Globe Valves for Power Generation

The alloy steel specified for HP steam globe valves depends on the steam temperature at the valve's operating point: (1) Up to 480°C: ASTM A216 WCB (carbon steel) is used for lower-temperature steam applications — economiser bypass, extraction steam at low-pressure stages. Inexpensive and widely available. (2) 480°C–565°C: ASTM A217 WC6 (1.25% Cr, 0.5% Mo cast alloy steel) is the standard for intermediate-temperature steam service. Used for feedwater heater drain valves, extraction steam at medium pressure stages. (3) 565°C–593°C: ASTM A217 WC9 (2.25% Cr, 1% Mo) for high-temperature HP steam lines. Standard for turbine bypass valves in subcritical and supercritical boilers (below 250 bar, 593°C). (4) Above 593°C (ultra-supercritical): ASTM A182 F91 (P91 — 9% Cr, 1% Mo, V, Nb) forged body — the primary material for ultra-supercritical steam turbine bypass and main steam isolation. F91 provides superior creep rupture strength at temperatures where WC9 begins to lose strength. (5) Above 620°C: ASTM A182 F92 (P92 — 9% Cr, 2% W) for the most advanced ultra-supercritical boilers. All steam globe valves should have Stellite 6 hard-faced seats and disc faces regardless of body material.

A stop-check globe valve (also called a non-return stop valve or screw-down non-return valve) combines the functions of a globe stop valve and a check valve in a single body. The disc is not positively connected to the stem: (1) When the handwheel is raised (stem backed off), the disc is free to act as a check valve — it opens when upstream pressure exceeds downstream pressure and closes when flow reverses or stops; (2) When the handwheel is screwed down, the disc is positively forced against the seat, providing manual isolation regardless of pressure direction. This dual function makes stop-check globe valves standard at: (1) Steam turbine extraction steam connections — prevents backflow from the feedwater heater into the turbine when extraction pressure drops (check function), and provides positive isolation for turbine maintenance (stop function); (2) Boiler steam drum outlets — the check function prevents reverse flow from the steam main into the drum when drum pressure drops; (3) Boiler feed pump discharge connections — prevents reverse rotation of the pump on pump trip (check function) and provides pump isolation (stop function). Always specify 'stop-check' or 'non-return stop valve' explicitly in the specification — standard globe valves (with the stem positively attached to the disc) cannot perform the check function.

Pressure class for power plant globe valves is selected from ASME B16.34, which defines pressure-temperature (P-T) ratings for each material group. The process: (1) Determine the design pressure and design temperature at the valve location per ASME B31.1 (usually the upstream piping system design conditions plus a safety margin); (2) Select the body material based on temperature (as above — WCB, WC6, WC9, F91); (3) Look up the ASME B16.34 P-T table for that material group at the design temperature — find the lowest pressure class whose rated pressure at that temperature exceeds the design pressure; (4) Add a safety margin: in practice, most engineering specifications require the valve's rated pressure to be at least 10% above the piping system MAWP. Typical power plant pressure class assignments: feedwater and condensate (20–40 bar, 120°C) → Class 150 or Class 300; intermediate steam extraction (40–80 bar, 300–400°C) → Class 300 or Class 600 WC6; HP steam (80–200 bar, 540–565°C) → Class 600 or Class 900 WC9; ultra-HP steam (200–300 bar, 600°C+) → Class 1500 or Class 2500 F91.

The Indian Boiler Regulations (IBR) are issued under the Indian Boilers Act, 1923, and specify the design, fabrication, testing, and certification requirements for boilers and all equipment connected to them — including steam piping, valves, fittings, and pressure vessels. IBR Form III C is the inspection certificate issued by an IBR-recognised inspecting authority (Chief Inspector of Boilers office, or private IBR-approved inspection agencies like Lloyd's Register, TÜV India, DNV India) for a pressure part that has passed IBR inspection and testing requirements. IBR certification is mandatory (by law) for all valves, fittings, and piping installed on: (1) Boiler steam drums and all steam outlet connections; (2) All steam pipes connected to the boiler up to the first isolation valve from the boiler; (3) Boiler feedwater piping from the feed pump to the boiler; (4) Steam pipes in the Category A (high-pressure) scope as defined by the Chief Inspector of Boilers. Non-IBR valves cannot be legally installed in IBR scope piping in India — installation of non-IBR pressure parts is a violation of the Indian Boilers Act and can result in the boiler's operating license being revoked. When procuring steam globe valves for Indian power plants: specify 'IBR Form III C certification required' in the purchase order; the manufacturer must be on the IBR-approved manufacturer list or submit designs for IBR approval; pressure testing must be witnessed by an IBR-recognised inspector.