Double Block-and-Bleed (DBB) Valves: Instrument Isolation & Bleed in One Body
A double block-and-bleed (DBB) valve is an instrumentation isolation valve. It combines two independent in-line block seats with a bleed or vent point between them, all housed in a single compact body. That design lets a gauge or process take-off be positively isolated, and lets the isolation be proven before the instrument is serviced. On this page, "double block and bleed valve" refers to a small-bore instrumentation DBB, used for gauge, transmitter, DP-cell, and analyzer take-off service. Not a large-bore pipeline API 6D gate-valve DBB. That's a distinct class of device, governed by API 6D.
Crestflo manufactures instrument double block-and-bleed valves for customers across the US, built in 316/316L and extending up through high-nickel and corrosion-resistant alloys. Every valve is designed to ASME B16.34, available with flanged or screwed (NPT) ends, and comes with full material traceability. Standard items ship from ready stock. Specials are made to order, with delivery running 6-8 weeks. Reviewed by the Crestflo Engineering Team.
What Is a Double Block-and-Bleed (DBB) Valve? Function & Purpose
A double block-and-bleed valve gives you two block (seat) isolations in series, with a bleed or vent take-off sitting in the cavity between them, all cut into a single body. Close both blocks and the instrument or take-off point is isolated twice over. Open the intermediate bleed and the trapped cavity depressurizes, which lets the operator confirm the upstream block is actually holding before anyone pulls the downstream device. That double isolation, backed by a bleed you can prove, is why DBB valves take the place of two separate isolation valves plus a bleed valve on critical or hazardous instrument hook-ups, including PED-rated ones.
Let's untangle the terminology first. A single block-and-bleed pairs one block seat with a bleed. Double block-and-bleed, or DBB, adds a second block, giving you redundant isolation you can actually test. DBBB, short for double-block-double-bleed, takes it further, with two bleed points instead of one. And double isolation and bleed, DIB, describes that same two-seat isolation intent, just under a different name. Crestflo's focus here is the instrument double block and bleed valve, sized from 1/16″ to 1″ for instrumentation work. You'll also see it called double block & bleed, double block-and-bleed, or double block bleed valve, depending on who's writing the spec.
DBB Valve, DIB, DBBB & Double Isolation and Bleed: Also Known As
Engineers and buyers across the industry use several names for the same component. You'll hear it called a double block-and-bleed valve, double block & bleed valve, double block bleed valve, DBB, or DBB valve. On drawings and data sheets it often shows up as an instrument double block and bleed valve, double block and bleed instrument valve, or double block and bleed isolation valve. There are function-led variants too: the double isolation and bleed (DIB) valve and the double-block-double-bleed (DBBB) valve. Sorted by isolating element, you'll also see the double block and bleed ball valve and the double block and bleed needle valve. Every one of these names points to the same thing: the two-block, one-bleed instrumentation isolation valve covered on this page.
How a Double Block-and-Bleed Valve Works: Working Principle, P&ID Symbol & Diagram
In service, both block seats sit open so flow reaches the instrument normally. To isolate the line, the operator shuts the first block, the one on the process side, then the second, the instrument-side block, and opens the bleed between them. Any pressure trapped in that intermediate cavity vents out to atmosphere or a test point. If the cavity holds nothing once the bleed is open, the first block has proven itself leak-tight, and the downstream device can come off, get calibrated, or be tested without worry. It's the bleed port that turns two seats into a double isolation you can actually verify, not just hope for.
On a piping and instrumentation diagram (P&ID), the double block-and-bleed symbol appears as two block-valve symbols in series, with a bleed or vent take-off tapped into the line between them. That's the standard schematic representation. It holds regardless of whether the internal blocks are ball or needle elements. A DBB valve diagram, then, packs three functions, block, block, and bleed, into one body sitting between the process connection and the instrument.
