Understanding ASTM A105 BL Flanges - Applications and Benefits

An ASTM A105 BL flange is a blind flange forged from carbon steel to ASTM A105 and used to close the end of a pipe run, a vessel nozzle or a valve outlet. In the 150LB (Class 150) raised-face version at 2-1/2 in it is a standard line component, and the seal it forms depends as much on the flange gaskets and the bolting as on the flange itself.

What the ASTM A105 BL Designation Means

ASTM A105 covers forged carbon steel piping components for pressure service: flanges, fittings and valves. BL marks the blind type, a solid plate with no bore that is bolted across an opening to stop flow and isolate a section of the system. Because it has no bore, the blind flange takes the full pressure end load, so its thickness and its bolting follow the same rules as other ASME B16.5 pressure classes.

The 150LB marking is the ASME B16.5 Class 150 designation, and 2-1/2 in is a common line size in process piping. Together they fix the bolt pattern and the gasket dimensions, not a single working pressure: the allowed pressure and temperature come from the ASME B16.5 pressure-temperature rating for A105.

Note: the Class 150 pressure-temperature rating depends on the material and the service temperature, so read the ASME B16.5 tables before the part is specified.

Where Blind Flanges Are Used

A blind flange is fitted wherever a line must be closed safely and stay closed. On the pipe flanges used in oil and gas service, from crude oil to natural gas and other hydrocarbons, it blanks spare pump and meter connections and isolates sections during maintenance. On offshore drilling and production systems it keeps topside piping sealed against pressure, vibration and weather.

• Oil and gas: pump stations, metering skids and spare connections on crude, gas and hydrocarbon lines.
• Chemical processing: reactor, heat exchanger and storage tank nozzles where acids or solvents are handled and the joint must stay leak-free.
• Offshore and marine: topside piping and utility systems where the closure faces pressure, vibration and a corrosive atmosphere.
• Plant utilities: pump discharges, filter vessels and blanked-off branches in water and process piping.

A raised face concentrates the bolt load on a smaller gasket area, so the joint seals reliably with a flat gasket and moderate, evenly applied torque.

Gasket Selection and Installation

The joint is only as tight as the gasket and the bolting behind it. Select the gasket for the medium and the temperature first, then match its thickness and its dimensions to the raised face.

• Gasket material: non-asbestos fibre, graphite or PTFE sheet for general hydrocarbon and chemical service, matched to the medium, the temperature and the corrosion allowance.
• Surface condition: clean the raised face, remove old gasket residue and check for radial scratches or corrosion before assembly.
• Bolting: use stud bolts and nuts of the grade specified for the pressure class and tighten in a cross pattern over several passes.
• Torque: apply the value for the bolt grade, the gasket material and the diameter; over-tightening crushes the gasket and distorts the blind plate.

Since the blind flange carries the full end load, re-check the bolts after the first pressure or thermal cycle and tighten any that have settled so the gasket load stays even.

Inspection, Maintenance and Standards

A blind flange usually stays in place for years, so condition checks matter more than routine work. Inspect the joint for weeping, corrosion staining and bolt rust, and verify the seal with a pressure test before the line returns to service.

• Visual check: look for weeping, rust staining along the gasket line and any deformation of the blind plate.
• Fastener condition: replace corroded stud bolts and nuts, because weakened fasteners cannot hold the gasket load.
• Pressure test: test the isolated section to the value required by the plant procedure before it goes back into service.
• Records: note the size, the pressure class, the ASTM A105 material and the ASME B16.5 dimensions so the part stays traceable.

Two references matter when the part is specified: ASTM A105 for the material and the mechanical properties of the forging, and ASME B16.5 for dimensions, tolerances and pressure-temperature ratings. Holding both keeps the blind flange interchangeable with the rest of the piping components.

Frequently Asked Questions

What does BL mean in an ASTM A105 BL flange?

BL marks the blind flange type: a solid plate with no bore that closes a line end or a nozzle and stops flow through the opening.

Is a Class 150 blind flange limited to 150 psi?

No. 150LB is the ASME B16.5 class designation, and the permitted pressure depends on the material and the service temperature, so use the Class 150 pressure-temperature table for A105.

Which gasket fits a raised-face ASTM A105 blind flange?

A flat gasket cut to the raised-face dimensions: non-asbestos, graphite or PTFE sheet for typical hydrocarbon and chemical media, always matched to the medium and the temperature.

How should a blind flange be bolted up?

Fit stud bolts and nuts of the specified grade, tighten in a cross pattern over several passes to an even torque, and re-check the bolts after the first pressure or thermal cycle.

Can a blind flange stay as a permanent closure?

Yes, when the pressure class, the material and the gasket suit the service and the joint is inspected on the plant's normal schedule.