Abstract: The hydraulic valve and mechanical breather valve are the important equipme nts to assure the safe operation of oil tank, and the former can immediately work under the condition of malfunction of the latter. If the height data of liquid seal of hydraulic valve is obtained by a correct calculation, both the volatilization loss of light oil and overpressure or negative pressure of oil tank will be avoi ded. The author provides systematical equations to compute the height and describes the derivation proces s of the equations as well as its applied effect.