DBB Valve Types & Configurations: Ball, Needle, Flanged, Screwed & DBBB
Instrument DBB valves get configured two ways: by isolating element and by end connection. Block seats come as ball elements for quarter-turn, full-bore isolation, or as needle/OS&Y-bonnet elements built for fine-metering-grade shut-off. The body itself can be a close-coupled bar-stock design, or a flanged block that mounts straight onto a process connection. A monoflange valve packs that same double-isolation-and-bleed function into a single flanged body; check the monoflange valve page for that variant.
| Configuration | Isolating elements | Typical use |
|---|---|---|
| Double block and bleed ball valve | Two ball seats + bleed | Full-bore quarter-turn isolation of gauges and take-offs |
| Double block and bleed needle valve | Two needle seats + bleed | Fine shut-off, high-pressure and small-bore instrument service |
| Flanged DBB | Two blocks + bleed, flanged ends | Direct-mount to a flanged process connection |
| Screwed / NPT DBB | Two blocks + bleed, threaded ends | Threaded instrument hook-ups and manifolded take-offs |
| DBBB (double-block-double-bleed) | Two blocks + two bleeds | Where each isolation stage needs its own vent |
| Close-coupled / integral bar-stock | Two blocks + bleed, one body | Compact replacement for two valves and a bleed |
A "double block and bleed gate valve" usually points to a large-bore pipeline (API 6D) product. That's a separate product class from the instrument DBB valves shown here. End connections can be built to your requirement, so specify the isolating element and end arrangement on the RFQ.
Double Block-and-Bleed Valve Pressure Rating (to 3,000 psi), Temperature & Sizes
Crestflo builds its instrument DBB valves to ASME B16.34, rated for a working pressure up to 3,000 psi, and offers them with flanged or screwed (NPT) ends across the 1/16″–1″ instrumentation valve range, in both imperial and metric sizes. The sealing element sets the temperature limits. Metal or hard seats cover the widest span, reaching all the way to the body material's limit. PTFE soft seats stay within the standard PTFE service window of roughly −54 °C to +232 °C (−65 °F to +450 °F), and PEEK seats push past that range for elevated-temperature duty.
| Parameter | Basis / value |
|---|---|
| Design standard | ASME B16.34 pressure-temperature rating |
| Working pressure | Up to 3,000 psi |
| Size range | 1/16″–1″ instrumentation range, imperial and metric |
| End connections | Flanged (raised-face) and screwed / NPT (ASME B1.20.1) |
| Soft-seat temperature | PTFE approx. −54 °C to +232 °C (standard PTFE limit); PEEK higher |
| Hard/metal-seat temperature | To the body-material limit per ASME B16.34 |
Values follow the applicable standard and can be customized to your requirement. For certified ratings, flange class, and per-size data matched to your exact configuration, request them from the engineering team.
DBB Valve Seats & Materials: 316 Stainless, Duplex, Inconel, Monel & Hastelloy
The standard stainless steel double block-and-bleed valve is built in 316/316L, and it comes with soft (PTFE/PEEK) or hard/metal seat options plus a full corrosion-resistant-alloy ladder for chloride, sour, and aggressive-media service. Sour-service builds ship in NACE MR0175 / ISO 15156 compliant materials on request. Choose an exotic alloy when chloride pitting, sour partial pressures, or elevated temperature push past the 316 envelope. No brass is offered on the US range.
| Grade | UNS | Family | Instrumentation form |
|---|---|---|---|
| 316 / 316L | S31600 / S31603 | Austenitic stainless | 316/316L valves |
| 304 / 304L | S30400 / S30403 | Austenitic stainless | 304/304L |
| 904L | N08904 | Super-austenitic | 904L valves |
| Duplex 2205 | S31803 / S32205 | Duplex | Duplex 2205 |
| Super Duplex 2507 | S32750 | Super duplex | Super Duplex 2507 for chlorides |
| 254 SMO | S31254 | 6Mo super-austenitic | 254 SMO |
| AL-6XN | N08367 | 6Mo super-austenitic | AL-6XN |
| Alloy 20 | N08020 | Nickel-alloy | Alloy 20 |
| Inconel 625 | N06625 | Nickel-chromium | Inconel 625 |
| Inconel 825 | N08825 | Nickel-iron-chromium | Inconel 825 |
| Monel 400 | N04400 | Nickel-copper | Monel 400 valves |
| Hastelloy C-276 | N10276 | Nickel-molybdenum | Hastelloy C-276 |
| Titanium Gr2 | R50400 | Titanium | Titanium Gr2 |
UNS designations serve as the standard published references for each grade. Finishes cover material finish, electropolish, passivation, and silver anti-galling plating, and oxygen and ultra-high-purity cleaning is available for clean-service builds. Explore the full instrumentation valve material range or specify your grade on the RFQ.
