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Gauge Block-NonoFlange DBB
Quick Specs 1/2" - 2" (DN15 - DN50) Class 150 - 1500 / up to 10,000 psi (needle-type configurations) API 6D, API 607, ASME B16.34, ISO 15848 A182 F316, A182 F51, A182 F53 PTFE, PEEK, metal-to-metal (Stellite 6) Gauge block body with nonoflange (no-flange) connection, double block & bleed with single bleed port, compact instrumentation valve What a Gauge Block DBB Doesââand Why Instrument Engineers Specify It Pressure gauges, transmitters, and instrument taps need isolation for calibration, replacement, and verification without shutting down the main process line. That isolation has to be doubleââone blocking seat isn't enough when the process line carries 1500 psi of hydrocarbon or 10,000 psi of hydraulic fluid. A single seat could leak. A single seat could fail. And if it does, the process media reaches the instrument and the bleed port simultaneouslyââthe technician gets hit with live process pressure while trying to verify zero. That's why the API 6D definition of Double Block and Bleed requires two independent sealing boundaries with a bleed cavity between them: the first seat blocks upstream pressure, the second seat blocks downstream pressure toward the instrument, and the bleed cavity between the two seats gives you a verified zero-pressure zone where you can confirm that both seats are holding before you open the instrument side. A gauge block DBB valve puts both blocking seats and the bleed cavity into a single compact body that mounts directly at the instrument connection pointââno separate valves, no tee-fitting assemblies, no multiple potential leak paths between the blocking and bleeding functions. NonoflangeââNo Flange, No Bolting, Direct Thread-On Installation The nonoflange connection means the valve body threads directly onto the instrument connection pointââtypically a 1/2" or 3/4" NPT threaded tap on the pipe or vessel wallââwithout any flange bolting between the valve and the process. That direct thread-on mounting eliminates the flange gasket as a potential leak path (flanges leak at the gasket; threaded connections don't have gaskets), reduces the installation envelope (no flange bolting circle, no gasket alignment, no bolt tensioning procedureââjust thread the valve onto the tap and wrench-tighten), and keeps the overall assembly height down to the minimum practical dimension for instrument installation. Flanged gauge block DBBs exist (FLOWKS manufactures those tooââsee the Flange DBB series), and they serve applications where the instrument connection point is flanged rather than threaded. But for the majority of instrumentation taps on process pipingââwhich are NPT-threaded pipe bosses welded onto the main lineââthe nonoflange version is the simpler, more compact, and more direct installation option. No flange to bolt. No gasket to align. No flange bolting procedure to execute. Thread it on. Wrench it tight. Done. The compact body geometry is a functional requirement, not a cosmetic preference. Instrument installations have limited physical spaceââthe gauge or transmitter sits on the end of the valve, and both have to fit within the pipe rack or vessel skirt clearance envelope. A gauge block body with nonoflange threading keeps the total assembly length (process connection to instrument flange or thread) at the minimum dimension that still houses both blocking seats and the bleed cavity. Shorter assembly length means less protrusion from the pipe wall, less vulnerability to impact damage from maintenance activity in congested pipe rack areas, and less weight on the threaded pipe boss (threaded connections carry bending loads from the assembly weightââshorter and lighter means lower bending stress on the thread engagement). Two Seats and a BleedââThe Functional Architecture of DBB Isolation The gauge block body contains two independent seating elements arranged in series along the flow pathââthe upstream (process-side) seat and the downstream (instrument-side) seat. Each seat seals independently: the upstream seat blocks process pressure from entering the bleed cavity between the two seats, and the downstream seat blocks any pressure that might leak past the upstream seat from reaching the instrument connection. The bleed port sits between the two seatsââa small-diameter port (typically 1/4" NPT) that vents the cavity to atmosphere or to a closed drain system. In operation: both seats closed, bleed valve openââthe bleed port verifies zero pressure in the cavity between the seats, confirming that both blocking boundaries are holding. If the upstream seat leaks, process pressure enters the cavity and vents through the bleed portââthe technician sees live pressure at the bleed and knows the upstream seat isn't holding. If the downstream seat leaks, pressure from the cavity (if any) reaches the instrument sideââthe technician sees pressure at the instrument connection and knows the downstream seat isn't holding. The bleed port makes seat integrity visible and verifiableââyou don't assume both seats are holding, you verify it by checking the bleed port pressure before opening the instrument side. The two seats don't share any common failure modeââthey're independent sealing elements with independent operating mechanisms (the upstream seat closes when the upstream stem rotates to closed position; the downstream seat closes when the downstream stem rotates to closed position; the bleed port opens when the bleed needle valve stem backs off its seat). A single-point failure (one seat leaks) doesn't compromise the other seat's sealing integrityââthe remaining seat still blocks pressure from the failed side, and the bleed port vents the cavity to reveal which seat has failed. That independence is the fundamental safety advantage of DBB over single-block isolation: one seat can fail and the system still provides verified isolation from the remaining seat, with visible evidence of the failure through the bleed port. Seat Material OptionsââPTFE, PEEK, and Metal-to-Metal for Different Process Conditions PTFE seats: the standard choice for general instrumentation isolation at moderate temperatures and pressures. PTFE provides tight shutoff (Class VI achievable), low friction operation, and wide chemical compatibility across hydrocarbon, water, steam, and chemical process media. PTFE's temperature ceiling (approximately 200°C) covers the majority of instrument tap operating temperatures where gauge block DBBs are installed. PEEK seats for elevated temperature and higher mechanical stress applicationsââPEEK maintains dimensional stability and seating surface integrity at temperatures where PTFE softens and deforms under sustained load, and at higher pressure ratings where PTFE's mechanical strength becomes marginal. Metal-to-metal seats (Stellite 6 hard-facing on the ball and seat sealing surfaces) for high-pressure, high-temperature, and erosive media applications where soft seats can't surviveââmetal seating delivers Class IV leakage (not bubble-tight, but durable under conditions that destroy PTFE and PEEK). The seat material choice follows the same process-matching logic as across the FLOWKS product series: match the sealing surface to what the media and the operating conditions will do to it over the intended service duration. FAQ Nonoflange: direct NPT thread-on connection to the instrument tapââno flange bolting, no gasket, minimum assembly length. Flanged: flanged connection between the valve and the instrumentââfor applications