SISTEMAS PRESION/VACIO

SISTEMAS PRESION/VACIO

4.0 PRESSURES  

4.1 Responsibilities 

Goddard Space Flight Center Code 750.0 has primary responsibility for the safety of facility users working under their jurisdiction at GSFC and its off-site facilities while working with all operations involving pressure systems.  The following procedures and requirements must be met. 

4.2 Relevant Standards/Documents 

The pressure requirements and standards listed below are intended to ensure an acceptable level of compliance.  It is the responsibility of project management however, to obtain and adhere to all applicable pressure requirements.  

American Society of Mechanical Engineers Boiler and Pressure Vessel Code Section VIII (metallic vessels), Section X (non-metallic vessels) 

Design, Inspection, and Certification of Pressure Vessels and Pressurized Systems 

GMI 1710.4A   Design, Inspection, and Certification of pressure vessels and     pressurized Systems (RECERT) 

NASA Aerospace Pressure Vessel Safety Standard 

NHB 1700.6   Guide for In-service Inspection of Ground-Based Pressure Vessels     and Systems. 

NHB 1700.1 (V-l)  NASA Safety Policy and Requirements Document 

NHB 1700.6, NMI 1710.3, NSS/HP- 1740.1  Guide for In-service Inspection of Ground-Based Pressure Vessels and Systems. 

Safety Program for Pressure Vessels and Pressurized Systems 

 4.3 Definitions  

The following is a list of various definitions pertaining to pressure systems found in this chapter: 

Design Burst Factor (DBF) - The ratio of the ultimate burst pressure to the Maximum Operating Pressure (MOP). 

Design Burst Pressure (DBP) - The test pressure that pressurized components and vessels must withstand without rupture to demonstrate its design adequacy in a qualification test. 

Factor of Safety/Safety Factor - The ratio of the ultimate strength of the material to the allowable stress. 

Fault tolerance - Failure tolerance is the basic safety requirement that shall be used to control most payload hazards. The payload must tolerate a minimum number of credible failures and/or operator errors determined by the hazard level. This criterion applies when the loss of a function results in a potentially hazardous event. 

Maximum Allowable Working Pressure (MAWP) - The Maximum operating pressure permissible for a pressure component at the operating temperature specified for that pressure. It is a component's pressure rating based on structural stress values and functional capability. This is usually determined by the manufacturer. 

Maximum Operating Pressure (MOP) - The maximum pressure a system will actually operate at in a particular application. MOP is synonymous with MEOP. 

Proof Pressure Test - The test pressure that pressurized components shall sustain without detrimental deformation. The proof pressure is used to give evidence of satisfactory workmanship and material quality. 

4.4 Pressure / Vacuum Systems 

In order to ensure the safety of personnel, facilities, and equipment, it is necessary to have pressure systems designed and tested to meet applicable standards.  In general, systems must be verified to handle the pressures that will be applied to them. This section establishes the operational requirements such as procedures and training as well as design requirements for a pressure system to enter the GSFC facility. 
 

This section applies to all pressure vessels and pressure systems that will have one or more of the following: 

1. Maximum Operating Pressure (MOP) of greater than 150 psi.
2. Vessels or components that use potentially hazardous fluids (cryogenic, flammable, etc.);
3. Stored energy of 14,240 foot-pounds or greater based on adiabatic expansion of a perfect gas;

 Potentially hazardous materials such as hydrazine, may not be brought into the facilities. Other types of potentially hazardous materials will require an Operational Hazard Analysis (OHA) to ensure the risk to personnel and facilities have been controlled. Systems shall be designed to be 2 fault tolerant against catastrophic hazards unless a waiver, deviation, or variance is approved by Code 754.0. 

If a non-flight pressure vessel or pressure system is ground based permanently at Goddard, it must be approved by Code 750.0. These systems are handled separately. The system must be compliant with NHB 1700.6 (Guide for In-service Inspection of Ground-Based Pressure Vessels and Systems). 

Systems that include hydraulics, cryogenics, vacuum, or potentially hazardous fluids have additional requirements covered in section 4.15 Special Requirement 

4.5 Analysis Requirements 

Structural and stress (static and dynamic loads that include thermal stresses), fatigue life, and fracture mechanics analyses shall be performed to ensure the structural adequacy of  pressure vessel that are to be used at Code 750.0.  A Fracture Control Plan shall be developed.  Fracture control pressure vessels shall be designed to avoid service failure caused by the propagation of flaws. 

