DBB vs Monoflange: How to Choose Between a Double Block & Bleed and a Monoflange Valve

A double block & bleed (DBB) describes an isolation function: two block valves in series with a bleed or vent between them, all on a single instrument tap. A monoflange, by contrast, is a compact construction, it packages block-and-bleed into one forged body that bolts directly to the process flange or instrument. Many monoflanges are, in fact, DBBs. But a DBB can just as easily be built from discrete valves or a valve manifold. Pick a monoflange when weight, footprint and leak-path count drive the decision, think offshore topsides, transmitter-mounted taps. Pick a conventional DBB or manifold when you need higher ratings, more configuration freedom, or valves that can be serviced individually.

On this page, "DBB" and "monoflange" refer to instrumentation isolation valves at gauge, transmitter and sampling tapping points. They aren't large-bore inline or piggable pipeline DBB valves, and the term has nothing to do with the "monoblock" engine sense of the word. Crestflo makes both assemblies, so questions about metallurgy, ratings and sourcing all get answered in one place.

Reviewed by the Crestflo Engineering Team

Also Known As: DBB, Double Block & Bleed and Monoflange Valve Designations

Engineers and piping drawings refer to these two assemblies by several names, and the terminology overlaps quite a bit. A double block & bleed shows up as DBB, double-block-and-bleed, double block bleed, DBB valve or DBB manifold. It belongs to the wider block-and-bleed valve family, alongside single block & bleed (SBB) and double isolation & bleed (DIB). DBB valve types span a range, from single block & bleed up to double block & bleed. A monoflange, meanwhile, goes by mono flange, monoflange valve, monoflange isolation valve, instrument monoflange or flange-mounted block-and-bleed. Its valve types, single (SBB) and double (DBB), carry that same block-and-bleed function but in one flanged body. "Monoblock valve" is a common synonym for a monoflange in fluid-system work, and it shouldn't be confused with the automotive engine term. Both assemblies function as primary isolation valves at instrument tapping points.

DBB vs Monoflange at a Glance: Isolation, Leak Paths, Weight & Cost

They aren't mutually exclusive. A monoflange is a way of building a block-and-bleed, and a DBB is the isolation arrangement it most often implements. The table puts the practical differences a specifying engineer weighs right up front.

The ratings shown here are standard published values for this assembly type, based on the applicable standard, and they can be tailored to what you need. Want exact certified ratings, ordering codes or connection schedules for a specific configuration? Those are available on request.

What Is a Double Block & Bleed (DBB) Valve?

A double block & bleed valve gives you two independent block, or isolation, valves in series on a single instrument tapping, with a bleed or vent valve sitting in the cavity between them. Close both blocks and the instrument is isolated from the process. Open the bleed and the trapped cavity drains and depressurises, so you can confirm zero energy before pulling, calibrating or servicing the instrument. That provable, vented isolation is why DBB gets specified anywhere a single valve isn't judged safe enough: pressure and differential-pressure transmitters, gauges, sampling points, and chemical-injection isolation on a PED-rated system.

A DBB isn't the same thing as a single block & bleed (SBB), which relies on one block valve plus a bleed. The word "double" points to two blocks in series, giving isolation that's redundant and verifiable. You can build a DBB from discrete needle or ball valves, package it as a valve manifold, or integrate it into a monoflange body. The construction changes; the function doesn't. Crestflo double block & bleed valves are designed per ASME B16.34 with flanged and screwed ends and a working pressure up to 3,000 psi. For configurations, see the double block & bleed valve product page; for the block and vent components, check the bleed valve and needle valve pages.

What Is a Monoflange Valve, and How Does It Work?

A monoflange valve packs block-and-bleed isolation into a single forged block, bolted straight onto a process flange, orifice tapping or flanged transmitter, with an outlet on the opposite face feeding the instrument. Replace a whole train of separate valves, adaptors and tubing with that one machined body, and the joints between components disappear too. Fewer joints means fewer leak paths, less weight and a shorter installed height. Monoflanges come in two configurations: single block plus bleed, known as SBB, and double block plus bleed, known as DBB. Same basic construction either way, just a different isolation arrangement.