where the instrument tap is flanged rather than threaded, or where flanged connections are specified for maintenance accessibility. Same DBB functional architecture (two seats + bleed cavity), different connection interface. Single seat isolation provides one blocking boundary. If that single seat leaks, process pressure reaches the instrument and the bleed port simultaneouslyââthe technician gets hit with live process pressure. DBB provides two independent blocking boundaries with a verified zero-pressure cavity between them. If one seat fails, the other still blocks, and the bleed port reveals which seat has failed. Two seats aren't redundantââthey're independent, with independent failure modes and visible verification through the bleed cavity. Class 150 through Class 1500 for flanged connection versions. Up to 10,000 psi for needle-type configurations with threaded connectionsââthe compact body geometry and forged material construction handle high internal pressure at small bore sizes where flanged pressure ratings exceed practical limits for instrument connection dimensions. It vents the cavity between the two blocking seats to atmosphere or a closed drain system. When both seats are closed and the bleed valve is open, zero pressure at the bleed port confirms both seats are holding. If process pressure appears at the bleed, the upstream seat is leaking. If pressure appears at the instrument connection, the downstream seat is leaking. The bleed port makes seat integrity visibleââyou verify isolation, you don't assume it. YesââAPI 607 fire-safe certification available with metal-to-metal seating (Stellite 6). PTFE and PEEK seats burn away in fire exposure; metal seats maintain sealing through fire and return to effective shutoff afterward. For installations on flammable gas and liquid process lines where fire-safe isolation is required, metal seating is the mandatory specification.
Gauge Block NonoFlange SBB
Quick Specs 1/2" - 2" (DN15 - DN50) Class 150 - 1500 / up to 10,000 psi (needle-type configurations) API 6D, API 607, ASME B16.34, ISO 15848 A182 F316, A182 F51, A182 F53 PTFE, PEEK, metal-to-metal (Stellite 6) Gauge block body with nonoflange connection, single block & bleed with bleed verification port, compact instrumentation valve SBB vs DBBââOne Seat or Two, and When One Is Enough Double Block and Bleed (DBB) provides two independent sealing boundaries with a verified zero-pressure cavity between themââthe maximum isolation assurance available in a single valve body. But not every instrument isolation application needs maximum assurance. Some applications need verified isolation with a bleed check, but the process conditions don't demand two independent blocking boundaries: low-pressure utility lines, non-hazardous media (water, air, nitrogen), instrument taps on non-flammable service where the consequence of a single seat leak is a gauge reading error, not a safety incident. For those applications, Single Block and Bleed (SBB) provides one blocking seat and one bleed verification portââenough isolation to remove the instrument safely (the seat blocks process pressure, the bleed port verifies the seat is holding before the instrument side gets opened), but with only one sealing boundary instead of two. The trade-off is straightforward: one seat instead of two means fewer components, shorter assembly length, lower cost, and simpler operationââbut only one blocking boundary, not two independent ones. If that single seat fails, there's no backup seat to maintain isolation. The bleed port reveals the failure (process pressure appears at the bleed), but the isolation is gone until the seat gets repaired. FLOWKS gauge block SBB valves use the same compact body geometry as the DBB versionâânonoflange thread-on mounting, direct NPT connection to the instrument tap, minimum assembly heightââbut with one blocking seat instead of two. The bleed port sits on the downstream side of the single blocking seat, providing the verification function that distinguishes SBB from simple single-block isolation without bleed capability. With the blocking seat closed and the bleed valve open, zero pressure at the bleed port confirms the single seat is holding. If process pressure appears at the bleed, the seat has failedââthe isolation is compromised and the instrument side gets exposed to live process pressure. That verification capability (bleed-check before opening the instrument side) is what makes SBB a safer isolation method than a simple single-block valve without bleedââthe technician doesn't assume the seat is holding, the technician verifies it through the bleed port. But it's still only one seatââif the verification reveals a failure, there's no backup. When SBB Is the Right Specificationââand When It Isn't The decision between SBB and DBB isn't about preference or cost optimizationââit's about consequence analysis. What happens if the single blocking seat leaks or fails? If the consequence is a gauge reading error or a transmitter calibration drift on a non-hazardous utility lineââSBB is adequate. The bleed port reveals the failure, maintenance replaces the seat, and the process continues. If the consequence is exposure to toxic, flammable, or high-pressure media that would create a safety incident or environmental releaseââDBB is mandatory. The single seat in SBB provides verified isolation when it's holding, but it provides no backup when it fails. On hazardous service, that's unacceptableââthe second seat in DBB provides the backup that maintains isolation even when the first seat fails, and the bleed cavity reveals which seat has failed without losing overall isolation capability. Regulatory frameworks and industry standards increasingly default to DBB for process isolation on hydrocarbon, chemical, and high-pressure pipelinesââthe API 6D definition of Double Block and Bleed is the baseline specification for new installations on flammable and toxic service. SBB remains appropriate for non-hazardous utility service (instrument air, nitrogen blanketing, low-pressure water, HVAC systems) where the isolation consequence is operational inconvenience, not safety risk. On those applications, SBB's simpler architecture (one seat, one bleed, fewer components, shorter assembly) provides verified isolation with adequate safety marginââthe cost and complexity reduction of eliminating the second seat is justified by the lower consequence of a single-seat failure. The Bleed PortââSame Verification Function, Positioned After One Seat Instead of Two In a DBB valve, the bleed port sits between two blocking seatsââit vents the cavity between them and verifies that both upstream and downstream seats are holding simultaneously. In an SBB valve, the bleed port sits downstream of the single blocking seatââit vents the space between the seat and the instrument connection, and verifies that the single seat is holding before the instrument side gets opened. The verification procedure is the same in both configurations: close the blocking seat(s), open the bleed valve, check for zero pressure at the bleed port. Zero pressure confirms the seat(s) are holding. Live pressure at the bleed reveals a seat failure. The difference is that DBB verifies two independent seats simultaneously, while SBB verifies one seatââthe verification reveals the same truth (seat integrity status), but the consequence of a revealed failure is different (DBB maintains isolation from the remaining seat; SBB loses isolation entirely). The bleed port on the SBB configuration