4.6  Test Requirements 

The proof pressure level shall be selected to demonstrate that the vessel is free of flaws larger than the permissible size. If proof does not provide definitive flaw detection, other flaw detection techniques shall be required. Proper loads and environments shall be sequenced to simulate the operational service. As a minimum all pressure vessels shall be subjected to an acceptance proof test. For pressure vessels not designed for general yielding, the maximum allowable proof-test stress shall be equal to the yield stress producing 0.2 % permanent strain.  

Pre-proof test inspection shall be performed to establish the initial condition of the structure. Post-proof test inspection is mandatory where the proof test does not provide, by direct demonstration, assurance of satisfactory performance over the specified service life. 

4.7 Operations, Maintenance, and Certification Requirements 

Documentation is required on the inspection interval for the vessel based on fatigue analyses.  Procedures are required for operation of the pressure vessel and its associated system.  Documentation is required on the probable location and character of defects expected for the pressure vessel. 

Documentation logs shall be developed and maintained throughout the life of the pressure vessel.  A recording system must be maintained which accounts for all maintenance operations, tests, and inspections performed with or on the pressure vessel. It shall include pressure and temperature history, and results of inspections conducted. 

Pressure relief devices shall be used to limit the maximum pressure to below MAWP. 

4.8 Relief Devices 

Relief devices are required on all pressure vessels unless they have potentially hazardous fluids. Vessels containing potentially hazardous fluids must have a design pressure that exceeds 4 X worst case pressure due to failure (2 fault tolerant) . 

Relief devices are required downstream of all regulators when the upstream pressure exceeds downstream design pressure. 

The following is a list of relief device design requirements: 

1. Relief devices shall be set no greater than 110 % of MOP.
2. Relief device settings shall not exceed the MAWP of the pressure vessel or system.
3. Relief device must be able to handle the worst case capacity.
4. There shall be no shutoff devices between the relief devices and the pressure source.
5. Relief devices shall be adequately supported for discharge thrust impulse forces.
6. Relief devices shall vent out of the work area.
7. Relief valves shall be inspected and calibrated every 2 years. All GSE relief valves shall have identification of the calibration and inspection dates.

 4.9 Gauges 

Gauges are required downstream of all regulators, on all storage vessels (GSE), and between isolation valves (trapped volumes). Trapped volumes can over pressurize causing failure of system. The requirement for gauges can be approved (waiver, deviation, or variance) by safety if isolated volumes, by design, cannot over pressurize. 
 

The following is a list of gauge design requirements: 

1. The system MOP shall be less than or equal to 75 % of the gauge scale.
2. Gauges shall have a one piece shatter-proof window and a blow out back (or equivalent) to allow unrestricted venting.
3. All gauges shall be inspected and calibrated every 2 years. All relief valves shall have identification of the calibration and inspection dates.

 4.10 Flex hoses and rigid tubing 

The facility user must ensure that all flex hoses and tubing meet the following requirements: 

1. Flex hoses and rigid tubing shall have a minimum design burst factor of 4.
2. ASME/ANSI B31. Piping or rigid tubing shall be used in lieu of flex hose wherever possible.
3. Flex hose shall be restrained properly at every 6 feet and at both ends for pressures greater than 150 psi.
4. Flex hose shall be proof tested annually to 1.5 X MAWP and tagged in accordance with paragraph 3.10.
5. Piping and rigid tubing shall meet ANSI/ASME B31.1 and B31.3.

 4.11 Vents 

Pressure systems shall be designed so that pressure cannot be trapped in any part of the system without vent capability.   

The following vent design requirements must be meet: 

1. Vent system outlets shall be in a location normally inaccessible to personnel and shall be identified/marked.
2. Incompatible venting fluids shall be separated sufficiently to prevent mixing.
3. All vent outlets shall be designed so as to prevent accumulation of vented fluid in dangerous concentrations.
4. Vent lines shall be adequately supported for discharge thrust impulse forces .
5. Each line venting into a multiple use vent system shall be protected against back pressurization. All venting fluids shall be compatible.

 4.12 Bonding and grounding 

All pressure systems containing flammable/combustible fluids shall be bonded and grounded. 
 
 Bonding and grounding design requirements: 

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