Bmoorthy, as implied by mr Anegri if you read both the 'Shell spec 77/312' and 'TA LUFT' both require testing for extrenal leakage only. Seat leakage is covered by so many other specifications - API 598,BS6755 Pt1 etc.
 The legislation based around emmissions of certian medias, the risk assessment of health and safety issues and the companies location may all be a deciding factor. i.e. leakage of phosphoric acid, chlorine etc would constitute a health and safety valve issue, the location the company is in may deem this as unacceptable. Leakage of hydrocarbons and other medias is monitored by various government agencies and if set levels are breached then the company can either be fined or shut down until they can prove that changes have been applied. I believe that most engineers play it safe. don't forget your equipment may be around for sometime and future compliance may also play a part.

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