high-temperature and pressure environments. However, its rigidity means it may not always compensate for seat misalignments or changes in temperature that affect the valve body and seating surfaces.

2. Flexible Wedge

A flexible wedge is designed with a cut around its perimeter or a special shape that allows the wedge to flex as it seats. This design helps accommodate changes in valve body dimensions due to thermal expansion or contraction, improving the seal in varying temperature conditions. Flexible wedges are particularly useful in steam systems where temperature fluctuations are common. However, they can be less suitable for applications involving high vibration or thermal cycling, which may lead to fatigue cracks.

3. Split Wedge Or Parallel Disks

The split wedge, or parallel disk design, consists of two solid pieces that are hinged together or use a mechanism to keep them in alignment. This design allows the wedge to adjust to variations in the angle between the seats and the wedge surfaces, enhancing sealing effectiveness. Split wedges are advantageous in applications where thermal binding (sticking due to differential thermal expansion) is a concern.

4. Slab Gate

Slab gate valves use a flat gate that slides between two parallel seats, providing a tight seal. While not a wedge in the traditional sense, the slab gate functions similarly by blocking or allowing flow. This design is particularly favored in the oil and gas industry for pipeline valves because it provides a full-bore, low-friction path for the fluid, making it ideal for transporting viscous fluids like oil.

Wedge Selection Considerations

Choosing the appropriate wedge type depends on several factors:

• Operating Conditions: Temperature fluctuations, pressure range, and the presence of vibrations can affect wedge selection.
• Fluid Characteristics: Slurry services may require a specific wedge type to prevent particle trapping.
• Sealing Requirements: Some applications demand tighter seals, influencing the choice of wedge design.

Understanding the different types of wedges in gate valves and their respective advantages and limitations is essential for selecting the right valve for a specific application, ensuring optimal performance and longevity.

The image below shows how the gate valve wedge opens and closes the flow of the fluid by application of a vertical movement (which can be manual or operated by an actuator).

The wedge is positioned between two parallel (or oblique) seats that are perpendicular to the flow. The fluid flows horizontally through gate valves and is not subject to pressure drops.

The image below shows the different types of wedges used in gate valves:

• “solid wedge” (in this case, the wedge is manufactured with a solid piece of steel)
• “flexible-wedge” (in this case the disc features cuts around its perimeter to enhance the ability of the valve to correct changes in the angle between the seats)
• “split-wedge” (two pieces construction disc, to enforce self-alignment of the wedge on the seats)
• “parallel-slide wedge“

GATE VALVE STEM TYPES

Gate valves control fluid flow by lifting a barrier (gate) out of the fluid path, and this operation is facilitated by the valve’s stem. The stem, which connects the actuator (e.g., handwheel, lever, or electric motor) to the gate, is a critical component in translating the actuator’s motion into the opening or closing of the valve. There are several types of stems used in gate valves, each with its specific design and operational characteristics:

1. Rising Stem (OS&Y – Outside Screw And Yoke)

The rising stem design features a stem that moves up and down along with the gate, providing a visual indication of the valve’s position (open or closed). In this configuration, the stem’s threads are external, located outside the valve body, and interact with the yoke, which is part of the actuator assembly. The rising stem design is advantageous for visual inspection and understanding the valve’s status, but it requires more vertical space for operation.

• Advantages: Visible indication of valve position; reduced risk of thread contamination by the fluid.
• Applications: Widely used in applications where valve status indication is important and where there is sufficient space for stem movement.

2. Non-Rising Stem (NRS)

In a non-rising stem design, the stem remains stationary in the vertical direction while the gate moves up and down. This is achieved by having the stem threads inside the valve body, engaging directly with the gate. Non-rising stem valves are compact and suitable for applications with limited vertical space.

• Advantages: Requires less vertical space; suitable for underground installations or tight spaces.
• Applications: Common in water, wastewater, and gas services where space constraints exist.

Rising and non rising stem of gate valves

3. Sliding Stem

Though not as common in gate valves, a sliding stem design can be found in some specialized gate valves where the stem slides in and out of the valve body without rotating. This design is similar in principle to the non-rising stem but is distinguished by the mechanism of stem movement.

4. Rotating Rising Stem

A rotating rising stem combines the visual position indication of a rising stem with a rotation mechanism. As the valve is opened or closed, the stem not only rises or lowers but also rotates. This rotation can help reduce the wear on the seating surfaces, extending the valve’s service life.

• Advantages: Visual position indication and reduced seat wear due to rotation.
• Applications: Useful in applications requiring durability and clear valve position indication.

Stem Selection Considerations

Choosing the right stem type for a gate valve involves several factors:

• Space Availability: Non-rising stems are preferred in limited vertical space applications while rising stems are chosen when visual position indication is crucial and space permits.
• Environment: External stem threads (rising stems) are less prone to contamination in clean environments, whereas internal threads (non-rising stems) are protected from the external environment but can be exposed to the process fluid.
• Operation and Maintenance: Considerations include ease of operation, maintenance requirements, and the need for clear valve position indication.