serves a secondary function: venting trapped pressure between the seat and the instrument connection before the instrument gets removed. Even if the seat is holding perfectly, residual pressure trapped between the closed seat and the closed instrument connection (from previous process exposure, thermal expansion, or instrument bleed-down) needs a vent path before the instrument flange or thread gets disassembled. The bleed port provides that ventââthe technician opens the bleed valve, the trapped pressure vents safely, and the instrument gets removed with zero residual pressure on the connection side. That venting function is independent of the seat verification functionââthe bleed port vents trapped pressure even when the seat is holding, and it reveals seat failure when the seat isn't holding. Two functions, one port. Compact Instrumentation GeometryââSame Mounting, Shorter Than DBB The SBB body is shorter than the DBB body because it houses one blocking seat instead of twoââthe elimination of the second seat assembly reduces the body length by the distance between the two seat positions. On instrument installations with tight physical clearance (pipe rack congestion, vessel skirt interference, limited protrusion space from the pipe wall), that shorter body dimension can make the difference between a valve that fits and a valve that doesn't. The nonoflange thread-on connection keeps the mounting interface identical to the DBB versionââsame NPT thread engagement on the instrument tap, same direct-mount installation, same elimination of flange bolting and gasketing. The difference is purely in the internal seat architecture: one seat instead of two, shorter body, lower weight on the threaded pipe boss. FAQ SBB: one blocking seat + one bleed verification port. DBB: two blocking seats + one bleed port between them. SBB provides verified isolation from one seatââthe bleed confirms the seat is holding, but if it fails, there's no backup. DBB provides verified isolation from two independent seatsââif one fails, the other maintains isolation, and the bleed reveals which one failed. SBB is appropriate for non-hazardous service where single-seat failure consequence is operational, not safety-related. DBB is mandatory for hazardous service where single-seat failure consequence is a safety incident. The DBB body is longer than the SBB body (it houses two seats instead of one). If the installation has enough clearance for the longer DBB assembly, the upgrade is a simple valve swapââsame nonoflange NPT thread mounting interface, same instrument connection point, just a longer body with two seats instead of one. If the clearance doesn't accommodate the longer DBB body, the SBB configuration is the physical limit for that installationââyou can't fit two seats where the space only allows one. Typically 1/4" NPTââstandard instrument bleed connection size across DBB and SBB gauge block valves. The bleed port accepts a standard needle valve, a bleed plug, or a vent tube to atmosphere or closed drain. The connection is the same whether the valve is DBB or SBB. With metal-to-metal seats (Stellite 6)ââyes, API 607 fire-safe certification available. PTFE and PEEK seats burn away in fire exposure; metal seats maintain sealing through fire. For non-hazardous service where fire-safe isn't a regulatory requirement, PTFE or PEEK seats provide adequate performance. For installations on flammable service (even at low pressure), metal seating is the correct specification regardless of whether the valve is SBB or DBB. Because verified isolation is safer than assumed isolation. A simple isolation valve without a bleed port doesn't provide any way to confirm the seat is holding before the instrument side gets opened. The technician assumes the seat is holding based on the valve position indicatorââbut position indicators can be wrong, and seats can leak even when the stem indicates closed. The bleed port in SBB provides a direct, visible verification methodââopen the bleed, check for zero, confirm the seat is actually holding before proceeding. That verification step takes seconds and eliminates the assumption that isolation is intact.
Trunnion Double Block & Bleed DBB
Quick Specs 2" - 24" (DN50 - DN600) Class 150 - 1500 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 SeatââBecause Pipeline Consequences Don't Allow It A pipeline isolation valve sits between sections of process piping carrying hydrocarbon, chemical, or high-pressure gasââmedia 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 holdingââbecause 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 themââthe 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 pinsââit 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 simultaneouslyââthe 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 sizesââa 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 worksââit provides two blocking boundaries and a bleed verification portââbut 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 valvesââeach connection is a potential external leak path. A single-body DBB eliminates all the inter-valve connectionsââboth 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 DBBââthe 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 bodyââmore steel in the pipe rack, more weight on pipe supports, more structural loading. Weight reduction isn't just a convenienceââon 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 timeââany 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 cavityââsimpler operation, fewer steps, less opportunity for sequence error. Bleed Port ConfigurationââVented to Atmosphere or Closed Drain The bleed cavity between the two blocking seats vents through a designated port on the valve bodyââtypically 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 plugââused 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 mediaââused 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 regulationsââyou don't vent HâS 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 CertificationââDBB 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 seriesââgraphite packing with live-loaded gland design for ISO 15848 compliance. The stem emerges from the body at the bonnet-to-body interfaceââthat'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 requirementsââthe 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, HâS exposure thresholds), ISO 15848 certification isn't an optional upgradeââit's a specification requirement driven by regulatory compliance. FAQ Size and application. Gauge block DBB valves are compact instrumentation valves (1/2"-2" bore) for instrument isolationââdirect 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 segmentsââfull-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 directionââthe 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 pinsââboth 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 isolationââtwo independent boundaries, two independent failure modes, verified through the cavity between them. With soft seats (PTFE, Nylon, PEEK)ââClass VI (bubble-tight) achievable on both seats. With metal-to-metal seats (Stellite 6)ââClass 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 independentlyââboth seats use the same material specification, but each seat's sealing performance is verified separately through the bleed cavity. 1/2" or 1"ââflanged 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. YesââAPI 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 requirementââmetal seating is mandatory, not optional.
