Abstract: Selection of design pressures for high pressure equipment is critical to the safe production of hydrogenation units. If the design pressure is too low, the compressor shutdown will cause a high probability of safety valve take-off, and the operating pressure of individual equipment may exceed the design pressure, resulting in lower safety factor of the system; if the design pressure is too high, the corresponding investment will be increased. This paper presents a method for calculating the stagnation pressure of recycle hydrogen loop and determining the design pressure of cold high pressure separator based on the API 521 (6th edition) standard. By organizing and analyzing the on-site measured data collected during two interlock shutdown accidents of the recycle hydrogen compressor in a certain residue oil hydrogenation unit and comparing them with the results obtained by using this method, it is found that the calculated results from this method are highly consistent with the operational data collected from the actual production, and the stagnant pressure of the recycle hydrogen return circuit can be determined relatively accurately. The method provides guidance on how to determine the design pressure for new and revamped hydrogenation units.

Key words: cycle hydrogen return circuit, stagnation pressure, cold high pressure separator, design pressure