Extraction Steam Check Valve
Non-return protection on turbine extraction lines. The design case is not normal operation - it is what happens in the two seconds after a turbine trip.
- Size NPS 6-48 (DN 150-1200)
- Pressure class Class 150-900
- Design temperature 150-450 °C by extraction stage
- Body material A105, A182 F11, F22; WCB/WC6 cast where specified
Overview
On a turbine trip, the water and steam inventory in the feedwater heater flashes and drives steam back toward the turbine. Reverse steam through the extraction nozzles can overspeed the rotor. The extraction check valve exists for that single event, which is why closure speed matters far more than pressure drop, and why power-assisted closing is common on the larger extractions.
Target systems
- HP, IP and LP turbine extraction lines to feedwater heaters
- Deaerator extraction lines
- Auxiliary steam take-offs from extraction points
- Treating as a standard swing check. The closure requirement comes from a turbine-protection study, not from a catalogue.
- Selection without the extraction line geometry and heater inventory - closure time depends on the volume that has to be swept.
- Service where the turbine OEM has specified a particular valve arrangement that we have not been shown. Ask for it and send it with the enquiry.
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 6-48 (DN 150-1200) Extraction lines are large-bore; the upper end depends on the specific machine. | per project |
| Pressure class | Class 150-900 Extraction pressures are moderate - the demanding parameter is closure speed, not class. | standard |
| Design temperature | 150-450 °C by extraction stage | typical |
| Type | Swing check with external lever, counterweight and optional air cylinder | standard |
| Closure assistance | Counterweight, spring or air-assisted power closure | per project |
| Closure time | Per turbine protection study We do not publish a closure time. It is derived from your machine and line, and we will calculate it against your data. | per project |
| Body material | A105, A182 F11, F22; WCB/WC6 cast where specified | standard |
| Position indication | Limit switches, proximity sensors on request | standard |
| Testability | Partial-stroke test provision on request | per project |
Design
Power-assisted closure
An air cylinder acting on the external lever drives the disc shut on a trip signal rather than waiting for reverse flow to do it. On large extractions this is the difference between the valve closing in under a second and closing after the rotor has already accelerated.
External lever and counterweight
The lever gives access to the disc position from outside the pressure boundary, allows the closing torque to be tuned, and provides a mounting point for position indication and the actuator.
Position indication for the protection system
Limit switches confirm the valve has actually closed. A protection device with no feedback is an assumption, and turbine protection is not an appropriate place for assumptions.
Low-loss disc geometry in the open position
Extraction lines carry large volumes at low pressure differential, so any restriction is a direct heat-rate penalty. The disc clears the flow path fully when open.
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 | Disc | A105 + Stellite 6 | F11 / F22 + Stellite 6 | F91 + Stellite 6 |
| 4 | Hinge arm | A182 F6a | A182 F6a | A182 F6NM |
| 5 | Hinge pin | A182 F6a, hardened | A182 F6a, hardened | A182 F6NM / Inconel 718 |
| 6 | Bushing | Stellite 6 / hardfaced | Stellite 6 / hardfaced | Stellite 6 / hardfaced |
| 7 | Cover (bonnet) | A105 / A105N | A182 F11, F22 | A182 F91, F92 |
| 8 | Pressure seal ring | Flexible graphite + SS liner | Flexible graphite + SS liner | Flexible graphite + Inconel liner |
| 9 | Thrust ring (segmental) | A105 | A182 F11 / F22 | A182 F91 |
| 10 | Retainer segments | A105 | A182 F11 / F22 | A182 F91 |
| 11 | Cover plug / vent | A105 | A182 F11 / F22 | A182 F91 |
| 12 | 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. |
| API 594 | Check valve standard - applicability to this specific construction is confirmed per design, not assumed. |
| 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-25 | Marking of the body and the nameplate - size, class, material heat, figure number and flow direction where applicable. |
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
- Functional stroke test of the lever, counterweight and actuator assembly
- Limit switch setting and verification
- Closure time measurement where a closure requirement is specified
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
- Closure time test record
- Actuator and limit switch datasheets
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
Do I need a power-assisted extraction check valve?
It depends on the extraction volume, the heater inventory and the turbine manufacturer requirements. Small LP extractions often do not; large HP extractions on a big machine usually do. This is decided by the turbine protection study, and if you have one, send it - we will select against it rather than guess.
How fast will the valve close?
We will not publish a number, because it is meaningless without your line geometry, disc size and actuating arrangement. Give us the extraction line data and we will calculate a closure time and state the assumptions behind it.
Can the valve be tested without shutting the unit down?
Partial-stroke testing can be provided so the disc is moved through part of its travel with the unit running, confirming it is free. Full closure testing requires an outage. We will fit the provision if you ask for it at the enquiry stage; retrofitting is much harder.