When hydraulic pressure test on the pressure vessel is conducted, do not remove the safety valve installed on it. Increase internal pressure of the vessel from 80% to 90% of the designed pressure of the valve. Mount the test gag, and hold the end of the stem gently in order to protect the valve from damage caused by undue load.
The gag shall also be removed at 80% to 90% of the designed pressure.
When several valves are installed on a vessel or a pipe line and one of the valves is being adjusted, the same caution should be taken when the test gag(s) is(are) being mounted on the other valve(s) to prevent it(them) from blowing off.
5.1 Cautions
The test gag cannot be mounted on ordinary type of caps. Therefore, the caps should be replaced with those made for such purpose.
In case of pressure test for pressure vessels without using the test gag, test them without the safety valves for the safety of it.
A removed valve should be covered at the flange parts against intrusion of dusts, etc.
Replace the plug and gasket of a closed top (or take them off for an open top) after the test, otherwise the valve will not operate, and hazard may occur thereby.
5.2 Test gag
(1)   Test gag should be used only when hydrostatic testing is performed at field;
(2)   Safety valve & safety relief valve are usually shipped with the test gag;
(3)   The test gag should be furnished in accordance with following diagram: