Dark-shaft Gate Valve | Non-Rising Stem | DN50–DN1000

The Dark-shaft Gate Valve (Inside Screw Non-Rising Stem Gate Valve) is a shut-off valve whose stem is hidden inside the body, rotating in place without axial lifting during operation. It cuts off or connects pipeline medium with low flow resistance, good sealing, and strong corrosion resistance. Unlike rising stem valves, the dark-shaft design eliminates extra vertical space, making it ideal for buried installation, narrow pipe wells, basements, and space-constrained locations. Available in ductile iron, carbon steel, and stainless steel with elastic or metal seals, flanged/threaded/welded ends, and manual/electric/pneumatic/gear drives for diverse conditions.

1. Space-Saving Compact Non-Rising Stem Design: The valve stem is fully hidden inside the valve body with a fixed overall height that does not increase during opening and closing - unlike rising stem (OS&Y) valves whose stem extends upward when open. This effectively saves vertical installation space, making it perfect for buried pipelines, underground valve wells, equipment mezzanines, basements, and other narrow spaces where headroom is limited. The constant height also simplifies insulation and piping layout design in compact installations.

2. Reliable Bidirectional Zero-Leakage Sealing: Adopts elastic seat or metal-to-metal sealing structure (optional). The elastic seat made of EPDM, NBR, or PTFE compresses to compensate for slight deviations in the sealing surface, achieving bidirectional zero leakage suitable for water, sewage, and heating systems. The metal hard seal option is suitable for high-temperature and high-pressure harsh conditions, ensuring no medium leakage even under pressure fluctuations and thermal cycling. Both seal types provide long-term reliable shutoff performance.

3. Durable & Corrosion-Resistant Construction: Valve body and bonnet are made of ductile iron (QT500-7), carbon steel (WCB), or stainless steel (304/316), with integral casting forming to ensure overall strength and sealing integrity. The inner and outer surfaces are treated with epoxy coating or anti-corrosion plating to resist rust and corrosion, and the valve stem thread is subjected to anti-corrosion treatment (chroming or nitriding) to avoid erosion by media, rain, dust, and salt spray - significantly extending service life in outdoor, underground, and corrosive environments.

4. Low Flow Resistance & Full-Bore Flat-Bottom Channel: Equipped with a full-diameter flat-bottom flow channel, the gate is completely separated from the flow path when fully open, with flow resistance close to that of a straight pipe section, ensuring smooth media circulation and minimal pressure loss. The flat-bottom design without recessed gate pockets prevents debris, sediment, and sand accumulation at the valve bottom - a common failure point in conventional gate valves. The stem-and-gate thread transmission provides stable, controllable operating torque for smooth, effortless operation.

5. Back-Seal Structure for Pressure-Free Maintenance: Models DN≤300 are designed with a back-seal (backseat) structure that allows replacement of O-rings at the gland without shutting off the water supply and under any valve opening degree, dramatically improving maintenance convenience and reducing downtime for continuous-service water systems. This is a critical advantage for municipal water supply networks where planned shutdowns affect large numbers of customers. The back-seat also provides an additional sealing layer when the valve is fully open, protecting the packing from medium pressure.

6. Anti-Theft Lock & Multiple Drive Options: An optional anti-theft lock device prevents unauthorized operation, ensuring the safety and integrity of public pipeline systems such as municipal water supply, drainage, and fire protection networks - especially important for valves installed in public areas or underground vaults. Multiple drive modes are available including manual handwheel, electric actuator, pneumatic actuator, and gear transmission, with electric and pneumatic drives supporting remote control and SCADA/DCS integration for automated pipeline systems. All drives are interchangeable to meet project-specific automation requirements.

The Dark-shaft Gate Valve operates on the inside-screw non-rising stem wedge principle. Rotating the handwheel turns the stem in place - no axial movement, so height stays constant. Internal Acme threads engage a nut in the wedge, lifting or lowering it between seats: open, it retracts above the full-bore flat-bottom channel for unobstructed flow with no debris traps; closing, it seats tightly with elastic EPDM/NBR/PTFE for bidirectional zero leakage or metal seal for high pressure. Internal threads are protected from corrosion, ideal for buried service.

Municipal Infrastructure: Urban water supply and drainage pipelines, sewage treatment plants, municipal heating pipe networks, fire protection systems, and underground pipe corridors. The buried installation-friendly design and corrosion-resistant epoxy coating make it adapt to long-term underground humid environments, while the optional anti-theft lock protects public water assets from unauthorized operation.

Water Conservancy & Agricultural Irrigation: Reservoirs, dams, irrigation canals, farmland irrigation main pipelines, and pump station outlets. It can withstand high water pressure impact and adapt to outdoor wind, sand, rain erosion, and sediment-containing media. The flat-bottom debris-free channel prevents clogging from silt and debris common in irrigation and raw water systems.

Industrial Pipeline Systems: Petroleum chemical industry (crude oil and chemical media transportation), power industry (cooling water and condensed water control), metallurgical industry (blast furnace cooling water and hydraulic pipelines), and mining tailings treatment (slurry transportation). Metal hard seal models handle high-temperature and corrosive industrial media reliably.

