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Dead legs/purging point, vent point of the pipelines, pressure vessels, heat exchangers are generally consist of a valve and threaded cap at the end. During turnaround is it mandatory to check the thread of the nipple whether it is thinned or thread damaged especially when it is found the thickness of the pipe is satisfactory.
 
Answers
23/09/2014 A: S Sivasankaran SABANAYAGAM, CPCL, cpclsiva@yahoo.com
It is a better practice (which is being now followed) to provide a flange connection and Blind the same with BLRF flange thereby avoiding threaded connections. The existing threaded connections may also be replaced with flange end connections in phased manner
04/09/2014 A: keith bowers, B and B Consulting, kebowers47@gmail.com
In my opinion, a better arrangement is 1" sch 160 or 'double extra strong' 'weldolet' , threaded into and seal welded to a 'bar stock' internal threaded gate valve, with a SOLID threaded pipe plug in the outlet.
The philosophy is thick, impact resistant attachment to the piping, a valve resistant to vibrating open , with a solid body and very robust plug. It is good practice to dip the solid plug in corrosion inhibiting oil just before screwing it in tight (+/- 100 #/ft). This keeps any corrosion causing material that seeps past the valve from getting to any 'thin' portion of the closure.
Some folks also put a stripe of durable paint on the valve body and plug to indicate it has been so serviced and secured. The color of the paint is changed every inspection interval--red this year, blue next time ,etc.
Condition of such a connection can be quickly assessed with a trained 'hammer strike.' A good condition gives a clear characteristic ring while any debris or internal corrosion will dramatically 'muddy' the sound. We built 'training and calibration' test arrays of 'great, very good, marginal,and bad connections. We moved an array to each location where such inspection was to be done. Each inspector , each shift, 'checked his calibration' against the array.
A screwed pipe cap is , IMHO, not acceptable for any piping system containing petroleum or any solution other than potable water.