Flight Hardware Pneumatic System Design Requirements.
Specific requirements for the design of flight hardware pneumatic systems and specific pneumatic system components are described below:
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12.6.1. Flight Hardware Pneumatic System Piping
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12.6.1.1. NPT connectors shall not be used in hazardous pressure system piping.
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12.6.1.2. Socket-welded flanges shall not be used in hazardous pressure system piping.
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12.6.2. Flight Hardware Pneumatic System Tubing. All pipe and tube welded joints shall be 100 percent radiographically inspected. All joints shall be inspected by surface NDE techniques after system acceptance pressure testing. Where post-proof test surface NDE is impractical, visual inspection will be allowed with justification and PSWG and Range Safety approval.
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12.6.2.1. Welded connections shall meet the requirements of AWS D17.1, Specification for Fusion Welding for Aerospace Applications, as prescribed by NASA-STD-5006, General Fusion Welding Requirements for Aerospace Materials Used in Flight Hardware.
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12.6.2.2. Tube and fitting welded joints shall meet the inspection requirements of AIA/NAS 1514, Radiographic Standard for Classification of Fusion Weld Discontinuities, and ASTM E1742, Standard Practice for Radiographic Examination. Qualification of visual inspection personnel shall be a minimum of VT Level II as per SNT-TC-1A. Surface inspection, if applicable, shall meet the requirements of ASTM E1417, Standard Practice for Liquid Penetrant Inspection.
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12.6.3. Flight Hardware Pneumatic System Regulators
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12.6.3.1. Regulators shall be selected so that their working pressure falls within the center 50 percent of their total pressure range if it is susceptible to inaccuracies or creep at either end of its pressure range.
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12.6.3.2. Pressure regulator actuators shall be capable of shutting off the fluid when the system is at the maximum possible flow and pressure.
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12.6.3.3. Designs using uncontained seats are unacceptable.
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12.6.3.4. Systems that contain regulators that are remotely operated during prelaunch operations shall be designed to be fail-safe if pneumatic or electric control power to the regulator is lost.
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12.6.4. Flight Hardware Pneumatic System Valves
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12.6.4.1. Valve actuators shall be operable under maximum design flow and pressure.
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12.6.4.2. Manually operated valves shall be designed so that overtorquing the valve stem cannot damage soft seats to the extent that seat failure occurs.
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12.6.4.3. Designs using uncontained seats are prohibited.
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12.6.4.4. Valves that are not intended to be reversible shall be designed or marked so that they shall not be connected in a reverse mode.
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12.6.4.5. All electrical control circuits for remotely actuated valves shall be shielded or otherwise protected from hazardous stray energy.
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12.6.4.6. Remotely controlled valves shall provide for remote monitoring of OPEN and CLOSED positions during prelaunch operations.
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12.6.4.7. Systems that contain remotely operated valves shall be designed to be fail-safe if pneumatic or electric control power to the valve is lost during prelaunch operations.
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12.6.4.8. Check valves shall be provided where back flow of fluids or media would create a hazard.
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12.6.4.9. Special care shall be taken in the design of oxygen systems to minimize the heating effect due to rapid increases in pressure. Fast opening valves that can produce high velocity kinetic effects and rapid pressurization shall be avoided.
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12.6.4.10. Valve stem travel on manual valves shall be limited by a positive stop at each extreme position.
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12.6.4.11. The application or removal of force to the valve stem positioning device shall not cause disassembly of the pressure-containing structure of the valve.
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12.6.5. Flight Hardware Pneumatic System Pressure Indicating Devices
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12.6.5.1. A pressure indicating device shall be located on the downstream side of each pressure regulator and on any storage system.
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12.6.5.2. These pressure indicating devices shall be designed to be remotely monitored during prelaunch operations.
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12.6.6. Flight Hardware Pneumatic System Flexible Hoses. Flexible hose requirements are specified in 12.1.10.4.
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12.6.7. Flight Hardware Pneumatic System Pressure Relief Devices
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12.6.7.1. Pressure relief devices shall be installed on all systems having an on-board pressure source that can exceed the MAWP of any component downstream of that source unless the system is single failure tolerant against overpressurization during prelaunch operations.
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12.6.7.2. Flight systems that require on-board pressure relief capability shall be designed to the following minimum requirements:
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12.6.7.2.1. The pressure relief device shall be installed as close as practical downstream of the pressure reducing device or source of pressure such as a compressor and gas generator.
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12.6.7.2.2. Pressure relief devices should be set to operate at a pressure not to exceed 110 percent of the system MOP.
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12.6.7.2.3. The relieving capacity of the relief device and any vent outlet piping shall be equal to or greater than the maximum flow capability of the upstream pressure reducing device or pressure source and should prevent the pressure from rising more than 20 percent above the system MOP.
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12.6.7.2.5. All relief devices and associated piping shall be structurally restrained to minimize any thrust effects on the pressure system vessels or piping.
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12.6.7.2.6. The effects of the discharge from relief devices shall be assessed and analyzed to ensure that operation of the device shall not be hazardous to personnel or equipment. Items to be analyzed are thrust loads, noise, impingement of high velocity gas or entrained particles, toxicity, oxygen enrichment, and flammability.
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12.6.7.2.7. All pressure relief devices shall be vented separately unless the following can be positively demonstrated:
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12.6.7.2.7.1. The creation of a hazardous mixture of gases in the vent system and the migration of hazardous substances into an unplanned environment is impossible.
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12.6.7.2.7.2. The capacity of the vent system is adequate to prevent a pressure rise of more than 20 percent above MOP when all attached pressure relief devices are wide open and the system is at full pressure and volume generating capacity.
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12.6.7.2.8. No obstructions shall be placed downstream of the relief device.
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12.6.7.2.9. Relief devices shall be located so that other components cannot render them inoperative.
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12.6.8. Flight Hardware Pneumatic System Vents
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12.6.8.1. Pressure systems shall be designed so that pressure cannot be trapped in any part of the system without vent capability.
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12.6.8.2. Vent system outlets should be in a location normally inaccessible to personnel or shall be conspicuously identified.
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12.6.8.3. Vent outlets shall be protected against rain intrusion and entry of birds, insects, and animals.
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12.6.8.4. Oxidizer and fuel vent outlets to the atmosphere shall be separated sufficiently to prevent mixing of vented fluids.
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12.6.8.5. All vent outlets shall be designed to prevent accumulation of vented gases in dangerous concentrations (oxygen rich) in areas frequented by unprotected personnel.
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12.6.8.6. Hydrogen vents shall discharge to atmosphere through an approved burner.
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12.6.8.7. Special attention shall be given to the design of vent line supports at vent outlets due to potential thrust loads.
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12.6.8.8. Each line venting into a multiple-use vent system shall be protected against back pressurization by means of a check valve if the upstream system cannot withstand the back pressure or where contamination of the upstream system cannot be tolerated.
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