This saving in weight and leak paths is why monoflanges win out on offshore platforms and topside skids, where every kilogram and every potential leak point comes at a premium, and on transmitter-mounted installations, where a compact, self-supporting isolation body beats a valve train hanging off the tap. Because it's one body, a monoflange can be serviced in place. A damaged body gets swapped as a single unit, not valve by valve. Crestflo builds monoflanges with the process flange rated to its ASME B16.5 flange class and instrument-side ports set to standard working pressures, with values per the applicable standard and customizable to requirement. See the monoflange valve product page for connection and configuration options.

Figure: DBB vs monoflange diagram, the block-block-bleed flow path built from discrete valves in a double block & bleed (DBB) assembly (left) versus the same flow path integrated into one monoflange body (right).

Single vs Double Block & Bleed: and Where the Monoflange Sits

Single block and bleed (SBB) uses one block valve plus a bleed. Double block and bleed (DBB) puts two blocks in series with a bleed between them, which gives you verifiable double isolation. Redundancy and provability mark the real difference here. With two blocks and a vented cavity, you can confirm both seats are holding before breaking into the line. A monoflange comes in either form: a single monoflange delivers SBB, and a double block and bleed monoflange delivers DBB. That means the SBB-versus-DBB choice comes down to the isolation you need, while the monoflange-versus-discrete choice comes down to how the thing is built. Some manufacturers use the rarer term OBB, one block bleed, for the single-block arrangement.

DBB vs DIB: Double Block & Bleed vs Double Isolation & Bleed

DBB and DIB get mixed up constantly, and both happen to use two seats. A double block & bleed (DBB) bleeds the cavity between two block valves to prove isolation. The point is the vented, verifiable cavity, not the valve count. A double isolation & bleed (DIB) works differently: it gives you two independent, self-sealing seat isolations, arranged either unidirectionally or bidirectionally, and each seat can hold on its own. In API terms the distinction is about sealing philosophy, a proven-vented cavity versus two independent self-relieving or non-relieving seats. It's not simply the number of valves. Specify DBB when you want to verify isolation by venting the cavity. Specify DIB when each seat has to independently seal against process pressure.

When to Use a DBB vs a Monoflange: Selection Guide

Start with the installation constraints and let them drive the decision. Then pick whichever construction clears them at the lowest weight and joint count.

Pick a monoflange when weight, footprint, and leak-path count top the priority list: offshore jobs, topside work, and transmitter-mounted taps are the usual fit. Need configuration freedom instead? A conventional DBB or a valve manifold works better when you want individually serviceable valves or packaging suited to a differential-pressure hook-up. Two resources cover the adjacent comparisons in more depth: monoflange vs manifold and the 2 vs 3 vs 5-valve manifold guide. For gauge isolation, check the gauge root valve page. Weight tends to drive the selection in offshore, subsea, and midstream oil & gas service.

DBB and Monoflange vs a Valve Manifold

A valve manifold, whether it's a 2, 3 or 5-valve setup, offers another way to combine block, equalise and bleed functions. This shows up most often on differential-pressure transmitters, where an equalise valve is needed. A monoflange or a discrete DBB isolates and vents a single tap. Group the block, equalise and vent valves needed to mount and calibrate a DP transmitter, though, and you're looking at a manifold instead. The choice comes down to the instrument: single-tap isolation points to a monoflange or DBB, while a DP transmitter hook-up usually points to a 3 or 5-valve manifold. Compare the two in monoflange vs manifold, and browse the 2-valve, 3-valve and 5-valve manifolds.

DBB & Monoflange Materials: 316L, Duplex, 254 SMO, Inconel & Monel

Both assemblies share the same instrumentation alloy menu, and which one you pick comes down to service severity, not valve type. Match the alloy to the medium, the chloride level and the temperature. The material-selection-by-service guide and the materials hub walk you through that decision, and Crestflo built both resources with exactly this kind of judgment call in mind.

For sour (H2S-bearing) service, both the DBB and monoflange assemblies come in options that meet NACE MR0175 / ISO 15156. Start with the sour-service selection guide, then work through ISO 15156 itself. Compositions and mechanical properties follow standard published values under the applicable ASTM specifications. They're customizable to requirement, though.

