How It Works: Gate Valves

Stem Mechanism

Gate valves are typically available in two designs when it comes to stems: rising and non-rising. Rising stems are directly connected to the gate, providing a clear visual representation of the valve’s status. In contrast, non-rising stems are usually threaded into the top part of the gate and feature a pointer to indicate their position. Non-rising designs are particularly effective in scenarios where space is limited vertically, such as well applications, and in cases where scraping or pigging of the valve isn’t necessary.

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Each gate valve is equipped with a sealing mechanism, ensuring a tight seal around the stem. Our innovative single loaded-spring (SLS) stem seal design, which is incorporated in Saf-T-Seal slab gate valves as well as WKM Pow-R-Seal double expanding gate valves, offers enhanced leak protection. It also features a self-adjusting seal designed to minimize maintenance needs.

Bonnet Types

There are four common types of bonnets utilized in gate valves to secure fluids and prevent leakage. Screw-in bonnets present a durable solution that relies on pressure to form a seal. Union bonnets are designed for easy access to the valve body, making them suitable for applications that require regular maintenance. Larger valves meant for higher pressure applications typically adopt bolted bonnets. Lastly, pressure seal bonnets are specifically crafted for services that exceed 2,250 psi [15 MPa].

Application Range

Due to the versatility of their construction materials, trim choices, and design options, gate valves are suitable for an array of applications. They are capable of operating in diverse environments — from high-temperature coking units to food and pharmaceutical processing, gate valves deliver dependable performance.

The protected seat-face design prevalent in double expanding and slab gate valves serves to shield the seat from wear caused by debris in the process fluid, enhancing their effectiveness in liquid applications. In situations where pipeline operational integrity is crucial, especially near waterways and municipal areas, double expanding gate valves present a smart option.

The smaller 2- to 4-inch non-rising stem variant of our Pow-R-Seal API 6A expanding gate valve is frequently deployed in wellhead manifold systems, prized for its robust mechanical sealing and impressive pressure capabilities.

Drilling manifold systems can also accommodate specific gate valve models, like the Cameron DEMCO valve DM series, which features space-efficient mounting designs.

In the power sector, NEWCO's gate, globe, and check valves, along with DOUGLAS CHERO's forged-steel counterparts, excel in both standard and critical applications, such as steam distribution in power facilities. By employing a welded connection that eliminates potential leak paths, the design minimizes weight while simplifying the exterior insulation process. Coupled with the forged steel body, this design ensures maximum sealing integrity.

For extreme subsea environments characterized by high pressures and low temperatures, subsea manifolds incorporating valves and interface panels are essential for ensuring critical isolation. The straightforward design of the Cameron RING-O subsea valve is highly compatible with these systems, capable of being actuated manually, via ROV, or through hydraulic means for user-friendliness.

Gate valves are the go-to choice for shut-off applications. Unlike ball valves that operate with a quarter-turn, gate valves function by opening and closing with multiple turns of the hand wheel.

Commonly deployed in wastewater treatment facilities, power plants, and processing systems, the threaded design of gate valves grants them an advantage over the quarter-turn functionality seen in ball and butterfly valves in larger applications.

When fully open, gate valves allow fluid to pass with minimal pressure loss.

This article covers essential aspects including the functions, symbols, benefits and drawbacks, and types of gate valves.

Defining a Gate Valve

Gate valves, also referred to as sluice valves or knife valves, serve as control valves that either permit or restrict the flow of materials completely. They employ a flat gate to obstruct flow between pipe flanges.

As full port valves, gate valves maintain an internal diameter equal to that of the pipe, minimizing pressure losses during operation. Moreover, this design allows for easy movement of pigs within the pipeline for cleaning and inspection purposes.

While butterfly valves can manage flow regulation and serve on/off functions, gate valves are best suited only for on/off applications and should not be utilized for throttling.

Renowned for their straightforward construction and ability to perform across various low-pressure scenarios, gate valves rank among the most common types in the industry.

Gate Valve Symbolization

The representation of a gate valve consists of two triangles meeting at a point, with a vertical line placed centrally between them, extending solid horizontal lines from opposing sides of the triangles.

