Blind Flange

The blind flange depicted in Figure 4.24 serves a function similar to that of a plug or cap. It is used to terminate the end of a piping system. The blind flange is basically a flange that does not have a hub or a bored center. Blind flanges have the face thickness of a flange, a matching face type, and similar bolting pattern. Blind flanges can also be used to seal a nozzle opening on a pressure vessel. Because it is bolted, the blind flange provides easy access to the interior of a vessel or pipe, unlike a cap that is welded. Figure 4.25 represents the drawing symbol for the blind flange.

1.

For flat heads, tubesheets, and blind flanges, the thickness used for each of the respective thickness' is that thickness divided by 4.

2.

For corner, fillet, or lap-welded joints, the thickness used shall be the thinner of the two parts being joined.

3.

For butt joints, the thickness used shall be the thickest joint.

4.

For any Code construction, if the vessel is stress relieved and that stress relieving was not a Code requirement, the MDMT for that vessel may be reduced by 30 ° without impact testing.

1.

For flat heads, tubesheets, and blind flanges, the thickness used for each of the respective thickness’ is that thickness divided by 4.

2.

For corner, fillet, or lap-welded joints, the thickness used shall be the thinner of the two parts being joined.

3.

For butt joints, the thickness used shall be the thickest joint.

4.

For any Code construction, if the vessel is stress relieved and that stress relieving was not a Code requirement, the MDMT for that vessel may be reduced by 30° without impact testing.

The fluid force is due to the fluid pressure acting on the end caps and can be expressed as follows:

The restraint force (anchor force) is zero for unrestrained pipeline:

The wall force at the capped end is due to the internal and external pressures:

When an unrestrained pipeline expands due to a temperature increase, there is a strain change due to the temperature effect but with no stress change due to the temperature effect. The wall force along the pipeline is constant because no restraint force is acting along the pipe and the longitudinal stress of unrestrained pipeline is

The hoop stress and radial stress are calculated from Eq. [9.12].

The end expansion movement at the free end is

1.

Positive isolation should be by one of the following:

a.

The removal of a flanged spool piece or valve and the fitting of blind flanges to the open-ended pipes.

b.

Line blind.

c.

A spade in accordance with standards. The arrangements of spading points, together with venting, draining, and purging facilities, should enable a section of line containing a spade to be checked as free from pressure before spade insertion or removal.

2.

Positive isolation methods should be provided:

a.

To permit isolation of major items of equipment or group of items for testing, gas freeing, making safe, etc. A group of items containing no block valves in the interconnecting pipework may be considered as one item of equipment.

b.

To isolate a section of plant for overhaul.

c.

To isolate utility services, e.g., fuel gas, fuel oil, atomizing, snuffing, or purge steam to individual fired heaters.

d.

To prevent contamination of utility supplies, e.g., steam, water, air, and nitrogen where permanently connected to a process unit.

e.

Steam and air connections for regeneration and steam/air decoking should be positively isolated from the steam and air systems, preferably by spool pieces or swing bends.

The direct cause of the accident was overpressure in one of the two parallel condensate pumps, which could not be withstood by an improperly fitted temporary blind flange. The first pump, originally called upon, failed. The second, subsequently called upon, had its pressure safety valve removed and replaced by the loosely fitted blind flange by an earlier shift as part of partially completed routine maintenance. Gas then leaked out of the pump/flange and was ignited, causing an explosion which blew through a firewall designed to withstand fire, but not explosions. Some other issues associated with the accident were as follows, together with suggestions of how a rigorous regulatory program might have averted the disaster.

•

Ineffective PTW process. A PTW covering the status of the pump undergoing maintenance was in place. It was generated by day shift personnel and stated that the pump was not ready and should not be switched on under any circumstances. Unfortunately, the situation was not discussed directly with the incoming night shift foreman. Instead, the permit, which could not subsequently be found, was simply left in the control center.

PTW systems and procedures are key elements of safety cases and are also mandated by other regulatory requirements. PTW procedures are usually required to be part of the formal documentation submitted for review and approval to the regulator, classification society, and/or CA. Procedures generally require sign-off at all stages of the work, including handover from one group or shift to the next. The argument could be made that had the night shift foreman been required to formally sign-off all active work permits in place during his upcoming shift, the disaster may have been averted.

Communication is also a key element of the safety case process. Usually a communications plan is required, which would include, for example, short tool-box style meetings between personnel ending and beginning their shifts. It is quite possible that had such a meeting taken place, the condition of the pump undergoing maintenance would have been known to the night shift staff.

•

Unacceptable design. A contributory factor in the disaster was the inability of the firewall to withstand an explosion. In fact one panel from the firewall was violently dislodged and caused a rupture in another pipe and subsequently another fire. Design review and approval of all safety-related structures and equipment are primary elements of all regulatory programs, both prescriptive and those relying on safety cases. It is quite likely that the unsuitability of the firewall for the facility's current configuration would have been picked up by the design appraisal process.

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Notification of changes to the facility. Regulatory programs require operators to advise the regulator, classification society, and/or CA when any significant changes are made to the facility. The Piper Alpha platform was originally intended to produce oil only, but was reconfigured to add gas production. This would certainly constitute a significant change.

One of the significant changes associated with this would have been the upgrade of the firewall to a blast wall. Had there been a rigorous process in place to review all aspects of the changes it is quite possible that the effect of this contributory factor could have been minimized or eliminated altogether.

A second and more fundamental result of the change from oil only to oil and gas production was the breaking of the basic safety concept that the most dangerous operations should be kept as far away as possible from personnel areas. The change resulted, for example, in gas compression equipment being located next to the control room. This change was also considered to be a contributory factor in the disaster.

•

Ineffective interfield communications and levels of authority. The fire would have burned out more quickly except that oil from two adjacent platforms, Tartan and Claymore, continued to feed it. The managers of the adjacent platforms either believed they had no authority to stop production, or they were directly instructed by their superiors to continue production.

Regulatory programs and safety cases have requirements for clearly defined, submitted, and approved communications plans, level of authority matrices and similar. It is quite likely that such plans would provide for communications protocols in emergency situations and give offshore installations managers the complete authority to take any appropriate steps they considered necessary to safeguard life and property in the event of an incident. This would have been an obvious mitigative step in dealing with the disaster.

If the centrally located opening exceeds ½ the inside diameter of the vessel, then the design procedure reverts to a standard flange design.