An underground fully welded ball valve is a pipeline isolation valve designed for buried or direct-burial installation. Its welded body construction reduces the number of external bolted joints, while an extended stem allows the valve to be operated and maintained from above ground.
These valves are commonly considered for natural gas pipelines, city gas networks, district heating systems, oil and utility pipelines where reliable shutoff, low external leakage risk and limited surface infrastructure are important.
What Is an Underground Fully Welded Ball Valve?
An underground fully welded ball valve is a ball valve with a welded pressure-boundary construction intended for underground or direct-burial service.
Unlike a conventional flanged ball valve, the body does not rely on external bolted body or bonnet joints. This reduces potential external leakage paths and removes exposed flange connections that could otherwise require protection in a buried environment.
The valve can be supplied with an extended stem, gearbox or other operating arrangement so that the valve can be operated from a ground-level access point.
Fully welded ball valves are commonly used as isolation valves rather than throttling valves.
Underground Fully Welded Ball Valve Technical Specifications
Typical supply configurations include:
| Item | Typical specification |
|---|---|
| Valve type | Fully welded ball valve |
| Installation | Underground / direct burial |
| Design standards | API 6D, ISO 14313, GB/T 12237 or project-specific standard |
| Size range | 1/2"–48" (DN15–DN1200), depending on design |
| Pressure rating | PN1.6–PN42.0 MPa or ASME Class 150–2500, depending on design and applicable standard |
| Body construction | Fully welded forged or fabricated construction, depending on model |
| Body materials | Carbon steel, low-temperature steel, stainless steel or project-specific alloys |
| End connections | Butt-welding ends or other specified configurations |
| Operation | Manual, gearbox, pneumatic, electric or other actuator arrangements |
| Stem | Extended stem for buried installation |
| Seat | PTFE-based, reinforced polymer, PEEK, nylon or other project-specific materials |
| Testing | API 6D, API 598, GB/T 26480 or specified project requirements |
The actual size-pressure combination must be confirmed against the valve datasheet and applicable design standard. A product range should not be interpreted as meaning that every nominal size is available at every pressure rating.
Custom-designed underground full welded ball valve drawings for a customer in the UAE
underground fully welded ball valve Main Overall Dimension: (mm)
| DN | L (RF) | L (WE) | D1 | D2 | H1 | H2 | W |
|---|---|---|---|---|---|---|---|
| 150 | 403 | 457 | 219 | 273 | 246 | 193 | 280 |
| 200 | 502 | 521 | 273 | 351 | 285 | 238 | 320 |
| 250 | 568 | 560 | 325 | 426 | 323 | 278 | 320 |
| 300 | 648 | 635 | 377 | 508 | 365 | 315 | 400 |
| 350 | 762 | 762 | 426 | 580 | 410 | 370 | 400 |
| 400 | 838 | 838 | 480 | 675 | 457 | 425 | 450 |
| 450 | 914 | 914 | 530 | 740 | 495 | 465 | 450 |
| 500 | 991 | 991 | 630 | 805 | 540 | 495 | 500 |
| 600 | 1143 | 1143 | 735 | 975 | 640 | 580 | 500 |
| 700 | 1346 | 1346 | 830 | 1130 | 715 | 660 | 500 |
| 800 | 1524 | 1524 | 930 | 1270 | 785 | 730 | 500 |
| 900 | 1727 | 1727 | 1036 | 1425 | 860 | 820 | 800 |
| 1000 | 1900 | 1840 | 1236 | 1625 | 970 | 920 | 800 |
Component Materials
The following configuration is an example based on a specific product design. Actual materials may change with valve size, pressure class, temperature and service medium.
| Component | Typical material |
| Body | ASTM A105 or project-specified carbon steel |
| Ball | Carbon steel or stainless steel with ENP / other coating where specified |
| Stem | ASTM A182 F6a or other specified stem material |
| Seat | Reinforced PTFE, PEEK, nylon or other service-specific material |
| O-rings | FKM (Viton®) or other compatible elastomer |
| Fire-safe sealing | Stainless steel + graphite, where required |
| Bolting | ASTM A193 B7 / ASTM A194 2H or project-specified equivalent |
| Spring | 17-7PH or other suitable spring material |
| Bearings | PTFE-based or stainless-steel-backed bearing materials |
The exact material list should be issued with the valve datasheet or approved drawing. It should not be treated as a universal material specification for every underground fully welded ball valve.
Structural Features
1. Fully Welded, Integrally Molded Body
The valve body is made of the same material as the pipeline (carbon steel or stainless steel) and integrally welded, eliminating bolted connections and completely preventing external leakage. Compared with cast-steel flanged ball valves, this reduces three potential leakage points, eliminates defects such as sand holes and pores in the cast body, and extends the service life of the pipeline. Because the body and pipe are the same material, the risk of weld cracking from uneven stress is avoided, and the valve adapts to demanding environments such as seismic zones and vehicle traffic.
