What is a spectacle blind flange?

 
A spectacle blind – also known as a figure-8 blind – is generally a piece of metal that is cut to fit between two pipe flanges and usually sandwiched between two gaskets. A spectacle blind is often made from two metal discs that are attached to each other by a small section of steel.

The shape is similar to a pair of glasses or “spectacles” – hence the name spectacle blind. One end of the blind will have an opening to allow flow through the pipe during operation and the other end is solid to block flow during maintenance. They are generally installed as a permanent device to separate process piping systems.

The whole idea of a spectacle blind is so that one can be absolutely sure that an item of equipment is properly and securely blanked off with no possibility of any flow or leakage through a valve during maintenance. The flow can be diverted to other items of equipment but not just bypassed around the blank as this would compromise the safety function of the spectacle blind. The flow has to valved off and pressure relieved prior to loosening flange bolts and swinging the blind.

All Dims Per ASME B16.48

1. For use with Flat or Raised Face Flanges
2. Specify Surface in Microinch ASA 500, 250, 125, 63 or 32. If Not Specified Mill Finish Supplied.
3. Available with Phonographic Serrated Surface or Ground Finish. Available with Ring Type Joint
4. Normally Supplied in A285 Gr. C, A515 Gr 70, A516 Gr. 70, 304SS, 316SS. Other Materials Available

Dimensions of spectacle blinds according to ASME B16.48

Dimensions of spectacle blinds according to ASME B16.48 for installation between ASME B16.5 flanges.

Material Symbol	C	Mn	P	S	Si	Ni	Cr	Mo	Cu	N	V	Co
ASTM A105	0.35	0.60-1.05	0.035	0.04	0.10-0.35	0.4	0.3	0.12	0.4	_	0.08	_
ASTM A350 LF2	0.3	0.60-1.35	0.035	0.04	0.15-0.30	0.4	0.3	0.12	0.4	_	0.08	0.02
ASTM A182 F304	0.08	2	0.045	0.03	1	8.0-11.0	18.0-20.0	_	_	_	_	_
ASTM A182 F304L	0.03	2	0.045	0.03	1	8.0-13.0	18.0-20.0	_	_	_	_	_
ASTM A182 F316	0.08	2	0.045	0.03	1	10.0-14.0	16.0-18.0	2.00-3.00	_	_	_	_
ASTM A182 F316L	0.03	2	0.045	0.03	1	10.0-15.0	16.0-18.0	2.00-3.00	_	_	_	_
ASTM A182 F321	0.08	2	0.045	0.03	1	9.0-12.0	17.0-19.0	_	_	_	_	_
EN 10025 S235JR	0.2	1.4	0.045	0.045	_	_	_	_	_	0.01	_	_
20Mn	0.17-0.23	0.70-1.00	0.035	0.035	0.17-0.37	0.3	0.25		0.25
20	0.17-0.23	0.60-1.00	0.03	0.02	0.17-0.37	0.25	0.25		0.25
16Mn	0.13-0.19	1.20-1.60	0.03	0.02	0.20-0.60	0.3	0.3		0.25

Material Symbol	Tensile Strength, min, MPa	Yield point, min, Mpa	Elongation, min, %	Reduction of area, min, %	Hardness, max, HB	Min. Impact	Test Temperature, °C
ASTM A105	485	250	22	30	187
ASTM A350 LF2	485-655	250	22	30		20	-46
ASTM A182 F304	515	205	30	50
ASTM A182 F304L	485	170	30	50
ASTM A182 F316	515	205	30	50
ASTM A182 F316L	485	170	30	50
ASTM A182 F321	515	205	30	50
20Mn	450	275	24	50	197
20	410	245	25	55	156

Raised Face Joint Blanks

Raised Face Joint Blanks

• ASME B16.48 says: The gasket seating surface finish and dimensions for raised face line blanks shall be in accordance with ASME B16.5. A raised face may be specified at the option of the purchaser.
• A Raised Face would add machining time and complexity so the price would be higher. Call this the Bugatti or Rolls Royce of the line blind world.
• Personally, I have seen that version once or twice somewhere. The version without Raised Face is the default and is used everywhere.