Main Steam Valve
Isolation on the hottest, highest-pressure line in the plant. Material grade - not valve type - is the first selection decision here.
- Size NPS 2-24 (DN 50-600)
- Pressure class Class 1500 - 4500
- Design temperature 540 - 650 °C for USC main steam
- Body material A182 F22, F91, F92; WC9 where cast is specified
- End connection Butt weld to ASME B16.25, matched to pipe schedule
Overview
Isolating the superheated steam line between the boiler outlet and the turbine stop valve. This is the most demanding valve position in a conventional plant: full design pressure, peak metal temperature, and thermal transients on every start and stop. The valve type is usually gate for full-bore isolation, globe for drains and warm-up, and stop-check where isolation and non-return are both needed.
Target systems
- Boiler outlet to turbine main steam line
- Hot and cold reheat lines
- Main steam drains and warm-up bypass
- Superheater outlet header connections
- Turbine bypass isolation
- Substituting a lower-grade body material to meet a price target. At 566 °C the difference between F22 and F91 is not a commercial preference - it is whether the valve holds its rating.
- Selection from a size and class alone. Main steam requires the design temperature and the transient profile, not just the nameplate conditions.
- Service without a documented warm-up procedure. A cold valve opened into hot steam distorts, and the failure will be blamed on the valve.
Technical data
Every value is marked standard for the catalogue range, typical where it depends on material or configuration, or per project where it can only be confirmed against your specification.
| Parameter | Value | Basis |
|---|---|---|
| Size | NPS 2-24 (DN 50-600) | standard |
| Pressure class | Class 1500 - 4500 Class 4500 for ultra-supercritical duty, confirmed per item. | standard |
| Design temperature | 540 - 650 °C for USC main steam Above 566 °C, F91 or F92 is normally required - confirm against the B16.34 rating table for the actual grade. | typical |
| Design pressure | 17-31 MPa typical | typical |
| Medium | Superheated steam | standard |
| Body material | A182 F22, F91, F92; WC9 where cast is specified | standard |
| Valve type | Pressure seal gate, globe or stop-check | standard |
| Trim | F6NM / 422SS, Stellite 6 hardfaced | standard |
| End connection | Butt weld to ASME B16.25, matched to pipe schedule Flanged ends are unusual on main steam and need justification. | standard |
| Operation | Bevel gear or electric actuator | standard |
| PWHT | Required on F91/F92 welds, with hardness survey | standard |
| Weld procedure qualification | Per project specification and ASME IX | per project |
Design
Material selected from the creep range, not the room-temperature rating
At main steam temperature the governing property is creep rupture strength, not tensile strength. F22 loses allowable stress rapidly above about 540 °C; F91 and F92 are the reason modern supercritical plants can run where they do. The material matrix page shows the crossover points.
Controlled PWHT with recorded hardness
F91 and F92 are only as good as their heat treatment. Under-tempering leaves the weld hard and brittle; over-tempering destroys creep strength permanently and cannot be corrected by re-treating. We supply the charts and the hardness survey with the valve.
Flexible wedge for thermal transients
On the gate configuration the flexible wedge absorbs body distortion during startup and shutdown. Main steam valves see this cycle far more often than the design life assumptions usually admit, particularly on plants now running two-shift.
Butt weld ends matched to the actual pipe
The bevel and bore are prepared to ASME B16.25 for the specific pipe schedule and material on your line, not for a nominal size. Send the pipe specification with the enquiry - a mismatch found at site is expensive.
Warm-up provision where specified
An integral bypass or a separate warm-up valve lets the downstream side be brought up to temperature before the main valve opens. Whether you need one depends on the line and the operating procedure; we will ask rather than assume.
Standard material specifications
Part numbering follows the general arrangement drawing. Three columns cover the common temperature bands; the actual configuration for your order is confirmed on the datasheet.
| No. | Part name | Carbon steel | Low alloy | Creep grade |
|---|---|---|---|---|
| 1 | Body | A105 / A105N | A182 F11, F22 | A182 F91, F92 |
| 2 | Seat ring | 13Cr + Stellite 6 | 13Cr + Stellite 6 | F91 + Stellite 6 |
| 3 | Wedge (flexible) | A105 + Stellite 6 | F11 / F22 + Stellite 6 | F91 + Stellite 6 |
| 4 | Stem | A182 F6a | A182 F6a Cl.2 | A182 F6NM / 422SS |
| 5 | Bonnet | A105 / A105N | A182 F11, F22 | A182 F91, F92 |
| 6 | Pressure seal ring | Flexible graphite + SS liner | Flexible graphite + SS liner | Flexible graphite + Inconel liner |
| 7 | Thrust ring (segmental) | A105 | A182 F11 / F22 | A182 F91 |
| 8 | Retainer segments | A105 | A182 F11 / F22 | A182 F91 |
| 9 | Gland | A105 | A182 F11 / F22 | A182 F91 |
| 10 | Gland flange | A105 | A105 | A182 F91 |
| 11 | Stem packing | Flexible graphite | Flexible graphite | Flexible graphite, low-leakage set |
| 12 | Gland eyebolt / pin | A193 B7 / A194 2H | A193 B16 | A193 B16 |
| 13 | Yoke | A105 / WCB | A105 / WCB | A105 / WCB |
| 14 | Yoke bushing | Ni-Al bronze | Ni-Al bronze | Ni-Al bronze |
| 15 | Back seat | 13Cr overlay | 13Cr overlay | Stellite overlay |
| 16 | Handwheel | Ductile iron / carbon steel | Ductile iron / carbon steel | Ductile iron / carbon steel |
| 17 | Nameplate | AISI 304 | AISI 304 | AISI 304 |
Standards
What each standard actually governs on this valve. A standard listed without a scope is decoration.