Flange Block-Bleed DBB
Quick Specs 2" - 20" (DN50 - DN500) Class 150 - 600 API 6D, API 607, ASME B16.34, ISO 15848 A216 WCB, A351 CF8M, A182 F51 PTFE, Nylon, PEEK, metal-to-metal (Stellite 6) Flanged end connections (RF/RTJ), bolted bonnet, double block & bleed in single body, manual or actuated operation Flanged ConnectionsââWhere the Pipeline Specification Calls for Bolted End Connections The flanged DBB valve serves the same isolation function as the trunnion DBBââtwo independent blocking seats with a verified bleed cavity between themââbut with flanged end connections to the pipeline instead of butt-weld. Flanged connections (raised face or ring-type joint per ASME B16.5) are the standard pipeline connection method for Class 150 through Class 600 in many facility specificationsââparticularly on process piping systems where maintenance accessibility requires bolted connections that can be disassembled without cutting and re-welding the pipe. A butt-weld DBB valve (like the FLOWKS all-welded DBB series) permanently joins the valve to the pipelineââremoval requires cutting the weld and re-welding a replacement. A flanged DBB valve unbolted from the pipeline flangesââfour bolts per flange (on smaller sizes) or dozens on larger sizes, but the principle is the same: unbolt, remove, replace, re-bolt. No welding. No hot work permit. No radiographic inspection of replacement welds. That bolted-connection accessibility is the primary reason facilities specify flanged DBB valves on Class 150-600 pipingââthe maintenance logistics of bolted connections are simpler, faster, and cheaper than welded connections on the pressure classes where flanged joints are structurally adequate. Above Class 600, butt-weld connections become the standard specification because flanged joints at Class 900 and above require heavier bolting, larger gasket surfaces, and higher bolt tensioning loads that push the flanged-connection maintenance complexity beyond practical limits for many installations. The FLOWKS flanged DBB covers Class 150 through Class 600ââthe pressure range where flanged connections serve the majority of process piping installations and where the bolted-connection maintenance advantage is most valuable. Bolted BonnetââMaintenance Access Without Removing the Valve from the Pipeline The bonnet (the valve body section that contains the stem, packing, and the upper trunnion bearing) attaches to the body through a bolted bonnet-to-body connectionââbolted, not welded. That bolted bonnet design provides maintenance access to the stem packing, the upper trunnion bearing, and the ball-and-seat assembly without removing the entire valve from the pipeline flanges. The maintenance procedure: unbolt the bonnet, lift the bonnet assembly (stem, ball upper pin, packing) out of the body, access the seat rings and ball for inspection or replacement, re-install the bonnet with new gasket, re-bolt, re-torque. The pipeline flanges stay boltedââthe valve body stays connected to the pipeline throughout the maintenance operation. That in-line maintenance capability is the same principle that FLOWKS top-entry ball valves use for buried and welded pipeline serviceââthe difference is that the flanged DBB's bolted bonnet serves flanged-connection installations where the pipeline flanges themselves are the primary disassembly point, and the bonnet bolts are the secondary access point for internal trim maintenance. The bonnet-to-body gasket is a critical sealing elementââit's the pressure boundary between the body cavity (exposed to pipeline pressure) and the bonnet cavity (exposed to packing and stem). The gasket specification matches the process media and temperature conditions: spiral-wound gaskets (stainless winding with graphite or PTFE filler) for general process service, metal-ring gaskets for high-temperature and fire-safe applications where spiral-wound gaskets would degrade under sustained thermal loading. The gasket gets replaced every time the bonnet gets unboltedââgaskets are one-time sealing elements, not reusable. That replacement requirement is standard maintenance practice across bolted-bonnet valve designsââyou don't re-use a gasket on a pressure boundary, you install a new one every time the joint gets re-assembled. Double Block and BleedââSame Functional Architecture, Flanged Installation Package The DBB isolation architecture inside the flanged body is identical to the trunnion DBB: two independent spring-loaded seats pressing against the trunnion-mounted ball from opposite sides, with a bleed cavity between them that vents through a designated port on the body. The upstream seat blocks process pressure from the active pipeline section. The downstream seat blocks pressure toward the isolated section. The bleed cavity verifies both seats simultaneouslyââzero pressure at the bleed confirms both blocking boundaries are holding. The functional behavior is the same regardless of the connection type (flanged or butt-weld): the seats seal, the bleed verifies, the isolation is double and independent. The difference between the flanged DBB and the butt-weld DBB is entirely in the installation interfaceââflanged ends for bolted pipeline connection with maintenance accessibility, versus butt-weld ends for permanent pipeline integration with maximum structural integrity and minimum external leak paths. Both configurations deliver the same DBB isolation performance. The choice between them is driven by the facility's piping specification (does the line use flanged or welded connections at this size and pressure class?), maintenance philosophy (is in-line removal a requirement or is permanent installation preferred?), and the specific application's structural and regulatory requirements. FAQ Flanged DBB: bolted end connections (RF/RTJ flanges)ââremovable from the pipeline by unbolting without cutting pipe. Butt-weld DBB: welded end connectionsââpermanently joined to the pipeline, removal requires cutting the weld. Same DBB isolation function inside the body. Flanged for maintenance accessibility on Class 150-600 piping. Butt-weld for permanent installation and minimum external leak paths on Class 900+ piping or buried service. Above Class 600, flanged connections require heavier bolting, larger gasket surfaces, and higher bolt tensioning loads that make flanged maintenance less practical than butt-weld. Butt-weld connections are the standard specification for Class 900-1500 pipingââthe structural integrity and leak-path elimination of welded joints outweighs the maintenance accessibility advantage of flanged connections at those pressure ratings. FLOWKS offers butt-weld and all-welded DBB configurations for Class 900-1500 applications. In-line maintenance access to the stem packing, trunnion bearings, and seat/ball assembly without removing the valve from the pipeline flanges. Unbolt the bonnet, lift it out, service the internals, re-bolt with a new gasket. The pipeline stays connected throughoutââno hot work, no cutting, no re-welding. That accessibility is the maintenance advantage of bolted-bonnet design over welded-bonnet design. YesââAPI 607 with metal-to-metal seats (Stellite 6). Flanged connections with RTJ gaskets maintain sealing through fire exposure alongside the valve body's metal seating. On flammable service pipelines, fire-safe certification is mandatory regardless of the connection typeââflanged or welded, the seating system has to hold through fire. 1/2" or 1" flanged or threaded connection on the bodyââvented to atmosphere or closed drain depending on the process media. Hazardous media (HâS, hydrocarbon, VOC) requires closed drain capture. Non-hazardous utility service vents to atmosphere through a needle valve. The bleed port size scales with the valve boreââlarger bores use larger bleed ports to accommodate higher potential bleed flow rates under seat-failure conditions.
Flange To Screw Needle Type DBB
Quick Specs 1/2" - 1" (DN15 - DN25) up to 10,000 psi (Class 150 - 2500 equivalent) API 6D, API 607, ASME B16.34, ISO 15848 A182 F316, A182 F51, A182 F53 PTFE, PEEK, metal-to-metal (Stellite 6) Flanged process-side connection + threaded (NPT) instrument-side connection, needle-type DBB with compact body, double block & bleed with single bleed port Flange on One End, Screw on the OtherââWhere the Pipeline Is Flanged but the Instrument Is Threaded Process piping at the instrument connection point often has a flanged isolation valve on the pipeline side and a threaded (NPT) connection at the gauge or transmitter sideââthe pipeline specification calls for flanged connections above 1" bore for structural integrity and maintenance accessibility, but the instrument connection (gauge, transmitter, pressure switch) is almost always 1/2" NPT threaded because instruments don't have flanged process connections. That mismatchââflanged on one side, threaded on the otherââis exactly what this valve addresses. The process-side (upstream) connection is a flanged end (RF or RTJ per ASME B16.5) that bolts directly to the pipeline flange. The instrument-side (downstream) connection is a threaded NPT port that the gauge or transmitter threads onto directly. Two blocking seats and a bleed cavity sit between the flanged upstream connection and the threaded downstream connectionââthe DBB isolation function spans the entire flow path from flanged pipeline interface to threaded instrument interface, with no additional fittings, adapters, or interconnection leak paths between them. Without this flange-to-screw configuration, the installer has to cobble together a flanged isolation valve, a reducing fitting (flange-to-thread adapter), and a separate bleed valveââthree components with two interconnection points (flanged-to-adapter, adapter-to-instrument) that are both potential external leak paths. Each interconnection adds a gasket or thread seal that has to be maintained independently. The flange-to-screw needle-type DBB eliminates both interconnection pointsââthe flanged end bolts directly to the pipeline, the threaded end accepts the instrument directly, and the DBB isolation is inside the single body between them. No adapter. No extra fitting. No interconnection leak paths. One valve body spans the entire connection mismatch. Needle-TypeââCompact Body for High-Pressure Instrumentation Taps The needle-type designation means the blocking seats are needle-valve trim rather than ball-valve trimââa tapered needle plug that seats into a matching cone-shaped seat ring, rather than a rotating ball that seals against flat or contoured seat rings. Needle trim has two practical advantages at small bore sizes and high pressures: First: the needle-and-cone seating geometry seals progressivelyââthe needle seats deeper into the cone as closing force increases, creating a line-contact seal that tightens with additional stem torque. That progressive seating behavior gives needle-type valves a mechanical advantage at high pressures where ball-valve seats (which rely on surface-to-surface contact at a defined seat geometry) can struggle to maintain bubble-tight shutoff at pressures above Class 1500. The needle's tapered geometry creates a wedging effectââthe more closing torque you apply, the tighter the needle wedges into the cone, and the seal gets progressively tighter rather than reaching a limit defined by the seat's elastic deformation. Second: the needle trim's compact geometry fits inside a body that's smaller than a ball-valve body at the same bore sizeââthe needle assembly (stem, tapered plug, seat cone) occupies less internal volume than a ball assembly (ball, two