Pressure, Standards & Proof: ASME B16.34, API, NACE & Traceability

Isolation valves meet the qualification standards for valves themselves, while the connection ends are qualified separately, under whatever standard applies to that connection type. The table shows exactly what governs each part of a DBB or monoflange isolation assembly.

Crestflo double block & bleed valves are built per ASME B16.34, and the isolation valves are manufactured and qualified to the applicable ASME B16.34 / API standards. ASTM F1387 governs the compression tube-fitting ends only, not the valve body itself. Each part carries a heat-number mark, backed by EN 10204 3.1 material test certificates (3.2 available on request), so full traceability is never in question. Crestflo holds ISO 9001 / 14001 / 45001 certification and is also PED certified.

Cost, Weight & Lead Time: DBB vs Monoflange

Crestflo's monoflange has to trade a bit of flexibility for compactness. For a single, compact tap, a monoflange usually beats a conventional DBB or manifold on installed weight and cost, since it does away with the separate valves, adaptors and tubing a discrete build needs. Where a conventional setup wins is configuration freedom, and the ability to service one valve without touching the rest of the assembly. Neither option is cheaper in every case. The right pick is whichever meets the isolation duty with the lowest weight and joint count for that particular installation.

Across both types, Crestflo quotes 6-8 weeks for delivery, against the 20-plus weeks typical of the major brands. Minimum order quantities are small, and private-label programs are available too. Standard items ship from stock already on hand; specials get made to order.

Get DBB & Monoflange Isolation Valves from Crestflo

Crestflo makes both assemblies under comparison here: double block & bleed valves and monoflange valves. It also produces the manifolds, bleed valves and root valves that go alongside them, all in stainless, high-nickel and CRA alloys. DBB valves follow ASME B16.34 and handle working pressure up to 3,000 psi. Monoflanges, meanwhile, are built to whatever flange class and valve standard the application calls for. Compression tube-fitting ends are tested per ASTM F1387 for drop-in tube connections, and NACE MR0175 material options come standard, with heat-number traceability running through the whole line. Behind all this sits a four-decade-old export house recognized by the Government of India, one with a solid delivery track record across the Middle East; Crestflo is its instrumentation division.

• Availability: Ready stock for standard items · made-to-order for specials · 6–8 week delivery.
• Origin: Indian origin; US / European melt available on request, with full material traceability.
• Contact: sales@crestflousa.com · +1 346 594 9005 · Houston, TX.

Datasheets and CAD are available to customers on request. Take a look at the double block & bleed valve and monoflange valve pages, or browse the wider instrumentation valves range. Need to match a Swagelok®, AS-Schneider® or WIKA® monoflange, or an existing brand's DBB assembly? Don't work from a published part chart. Instead, request your cross-reference from the engineering team. Crestflo checks each equivalent build case by case, and every brand name belongs to its respective owner. From there, request a quote or request a sample. Distributors and OEMs can also look over the private-label program.

Why Crestflo for DBB & Monoflange Isolation

Crestflo writes this comparison as the maker of both assemblies, not just an observer, and it pairs the metallurgy with a direct path to specify and buy: ASME B16.34-designed DBB valves, monoflanges built to the applicable standard, ASTM F1387-qualified compression ends for drop-in tube connections, NACE MR0175 sour-service options, EN 10204 3.1 / 3.2 certificates with heat-number traceability on every part, and imperial and metric sizes throughout. Lead time runs 6–8 weeks. Compare that to the 20-plus weeks typical of the majors. Small MOQs and private label are available too. The comparison stays neutral. What matters is the construction that clears your duty at the lowest weight and joint count, not the most expensive one, because the goal here is the right specification, delivered. We Engineer Confidence.

Frequently Asked Questions: DBB vs Monoflange

Related Comparisons & Resources

• Monoflange vs manifold
• 2 vs 3 vs 5-valve manifold selection
• Needle vs ball valve
• How to specify instrumentation fittings & valves
• Material selection by service
• Sour-service (NACE MR0175) selection
• Double block & bleed valve · Monoflange valve