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Pressure and Vacuum Systems Safety Supplement Part
7: Vacuum Systems |
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All vacuum systems and components are considered pressure systems and components. Users, fabricators, and designers of vacuum systems shall be aware of the potential hazards vacuum systems pose especially when performing operations such as backfilling or when working near a vacuum chamber with a thin section or window. Such hazards include rupture from overpressure during a backfill operation, buckling collapse, or implosion do to failure of a component such as a thin window.
Vacuum systems with a design initiated on or after January 4, 2016 shall meet the requirements of Part 7: Vacuum Systems of this Supplement. For vacuum systems under construction prior to the above start date, the assigned DA or Vacuum Engineer/Technician shall choose to either follow Part 7: Vacuum Systems of this Supplement or continue following Revision 3.3 of ES&H Manual Chapter 6151 Pressure and Vacuum Safety Program and its Technical Appendices. If Revision 3.3 is followed and the vacuum system is a Category 3 system as defined below, the DA must still complete Form PS-1, Form PS-4 and Form PS-5 of this Supplement; create and file a P&ID; and complete the steps defined in Part 2: Section 2.10 Inspections and Part 2: Section 2.11 System Turnover.
A qualified Vacuum Technician or Responsible Vacuum Engineer shall determine the proper category for a proposed vacuum system design, repair, or alteration.
A vacuum system shall be classified into one of the following categories:
A vacuum system that is not connected to any credible pressure source exceeding 15 psig may be considered a Category 0 system if all of the following conditions are met.
· Total system volume is less than 35 ft3 or the largest internal cross sectional area does not exceed 33 in2 regardless of length.
· The pressure source relieving the vacuum cannot exceed 15 psig in any failure mode. A regulator attached to a building nitrogen supply at 60 psig, for example, does not meet this requirement.
· The system is not an insulating vacuum for cryogenic components; examples of this include cryostats, cryogenic transfer line vacuums, and storage vessels for LHe or LN2.
· The system has no component or section with an unreinforced wall thickness less than 0.02 inches more than 6 inches in diameter.
Vacuum systems not used to insulate cryogenic surfaces in which the differential operating pressure can never exceed 15 psi but have a volume larger than 35 ft3 and a cross sectional area larger than 33 in2. Examples of Category 1 vacuum systems include HMS and HRS vacuum systems, injector chambers and dog-leg chambers.
Vacuum systems attached to or containing a credible pressure source that can exceed 15 psig and are protected from pressurization exceeding 15 psig through engineering controls (e.g. pressure relief devices). Insulating vacuums for cryogenic systems shall be considered Category 2 vacuum systems. Examples of Category 2 vacuum systems include cryostats, cryogenic transfer lines, cold boxes, and target scattering chambers.
Vacuum systems attached to or containing a credible pressure source that can cause the vacuum system pressure to exceed 15 psig and where this system cannot be protected from pressurization exceeding 15 psig shall be considered a pressure system with a design pressure (or MAWP) exceeding 15 psig and subject to the full rigor of ES&H Manual Chapter 6151 Pressure and Vacuum Systems Safety Program and this Supplement.
Note: Thin windows installed on Category 3 systems shall be designed in accordance with the applicable ASME Code and shall not be designed following the rules give in Part 7: Section 7.7 Requirements for Components with Thin Windows..
Vacuum systems meeting the criteria for Category 0 shall be designed by a qualified vacuum technician. The vacuum technician shall assume responsibility for the safe construction, alteration, or repair of the system following sound engineering principles. No further requirements (including the generation of a PS number and folder structure) apply.
Vacuum systems meeting the criteria for Category 1 shall be designed by a qualified Responsible Vacuum Engineer experienced in the design of vacuum systems and vessels. The design, fabrication, testing, repair, and alteration of Category 1 systems shall comply with all the requirements of this section. A PS number and folder structure are not required.