DBB Standards & Compliance: ASME B16.34, B1.20.1 & NACE MR0175
Crestflo double block-and-bleed valves are built to the ASME B16.34 pressure-temperature standard. NPT threads follow ASME B1.20.1, and materials compliant with NACE MR0175 / ISO 15156 are available for sour service. Instrument valves qualify under the applicable valve standards, namely ASME B16.34 and API pressure-test/qualification methods. They don't qualify under the compression-fitting standard ASTM F1387. The company holds ISO 9001, ISO 14001, ISO 45001 and PED certification, along with the standard approvals that apply to instrumentation valves of this type.
| Standard | Scope | Applies to |
|---|---|---|
| ASME B16.34 | Valve pressure-temperature ratings & design | Body design, working pressure |
| ASME B1.20.1 | NPT pipe threads | Screwed / NPT end connections |
| NACE MR0175 / ISO 15156 | Sour-service materials | Alloy selection for H₂S service |
| ISO 9001 / 14001 / 45001, PED | Quality, environmental, safety, pressure equipment | Manufacturing system |
| EN 10204 | Material test certificates | 3.1 standard / 3.2 on request |
DBB Valve Testing, Traceability & Quality
Every double block-and-bleed valve carries heat and lot-number marking, and it ships standard with an EN 10204 3.1 material test certificate; 3.2 certification is available on request. Each block's seat integrity gets checked through the bleed port, alongside seat and shell leak testing, hydraulic proof and burst, and pneumatic testing. Positive material identification (PMI) confirms the alloy grade. In-house NDT backs up the build. Destructive testing happens at ISO/IEC 17025-accredited laboratories, and third-party inspection can be arranged on request. That bleed-port check, the one that defines DBB function in the first place, doubles as the quality check that proves each isolation stage before the valve ships.
Double Block-and-Bleed Valve Applications & Industries
Instrument DBB valves isolate pressure gauges, transmitters, and DP-cells. They also secure sample and take-off points where a single isolation isn't enough. Positive, provable isolation before an instrument comes out under pressure: that's the requirement across oil and gas, offshore, refining, and petrochemical service, and it's where these valves get specified. On sour and high-chloride duty, the DBB body is built in a NACE-compliant duplex or nickel alloy, so the isolation function and the corrosion allowance are specified together.
DBB Ordering, Cross-Reference & Request a Quote
Crestflo makes dimensional drop-in double block-and-bleed valves designed to swap directly for the instrument DBB valves you already specify. No need to mix brands: give us your existing part number, and you'll get an equivalent Crestflo build in return. Just ask the engineering team for a cross-reference against your Swagelok®, Parker®, or Hoke® part. Full specifications sit right on the page, and the catalog itself is available for download. Datasheets and CAD models for individual products go out to customers on request. Standard items ship from stock already on hand. Specials are built to order, with delivery running 6-8 weeks.
Contact Crestflo US: sales@crestflousa.com · +1 346 594 9005 · Houston, TX. Request a quote or request a sample; distributors can review the distributor program and OEMs the private-label program.
Why Crestflo for Double Block-and-Bleed Valves
Crestflo is the instrumentation division of a Government-of-India-recognized export house with four decades behind it and a proven delivery record across the Middle East. It now serves the US market with drop-in instrumentation valves. Double block-and-bleed valves come in stainless and high-nickel alloys, in both imperial and metric sizes. Every part carries heat-number traceability and an EN 10204 material certificate.
Standard product is of Indian origin. US or European melt is available on request, with full material traceability. Small minimum order quantities, private-label and made-to-print options, and a lead time of 6-8 weeks make Crestflo a low-risk second-source supplier and manufacturer for distributors, OEMs, equipment manufacturers, and end users.