seat rings, trunnion pins) at 1/2" bore. That compact geometry keeps the overall body dimensions shorter and lighterââcritical for instrumentation installations where space allocation at the pipeline tap is limited, and where the weight on the flanged connection has to stay within the flange's structural capacity. The two blocking seats in the needle-type DBB are two independent needle-valve assembliesââone upstream (process-side) needle and one downstream (instrument-side) needle, each with its own stem, seat cone, and operating handle. The bleed port sits between the two needle seatsââa third, smaller needle valve that vents the cavity between the two blocking needles. Three stems, three handles, one body. Each needle operates independentlyââclose the upstream needle first, open the bleed needle to verify zero, then close the downstream needle. The sequence is the same as any DBB operation, but the seating mechanism is needle-and-cone rather than ball-and-ring. Two Needles and a BleedââIndependent Sealing, Independent Verification The upstream needle blocks process pressure from the flanged pipeline connection. The downstream needle blocks any pressure that might leak past the upstream needle from reaching the threaded instrument connection. The bleed needle vents the cavity between the two blocking needles to atmosphere or closed drain. Same DBB verification logic: close both blocking needles, open the bleed needle, check for zero pressure at the bleedââzero confirms both needles are holding. Process pressure at the bleed reveals an upstream needle failure. Pressure at the instrument connection reveals a downstream needle failure. The three-stem architecture makes the isolation status directly visible and independently operableââno need for a single actuator that closes both seats simultaneously (that's the ball-valve DBB approach), each needle gets closed and verified individually by the technician performing the isolation procedure. That individual operation capability is particularly valuable on high-pressure instrumentation taps where the closing sequence matters: the upstream needle gets closed first (isolating the process side), then the bleed gets opened (venting any residual pressure in the cavity), then the downstream needle gets closed (isolating the instrument side). The sequence can't be shortcutââeach step depends on the verification from the previous step. Three separate stems make the sequence explicit and unambiguousââyou can't accidentally close both blocking needles and skip the bleed verification because the three handles physically separate the three operations. High Pressure CapabilityââNeedle Geometry Handles What Ball Geometry Can't at Small Bore The needle-and-cone seating geometry's progressive wedging action enables sealing at pressures where ball-valve seat surface contact reaches its mechanical limit. FLOWKS flange-to-screw needle-type DBB valves handle pressures up to 10,000 psi at 1/2" boreââthe needle's tapered wedging seal maintains bubble-tight shutoff at those pressures because additional stem torque drives the needle deeper into the seat cone, creating progressively tighter contact rather than relying on elastic deformation of a flat seat surface. At Class 2500 and above (6000+ psi), ball-valve seating geometry at 1/2" bore requires extremely precise seat machining and high closure force to achieve Class VIââthe needle's progressive wedging provides the same shutoff class with less machining precision requirement and more forgiving closure torque behavior. The body materials (A182 F316, F51, F53 forged alloys) handle the internal pressure at small bore dimensions where the wall thickness-to-bore ratio provides substantial structural margin at 10,000 psi. FAQ Seating mechanism. Needle-type: tapered needle plug seats into a cone-shaped seat ringââprogressive wedging seal that tightens with additional stem torque. Ball-type: rotating ball seals against flat or contoured seat ringsââsurface-to-surface contact at a defined geometry. Needle-type handles higher pressures (up to 10,000 psi) at small bore sizes because the progressive wedging seal maintains tight shutoff where ball seating reaches its mechanical limit. Ball-type provides faster operation (90° quarter-turn vs. multiple stem turns) and is standard at larger bore sizes. Because process pipelines use flanged connections above 1" bore, but instruments (gauges, transmitters) use 1/2" NPT threaded connections. The flange-to-screw body spans that connection mismatch without requiring separate adaptersââthe flanged end bolts to the pipeline, the threaded end accepts the instrument, and the DBB isolation is inside the single body between them. Up to 10,000 psi on needle-type configurations with forged body materials (A182 F316, F51, F53). The needle-and-cone seating geometry's progressive wedging maintains tight shutoff at pressures where ball-valve seating at small bore sizes becomes marginal. For lower pressure applications (Class 150-600), ball-type DBB configurations are also available with flanged connections on both ends. Sequential DBB procedure: close the upstream (process-side) blocking needleââopen the bleed needle and verify zero pressure at the bleedââclose the downstream (instrument-side) blocking needle. The three separate stems make each step explicit and independently verifiable. You can't shortcut the sequenceââeach handle physically separates each operation, forcing the correct verification sequence. YesââAPI 607 with metal-to-metal seating (Stellite 6 needle and cone). PTFE and PEEK seats don't survive fire exposure; metal seating maintains isolation through fire. On flammable gas and liquid instrument taps, fire-safe metal seating is mandatory regardless of the blocking mechanism type (needle or ball).