Understanding the different types of stems and their operational characteristics is essential for selecting the appropriate gate valve for a specific application, ensuring optimal performance and longevity.

OS&Y VS. IS&Y DESIGN

It is very frequent to see the term “OS&Y” associated with gate valves.
This term means that when the handle of a gate valve is rotated to open or close the valve, it directly raises and lowers the disc by interacting with the stem of the valve.

In an “OS&Y gate valve”, the stem of the valve itself raises and lowers outside the body of the valve in a very visible way, while the handle remains in a fixed position.

When the stem raises, the disc inside the body of the valve rises from the seat letting the fluid flow through the valve (valve in open position).
Therefore with an OS&Y gate valve, the actual position of a valve (closed or open) is always evident to the operators.

Differently, the valve position is not immediately visible for “IS&Y gate valves” (inside screw and yoke), as the stem of the valve does not raise or lowers outside the valve when the handle is rotated.

GATE VALVE MATERIALS

BODY

The body of gate valves below 2 inches is generally made of forged steel (the most common body material grades are ASTM A105 for high-temperature service, ASTM A350 for low-temperature service, and, ASTM A182 F304/F316 for corrosive service).

The bodies of gate valves of bore sizes above 2 inches are, instead, made of cast steel (the main cast grades are ASTM A216 WCB for high-temperature service, ASTM A351 for low-temperature conditions, and ASTM A351 CF8 and CF8M – i.e. stainless steel 304 and 316 gate valves).

TRIM

The removable and replaceable parts of the valve are collectively defined as “trim” (for a gate valve: seat, disc, backseat, and, stem).
The API 600 specification foresees several standard trim combinations, as illustrated below

API TRIM #	BASE MATERIAL	MATERIAL FOR SEAT	MATERIAL FOR DISC	BACKSEAT MATERIAL	MATERIAL FOR STEM
1	410	410	410	410	410
2	304	304	304	304	304
3	F310	310	310	310	310
4	Hard 410	Hard 410	410	410	410
5	Hard faced	Stellite	Stellite	410	410
5A	Hard faced	Ni-Cr	Ni-Cr	410	410
6	410 and Cu-Ni	Cu-Ni	Cu-Ni	410	410
7	410 and Hard 410	Hard 410	Hard 410	410	410
8	410 and Hardfaced	Stellite	410	410	410
8A	410 and Hardfaced	Ni-Cr	410	410	410
9	Monel	Monel	Monel	Monel	Monel
10	316	316	316	316	316
11	Monel	Stellite	Monel	Monel	Monel
12	316 and Hardfaced	Stellite	316	316	316
13	Alloy 20	Alloy 20	Alloy 20	Alloy 20	Alloy 20
14	Alloy 20 and Hardfaced	Stellite	Alloy 20	Alloy 20	Alloy 20
15	304 and Hardfaced	Stellite	Stellite	304	304
16	316 and Hardfaced	Stellite	Stellite	316	316
17	347 and Hardfaced	Stellite	Stellite	347	347
18	Alloy 20 and Hardfaced	Stellite	Stellite	Alloy 20	Alloy 20

MATERIAL SELECTION

TRIM	RECOMMENDED SERVICE
13% Cr, Type 410 Stainless Steel	For oil and oil vapors and general services with heat treated seats and wedges.
13% Cr, Type 410 plus Hardfacing	Universal trim for general service requiring long service life up to 1100°F (593°C).*
Type 316 Stainless	For liquids and gases that are corrosive to 410 Stainless Steel, up to 1000°F (537°C).*
Monel	For corrosive service to 842°F (450°C) such as acids, alkalies, salt solutions, etc.
Alloy 20	For corrosive service such as hot acids -49°F to 608oF (-45°C to 320°C).
NACE	Specially treated 316 or 410 trim combined optionally with B7M Bolts and 2HM nuts to meet NACE MR-01-75 requirements.
Full Stellite	Full hard-faced trim, suitable for abrasive & severe services up to 1200°F (650°C).

GATE VALVE DIMENSIONS

The tables show the dimensions and weights of API 600 gate valves (bolted bonnet / rising stem)

CLASS 150

Dimensions in inches (millimeters)

