I have come across pressure drawings of vendors where they give two values of Hydrotest pressure - one for field and one for shop. Why is it so? What is the basis for such calculation?

Arijit,

The hydrotest pressure is the pressure at the top of the vessel during the hydrotest. But due to the static head of the water the pressure at the bottom of the vessel is higher.

Typically the shop hydrotest is in horizontal condition. The field hydrotest is as the name suggests is done in the field, is performed in whatever the condition the vessel is in the field. For a vertical vessel it will be in vertical condition.

As, the total hydrostatic head of the liquid in vertical condition is more than that in the horizontal conditions. Hence, for the same level of the stress the test pressure would be a higher value for the horizontal hydrotest.

Refer to UG-99 in ASME Sec. VIII Div. 1 code for description on how to compute the hydrostatic test pressure.