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Title: The medium inside the jacket is steam; the design pressure is 0.5 Mpa. Both the inner cylinder and the jacket are made of Q245R, with a design temperature of 150°C. The allowable stress at the design temperature is 140 Mpa, and it is 148 Mpa at room temperature. The correct hydraulic test pressure PT for this device is ( ). A. 0.66 Mpa (inner cylinder), 0.75 Mpa (jacket) B. 0.75 Mpa (inner cylinder), 0.66 Mpa (jacket) C. 0.66 Mpa (inner cylinder), 0.66 Mpa (jacket) D. 0.75 Mpa (inner cylinder), 0.75 Mpa (jacket) Let’s provide a more detailed explanation for the question above. GB/T 150 specifies that when determining the calculated pressure for common elements, the maximum pressure difference between adjacent chambers should be taken into account. The design pressure parameters for the compressive element in the middle are selected based on the following different operating conditions: 1. The inner cylinder is at atmospheric pressure while the jacket is at positive pressure; the design pressure of the jacket is used as the external pressure for calculating the pressure acting on the inner cylinder ； The jacket hydrotest pressure is used as the calculated pressure for external pressure verification. 2. Positive pressure in the inner cylinder and positive pressure in the jacket – a design without differential pressure: it is not possible to ensure that both sides are under pressure at the same time. The design pressure of the inner cylinder is used as the calculation pressure to determine the wall thickness ； The calculated pressure is determined based on the jacket design pressure as the external pressure ； The jacket hydrotest pressure is used as the calculated pressure for external pressure verification. Pressure difference design: It ensures that both sides are under pressure at all times; the designed pressure difference is used to calculate the pressure on the intermediate pressure-bearing elements and to determine the wall thickness. When the pressure inside the inner cylinder is higher than that in the jacket, the calculations are based on the internal pressure ； When the pressure inside the inner cylinder is lower than the pressure in the jacket, calculate based on the external pressure ； The jacket hydrotest pressure is used as the calculated pressure for checking the external pressure on the intermediate pressure-bearing components. 3. The inner cylinder is under vacuum while the jacket is at positive pressure; the design pressure difference is used as the calculation pressure for the intermediate pressure-bearing elements to determine the wall thickness ； Use the jacket hydrotest pressure as the calculated pressure for external pressure verification. 4. The inner cylinder is at atmospheric pressure while the jacket is under vacuum; the design pressure difference is used as the calculation pressure for the internal pressure ； Use the jacket hydrotest pressure as the calculated pressure for external pressure verification. It should be noted that the calculated pressure for the element under compression in the middle is, numerically, neither the design pressure of the inner cylinder nor that of the jacket; rather, it is determined by selecting the most severe combination of the pressure conditions on both sides. Additional note: If the positive pressure in the jacket is high, it is possible to optimize the jacket design by using a semi-tube jacket, a football-leaf jacket, or a manifold structure; this will **reduce the thickness of the cylinder and lower the cost of the equipment.