Debasis Pneumatic air test from shell side under positive pressure of 0.5 KSC to 1.0 KSC is OK. Normally the tube to tube sheet should not be kept under pneumatic pressure for too long a time. Rule is not to keep pneumatic pressure condition too long even if done at lower positive pressure like 0.5 KSC. Also air pressure under ambient temp would be different if pneumatic air is tested above ambient temperature. With more time and a little higher air temperature may create problems like what you experience due to time of holding being longer and air pressure. Once leak is seen between tube to tube sheet, one practice is to roll tight the leaky tube. This is a mechanical way of sealing the leak and a re-test (but not held too long) will help in identifying if the leak seen has been arrested. Mechanical seal with even torque load with a calibrated torque wrench is preferred to arrest tiny leaks seen in air test. Advantage with this: it will not spread to next tubes during re-test. If leak still persists or at some other corner in the same tube to tube sheet groove, further rolling may help. Spot welding may give problems like what you experience in the same tube to tube sheet groove or spread to another close by tube to tube sheet groove. Taper plug design : One is to choose the taper length and taper gradient in a way to choose a narrow taper gradient than recommended ( normal taper = 1 in 120 gradient with size of taper plug generally under-size than the average tube inner diameter. Taper plug will give a line contact compared to strength groove rolling - which gives both line and strength contact while tight rolling. Another standard practice used by industry : Choose an under-size taper plug (Teflon or non metallic and ram it full inside the tube sheet) and then use or choose a second metal taper plug (slighltly over size than inner Dia of tube) and then with gradient less than normal 1 in 120. For very critical services like Hydrogen or Petro chemical explosion handling gas-to-gas exchangers a simple seal welding is done depending on the leak tightness tolerance needed for the intended process service. Trust this answers your query and of some help for your problem. C.V.Srinivasan Nishi Engineers Pvt Ltd India August 7, 2007 E-mail: nishi@vsnl.com >During s/d maintenance of HEx normally a plant air-line >pressure pneumatic test is carried out with soap bubble test >to check leak tightness of tube sheet - to - tube seal/ >strength joint as a whole. When any leaky condition >detected depending upon it's population a touch-up weld >built-up preferably by a low heat input welding process ( >like TIG) is done . > >But it has been a practical issue that when welder even >under v. close supervision attempts to weld a point leak at >oe place, another leaky condition appears in the next >pneumatic test from any side by tube-to-tube sheet weld. And >the game continues to repeat crossing planned s/d time and >often it becomes a state for severe manegement tension so >far as plant start up delayes are concerned. > >This new generation of leak has been tried to arrest by >putting taper brass plug removable inserts in the >surrounding tubes during the weld built to facilitate quick >heat transfer but still it is not rendered a reliable >comfort to maintenance and inspection engineers. > >If anyone has any established and standardised technique / >gadgets in this regards, may pl. feel open to share for he >common global interest of working maintenance and inspection >engineers. > >my regards