The main reason (required test pressure for hydrotest or pneumatic test) may be from safety, accuracy, purpose (pressure proof test or leak test), etc.
But the pneumatic test may be an alternative method according to the codes when the hydrotest is not available or the owner approved.

Please find the attachment.

1. Thomas J.C, Eun, ASME Section VIII, B31.3, and Section IX-Design and Construction Guidebook, p 200-201, Keyano College, 2005

Hope this helps,

Thomas J.C. Eun
thomaseun@hanmail.net (for Korean letter)


Hydrotest-Pneumatictest.pdf

I appreciated your reply and attached documents.
I missed that pneumatic test is an alternative.
Thanks.