Aircraft LOX systems are similar to gaseous oxygen systems except that the several cylinders of gaseous oxygen are replaced by one or more LOX converters. The use of more than one converter provides for an adequate supply of oxygen on long-range flights or where there is more than one crew member using the oxygen system. In addition to the converter( s), most LOX systems contain a heat exchanger, shutoff valves, and quantity indicating units. See figure 4-7 for a schematic diagram of a LOX system.

Container

The LOX converter consists of an inner and outer shell of stainless steel separated by a vacuum. A blowout disc provides a margin of safety from explosion if a leak occurs in the inner shell.

Figure 4-7.- LOX system schematic.

Filler Valve

The filler valve is a combination filler, vent, and buildup valve. The filler portion of the valve is essentially a spring-loaded check valve (fig. 4-8). When the servicing hose of the LOX cart is coupled to the filler connection, the poppet is displaced. This seals the supply port and allows container pressure to be relieved through the vent port. At the same time, oxygen flows through the filler connection and fill port to the container. When the container is full, the liquid flows from the container through the gas port and then through the vent port. In the normal position, the spring in the filler connection holds the poppet in place, forming a gastight seal. There is a check valve in the fill port that acts as a backup seal in the event the filler connection develops a leak. The vent port is also sealed in this position, allowing the gaseous boiloff (from the top of the container) to flow through the gas port to the supply port and into the oxygen system.

Pressure Control Valve

The pressure control valve used on most converters is a combination opening and closing valve (two valves contained within one housing).

These valves are controlled by spring-loaded bellows. The pressure closing valve is spring-loaded open and the pressure opening valve is spring-loaded closed. The pressure closing valve maintains operating pressure within the converter. The pressure opening valve controls the flow of gaseous oxygen into the supply line. If the pilot's demand for oxygen becomes greater than the capability of the pressure opening valve to deliver, there is a differential check valve that opens and allows liquid oxygen to flow directly into the supply line. It is transformed into gaseous oxygen during its passage through the oxygen system supply lines.

Relief Valves

A relief valve is provided in the converter to relieve excessive pressure buildup in the event of a malfunction in the pressure control valves. It also relieves normal pressure buildup when the system is not in use. This normal buildup pressure is caused by heat entering the system, and will cause a loss of 10 percent of the systems capacity every 24 hours. As an example, approximately 1 liter of loss will be experienced from a 10-liter converter.

Figure 4-8.- Filler valve