Sampling DBB Needle Valve
Quick Specs 1/4" - 1/2" (DN8 - DN15) up to 10,000 psi / Class 150 - 2500 equivalent API 6D, ISO 15848, NACE MR0175 (sour service configurations) A182 F316, A182 F51, A182 F53, Monel (NACE sour service) PTFE, PEEK, metal-to-metal (Stellite 6) Sampling needle valve with DBB isolation, compact body, threaded connections (NPT/BSP), sample port with double block protection Sampling from Live Process Linesââand Why the Sample Tap Needs DBB Protection A sampling valve extracts a small volume of process fluid from a live pipeline for laboratory analysis, quality verification, or regulatory compliance testingââthe sample has to be representative (same composition, same pressure, same temperature as the main process stream), and the extraction has to be safe (the technician drawing the sample shouldn't get exposed to the full pipeline pressure, and the pipeline shouldn't lose containment through the sample tap). A simple single-block sampling valve provides one isolation boundaryââwhen the valve is closed, the sample port is isolated from the pipeline. But that isolation is single-point: if the single seat leaks, pipeline pressure reaches the sample port and the sample container simultaneouslyââthe technician gets hit with live process pressure while handling a sample bottle that's supposed to be at atmospheric pressure. On hydrocarbon, chemical, and sour gas sampling applications where the process media is flammable, toxic, or high-pressure, that single-point isolation failure is a safety incidentâânot just a sample contamination issue. FLOWKS sampling DBB needle valves provide double block and bleed isolation at the sample connection pointââtwo independent needle-valve blocking seats with a bleed cavity between them, in a compact body that mounts directly at the pipeline sample tap. The upstream needle blocks pipeline pressure from the bleed cavity. The downstream needle blocks any pressure leaking past the upstream needle from reaching the sample port. The bleed cavity between the two needles vents to atmosphere or closed drainââzero pressure at the bleed confirms both blocking needles are holding before the sample port gets opened for sample extraction. The DBB architecture provides the same verified isolation logic at the sample tap that it provides at gauge and transmitter isolation points: you verify isolation through the bleed before you open the downstream side to atmosphere. On process sampling, that verification step is critical because the technician is about to open a sample container that's designed for atmospheric pressureââyou don't expose an atmospheric-pressure sample bottle to even a trace amount of live pipeline pressure without verifying that the isolation is intact. Needle TrimââThe Right Seating Mechanism for Small-Bore Sample Extraction Sample extraction volumes are smallââa few milliliters to a few hundred milliliters, drawn through a 1/4" or 1/2" bore sampling port. The bore size is small because the sample volume is small and the extraction rate needs to be controlled precisely (fast sample draw pulls non-representative fluid; slow, controlled draw gives representative composition). Needle trim at 1/4"-1/2" bore provides the flow control precision that sampling requiresââthe tapered needle plug modulates the flow area incrementally as the stem rotates, giving the technician fine control over the extraction rate rather than an on/off open-or-closed binary. That modulation capability is built into the needle geometryââthe needle doesn't just open and close, it throttles at partial positions where the flow area between the needle tip and the seat cone is precisely controlled by the stem rotation angle. For the DBB blocking seats (upstream and downstream), the same needle geometry provides progressive wedging sealingââtighter shutoff with additional stem torque, same as the flange-to-screw needle-type DBB. For the sampling extraction port (the downstream connection where the sample container attaches), the needle's throttling capability gives controlled extraction rate managementââthe technician doesn't open the sample port fully and let the process media rush into the container at pipeline velocity. Instead, the needle is opened incrementally, the sample flows at a controlled rate, the container fills slowly and steadily, and the composition stays representative because the extraction rate doesn't create turbulence or preferential-phase separation in the sample stream. Sample Extraction ProcedureââDBB Verification Before Opening to Atmosphere The sampling procedure with a DBB needle valve follows a defined sequence that the three-stem architecture enforces physically: Close both blocking needles (upstream and downstream)ââthe sample port is fully isolated from the pipeline. Open the bleed needleââvent any residual pressure in the cavity between the two blocking needles. Check for zero pressure at the bleed. Zero confirms both blocking needles are holding. If bleed shows zero: proceed with sample extraction. Attach the sample container to the sample port connection. Open the downstream (instrument-side) blocking needle incrementallyââcontrolled flow into the sample container. Close the downstream needle when the sample volume is collected. Close the bleed needle. The sample container is at atmospheric pressure and contains a representative process sample. If bleed shows live pressure: do NOT proceed. The upstream blocking needle is leakingââthe isolation is compromised. The bleed port has revealed the failure before the sample port gets opened. Stop, report, and replace or repair the upstream seat before attempting sample extraction. That sequence can't be shortcutââthe three separate handles physically separate each step, and the bleed verification between blocking and extraction forces the technician to confirm isolation before exposing the sample container to any potential process pressure. On toxic and flammable media sampling, that sequence is the safety protocolâânot a recommendation, not a best practice, but the required procedure that regulatory frameworks and company safety standards mandate for process sampling from live pipelines. NACE MR0175 Sour ServiceââSampling from HâS-Containing Process Streams Sour gas and sour crude sampling (process streams containing HâS at concentrations above the NACE MR0175 threshold) requires materials that resist sulfide stress crackingââthe hydrogen sulfide environment attacks standard stainless steels at the points where stress concentration and microstructural susceptibility coincide (thread roots, seat contact zones, stem-to-body penetration points). FLOWKS sampling DBB needle valves are available in NACE MR0175-compliant configurationsââbody materials specified per the NACE requirements for the specific HâS concentration, pH, and temperature conditions of the process stream (Monel for severe sour service, Inconel for high-temperature sour service, NACE-compliant F316 for moderate sour conditions where the standard alloy meets the hardness and heat-treatment requirements). The seat and trim materials are also specified per NACEââStellite 6 hard-facing on the needle and seat cone surfaces is NACE-compliant and provides the metal-to-metal sealing durability that sour service demands (PTFE and PEEK are chemically compatible with HâS, but metal seating is preferred for the mechanical durability under the repeated cycling that sampling valves experience). Sour service sampling is inherently more dangerous than non-sour samplingââHâS exposure thresholds are low (10 ppm for 8-hour