The vertical line denotes the blocking operation, while the horizontal lines illustrate the valve's two-way function for on/off operations.

The gate valve symbol used in piping and instrumentation diagrams (P&ID) modifies the standard valve symbol by excluding the vertical line separating the triangles.

The three symbols shown in an isometric diagram demonstrate connections for butt-welding, flanged ends, and socket ends.

Various Types of Gate Valves

Gate valves are categorized in several distinct ways:

Parallel vs. Wedge-shaped Gate Valves

Primarily, gate valves can be divided into two types — parallel and wedge-shaped. Parallel gate valves utilize a flat gate positioned between two parallel seats to shut off flow. The gate seals shut when pipe pressure is applied, making these valves favorable for low-pressure conditions.

• Knife gate valves represent a popular form of parallel gate valve, employing a pointed gate to block fluids containing suspended solids or slurries. The disc's design wipes as it traverses the seat rings.

• Slab gate valves, known as through-conduit valves, feature a bore in their construction. The slab gate aligns with the flow to minimize turbulence, enabling low pressure loss and exceptional performance for crude oil transmission.

• Wedge-shaped gate valves exist, shutting off fluid with a wedge-shaped gate that positions itself on two opposing inclined seats, thus tightening the seals through added pressure from the wedge seats. This design longevity results from reduced contact with the seats.

• The split wedge gate valve design enhances sealing performance due to its flexible characteristics; it avoids jamming despite misaligned inclined seats, making it effective with abrasive fluids.

• Solid wedge gate valves—denoting a robust structure—are versatile across various pipe configurations.

• Flexible wedge gate valves come in diverse forms, with each variant presenting unique benefits. For instance, a narrow flexible wedge boasts strength yet limited malleability compared to a wider gate which offers more flexibility, commonly found in steam systems.

Metal Seated vs. Resilient Seated Gate Valves

In metal seated gate valves, a conical gate enters a hole at the valve’s bottom upon closure, anchored by inclined seating. Despite their antiquated design making them vulnerable to impure substances, they are not as commonly used as resilient seated gate valves today.

Resilient seated gate valves, favoring a flat base, permit entry of impurities without accumulating them, unlike their metal seated counterparts. The rubber element in a resilient seated gate valve draws in impurities upon closure and cleanses them upon opening.

Rising vs. Non-Rising Stem Gate Valves

Rising stem gate valves position their stems outside the valve body, moving along with the gate, allowing ease of maintenance observation. This design is optimal for surface installations.

Conversely, non-rising stem designs rotate the stem synchronously with the gate from within the valve, featuring an indicator atop the stem to display position. This model is compatible with industrial use, suitable for both surface and underground installations.

Additional Gate Valve Types

Parallel Expanding Gate Valve

These valves utilize a combination of two slab gate designs to form effective seals at the gate seats, predominantly utilized in electrical systems and the petroleum sector.

Benefits and Drawbacks of Gate Valves

When utilized as control valves, gate valves exhibit valuable attributes in various industrial operations. Like other control valves, gate valves possess advantages and disadvantages. Notably, their merits include:

Advantages

Minimal Flow Resistance

Being a full port valve, the gate valve facilitates fluid passage with little resistance. When fully opened, the valve body does not hinder flow motion or direction.

Superior Sealing Performance

Gate valves outperform shutoff valves in sealing efficiency. Though opening and closing are slower than other valve types, they still operate quicker than traditional shutoff valves.

Versatile Applications

Gate valves demonstrate efficacy across various fluid types, including oil, steam, slurries, and suspended materials. They can also function as low vacuum system valves or venting valves.

Bidirectional Flow

Gate valves are not confined to a single flow direction. Their two-way directional capability means they can adapt irrespective of media direction, making them suitable for situations where fluid flow can change unexpectedly.

Further advantages of gate valves encompass:

• The gradual opening/closing motions of gate valves minimize the risk of a water hammer effect when the valve shuts.
• Within gate valves, moving fluids generally aid in washing away impurities.
• Gate valves can effectively handle higher pressure conditions compared to butterfly valves.
• They excel when dealing with high-viscosity fluids.
• For valve manufacturers, gate valves present straightforward construction, simplifying installation and replacement.
• Minimal force is required to operate the gate.

