Piggable Expanding and Slab Gate Valves for Pipelines and Terminals
In oil and gas pipelines and terminals, two specialized types of gate valves are often relied on for their excellent shut-off capabilities: expanding gate valves and slab gate valves. Both are through-conduit valves and maintain an unobstructed bore when fully open, making them ideal for pigging applications.
Beyond shut-off, swing check valves are frequently used to prevent backflow in pipelines to protect pumps from damage and ensure the continuous, unidirectional movement of the liquids or gas.
For essential valve supply and service, count on our team at Williams Valve. Ensure optimal pipeline integrity and flow control with our high-performance gate and check valves.
Slab vs. Expanding Gate Valves
Slab and expanding gate valves are widely used in midstream pipelines and terminal storage. Both types are full bore, bi-directional, capable of double block and bleed sealing, and ideal for pigging operations. The through-conduit design has a smooth, continuous bore that matches the internal diameter of the pipeline when fully open. This eliminates any cavities or obstructions that could trap debris or impede the passage of pipeline pigs. It also reduces turbulence and pressure loss across the valve.
These valves are a top entry design with floating seats, and are serviceable while installed in the flow line. They are manufactured to API-6D requirements and carry the API-6D monogram audited and issued by the American Petroleum Institute.
Which valve is better for your operational requirements depends entirely on the application. Here is a quick side by side comparison of their key features:
| Feature | Slab Gate Valve | Expanding Gate Valve |
| Sealing Mechanism | Primarily line pressure pushes floating seats against the gate; springs for low-pressure seal. | Mechanical expansion of two-piece obturator against seats. |
| Seal Integrity (Low P) | Relies on springs for low-pressure sealing. | Positive mechanical seal regardless of line pressure. |
| True Double Block and Bleed (DBB) | Achieves DBB through floating seats and pressure differential. | Provides a true, independent mechanical DBB. |
| Sealing Surface Protection | Less protected during stroke; more susceptible to abrasive fluids. | Sealing surfaces protected during travel; only engage at full open/close. |
| Complexity | Simpler design, fewer moving parts. | More complex two-piece obturator. |
| Cost | Generally more economical. | Premium product, typically higher cost. |
| Maintenance | Generally easier to repair in-line. | May require more specialized maintenance due to complex internals. |
| Ideal For | General pipeline block, less critical services, where cost is a factor. | High-pressure gas, critical isolation, dirty service, HIPPS, ESD. |
Expanding Gate Valves for Pipelines
Our expanding gate valve is a premium product designed for critical applications. While pipeline pressure helps seal, the primary seal comes from the mechanical action of the expanding gate against the floating seats. This provides superior security, especially with extreme pressure differences. It’s often the preferred valve for pigging operations where the gate is the sole pressure barrier to the atmosphere.
A key feature is its Double Isolation and Bleed (DIB) function. This allows you to vent the body cavity even with high pressure on one side of the valve. The mechanical contact between the gate and seats ensures a positive low-pressure seal and helps prevent debris from entering the body cavity. These valves also offer low-torque manual operation.
Expanding Gate Valve Features
This valve provides a positive mechanical seal to ensure a bubble-tight shut-off at both high and low pressure. The mechanical seal is unaffected by vibration or variation in line pressure. The valve seals bi-directionally and will achieve DBB per API-6D requirements.
Floating seats have a soft, resilient face, and the top entry bonnet design allows for inline maintenance and replacement of the resilient seat. After isolation of the valve and relief of any internal pressure, the top entry bonnet may be removed and the seats and/or gate assembly replaced with the valve installed in the flow line.
The assembly maintains positive mechanical contact and seals against the upstream and downstream seats simultaneously. In the event of damage to the soft O-ring and face seals, metal to metal contact between the gate, the downstream seat, and the body provides a secondary seal. Fittings are provided for either seat lubrication or emergency seat sealant injection. Alternative stem seal designs are available to meet your specific requirements.
Gate Mechanics
Our expanding gate valve features a unique gate and segment assembly. The gate has angled surfaces that perfectly match angles on the segment.
As the valve opens or closes, a centralizer mechanism keeps this assembly in a collapsed position. This mechanism, which includes centralizer arms, gate guides, and an internal cam, restricts the gate’s movement to only the fully open or fully closed positions. This design ensures the assembly remains collapsed during operation, significantly reducing wear on the valve seats and keeping operating torques low.
