Trunnion Double Block & Bleed DBB

2" - 24" (DN50 - DN600)

API 6D, API 607, ASME B16.34, ISO 15848

A216 WCB, A351 CF8M, A182 F51, A182 F53

PTFE, Nylon, PEEK, metal-to-metal (Stellite 6)

Trunnion-mounted ball with spring-loaded seats, double block & bleed in single body, side-entry or top-entry maintenance access

Pipeline Isolation That Doesn't Trust One Seat——Because Pipeline Consequences Don't Allow It

A pipeline isolation valve sits between sections of process piping carrying hydrocarbon, chemical, or high-pressure gas——media where the consequence of isolation failure isn't a gauge reading error, it's a safety incident, an environmental release, or a regulatory violation. On those applications, one blocking seat isn't sufficient even when it's holding——because the consequence of that one seat failing is unacceptable. A single-block valve provides isolation from one sealing boundary; if that boundary leaks, the media reaches the downstream section that was supposed to be isolated. A trunnion DBB valve provides isolation from two independent sealing boundaries with a verified zero-pressure cavity between them——the upstream seat blocks pressure from the active pipeline section, the downstream seat blocks pressure toward the isolated section, and the bleed cavity between the two seats gives you a verified zero-pressure zone that confirms both seats are holding before the downstream section gets opened for maintenance, inspection, or hot-tap operations. Two independent seats, two independent failure modes, one verified isolation status visible through the bleed port.

The trunnion-mounted ball architecture provides the structural platform for two independent spring-loaded seats within a single valve body. The ball is anchored by upper and lower trunnion pins——it doesn't float under pressure differential like a floating ball valve. The two seat rings sit on opposite sides of the ball, each independently spring-loaded against the ball surface. When the ball rotates to the closed position, both seats seal simultaneously——the upstream seat against upstream pressure, the downstream seat against downstream pressure, both independent, both verifiable through the bleed cavity. That trunnion architecture is what makes DBB feasible at larger bore sizes——a floating ball valve has one sealing direction (the ball pushes against the downstream seat under upstream pressure), but a trunnion valve has two independent sealing directions (each spring-loaded seat pushes against the ball from its own side). Two seats. Two springs. Two independent sealing forces. One verified isolation cavity between them.

Why Trunnion DBB Replaces the Old Three-Valve Assembly

Before single-body DBB valves became the standard pipeline isolation specification, the industry used three separate valves to achieve double block and bleed: two isolation valves (one upstream, one downstream) and one bleed valve in a tee fitting between them. That three-valve assembly works——it provides two blocking boundaries and a bleed verification port——but it has four inherent problems that a single-body trunnion DBB eliminates.

First: multiple potential leak paths. Three valves means three valve bodies, three sets of body-to-piping connections, three bonnet-to-body gaskets, and the tee fitting connections between the three valves——each connection is a potential external leak path. A single-body DBB eliminates all the inter-valve connections——both blocking seats and the bleed port are inside one body, with no external connections between the blocking and bleeding functions except the single bleed port to atmosphere or drain. Fewer external connections means fewer potential leak paths.

Second: larger installation envelope. Three separate valves with a tee fitting occupy significantly more pipe rack space than a single-body DBB——the three-valve assembly length is the combined length of two isolation valves plus the tee fitting, while the single-body DBB fits within a single valve body length. On congested pipe rack installations where space allocation is a constraint, the single-body DBB's compact footprint is a practical advantage that goes beyond aesthetics.

Third: higher total weight. Three valve bodies plus a tee fitting weigh more than one DBB body——more steel in the pipe rack, more weight on pipe supports, more structural loading. Weight reduction isn't just a convenience——on large-bore pipeline installations, the weight difference between a three-valve assembly and a single-body DBB can affect pipe support design and stress analysis calculations.