Disadvantages

Slow Operating Mechanism

Gate valves experience potential damage if operated partially. Therefore, the gate must retract fully into the upper valve section when opened and rest on the vale seat when closed to seal properly before fluid flow is established.

The prolonged motion from the upper to the lower valve areas contributes to gate valves' slow operational nature, rendering them unsuitable for quick process applications.

Frequent Contact Damage

As the valve gate consistently engages with the seat, wear can lead to surface damage over time, reducing the operational lifespan of gate valves. Surface damage is often challenging to rectify.

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Other disadvantages of gate valves include:

• They occupy more space than butterfly valves due to their larger size.

Functionality of Gate Valves

Gate valves primarily operate in on/off capacities, allowing or restricting flow. To maintain their effectiveness, gate valves should not be used with excessive frequency, as frequent movement can reduce their service lifespan.

With their capacity to minimize pressure losses while permitting fluid movement through pipelines and facilitating the operations of pigs for cleaning and inspection, gate valves are widely favored for isolation and blocking applications.

However, gate valves should not be employed for regulating flow, as constant contact with the gate can result in damage to the seat and disc, leading to a reduced functional lifespan.

They find extensive use in diverse industries, including petroleum refineries, power generation plants, manufacturing facilities, and chemical production. Gate valves prove effective under conditions of high temperature and pressure.

Components of Gate Valves

The main components of gate valves include the body, hand wheel, yoke, bonnet, packing gland, stem, and gate. Below we break down each part distinctly.

Body

The gate valve body houses the valve trim and permits fluid to flow from one end of the pipe to the other. The materials used for valve bodies can include metals, metal alloys, or various other nonmetals.

Commonly referred to as shells, the gate valve body can connect to pipe flanges via butt, socket, wafer, or threaded connections.

There are three primary designs for gate valve bodies based on their intended usage: Reduced Bore, Full Bore, and Cross Flow.

Bonnet

The bonnet of a gate valve serves as a cover and also provides support for key components, such as the disc, stem, and actuators. Additionally, it allows for stem access.

The bonnet connects to the body through threaded, bolted, or welded methods. Some designs incorporate bonnetless valves wherein the bonnet merges seamlessly with the body, typically constructed from the same material.

Trim

The valve trim consists of the valve disc, gland sleeve, gland packings, back seat, valve stem, and valve seat, controlling the functioning of the valve through direct contact with the flowing medium.

Stem

The stem controls the movement of the disc. When rotated, the stem lifts or drops the disc, allowing for precise flow control. It connects to the actuator (often via the yoke) at one end and to the valve disc at the opposite end.

Valve Seat

The valve seat acts as the resting point for the disc, to which it seals. While some valves feature a single seat, gate valves are equipped with two seats, one at each end of the pipe flanges. Failing to achieve proper sealing between the disc and seat can lead to leaks.

Backseat

The backseat assists in maintaining the stem in an open position while also preventing fluid from entering the packing chamber.

Disc

The disc is crucial to gate valve functionality, as it governs the flow through the pipeline, effectively blocking or allowing fluid movement. It seals against the valve seat when engaged.

Gland Packings, Sleeve, and Yoke

Gland packings are designed to prevent leaks from the stem, housed within a stuffing box while gland sleeves maintain the packings securely in place.

The yoke serves as a connection point between the actuator and stem, sometimes integrated with the bonnet design.

Mechanism of Gate Valve Operation

The operation of gate valves aligns closely with standard valve mechanisms. The stem is manipulated via the hand wheel or actuator which controls the gate. Operating the hand wheel requires multiple turns to transition the valve from closed to fully open or vice versa.

The actuator method is simpler and does not necessitate physical presence at the location, requiring only an actuator system (be it pneumatic or electric) at the site. By raising the gate to the fully open position, flow through the pipeline occurs unhindered.

