To maintain pressure on a separator, a liquid seal must be effected in the lower portion of the vessel. This liquid seal prevents loss of gas with the oil and requires the use of a liquid-level controller and a valve similar to those shown in Figs. 12.8 and 12.9. A lever-operated valve similar to the one shown in Fig. 12.10 can be used to maintain the liquid seal in a separator when the valve is operated by a float that is actuated by the oil level in the separator. The oil discharge control valve shown in Fig. 12.9 can be actuated by a float-operated pilot (not illustrated), by a floatless liquid-level controller similar to the one shown in Fig. 12.11, or by a torque tube-type (dis-placement) liquid-level controller similar to the one shown in Fig. 12.8.