Wedge vs Parallel Slide Gate Valve

Quick Answer

Wedge gate valves seal by forcing a tapered wedge into the seats with mechanical advantage. Parallel slide gate valves use two flat discs pushed against the seats by line pressure. Wedge designs dominate general service; parallel slide designs are preferred for high-temperature steam to avoid thermal binding.

Design Differences

The core distinction lies in the sealing mechanism. A wedge gate valve uses a tapered disc (solid, flexible, or split) that wedges tightly between inclined seat faces as the stem pushes it down. A parallel slide valve has two parallel discs separated by a spring; line pressure forces the downstream disc against its seat to create a seal.

Wedge valves rely on stem force for sealing. Parallel slide valves rely on fluid pressure. This difference drives their suitability for different services.

Comparison Table

Parameter	Wedge Gate Valve	Parallel Slide Gate Valve
Sealing mechanism	Stem force wedges gate into seats	Line pressure pushes disc against seat
Gate type	Solid, flexible, or split wedge	Two parallel flat discs with spring
Thermal binding risk	Yes (solid wedge in hot service)	No (parallel seats accommodate expansion)
Best for	General oil, gas, water service	High-temperature steam (540 degC+)
Standards	API 600, API 6D	BS 1414, API 600
Seat geometry	Inclined (tapered)	Parallel (flat)
Bidirectional sealing	Yes	Yes (pressure-assisted)
Typical sizes	2” to 60”	2” to 36”
Cost	Lower	Higher (more complex internals)
Maintenance	Simpler (fewer moving parts)	Spring replacement may be needed

Thermal Binding Explained

Thermal binding occurs when a solid wedge gate valve is closed at high temperature. As the system cools, the body contracts around the wedge, trapping it between the seats. Reopening requires excessive torque that can damage the stem or gearbox. Flexible wedge and split wedge designs reduce this risk, but parallel slide valves eliminate it entirely because the flat discs move freely without wedging action.

For steam systems above 400 degC, most EPC contractors specify parallel slide gate valves per BS 1414 (now incorporated into EN ISO 10434). Power generation plants routinely use parallel slide designs on main steam and hot reheat lines.

Selection Guidelines

Use wedge gate valves for ambient to moderate-temperature services: crude oil, natural gas, produced water, and general process fluids. Use parallel slide gate valves for superheated steam, boiler feedwater, and any service where thermal cycling is frequent.

Common Mistake

Specifying solid wedge gate valves for cyclic steam service. The valve may seize shut after thermal cycling and require field disassembly. Always check the service temperature range before selecting the gate type.

Both designs comply with ASME B16.34 pressure-temperature ratings and ASME B16.10 face-to-face dimensions. The choice between them comes down to operating temperature and the risk of thermal binding.

Read the full guide to valve types