Conversely, switching the valve to the closed position involves sealing against the valve seat, which preserves flow pressure and restricts media movement. The flow rate may fluctuate during closure, creating an unpredictable relationship between the valve and fluid flow rate.

It is essential to note that fluid pressure and flow can cause wear on both the seat and gate. Therefore, gate valves should not be deployed for flow regulation, as their continuous exposure to flow can result in accelerated damage to the gate and seat components.

Gate Valves: Installation and Maintenance

Installation Practices

Proper installation of gate valves is crucial for extending their operational lifespan and ensuring reliable performance. Adhere to the following practices:

• Ensure the flange is at room temperature during installation.
• Although gate valves can be positioned in multiple orientations, vertical installation is preferred to facilitate easy access to the handwheel.
• Check that pipe flanges are spaced appropriately to allow for valve insertion and removal during maintenance.
• Complete any welding processes ahead of valve installation.
• Cleanse the pipeline of sludges and waste prior to valve installation.
• Verify the alignment of the flange and valve, ensuring the pipe can support the weight of the valve during handling.
• Inspect all components, certifying no damage occurred during transportation or storage.
• Tighten the bolts securing the valve to the pipe appropriately, distributing pressure evenly.
• Ensure flanged areas contacting the valve are clean and free from debris.
• Test the valve's stability post-installation.

Maintenance Recommendations

Implement the following practices to preserve valve functionality and longevity:

• Disengage pressure from pipes before commencing maintenance on gate valves.
• Ensure all pressure is released prior to beginning maintenance work.
• Replace the gate valve disc by accessing from the top, removing the bonnet and stem first.
• Engage in regular cycling of the valve when it remains unused.
• Follow reverse steps for valve removal during disassembly.

Factors to Consider When Purchasing a Gate Valve

When selecting an appropriate gate valve for specific operational conditions, consider the material, size, and design. Key considerations include:

Material Selection

The material composition for gate valve production should cater to distinct conditions. Common materials used include cast iron, cast carbon steel, stainless steel, ductile iron, gun metal, bronze, steel alloys, and forged steel.

Essential selection criteria comprise the fluid service and design temperature. The following materials are frequently employed in gate valve construction:

Valve Body Materials

• Cast Carbon Steel to ASTM A216 Gr. WCB.
• Stainless Steel to ASTM A351 Gr. CF8/8M/3/3M.
• ASTM A351 Gr. CN 7M.

Gasket Materials

• Compressed Asbestos Fiber Flat Ring.
• Graphite Asbestos Flat Ring.
• SS 304/316/304L/316L Spiral Wound with PTFE/CAF/Grafoil Filler.

Trim/Working Parts Materials

Below are typical trim material grades used in gate valve production:

• AISI 410 (SS 410, 13% Cr. Steel).
• AISI 304.
• AISI 316.
• AISI 304L.
• AISI 316L.
• Hast Alloy B/C.
• ASTM A351 Gr, CN 7M.
• Incorporates stellite hard face seat and wedge face.

Trim materials are organized and designated with trim numbers or combination numbers. Utilizing the provided chart facilitates material ordering for a gate valve seat, disc, stem, and backseat aligned against a trim number in accordance with API 600 specifications.

Gland Packing Materials

• Graphite (Grafoil) Molded Ring.
• Graphite Asbestos Rope.
• Inconel Braided Grafoil with corrosion inhibitor.

Codes and Standards

Several codes and standards guide the design and selection of gate valves:

Pressure Testing

API 598.

Pressure Temperature Rating

API B16.34.

Face to Face Standards

ANSI B16.10.

Socket Weld End Standards

ANSI B16.25 and B16.11.

Screwed End Standards

ANSI B1.20.1 (BSP/NPT).

Final Thoughts

This article has elaborated on essential considerations regarding gate valves, encompassing their functions, parts, requirements for installation and maintenance, and necessary factors to consider when procuring an industrial gate valve.

Dombor Valve is a reputable manufacturer specializing in high-quality valve solutions tailored to meet market demands. With two decades of manufacturing expertise, we focus on crafting valves that perform efficiently in all environments. Contact us for the finest professional gate valves available.

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