Tappings for Temperature and Pressure Connections

It depends on the service and size. For example, refineries and petrochemical plants have many very large pipes, both low pressure and very high pressure, and temperatures approaching 1800F.

It is almost universal practice that any connection (such as pressure taps) that is smaller than 2" be the greater of the Piping Class of the piping or 2000# class. There is also a general specification of minimum 3/4" class connection to process, gas and steam lines and that be no less than 2000# class weld-o-let with 2000# socket-weld block valve and socket-weld 2000# nipple with external threads for the connection.

While this may seem excessive, sad experience of very costly fires and loss of life have resulted from sudden failure of small connections on process piping. I personally witnessed such a tragedy at very close range when a female thread 3/4" block valve blew off when closed and a solid bull plug was screwed in the outlet and tightened. This released an essentially unlimited quantity of oil at 350 psig and over 650 F. The valve was a 'water drain' for start-up. Post incident investigation revealed it had at some time suffered severe corrosion of the internal threads-which, could not have been seen unless it was removed. The two people performing the work were immediately burned to death when the hot oil auto-ignited instantly on contact with air.

The only reason the fire caused only $30 Million in property damage was the heat weakened a pipe hanger above it in less than 2 minutes, causing a 42" cooling water main to sag and rupture-dumping over 100,000 GPM on the fire and extinguishing it in about 5 seconds. (The cooling tower basins emptied in about 2 min.)

The Company required immediate (on a 24/7 basis) identification and good visual inspection, of all similar valves throughout its worldwide installations and replacement at first opportunity. Several similar condition valves were found, 4 of which exhibited trace leakage and on unit shutdown for emergency maintenance, could be knocked off the pipe with single hammer blows. Screwed threads were forbidden in flammable or hazardous liquid or gas service from then on and retrofitting was accomplished at the next opportunity. Tubing connections were allowed only if there were at least two independently accessible block valves ahead of the first tubing fitting.

In summary, small connections are more dangerous than large connections because corrosion can more easily weaken them and mechanical impact (from dropped equipment etc.) can more easily cause an uncontrolled release. As always, the 'devil is in the details' .