A, according to the balance condition of lever, if the gravity of heavy objects increases, the safety value of steam pressure will also increase, so A does not meet the requirements.

B, reduce the weight appropriately, according to the balance condition of the lever, the safe value of steam pressure decreases, so B meets the question.

C. If the weight is moved to the left, its arm of force will become shorter, and the safety value of the valve will decrease according to the balance condition of the lever, so C meets the meaning of the question.

D. If the weight is moved to the right, its arm of force will become longer, and the safety value of the valve will become larger according to the balance condition of the lever, so D does not meet the meaning of the question.

So choose BC.