Pricing for double block-and-bleed valves runs on a request-for-quote model; there's no published list price. Cost depends on the isolating element, the alloy, the size, and the certification scope. We Engineer Confidence.
Double Block-and-Bleed Valve FAQ
What does a double block-and-bleed (DBB) valve do?
It builds in two separate seat isolations along the line, with a bleed or vent point sitting between them. That layout lets you shut off an instrument or take-off twice over, then confirm the isolation actually holds by venting the space between the two blocks before anyone touches the device. All of that happens in one compact body.
What is the purpose of a block-and-bleed valve?
To isolate a process connection and vent the trapped pressure between the isolation points safely. The "double" version adds a second block, so isolation becomes redundant and testable. That's why DBB valves show up on critical, hazardous, or high-pressure PED instrument hook-ups.
What is the difference between single and double block-and-bleed?
A single block-and-bleed uses one block seat and a bleed. A double block-and-bleed, or DBB, adds a second: two block seats plus a bleed. That extra seat isn't there for show. It gives redundant isolation, and combined with the bleed between the two seats, it lets you confirm the first seat is actually holding before the downstream device comes off.
How does a DBB valve work and what is on the P&ID?
Close both blocks first, then crack the bleed to depressurize the cavity between them. Don't service anything downstream until you've confirmed the upstream block holds no pressure. On a P&ID, the double block-and-bleed symbol is two block-valve symbols set in series, with a bleed or vent take-off tapped in between.
What is a double block-and-bleed gate valve, is that what you supply?
Gate-type DBBs show up mostly in large-bore pipeline work under API 6D, and that's a separate product class entirely. Crestflo's DBB valves aren't that. They're small-bore instrumentation isolation valves, built to ASME B16.34, meant for gauge, transmitter, and take-off service across the 1/16″–1″ range.
What is a double block-and-bleed ball valve?
This DBB relies on two ball elements for the block seats, with a bleed positioned between them. That arrangement gives full-bore, quarter-turn isolation. Crestflo makes the configuration available with either ball elements or needle elements, so when you submit an RFQ, just note which isolating element you'd prefer.
What pressure and sizes are available for a DBB valve?
These instrumentation valves are rated for working pressure up to 3,000 psi under ASME B16.34, with flanged or screwed (NPT) ends. Sizes span 1/16″ to 1″, in both imperial and metric. Flange class and per-size ratings aren't fixed; they're tailored to your requirement. Ask for certified data on your exact build.
What materials can a Crestflo DBB valve be made from?
Standard construction is 316/316L stainless, with 304/304L, 904L, Duplex 2205, Super Duplex 2507, 254 SMO, AL-6XN, Alloy 20, Inconel 625/825, Monel 400, Hastelloy C-276 and Titanium Gr2 also available. For sour service, materials meet NACE MR0175 / ISO 15156 requirements. Brass isn't offered anywhere on the US range.
Are Crestflo DBB valves certified and traceable?
Yes. They're designed to ASME B16.34, and the company holds ISO 9001, 14001, and 45001 certification, plus PED. Each part carries heat and lot number marking. It ships with an EN 10204 3.1 material test certificate, with 3.2 available on request. Seat and shell leak testing are standard, as is PMI. Third-party inspection can be arranged on request.
Can Crestflo replace my current DBB brand, and how fast?
Crestflo builds dimensional drop-in instrumentation double block-and-bleed valves. Specify your existing part number on the RFQ and you'll get an equivalent build back. Standard items ship from stock on hand. Specials are made to order, with a 6-8 week delivery window, small minimum order quantities, and private-label options available. Reach out to the engineering team to request your cross-reference.
Related Instrumentation Valves & Resources
Take a look at the full instrumentation valves range, along with the related monoflange valve, bleed valve, gauge root valve, 2-valve manifold, 3-valve manifold and 5-valve manifold. Need help choosing between them? The DBB vs monoflange and monoflange vs manifold guides lay out the tradeoffs plainly. Also worth checking: the ASME B16.34 design standard and the NACE MR0175 sour-service standard.