Im just wondering if anyone can advise on how they determine the hole size for a failure of a gasket? Reading Cox, Lees and Ang and using a compressed fibre gasket for example, they recommend that the hole size is determined by the thickness of the gasket and the arc between two bolt holes to determine the area of a segment loss. I have no problem with this and it makes sense in a major failure situation. However, when you actually calculate this trough, the resulting zoning requirements are far in excess of what most guidance docs recommend. It does state that for lesser holes an area of 2.5mm2 can be used but the temptation is to always plan for worst case scenario although obviously the failure rates for major sectional losses are much less than a small aperture leak. Just wondering what other people do

For CAF the options are area of the segment loss between bolts or use (0.1 A). A normal Compressed Asbestos Fibre (CAF) gasket is retained by pressure alone and at all but very low pressures blowout of part of the gasket must be considered as being credible and giving rise to a fairly large orifice (Hence Zone 2). To prevent these blowouts through an increased area, the use of a spirally supported gasket should be considered. This is a gasket fabricated of similar material to CAF but with a spiral metal support wound within it. While this effectively prevents blowouts there is still the possibility of leaks between the pipe flanges and the gasket due to joint stress or relaxation. However, there is a considerable difference in orifice size and so a reduced hazardous area. I would typically calculate the distance resulting from CAF segment failure and suggest alternative methods of pipe connection (supported gasket or other) so as to reduce the hazardous area. Assuming gas or vapour at 6 barg pressure this has a difference of a 7 metre zone 2 down to a 2 metre zone 2

I would think segmental failure of any soft fibre gasket at higher pressures credible. You could also explore the use of flange guards or similar etc.

There are other factors to consider in addition to pressure. Release can take place due to over compression of the gasket due to over tightening (excess torque) at the flange bolts. Also consider bolt relaxation over time. Considering your solvent delivery system, the more likely scenario is that of liquid (jet) release under pressure and liquid pool formation at some distance from the flange. The hazardous area would be determined by the properties of the substance, actual evaporation rate of the flammable liquid taking into account the environmental conditions i.e air temperature, substrate temperature and ventilation. Again, if you provide flange guards this will prevent liquid jet and reduce extent of pool.

Jason. What industry are you in? Petrochemical or surface coatings or?. As you are talking solvent delivery at 2 bar there is no risk of a NCAF joint failure if you protect the pipeline against overpressure by means of an ROP or PRV. We have no zone round our flange joints as there is no Source of Release. We use BSEN 60079-10-1 for zoning. contact me if you are a low temp (<40c) /pressure < 5bar risk industry as most calculations grossly over zone causing operational problems and other methods and analysis are more suitable to existing plants. regards Peter