2. Extended Stem Design
The stem extends to the ground operating well, supporting above-ground operation without large valve chambers - only small shallow wells are needed for maintenance, saving more than 30% of construction cost. The adjustable stem length adapts to different burial depths for diameters from DN15 up to DN1200.
3. High-Precision Ball and Sealing System
The ball is machined under computer-controlled measurement, ensuring high precision and smooth, interference-free operation. The sealing ring uses a PTFE + 25% carbon-fiber composite combined with a disc-spring seat structure that automatically compensates for temperature and pressure changes, maintaining long-term zero leakage. The valve has passed the API 6D leakage test, confirming reliable sealing performance.
Applications
- Urban gas supply: direct burial saves urban space; combined with cathodic protection, the valve adapts to damp underground environments. It is applied to gas output pipelines, main lines and branch supply pipelines, ensuring safe shut-off.
- Heating networks: bidirectional sealing resolves pressure-fluctuation problems in ring-shaped heating systems, reducing heat loss by more than 15%. Suitable for large heating output pipelines, main lines, branch lines and heat-exchanger circuits.
- Petrochemical industry: safe shut-off of highly corrosive media in reactors, storage tanks and transmission pipelines; explosion-proof (ATEX) certified options.
- Industrial equipment: steel plants - fluid, exhaust-gas, gas and heat-supply and fuel-supply pipelines; nuclear and thermal power plants - rapid shutdown of high-temperature steam pipelines with response time no more than 3 seconds.
- Smelting industry: blast-furnace gas pipelines, where the valve maintains stable sealing performance under high-temperature service (select the body material and seat rating per the actual gas temperature and the governing code).
Inspection and Testing
For buried pipeline valves, inspection should cover both pressure integrity and functional performance.
Depending on the applicable standard and purchase specification, inspection may include:
Shell Test
The pressure boundary is tested under the specified hydrostatic test conditions.
Seat Test
The valve seats are tested for leakage according to the applicable test procedure.
Body and Weld Inspection
For welded-body construction, inspection requirements may include visual inspection and NDE of specified welds.
Possible NDE methods include:
Radiographic testing (RT)
Ultrasonic testing (UT)
Magnetic particle testing (MT)
Liquid penetrant testing (PT)
The exact NDE extent should be defined by the applicable standard and project specification.
Material Verification
Material certificates should be reviewed against the approved material list.
PMI may be required for stainless steel, alloy components or other critical materials.
Functional Testing
The valve and actuator assembly should be checked for:
Opening and closing operation
Operating torque
Position indication
Limit switches
Emergency shutdown function, where applicable
If a pneumatic or electric actuator is supplied, actuator testing should be included in the agreed inspection plan.
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FAQ
What is an underground fully welded ball valve?
It is a ball valve designed for buried pipeline installation, typically using a welded body and an extended stem or operating arrangement that allows the valve to be operated from above ground.
Why are fully welded ball valves used underground?
The welded body reduces external bolted joints and associated leak paths and can be well suited to buried pipeline construction. The final suitability depends on the valve design, coating system, burial conditions and project requirements.
Can a fully welded ball valve be directly buried?
Yes, suitable fully welded ball valves can be designed for direct burial. However, direct-burial suitability should be confirmed from the manufacturer's design, coating system, stem extension arrangement and project specification rather than assumed from the valve name alone.
What standards are used for underground fully welded ball valves?
API 6D and ISO 14313 are important standards for pipeline ball valves. Chinese projects may also specify GB/T 12237 for steel ball valves and GB/T 26480 for inspection and testing. Other standards may apply to dimensions, welding ends, flanges, actuators and pipeline installation.
Is API 6D the same as ISO 14313?
They are closely related pipeline valve standards, but they should not simply be treated as interchangeable. ISO 14313:2025 states that it supplements API 6D 25th Edition with additional requirements.
What is the difference between A105 and WCB?
ASTM A105 is a forged carbon steel specification for pressure-containing components, while ASTM A216 WCB is a cast carbon steel grade. They are different material specifications and should not be listed as equivalent body materials.
What seat material is best for an underground ball valve?
There is no single best seat material. PTFE, reinforced PTFE, PEEK, nylon and metal seats may be suitable for different pressure, temperature and medium conditions. The seat should be selected from the actual service conditions and project specification.
What is an extended stem used for?
An extended stem allows an underground valve to be operated from above ground. The required extension length should be based on the valve centerline elevation, burial depth and operating arrangement.
Does a fully welded ball valve need a valve chamber?
Not necessarily. Direct burial can reduce the need for a conventional large valve chamber, but the actual installation must still provide suitable access for operation, inspection and maintenance of the stem, actuator, drain, vent and other required components.
Is a fully welded ball valve suitable for natural gas?
Yes, fully welded ball valves are commonly used for gas pipeline isolation. The valve should be specified according to the applicable pipeline and valve standards, pressure and temperature, seat material, fire-safe requirements, corrosion protection and actuator requirements.
Can an underground ball valve be used for throttling?
Ball valves are generally selected for isolation rather than continuous throttling. If regular flow control is required, the pressure drop, velocity, erosion and seat design should be evaluated before selecting the valve.