| Standard | Applies to |
|---|---|
| ASME B16.34 | Pressure-temperature ratings, materials, wall thickness, testing and marking. This is the base rating standard for the valve - it is what fixes the allowable pressure at your design temperature. |
| ASME B16.10 | Face-to-face and end-to-end dimensions. Relevant when the valve replaces an existing one or has to drop into a fixed spool. |
| ASME B16.25 | Butt welding end preparation, including bore and bevel geometry for the matching pipe schedule. |
| API 600 | Bolted bonnet steel gate valves. It applies to the gate configuration where the specification calls for it - but it is not a blanket standard for every pressure seal valve, and we will not list it as one. |
| API 598 | Inspection and pressure testing - which tests are mandatory, test durations and allowable leakage. We state the shell and seat test scheme on the ITP. |
| MSS SP-61 | Alternative pressure testing scheme for steel valves, used where the project specification calls for it instead of API 598. |
| MSS SP-25 | Marking of the body and the nameplate - size, class, material heat, figure number and flow direction where applicable. |
| NB/T 47044 | Chinese power station valve specification. Applies where the project is built to Chinese standards. |
Dimensions
Face-to-face per ASME B16.10 where tabulated. Weights are indicative and vary with class, material and operator - confirm against the approved GA drawing.
| Size | DN | Face-to-face | Weight (typical) |
|---|---|---|---|
| NPS 2 | DN 50 | 368 mm | ~55 kg |
| NPS 3 | DN 80 | 470 mm | ~105 kg |
| NPS 4 | DN 100 | 546 mm | ~180 kg |
| NPS 6 | DN 150 | 705 mm | ~390 kg |
| NPS 8 | DN 200 | 832 mm | ~700 kg |
| NPS 10 | DN 250 | 991 mm | ~1150 kg |
| NPS 12 | DN 300 | 1130 mm | ~1750 kg |
| NPS 16 | DN 400 | 1410 mm | ~3200 kg |
Inspection
- Hydrostatic shell test to API 598 / MSS SP-61
- Hydrostatic or pneumatic seat test, leakage rate per specification
- Back seat test (gate and globe, where a back seat is supplied)
- PMI on all pressure-containing parts and welded overlays
- NDE per specification - RT, UT, MT or PT as applicable
- Hardness survey after post-weld heat treatment
- Dimensional inspection against approved GA drawing
- Material certification EN 10204 3.1 (3.2 on request)
- Visual and functional check, full stroke on the manual operator
- Hardness survey after PWHT on F91/F92, results recorded per weld
- Creep-grade material verification by PMI on every pressure-containing part
The actual scope, and which points are hold or witness, is set on the Inspection and Test Plan agreed before manufacture.
Documents supplied
- Product datasheet
- General arrangement (GA) drawing
- Inspection & Test Plan (ITP)
- Material test certificate (MTC) EN 10204 3.1
- Hydrostatic and seat test report
- PMI report
- NDE reports as applicable
- Heat treatment charts
- Dimensional report
- Packing list and preservation record
- Weld procedure specification (WPS) and PQR
- PWHT charts per weld
What each document proves, and which ones suppliers most often cannot produce
Manufacturing evidence
These positions are reserved for photographs of actual DERVOS VALVE production. They are shown as placeholders rather than filled with stock or rendered images - see the note on the company page about what this site will and will not claim.
Frequently asked questions
F22 or F91 for my main steam line?
It depends on the design temperature and the ASME B16.34 rating for the grade, not on preference. As a rough guide, F22 is usable to around 540 °C, above which its allowable stress falls away quickly; F91 covers to roughly 600 °C and F92 beyond. Send the design pressure and temperature and we will show you the rating calculation rather than just naming a grade.
What goes wrong with F91 valves in service?
Almost always heat treatment. F91 that has been over-tempered or welded without correct PWHT looks fine on delivery and fails years later by creep. This is why the hardness survey and the heat treatment charts matter more than the visual inspection, and why you should ask for them on every F91 order - from us or anyone else.
Gate, globe or stop-check for main steam?
Gate for the main isolation, because full-bore means minimum pressure drop on the highest-energy line in the plant. Globe for drains, vents and warm-up where throttling and tight shut-off matter. Stop-check where you need isolation plus non-return in one body - typically boiler outlet on multi-boiler installations.
How long is the lead time on a Class 2500 F91 main steam valve?
It is dominated by forging availability and heat treatment, not machining. We will quote a real date against the actual forging supply at the time of enquiry rather than a standard catalogue figure.