Piston Disc Valves

High-reliability isolation for steam, superheated water and thermal oil systems. Self-adjusting expanded graphite sealing ring — minimal maintenance and exceptional service life.

How piston disc valves work

Piston disc valves are linear-motion isolation valves in which a cylindrical plug (the piston or "disc") slides inside an expanded graphite sealing ring. Unlike valves with conventional packing glands, the graphite ring wraps around the piston at 360° creating a continuous radial seal on both the stem and the seating surface.

When the valve is closed, the piston engages the graphite ring and simultaneously seats against the lower conical seat, creating a double seal. The expanded graphite is self-lubricating and gradually conforms to the piston surface, maintaining a perfect seal for years without adjustment — a decisive advantage over traditional packed valves.

Roffia supplies piston disc valves in ductile iron, carbon steel and stainless steel, in sizes from DN 15 to DN 300 with pressure ratings up to PN 40 (ANSI Class 300). Ideal for steam systems, boiler houses, district heating networks and thermal oil circuits.

Piston disc valves for steam — Roffia srl

Body materials

  • Ductile iron GGG40 — standard for steam, PN 16–25
  • Carbon steel A216 WCB — high-pressure steam, PN 25–40
  • Forged steel A105 — high-pressure applications
  • Stainless steel CF8M / AISI 316 — corrosive fluids, chemical-pharmaceutical

Sealing and piston

  • Expanded graphite ring — self-adjusting, up to 425 °C
  • PTFE ring — for cold water and chemical fluids
  • Hardened stainless steel piston — wear resistant
  • Stellite-faced piston — maximum life, severe service
  • Double seal: radial + conical seat

Technical specifications

  • DN: 15–300 mm
  • PN: 16, 25, 40
  • Temperature: up to 425 °C
  • Connections: EN 1092-1 flange, ANSI B16.5, threaded
  • Certifications: PED 2014/68/EU

Typical applications

Steam headers and distribution

Piston disc valves are the optimal choice for isolation on main steam headers and branch lines. The graphite ring seal requires no packing adjustment and guarantees zero leakage even after years of frequent operation, reducing energy waste. Read more: choosing valves for steam systems.

Boiler houses and district heating

In centralised heating plants and district heating networks, ductile iron piston disc valves serve as main isolation valves on superheated water circuits (up to 180 °C). Robust construction ensures long-term reliability with virtually zero maintenance.

Thermal oil circuits

In thermal oil circuits up to 300 °C, carbon steel piston disc valves offer perfect sealing without the soft-seal deterioration issues typical of ball or butterfly valves at high temperatures.

Industrial process

Plants in the food & beverage, chemical-pharmaceutical and industrial sectors use AISI 316 piston disc valves for isolation on process steam lines and hot fluid services where corrosion resistance is required.

Roffia piston disc valve range

Ductile iron
DN 15–300, PN 16–25
Carbon steel
DN 15–200, PN 25–40
Stainless AISI 316
DN 15–150, PN 16–25
Bronze threaded
DN 15–50, PN 16
Pneumatic actuated
Automatic on-off control
Electric actuated
Modulating control
Handwheel
Standard manual operation
Position indicator
Open / closed / intermediate

Frequently asked questions

What is the difference between a piston disc valve and a ball valve?
A piston disc valve uses a cylindrical piston that slides inside a self-adjusting graphite or PTFE sealing ring, providing progressive and self-compensating sealing. It is ideal for steam, superheated water and thermal oil because the seal improves over time as it adapts to wear. A ball valve is a quarter-turn valve with plug-to-seat sealing: it is faster to operate but less suitable for hot fluids and frequent thermal cycling. For steam systems above 200 °C and services with frequent operation, the piston disc valve is the more reliable choice.
Why are piston disc valves preferred in steam systems?
Piston disc valves offer three key advantages in steam service: (1) virtually maintenance-free stem sealing thanks to the flexible graphite ring that compensates for wear without packing adjustment; (2) resistance to thermal cycling — the metal piston tolerates repeated expansion and contraction without degrading the seal; (3) low operating torque even at high pressures. They are recommended by Spirax Sarco for isolation on steam headers, upstream of steam traps and on high-pressure steam distribution lines.
How does the sealing work in piston disc valves?
The cylindrical piston slides inside an expanded graphite sealing ring (or PTFE for low-temperature applications). The ring wraps completely around the piston at 360°, creating a continuous radial seal on both the stem and the seat. When the valve is closed, the piston also sits on the lower conical seat, achieving a double seal. The expanded graphite is self-lubricating and gradually adapts to the piston surface, maintaining a perfect seal for years without maintenance. This principle is radically different from traditional packing glands that require periodic tightening.
What materials are available for piston disc valves?
Roffia supplies piston disc valves in the following materials: ductile iron GGG40 (standard for steam, PN 16–25), carbon steel A216 WCB (high-pressure steam up to PN 40), forged carbon steel A105 (PN 63–100), stainless steel CF8M / AISI 316 (corrosive fluids and chemical applications). The piston is typically hardened stainless steel or stellite-faced, and the sealing ring is pure expanded graphite or PTFE. Available connections: EN 1092-1 flange, ANSI B16.5, BSP/NPT threaded. Sizes from DN 15 to DN 300.
Do piston disc valves require maintenance?
Piston disc valves require minimal maintenance compared to other steam valve types. The graphite ring does not need packing adjustment and has a typical service life of 5–10 years in continuous operation. Routine maintenance consists of: (1) annual visual inspection to verify no external leaks; (2) sealing ring replacement when needed (can be done in-line without removing the body); (3) seat inspection every 5–10 years. Roffia supplies spare kits (sealing rings, gaskets, piston) for all sizes and materials.

Request a piston disc valve quote

Please provide DN, PN, material, operating temperature, fluid type and required connections. We will select the correct valve and send you a quotation within 24 hours.

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