exposure, 100 ppm for 15-minute exposure, 300 ppm IDLH), and the sampling procedure puts the technician in close proximity to the sample port where any leak or isolation failure directly exposes them to the process stream. The DBB verification step (bleed-check before opening the sample port) is even more critical on sour serviceââthe consequence of isolation failure isn't just a contaminated sample, it's an acute toxic exposure. NACE-compliant materials and DBB isolation architecture combine to make sour service sampling as safe as the valve design can achieveââbut the procedure discipline (sequence adherence, bleed verification, no shortcuts) is the human factor that the valve design supports but can't replace. FAQ Application context and downstream connection. Gauge block DBB: instrument isolationââthe downstream connection mates to a gauge or transmitter that stays permanently installed on the valve. Sampling DBB: sample extractionââthe downstream connection is a sample port where the technician attaches a temporary sample container for extraction, then removes it after the sample is collected. Same DBB isolation architecture (two blocking seats + bleed cavity), but the downstream side serves a different functionââpermanent instrument connection on gauge block, temporary sample extraction port on sampling valve. Throttling capability and bore size. Sampling requires controlled extraction rateââthe sample flows into the container slowly and steadily, not at pipeline velocity. Needle trim throttles at partial positions, giving the technician fine control over the extraction rate. Ball trim is on/offââfull flow when open, no flow when closed, no modulation between. At 1/4"-1/2" bore where sampling operates, needle trim also provides better shutoff at high pressures than ball trimââthe progressive wedging seal maintains tight closure where ball seating at micro-bore sizes is mechanically marginal. Body, trim, and seat materials specified to resist sulfide stress cracking in HâS-containing environments. Monel body for severe sour service. NACE-compliant heat-treated F316 for moderate sour conditions. Stellite 6 trim for metal-to-metal seating that's NACE-compliant and durable under repeated cycling. NACE compliance isn't optional on sour serviceââit's the material specification requirement that prevents catastrophic failure (sulfide stress cracking) in the HâS environment. Close both blocking needles, open the bleed needle, check for zero pressure at the bleed port. Zero confirms both needles are holdingââsafe to proceed with sample extraction. Live pressure at the bleed reveals upstream needle failureââdon't proceed, don't open the sample port, don't expose the sample container to process pressure. The bleed check takes seconds and it's the safety gate between isolation verification and sample extraction. Yesââwith appropriate body and trim materials (A182 F316L or Inconel for cryogenic temperatures) and extended bonnet designs that move the stem packing away from the cryogenic body temperature. LNG sampling, liquid nitrogen sampling, and other cryogenic process streams require material specifications and bonnet configurations that maintain mechanical properties and packing functionality at temperatures down to -196°C. The DBB isolation architecture and needle trim throttling capability apply at cryogenic temperatures with the correct material and design adaptations.
Technical Overview
Double block and bleed valves give you two independent sealing boundaries with a verified vent cavity between them. That sounds redundant until you've stood next to a pressure gauge on a 1500-psi hydrocarbon line and watched a single isolation seat weep process fluid toward the instrument connection. One seat can leak. One seat can fail. DBB eliminates that risk by requiring both an upstream and a downstream seat to hold simultaneously, with the bleed cavity between them vented to atmosphere or closed drain so you can visually confirm both seats are sealing before opening the instrument side. FLOWKS manufactures gauge block, flange, trunnion, and needle-type DBB configurationsâfrom compact 1/2" instrument isolation valves through full-bore pipeline DBB units. Gauge block DBBs mount directly at instrument taps with nonoflange or flanged connections. Flange and trunnion DBBs isolate pipeline sections for maintenance, inspection, and pressure verification. Sampling DBB needle valves add a sample port between the two blocking seats, allowing fluid extraction without breaking isolation. Every DBB configuration shares the same functional principle: two seats, one bleed, verified zero-pressure proof that both boundaries hold before you proceed.
FLOWKS double block and bleed (DBB) valves provide verified dual isolation with a vented cavity between two independent seating boundaries. The upstream seat blocks process pressure; the downstream seat protects the instrument or downstream line; the bleed cavity between them vents to atmosphere or closed drain, giving visual confirmation that both seats are holding before any downstream work begins. Gauge block nonoflange DBB valves thread directly onto instrument tapsâ1/2" through 2", Class 150â1500, compact installation without flange bolting or gasket alignment. Gauge block SBB (single block and bleed) valves provide one blocking seat plus a bleed vent for applications where dual isolation isn't mandated. Flange DBB valves bolt into pipeline flanged connections for larger-diameter isolation. Trunnion DBB valves integrate the double block and bleed function into a trunnion-mounted ball valve body for full-bore pipeline service. Flange-to-screw needle-type DBB valves combine flanged process connections with threaded instrument connections, bridging between piping standards. Sampling DBB needle valves add a sample extraction port within the bleed cavity, allowing fluid collection without breaking either isolation boundary. FLOWKS DBB valves meet API 6D requirements for double isolation, with ISO 15848 fugitive emission certification available. Seat options include PTFE, PEEK, and metal-to-metal (Stellite 6) for chemical compatibility and temperature range matching.
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Industries Using DBB Valves
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Oil & Gas
Upstream, midstream and downstream valve solutions meeting API 6D, API 607 fire-safe requirements. From wellhead to refinery, FLOWKS provides reliable valve solutions for the most demanding oil and gas applications.
LNG
Cryogenic valves engineered for liquefied natural gas processing and transportation. FLOWKS cryogenic service valves are tested to -196°C per BS 6364.
Refining
Heavy-duty valves designed for the extreme conditions of petroleum refining. FLOWKS valves handle high temperatures, corrosive media, and erosive service in refineries globally.
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Ball Valves
FLOWKS offers floating ball valve, trunnion mounted ball valve and top entry ball valve for oil & gas, petrochemical and power generation. Designed per API 6D, API 608 and ASME B16.34.
Gate Valves
FLOWKS gate valves for isolation service in piping systems. Flexible wedge, solid wedge, slab and expanding gate designs per API 600, API 602 and ASME B16.34.
Globe Valves
FLOWKS globe valves for throttling and isolation. Standard, angle, Y-pattern and bellows seal configurations per API 602 and BS 1868.
Check Valves
FLOWKS check valves prevent reverse flow. Dual plate, swing, tilting disc and lift check designs per API 594 and BS 1868.