Thermal Body Cavity Relief
In the event of variations in temperature, our expanding gate valve is designed with options for relief of any internal body pressure due to thermal expansion. This thermal relief configuration is installed according to your operational requirements.
An upstream bypass system with a needle valve and a check valve can be fitted if needed. This
provides for relief of excessive body pressure to the upstream flow conduit. The needle valve must be kept open while the valve is in service, and can be closed to test and service the check valve.
Slab Gate Valves for Pipelines
Our slab gate valve is the workhorse of pipeline valves thanks to its reliability, ease of maintenance, and price points. The simplest of all piggable pipeline valves, this design has the fewest moving parts. The large seat to seal area tolerates minor scratches better than ball valves, and unlike a trunnion ball valve, the long life cycle of a slab gate can be further extended thanks to its ease of repair. A heavy 3 Mil ENP coating protects surfaces from damage and corrosion.
These full-bore, through-conduit gate valves are fully piggable without seat damage. Precise machining of the slab and seats minimizes bore space, reducing pipeline turbulence. This valve is most often used as a pipeline block valve for applications requiring a full bore and API-6D DBB sealing.
Our slab gate valves feature an API-622 qualified low-emission braided graphite packing. A unique internal retainer allows in-service adjustment while the valve is under pressure, significantly extending packing life and eliminating downtime. For added security, the stem seal packing box includes fittings for emergency packing injection.
Since the packing is independently retained, the yoke can be removed even when the valve is under pressure, simplifying maintenance or actuator replacement. Reassembly is also straightforward thanks to an integrally machined locator on the yoke.
These valves feature floating metal seat rings equipped with soft face seals, held against the gate by a dynamically energized spring. Should a seat fail, emergency sealant can be pumped directly into a groove on the seat face through body fittings. For added safety, an optional fire-safe design ensures a temporary seal via graphite wire rod and metal-to-metal contact between the seat and gate, even if soft seals are compromised.
For maintenance, the top-entry bonnet can be removed and internal components replaced with the valve still in the flow line, after isolation and pressure relief.
Spring Energized Seat Design for Slab Gate Valves
Our spring energized seat design provides automatic relief of excess body pressure in a pipeline gate valve. During operation, an initial seal is formed by the raised plastic ring on the faces of the seats, and the seat inserts clean both sides of the gate each time the valve is opened or closed.
In low pressure situations the initial upstream seal is provided by the force of line pressure acting against the upstream seat, moving the seat against the gate and providing a tight plastic-tometal seal. At the same time, the O-ring forms a tight seal with the seat recess.
When body pressure exceeds line pressure from causes such as thermal expansion, the upstream seat is forced back into its recess and the excess pressure in the body is bled between the seat and the gate into the line.
As line pressure is applied to the valve, it acts on the gate, forcing it against the plastic ring on the downstream seat and compressing it until the gate rests against the steel seat. Thus, a double seal is formed; first, a plastic-to-metal seal then metal-to-metal. The seal is also forced firmly into its recess, and the O-ring prevents any downstream flow around the seat.
Swing Check Valves for Pipeline Backflow Prevention
Swing check valves are known for their simplicity and reliability. They have fewer moving parts than many other valve types, leading to less wear and tear and lower maintenance requirements. Their full-bore design also means they don’t obstruct flow, which minimizes pressure drop and energy loss.
Unlike other valves that are actively opened or closed, a swing check valve operates automatically, using the flow of the product within the pipeline. These valves have a disc that swings open when the fluid flows in the correct direction. This allows the product to pass through unimpeded. However, if the flow stops or tries to reverse, gravity and the back-pressure of the fluid push the disc shut, creating a tight seal. Without this backflow prevention, pumps or compressors could sustain damage, product contamination could occur, or dangerous pressure surges could occur.
Quality Valve Supply for Midstream Pipelines and Terminals
For reliable valves crucial to midstream pipelines and terminal operations, choose Williams Valve. We supply quality gate and check valves to support your pigging operations and protect your pump and compressor stations. Contact us now for valve solutions engineered for the high demand of oil and gas applications.
