Introduction: Several coal-fired power plants in the Philippines have experienced seat aging and increased internal leakage after 1–2 years of using soft-seat butterfly valves in high-temperature steam lines.
In auxiliary steam systems of coal power plants (e.g., soot blower lines, tracing lines), media temperatures often reach 350°C–425°C. Conventional soft-seat butterfly valves using PTFE, EPDM, or NBR seats have the following limitations:
PTFE seat: Long-term service limit around 230°C. Above this, material softening and creep reduce sealing contact pressure.
Elastomeric rubber seat: Accelerated aging above 150°C, loss of resilience, and permanent deformation after cycling.
Typical field failures include: audible steam leakage when the valve is closed, normal actuator torque but unable to shut off completely, and seat charring or carbonization found during disassembly.
The selected electric hard-seal butterfly valve (wafer type, body WCB/SS304, disc SS304) provides the following verifiable parameters:
| Parameter | Value | Significance vs. Soft Seal |
|---|---|---|
| Max. temperature | ≤425°C (PDF “Suitable temperature”) | Covers superheated steam temperature range of power plants |
| Seat seal material | Stainless steel + flexible graphite | No organic content, no carbonization |
| Shell test pressure | Up to 6.0 MPa (PN40 rating) | Margin above actual operating pressure (typically 1.6–2.5 MPa) |
| Gas seal test pressure | 0.5–0.8 MPa | Validates low-pressure tightness, prevents steam micro-leakage |
The hard-seal structure relies on contact pressure between the metal seat and disc to achieve shut-off, independent of elastic material deformation. Hence it maintains consistent sealing contact pressure during temperature cycling.
The body and seat are integrated components. The seat sealing surface is overlaid with temperature-resistant and corrosion-resistant alloy material. A multi-layer flexible laminated seal ring is fixed on the disc. Compared with traditional butterfly valves, this type of valve offers high temperature resistance, easy operation, frictionless opening/closing, and compensates sealing through increased torque from the transmission mechanism when closing, thus improving sealing performance and extending service life.
① The unique triple offset design enables frictionless transmission between sealing surfaces, extending valve service life.
② Elastic sealing generated by torque.
③ The ingenious wedge design provides an automatic “tighten-as-you-close” sealing function, with compensation and zero leakage between sealing surfaces.
④ Compact, lightweight, easy to operate, and convenient for installation.
⑤ Pneumatic or electric actuation can be configured per user requirements to meet remote and programmatic control needs.
⑥ Various connection types: wafer, lug, flanged, butt-welded.
In steam lines, high temperature causes clearance changes between stem and bushing. When using SONGO series electric actuators, match valve torque with a safety factor (typically 1.3–1.5×). For a DN150 valve at 1.6 MPa operating pressure, calculated torque ≈120 N·m → select SONGO-30 (rated 300 N·m, providing margin).
WCB body is suitable for saturated steam. If trace chlorides or acidic condensate exist, upgrade disc to SS316 (1Cr18Ni12Mo2Ti) to avoid pitting corrosion.
For wafer-type hard-seal butterfly valves, the disc should open at an acute angle to the pipe axis to avoid high-velocity steam erosion on the seat back face. Typically install stem horizontally with actuator upright.
After six months of replacement service:
On-line seal test (bubble method) at 425°C steam showed no visible leakage.
Actuator completed approximately 1,200 cycles with no torque drift.
Stem packing (flexible graphite + PTFE composite) showed no external leakage.
Estimated maintenance interval extends from annual seat replacement (soft seal) to once every three years for packing inspection.
For high-temperature steam lines (≤425°C) in Southeast Asian coal power plants, electric hard-seal butterfly valves (metal seat + flexible graphite) provide a direct replacement when soft-seat valves fail. Selection must verify temperature, pressure, torque, and material compatibility. WCB body is not recommended for sulfur-bearing or acidic condensate services – SS316 should be used instead.