Marine & Offshore: Ship ballast water systems, bilge pipelines, and offshore platform oil pipelines with seawater corrosion-resistant stainless steel or specially coated ductile iron construction. The compact non-rising stem design is well-suited to the confined spaces of engine rooms and below-deck piping, and the corrosion-resistant stem treatment withstands salt spray and marine atmospheres.

Building Services & HVAC: High-rise building water supply and drainage systems, basement pipelines, HVAC heating and cooling water circuits, and fire sprinkler isolation. The constant-height design fits within suspended ceiling plenums and mechanical rooms with limited vertical clearance, while the elastic seat provides quiet, drip-free operation for occupied building environments.

Every Dark-shaft Gate Valve is manufactured and tested per GB/T 12234, ISO 5208, optional API 600, ISO 9001. Raw materials are MTR-verified by heat number with chemical/mechanical tests for all body materials. Each valve undergoes dimensional inspection, shell test at 1.5×, seat test at 1.1×, full-bore verification. Epoxy coating tested for adhesion; stem threads verified for chroming/nitriding. Elastic seats undergo compression and aging tests. NDT includes MT/PT on castings and welds. Units ship with serialized MTR, test cert, 18-month warranty, BV/SGS/TUV on request.

Q1: What is a dark-shaft (non-rising stem) gate valve and how does it differ from a rising stem valve?

A: A dark-shaft gate valve, also called an inside-screw non-rising stem gate valve, has its valve stem completely hidden inside the valve body. When the handwheel is turned, the stem rotates in place but does NOT move vertically - the stem's internal Acme threads drive a nut embedded in the wedge gate, so the gate travels up and down internally while the stem height remains constant. In contrast, a rising stem (OS&Y) gate valve has the stem move upward through the bonnet as the valve opens, providing a visual indication of valve position but requiring additional vertical headroom. Dark-shaft valves are preferred for buried, underground, basement, and space-constrained installations; rising stem valves are preferred for above-ground industrial service where position visibility and external thread lubrication are important.

Q2: Can the dark-shaft gate valve be used for buried or underground installation?

A: Yes - this is one of its primary design advantages. Because the stem does not extend above the valve and all stem threads are internal and protected from the environment, the dark-shaft design is inherently suitable for direct burial, underground valve vaults, and buried water mains. The epoxy-coated body and chromed/nitrided stem provide corrosion resistance in damp underground conditions. For buried service, we recommend ductile iron body with epoxy coating, elastic EPDM seat, and optionally an extended stem or buried-type extension bonnet if the valve needs to be operated from ground level. Please specify buried service requirements at ordering so we can configure appropriate corrosion protection and extension options.

Q3: What is the difference between elastic seat and metal hard seal, and which should I choose?

A: Elastic seat (resilient seated) valves use EPDM, NBR, or PTFE rubber seals bonded to the wedge or body seat. The rubber compresses to compensate for minor surface irregularities, providing bubble-tight bidirectional zero leakage at lower operating torque. Elastic seats are ideal for water, sewage, heating water, and general non-corrosive media at temperatures up to 80°C (EPDM) or 120°C (special rubber). Metal hard seal valves use metal-to-metal contact (typically Stellite or stainless steel overlays) and can handle much higher temperatures (up to 425°C for carbon steel) and pressures, as well as abrasive or corrosive media that would damage rubber. Choose elastic seat for municipal water and HVAC service; choose metal hard seal for high-temperature steam, oil, chemical, and high-pressure industrial applications.

Q4: What maintenance does a dark-shaft gate valve require, and can packing/O-rings be replaced online?

A: Routine maintenance includes periodic external inspection for leaks or corrosion, operational cycling (open-close-open) to prevent seat seizure in infrequently used valves, and lubrication of accessible stem bushings. For models DN≤300 equipped with the back-seal structure, O-rings at the gland can be replaced WITHOUT shutting off the water supply and at any valve opening degree - simply open the valve fully to engage the backseat, then service the gland. For models without backseat or for major wedge/seat reconditioning, the valve must be removed from service. Because the stem threads are internal and immersed in medium, they are self-lubricating and generally require no external lubrication. Under normal conditions, these valves achieve 15–20+ years of reliable service with minimal maintenance.

Q5: What sizes, pressure ratings, and materials are available?

A: Standard sizes are DN50–DN1000 (NPS 2"–40"), customizable from DN25 to DN1200 for special projects. Standard pressure ratings are PN10, PN16, and PN25 (Class 125–150), with special customization up to PN100 (Class 600) for carbon steel and stainless steel models. Body materials include ductile iron (QT500-7) for municipal water service, carbon steel (WCB) for industrial and high-pressure service, and stainless steel (304/316) for corrosive, seawater, and food-grade applications. Seal materials include EPDM (≤80°C), NBR, PTFE for elastic seats, and Stellite/stainless for metal hard seals. End connections include flanged (GB/T 17241.6 / ISO 5208 / ANSI), threaded, and butt-weld. Operation modes include manual handwheel, gear, electric, and pneumatic.

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