Sizes	L	L1 (BW)	H (Open)	W
2″	7 (180)	8-1/2 (216)	14-1/2 (368)	8 (200)
2-1/2″	7-1/2 (190)	9-1/2 (241)	17 (432)	8 (200)
3″	8 (200)	11-1/8 (283)	18 (457)	8-7/8 (225)
4″	9 (230)	12 (305)	22 (559)	11 (279)
5″	10 (254)	15 (381)	26 (660)	12-3/4 (325)
6″	10-1/2 (266)	15-7/8 (403)	30 (762)	14 (356)
8″	11-1/2 (290)	16-1/2 (420)	38-1/2 (978)	14 (356)
10″	13 (330)	18 (457)	46 (1168)	18 (457)
12″	14 (356)	19 (502)	55-1/4 (1403)	20 (508)
14″	15 (381)	22 (559)	60 (1524)	21-1/2 (546)
16″	16 (407)	24 (610)	74-7/8 (1902)	24 (610)
18″	17 (432)	26 (660)	79 (2007)	27 (686)
20″	18 (457)	28 (711)	87-1/2 (2223)	28 (711)
24″	20 (508)	32 (813)	105 (2667)	31-1/2 (800)
30″	24 (610)	38 (965)	130 (3302)	43 (1092)
36″	28 (711)	44 (1118)	162 (4115)	51 (1295)
GEAR OPERATOR RECOMMENDED FOR SIZE 10″ AND ABOVE

CLASS 300

Dimensions in inches (millimeters)

SIZES	L/L1	H (OPEN)	W
2″	8-1/2 (216)	16 (407)	7-7/8 (200)
2-1/2″	9-1/2 (241)	17-3/8 (442)	7-7/8 (200)
3″	11-1/8 (283)	19-3/4 (501)	8-7/8 (225)
4″	12 (305)	23-3/8 (594)	9-7/8 (251)
5″	15 (381)	23-3/4 (603)	12-1/2 (318)
6″	15-7/8 (403)	32-1/8 (816)	14 (356)
8″	16-1/2 (420)	41 (1041)	15-3/4 (400)
10″	18 (457)	48-3/8 (1229)	17-3/4 (451)
12″	19-3/4 (501)	57 (1448)	20 (508)
14″	30 (762)	62-1/2 (1588)	22 (559)
16″	33 (838)	69 (1753)	25 (635)
18″	36 (914)	80-1/2 (2045)	28 (711)
20″	39 (991)	91 (2311)	35-1/2 (902)
24″	45 (1143)	120-1/2 (3061)	43 (1092)
GEAR OPERATOR RECOMMENDED FOR SIZE 8″ AND ABOVE

CLASS 600

Dimensions in inches (millimeters)

SIZES	L/L1	H (OPEN)	W
2″	11-1/2 (290)	16-1/2 (420)	7-7/8 (200)
2-1/2″	13 (330)	18 (457)	8-7/8 (225)
3″	14 (356)	20-1/8 (511)	9-7/8 (251)
4″	17 (432)	25 (635)	14 (356)
5″	20 (508)	30-1/2 (775)	15-3/4 (400)
6″	22 (559)	33-5/8 (854)	17-3/4 (451)
8″	26 (660)	42-3/8 (1076)	20 (508)
10″	31 (787)	49 (1245)	25 (635)
12″	33 (838)	68-1/2 (1740)	27 (686)
14″	35 (889)	69 (1753)	31-1/2 (800)
16″	39 (991)	74 (1880)	35-1/2 (902)
18″	43 (1092)	84-1/4 (2140)	43 (1092)
20″	47 (1194)	93-1/2 (2375)	51 (1295)
24″	55 (1397)	110 (2794)	51 (1295)
GEAR OPERATOR RECOMMENDED FOR SIZE 8″ AND ABOVE

CLASS 900

Dimensions in inches (millimeters)

SIZES	L/L1	H (OPEN)	W
2″	14-1/2 (368)	26 (660)	10-1/4 (260)
3″	15 (381)	26-3/8 (670)	11-1/2 (292)
4″	18 (457)	30 (762)	14 (356)
6″	24 (610)	40-3/4 (1035)	20 (508)
8″	29 (737)	51 (1295)	24 (610)
10″	33 (838)	61 (1549)	27 (686)
12″	38 (965)	69-1/2 (1765)	31-1/2 (800)
14″	40-1/2 (1029)	77 (1956)	35-1/2 (902)
16″	44-1/2 (1130)	82-3/4 (2102)	43 (1092)
GEAR OPERATOR RECOMMENDED FOR SIZE 6″ AND ABOVE

CLASS 1500

Dimensions in inches (millimeters)

SIZES	L/L1	H (OPEN)	W
2″	14-1/2 (368)	21-1/2 (546)	11-1/2 (290)
3″	18-1/2 (470)	27-1/8 (689)	14 (356)
4″	21-1/2 (546)	31-1/2 (800)	20 (508)
6″	27-3/4 (705)	45 (1143)	24 (610)
8″	32-3/4 (832)	53-1/2 (1359)	27 (686)
10″	39 (991)	65 (1651)	35-1/2 (902)
12″	44-1/2 (1130)	74 (1880)	43 (1092)
14″	49-1/2 (1257)	83-1/2 (2121)	51 (1295)
16″	54-1/2 (1384)	88 (2235)	63 (1600)
GEAR OPERATOR RECOMMENDED FOR SIZE 6″ AND ABOVE