Fourth: sequential operation complexity. Operating three separate valves requires a specific sequence: close the upstream isolation valve, open the bleed valve, verify zero pressure at the bleed, close the downstream isolation valve, then proceed with the downstream work. That sequence has to be executed correctly every time——any sequence error (closing the wrong valve first, opening the wrong valve during verification) can expose the downstream section to live process pressure. A single-body DBB valve operates with one actuator that closes the ball (both seats seal simultaneously) and one bleed valve that verifies the cavity——simpler operation, fewer steps, less opportunity for sequence error.

Bleed Port Configuration——Vented to Atmosphere or Closed Drain

The bleed cavity between the two blocking seats vents through a designated port on the valve body——typically a 1/2" or 1" flanged or threaded connection. Two vent configurations: atmosphere vent (the bleed port opens directly to atmosphere through a needle valve or plug——used on non-hazardous service where venting small quantities to atmosphere is acceptable) and closed drain vent (the bleed port connects to a closed drain system that captures the vented media——used on hazardous, flammable, or environmentally regulated service where atmospheric venting is prohibited). The bleed port configuration is specified based on the process media characteristics and the facility's environmental emission regulations——you don't vent H₂S or VOCs to atmosphere, you capture them in a closed drain system. On non-hazardous utility service, atmospheric venting through a needle valve is standard practice.

ISO 15848 Low-Leakage Certification——DBB Isolation and Fugitive Emission Compliance

The stem sealing system on the trunnion DBB valve follows the same low-leakage design philosophy as the FLOWKS trunnion ball valve series——graphite packing with live-loaded gland design for ISO 15848 compliance. The stem emerges from the body at the bonnet-to-body interface——that's the only external penetration point for the valve's pressure boundary, and it's the point where fugitive emissions can escape if the packing doesn't seal effectively. ISO 15848 defines acceptable emission rates per the valve's certification level (BH, AH, or CH for different temperature and cycle conditions). FLOWKS trunnion DBB valves are designed to meet ISO 15848 emission requirements——the packing system, the gland loading mechanism, and the bonnet-to-body gasket are all specified for low-leakage performance at the valve's rated temperature and pressure conditions. On environmentally regulated process lines (VOC emission limits, H₂S exposure thresholds), ISO 15848 certification isn't an optional upgrade——it's a specification requirement driven by regulatory compliance.

Size and application. Gauge block DBB valves are compact instrumentation valves (1/2"-2" bore) for instrument isolation——direct thread-on mounting at gauge and transmitter connection points. Trunnion DBB valves are pipeline isolation valves (2"-24" bore) for section isolation between active and maintenance pipeline segments——full-bore or reduced-bore flow passage, flanged or butt-weld connections to the pipeline, actuator-driven operation for automated isolation sequences. Same DBB functional architecture (two seats + bleed cavity), different scale and different application context.

Floating ball valves seal in one direction——the ball floats downstream under upstream pressure and pushes against the downstream seat. That single-direction sealing provides one blocking boundary, not two independent ones. Trunnion-mounted balls anchor the ball with upper and lower pins——both seat rings spring-load independently against the ball from opposite sides. Each seat seals independently in its own direction. That bidirectional independent sealing is the structural requirement for true DBB isolation——two independent boundaries, two independent failure modes, verified through the cavity between them.

With soft seats (PTFE, Nylon, PEEK)——Class VI (bubble-tight) achievable on both seats. With metal-to-metal seats (Stellite 6)——Class IV on both seats, not bubble-tight but durable under erosive and high-temperature conditions. The seat material choice determines shutoff class on each seat independently——both seats use the same material specification, but each seat's sealing performance is verified separately through the bleed cavity.

1/2" or 1"——flanged or threaded, depending on the valve bore size and the facility's drain system connection standard. Smaller bore DBBs typically use 1/2" threaded bleed ports. Larger bore DBBs (above 8") typically use 1" flanged bleed ports to accommodate higher potential bleed flow rates if a seat failure occurs at full pipeline pressure.

Yes——API 607 fire-safe with metal-to-metal seating (Stellite 6). Soft seats burn away in fire exposure; metal seats maintain sealing through fire and return to effective shutoff afterward. On flammable gas and liquid pipelines, API 607 fire-safe certification is a regulatory requirement——metal seating is mandatory, not optional.