Pressure Seal Check Valve
Non-return valve with a self-energizing cover joint. Swing type for low-loss service, piston lift type where fast closure and stability matter more than pressure drop.
- Size NPS 2-24 (DN 50-600)
- Pressure class Class 600 - 4500
- Design temperature up to 650 °C / 1200 °F
- Body material A105, A182 F11, F22, F91, F92
- End connection BW to ASME B16.25, SW, flanged RF or RTJ
Overview
Preventing reverse flow in high-pressure steam and water lines. The pressure seal cover replaces the bolted bonnet, so the cover joint tightens with line pressure. The choice between swing and piston lift is driven by how quickly the valve has to close and how much reverse flow the downstream equipment can tolerate.
Target systems
- Boiler feedwater pump discharge
- Main steam and reheat lines to the turbine
- Extraction steam lines to feedwater heaters
- Auxiliary steam headers
- HP heater drain lines
- Positive isolation - a check valve is not a shut-off valve and must not be used as one. Pair it with a gate or globe valve.
- Pulsating flow near the disc-lifting velocity, which causes disc flutter and seat wear. A piston lift or nozzle type may be needed instead.
- Vertical downward flow on a swing check. Orientation must be stated at enquiry, not assumed.
- Systems where non-slam closure is a stated requirement without a documented closure analysis - that is an engineered selection, not a catalogue pick.
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 600 - 4500 | standard |
| Design temperature | up to 650 °C / 1200 °F | typical |
| Type | Swing check, piston lift check, tilting disc on request | standard |
| Medium | Superheated steam, saturated steam, feedwater, condensate | standard |
| Body material | A105, A182 F11, F22, F91, F92 | standard |
| Trim material | 13Cr / F6a, hardfaced with Stellite 6 | standard |
| Hinge pin | Hardened, renewable, retained outside the flow path | standard |
| Installation | Horizontal line, cover up (swing); vertical upward flow (lift) Other orientations need confirmation - some require a counterweight or a different disc. | per project |
| End connection | BW to ASME B16.25, SW, flanged RF or RTJ | standard |
| Minimum lifting velocity | Application-specific Depends on disc mass, medium density and orientation. Ask us to calculate it for your line size and flow. | per project |
Design
Renewable hinge pin and bushings
The hinge pin runs in hardfaced bushings and is retained so it can be withdrawn without machining the body. The pin, arm and bushings are the wear parts in a swing check and they are specified as replaceable rather than as part of the body.
Full-open disc clearance
The body cavity is machined so the disc reaches the fully open position clear of the flow, rather than sitting partly in the stream where it flutters. This is what keeps head loss low and stops the hinge pin wearing prematurely.
Pressure seal cover
The cover uses the same self-energizing arrangement as the gate and globe valves. On a check valve this matters more than usual, because there is no stem and the cover joint is the only dynamic seal on the pressure boundary.
Piston lift alternative
Where reverse flow must be stopped quickly, a piston lift disc guided over its full stroke closes faster and is more stable in pulsating flow. It costs more pressure drop. We will tell you which one your duty points to rather than defaulting to whichever is cheaper.
Optional counterweight and damper
External lever with counterweight, or a hydraulic damper, where the closure characteristic has to be tuned. This is supplied against a stated closure requirement, not offered as a generic upgrade.
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. |
| ASME B16.25 | Butt welding end preparation, including bore and bevel geometry for the matching pipe schedule. |
| API 594 | Applies to check valves - but note it covers wafer, dual plate and lift check types. Whether it applies to your specific check valve construction must be confirmed against the actual 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
- Disc seating check and free-swing verification through the full stroke
- Hinge pin clearance record before final assembly
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
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
Swing or piston lift - how do I choose?
Swing check for steady flow where pressure drop matters, typically main steam and larger feedwater lines. Piston lift where flow reverses quickly or pulsates, and where the downstream equipment cannot tolerate a slug of reverse flow - pump discharge on parallel pumps is the classic case. Send us the line size, flow rate, medium and whether pumps run in parallel.
Can a swing check be installed in a vertical line?
In vertical upward flow, yes, with the correct disc and sometimes a spring or counterweight. In vertical downward flow, a standard swing check will not seat reliably under gravity. Tell us the orientation at enquiry - it changes the design, not just the tag.
Do you provide a non-slam closure analysis?
We provide the disc mass, geometry and closure characteristics needed for a surge analysis. The system-level surge calculation depends on pump inertia and line dynamics that only the project engineer has. We will not claim non-slam performance without that input.
What causes a check valve to fail in steam service?
In our experience the common ones are: disc flutter from sizing the valve for the line rather than the flow, hinge pin wear from continuous partial lift, seat erosion from wet steam, and installation in an orientation the valve was not designed for. Four of the five are selection and installation issues, not manufacturing ones.