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Helium Pressurization

Each pod pressurization system consists of a helium tank, two helium isolation valves, two dual pressure regulator assemblies, parallel vapor isolation valves on the regulated helium pressure to the oxidizer tank only, dual series-parallel check valve assemblies and pressure relief valves.

The helium storage tank in each pod has a titanium liner with a fiberglass structural overwrap. This increases safety and decreases the weight of the tank 32 percent over that of conventional tanks. The helium tank is 40.2 inches in diameter and has a volume of 17.03 cubic feet minimum. Its dry weight is 272 pounds. The helium tank's operating pressure range is 4,800 to 460 psia with a maximum operating limit of 4,875 psia at 200 F.

A pressure sensor downstream of each helium tank in each pod monitors the helium source pressure and transmits it to the N 2 , He , kit He switch on panel F7. When the switch is in the He position, the helium pressure of the left and right OMS is displayed on the OMS press left, right meters. This pressure also is transmitted to the CRT and displayed.

The two helium pressure isolation valves in each pod permit helium source pressure to the propellant tanks or isolate the helium from the propellant tanks. The parallel paths in each pod assure helium flow to the propellant tanks of that pod. The helium valves are continuous-duty, solenoid-operated. They are energized open and spring loaded closed. The OMS He press/vapor isol switches on panel O8 permit automatic or manual control of the valves. With the switches in the GPC position, the valves are automatically controlled by the general-purpose computer during an engine thrusting sequence. The valves are controlled manually by placing the switches to open or close.

The pressure regulators reduce the helium source pressure to the desired working pressure. Pressure is regulated by assemblies downstream of each helium pressure isolation valve. Each assembly contains primary and secondary regulators in series and a flow limiter. Normally, the primary regulator is the controlling regulator. The secondary regulator is normally open during a dynamic flow condition. It will not become the controlling regulator until the primary regulator allows a higher pressure than normal. All regulator assemblies are in reference to a bellows assembly that is vented to ambient. The primary regulator outlet pressure at normal flow is 252 to 262 psig and 247 psig minimum at high abort flow, with lockup at 266 psig maximum. The secondary regulator outlet pressure at normal flow is 259 to 269 psig and 254 psig minimum at high abort flow, with lockup at 273 psig maximum. The flow limiter restricts the flow to a maximum of 1,040 stan dard cubic feet per minute and to a minimum of 304 standard cubic feet per minute.

The vapor isolation valves in the oxidizer pressurization line to the oxidizer tank prevent oxidizer vapor from migrating upstream and over into the fuel system. These are low-pressure, two-position, two-way, solenoid-operated valves that are energized open and spring loaded closed. They can be commanded manually or automatically by the positioning of the He press/vapor isol switches on panel O8. When either of the A or B switches is in the open position, both vapor isolation valves are energized open; and when both switches are in the close position, both vapor isolation valves are closed. When the switches are in the GPC position, the GPC opens and closes the valves automatically.

The check valve assembly in each parallel path contains four independent check valves connected in a series-parallel configuration to provide a positive checking action against a reverse flow of propellant liquid or vapor, and the parallel path permits redundant paths of helium to be directed to the propellant tanks. Filters are incorporated into the inlet of each check valve assembly.

Two pressure sensors in the helium pressurization line upstream of the fuel and oxidizer tanks monitor the regulated tank pressure and transmit it to the RCS/OMS press rotary switch on panel O3. When the switch is in the OMS prplnt position, the left and right fuel and oxidizer pressure is displayed. If the tank pressure is lower than 234 psia or above 284 psia, the left or right OMS red caution and warning light on panel F7 will be illuminated. These pressures also are transmitted to the CRT and displayed.

The relief valves in each pressurization path limit excessive pressure in the propellant tanks. Each pressure relief valve also contains a burst diaphragm and filter. If excessive pressure is caused by helium or propellant vapor, the diaphragm will rupture and the relief valve will open and vent the excessive pressure overboard. The filter prevents particulates from the non-fragmentation-type diaphragm from entering the relief valve seat. The relief valve will close and reset after the pressure has returned to the operating level. The burst diaphragm is used to provide a more positive seal of helium and propellant vapors than the relief valve. The diaphragm ruptures between 303 and 313 psig. The relief valve opens at a minimum of 286 psig and a maximum of 313 psig. The relief valve's minimum reseat pressure is 280 psig. The maximum flow capacity of the relief valve at 60 F and 313 psig is 520 cubic feet per minute.

Curator: Kim Dismukes | Responsible NASA Official: John Ira Petty | Updated: 04/07/2002
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