Parallel Slide Gate Valves for Steam Main Isolation
Technical guide · Supreme Valves India · Updated 2026-10-25
On steam mains, boiler outlets and turbine isolation, many plants specify parallel slide gate valves instead of wedge gate valves. The reason is temperature: steam valves are opened and closed while hot, the line cools and reheats, and a wedge can lock in its seats. A parallel slide valve avoids that and keeps sealing well as temperature changes.
How a parallel slide valve works
Instead of one tapered wedge, the gate is two flat, parallel discs held apart by a spring between them. The body has two parallel seats. As the valve closes, the discs slide down across the seats; the spring keeps light contact, and line pressure pushes the downstream disc firmly onto the downstream seat. The seal comes from pressure, not from forcing a wedge between tapered seats.
- Self-cleaning: the discs slide across the seats on every stroke, wiping away scale and debris.
- No thermal binding: nothing is wedged, so contraction of the body after closing hot doesn't lock the discs.
- Floating discs: the discs adjust to minor seat distortion from piping loads and temperature.
Compared with wedge gate valves
| Parallel slide | Solid or flexible wedge | |
|---|---|---|
| Sealing force | Line pressure plus spring | Stem thrust wedging the gate |
| Thermal binding | Not prone | Solid wedge prone; flexible wedge reduces it |
| Low pressure sealing | Less tight | Good, from mechanical force |
| Seat wear | Sliding contact, self-cleaning | Damage concentrated where the wedge seats |
| Typical use | Steam mains, boiler and turbine isolation, feedwater | General process isolation |
Bypass and drain arrangements
- Bypass valve: on larger and higher-pressure valves, a small globe bypass warms and pressurises the downstream line before the main valve opens. It reduces thermal shock, condensate hammer and opening force.
- Body drain: removes condensate from the body cavity during warm-up and before opening.
- Position indicator and locking: often specified on boiler isolation for safe operation.
Installation
- Install in horizontal lines with the stem vertical and pointing up, unless the manufacturer allows another orientation.
- Drain the upstream line properly so condensate doesn't collect against the closed valve.
- Open slowly, using the bypass where fitted, to avoid water hammer.
- For butt weld chrome-moly valves, follow the preheat and PWHT procedure and protect the internals.
Materials and construction
Bodies are WCB for moderate temperatures and WC6 or WC9 (or forged F11 and F22) for higher temperature steam, with stellite-faced discs and seats. Bolted bonnets are used at lower classes and pressure seal bonnets at Class 900 and above. Indian boiler installations need IBR documentation.
What to include in an enquiry
Size, class, design pressure and temperature, body material, end connection with pipe schedule, bonnet type, bypass and drain requirements, operator (handwheel, gear or electric actuator) and the applicable code.
Frequently asked questions
Why don't parallel slide valves stick when they cool down?
The two discs are held against parallel seats by a spring and by line pressure, not wedged into tapered seats. When the valve and pipe cool after being closed hot, there is no wedge to be gripped by the contracting body, so the valve opens freely. A solid wedge gate valve closed hot can lock in its seats as it cools.
Do parallel slide valves seal at low pressure?
They seal mainly by line pressure pushing the downstream disc onto its seat, with the spring giving initial contact. At very low differential pressure the seal is less tight than at operating pressure, so they are best suited to steam and other services that run at meaningful pressure.
When is a bypass valve needed on a steam isolation valve?
On larger valves and higher pressures, a small bypass valve lets the downstream line be warmed and pressurised gradually before the main valve opens. That reduces thermal shock, water hammer from condensate, and the force needed to open the main valve against full differential pressure.
Related products & guides
Continue to RFQ
Contact / RFQ form · Quotation checklist · EPC supplier · AEO SEO GEO hub
Project RFQ: EPC · Fire · How to buy · Lead hub · Contact / RFQ