Order at a Glance
| Field | Detail |
|---|---|
| Product | Through conduit gate valves - slab gate and expanding gate |
| Project | Natural gas processing units construction project, China |
| Service | Gathering and inlet isolation, processing-unit isolation, export and flare headers |
| Year of supply | 2022 |
| Governing standards | API 6D, ASME B16.34, ASME B16.5 / B16.25 ends, API 607 fire-safe, NACE MR0175 / ISO 15156 |
| Design points | Full-bore piggable passage, self-relieving seats, bidirectional seal on expanding gates, sealant injection on large bores |
| Certification | EN 10204 3.1 / 3.2; TPI-witnessed shell and seat tests |
Scope of Supply
A gas processing plant divides its valves the same way the gas flows: wet gathering at the front, treated gas and conditioned product at the back. The gate valves on this package followed that split.
| Service | Design | Why this design |
|---|---|---|
| Gathering and inlet isolation | Slab gate, full bore | Piggable straight passage; tight shutoff against line pressure; lowest cost per bore in this class |
| Unit switching and isolation | Expanding gate | Bidirectional seal, independent of which side is pressurized; used where a treater or dehydrator is taken offline |
| Export, flare and vent headers | Slab gate | Self-relieving seats for trapped gas; fire-safe backup on soft-seated units |
Quantities, sizes and pressure classes are not published.
Why Through Conduit Gates Belong on These Lines
The valve's whole purpose is written in its name: the gate carries a bore that matches the pipe, wide open the passage is a straight tunnel, and a cleaning pig passes through without knowing the valve is there. Everything else about the design follows from that one requirement plus the fact that the line is full of gas.
The bore has to stay a bore. Gathering and inlet lines are pigged routinely - dust, liquid holdup, corrosion product. A pig's cups are destroyed by a single ledge in the flow path, so the gate port and the body bores are machined to match in one setting, and the alignment is checked as part of the acceptance test. This is also why the seat rings on a through conduit gate sit flush; a proud seat ring is the same problem as a misaligned flange, only harder to reach.
Closed, the cavity is a gas bottle. Shut a gate valve on a gas line and the body cavity holds pressurized gas with nowhere to go. Warm the valve - sun on a grade-level body, or a fire on the next one - and that cavity pressure climbs past the valve's rating. API 6D requires self-relieving seats or a relief device for exactly this reason, and on gas service the cavity-relief behaviour is reviewed during the shell test rather than assumed from the catalogue. It is one of the checks most often missing from quotations built on price sheets alone.
Unit switching is why expanding gates exist. Taking a treater or dehydration unit offline means isolating it from both directions - the feed side and the regeneration side. A pressure-assisted slab seat seals the upstream side and relies on line pressure to do it; an expanding gate forces two discs against both seats mechanically, so the seal holds whichever side is live. That is the whole reason the plant asked for expanding gates on the switching duties, and it is also why the same valve travels slower and costs more: the mechanism is doing work a pressure differential would otherwise do for free.
Wet Gas Upstream, Treated Gas Downstream
Before the treating units, the stream can carry H2S and free water together - the combination that defines sour service. Materials in contact with that stream follow NACE MR0175 / ISO 15156, with hardness held at or below 22 HRC on pressure-retaining and load-bearing parts, verified on the mill certificates rather than asserted. Downstream of treating, the duty moderates, but the fire-safe requirement does not: soft-seated valves carry API 607 certification, because a valve that is tight in normal service must still isolate after its seat has burned. Large-bore units carried sealant injection fittings as an emergency sealing path - a fitting that costs little at the works and is impossible to add economically once the line is in service.
Inspection and Certification
Shell tests ran at 1.5× rated pressure and seat tests at 1.1× per API 6D, witnessed by third-party inspection against the purchase order. The acceptance file carried the cavity-relief check on gas-service seats, bore alignment verification for the piggable lines, fire-safe type-test certificates for soft-seated designs, and NACE compliance records with hardness survey results. Every valve shipped with an EN 10204 3.1 material certificate, upgraded to 3.2 where the order called for TPI endorsement, and nameplates carried tag numbers so each unit traces back to its test record.
FAQ
What is a through conduit gate valve?
It is a pipeline gate valve to API 6D in which the gate carries a bored opening matching the valve bore exactly. Wide open, the flow passage is a straight full-bore tunnel with no step for the flow to cross, so pressure loss is minimal and a cleaning pig can pass straight through. This is what separates it from a conventional wedge gate, whose cavity interrupts the bore.
Why are through conduit gates specified on gas gathering and processing lines?
Gathering and inlet lines are pigged on a routine basis to move liquids, dust and inhibitor residue out of the line. Any ledge inside the bore tears a pig's cups or stops the run entirely, so the isolation valves on those lines must offer a true full-bore passage. Through conduit gates give that passage, hold tight shutoff when closed, and cost less than a trunnion ball valve of the same class and bore.
Why does cavity pressure relief matter on gas service?
When a gate valve closes, it traps pressurized gas in the body cavity. On a hot line or in the sun, that trapped gas expands and the cavity pressure can climb past the valve rating. API 6D therefore requires the seats to be self-relieving or the cavity to carry a relief device. On gas service this is checked during the shell test review, not left to chance.
What is the difference between a slab gate and an expanding gate?
A slab gate is a single plate with a bored port; sealing relies on the pressure acting on the upstream seat. An expanding gate mechanically spreads two discs against both seats when it closes, giving bidirectional tight shutoff independent of line pressure. Expanding gates cost more and travel slower, so they earn their place on unit switching and isolation duties where sealing must not depend on which side is pressurized.
How were sour gas and fire-safe requirements handled?
Upstream of the treating units the gas can still carry H2S and free water, so materials in contact with the stream follow NACE MR0175 / ISO 15156 with hardness at or below 22 HRC on load-bearing parts. Soft-seated designs carry fire-safe certification to API 607, with a metal backup seal that takes over if the soft seat is destroyed by fire.
What documentation comes with the valves?
EN 10204 3.1 material certificates, upgraded to 3.2 where third-party inspection applies; shell and seat test records to API 6D, including the cavity-relief check; fire-safe certificates for soft-seated designs; and NACE compliance records where the line class requires them. Nameplates carry tag numbers so each unit traces back to its test file.