Vacuum systems in Category 1 are not required to meet the full rigor of the ASME Codes. These systems shall be designed to ensure that the system maintains suitable safeguards against buckling collapse. This can be verified using applicable paragraphs from ASME BPVC Section VIII Div 1 or Div 2. Alternatively, vacuum system design may be verified using other peer approved methods. A buckling analysis with a minimum factor of safety of 2 shall be performed by a Responsible Vacuum Engineer and reviewed by another Responsible Vacuum Engineer or DA.
All welding and brazing performed on Category 1 vacuum components shall meet the requirements of the Welding and Brazing Supplement. Welded and brazed joints shall be designed using sound engineering principles supported by detailed calculations, testing, and/or service experience. These designs shall be technically reviewed. The welding/brazing procedures for these joints shall fully comply with ASME IX or AWS. All welders/brazers performing these procedures shall be qualified to them in compliance with the applicable Code. Materials of unknown origin shall not be used in structurally relevant application on these components. The Responsible Vacuum Engineer shall determine the method and extent of examinations and inspections to be performed.
All other fabrication of the system may be overseen by a qualified vacuum technician.
In addition to any leak testing to demonstrate performance, Category 1 systems shall require an evacuation test (i.e. negative pressure test) which shall be supervised and witnessed by the Responsible Vacuum Engineer. For ordinary vacuum systems, the test pressure shall be full atmospheric pressure differential. For vacuum systems not intended to be pumped out to the full atmospheric pressure differential, the test pressure shall be 110% of the maximum allowable external differential pressure, but not more than full atmospheric pressure. The process steps for completing this test shall be as given in Part 6: Section 6.1.3 Process Steps with the exception that Steps 5 and 6 may be omitted.
For a vacuum system within a pressure vessel, the test differential pressure shall be 110% of the maximum allowed working pressure differential. Thin windows and other delicate equipment may be removed while testing the vacuum system.
Documentation for Category 1 systems (design calculations, material certifications, etc.) shall be maintained by the Responsible Vacuum Engineer. A Vacuum System folder within the Pressure Systems webpage is available for documentation storage.
Category 2 systems with a volume less than 35 ft3 or having no cross section larger 33 in2 are only required to meet the requirements of Part 7: Section 7.5.1 Pressure Relief and shall be designed by a Responsible Vacuum Engineer. All other fabrication, repair, or alterations of the system may be overseen by a qualified vacuum technician.
All other Category 2 systems shall meet the full requirements of this section. A PS number and folder structure are not required.
For Category 2 systems, the Responsible Vacuum Engineer shall ensure, using sound engineering practices, that there is adequate pressure relief (relief capacity) for the entire system and that the maximum system pressure does not exceed 15 psid. Coded relief devices are not required. Non-coded relief devices shall be qualified through operability tests demonstrating function and flow capacity or calculations showing adequate flow capacity. Flow capacities provided for devices produced by a reputable manufacturer shall be acceptable. A properly designed and relieved purge system may be used provided that the users of such a system are fully trained. A Responsible Vacuum Engineer shall ensure that the purge system is designed to provide adequate capacity using sound engineering principles. It is recommended that relief capacities be determined using methods similar to those given in API 520 and 521.
A properly designed purging system may be used on any Category 2 system provided all of the following conditions are met:
· The purge system shall be flow limited. This limit shall be determined by calculations, flow testing, or manufacturer rating.
· A relief device, that is located between the backfill pressure source and the vacuum volume, having equal or greater capacity than the flow limit determined above, shall be installed in such a manner that the pressure in the vacuum system being purged cannot exceed 15 psig under any normal or credible fault condition.
All welding and brazing performed on Category 2 vacuum components shall meet the requirements of the Welding and Brazing Supplement (WBS). Welded and brazed joints shall be designed using sound engineering principles supported by detailed calculations, testing, and/or service experience. These designs shall be technically reviewed. The welding/brazing procedures for these joints shall fully comply with ASME IX or AWS. All welders/brazers performing these procedures shall be qualified to them in compliance with the applicable Code. Materials of unknown origin shall not be used in structurally relevant application on these components. The Responsible Vacuum Engineer shall determine the method and extent of examinations and inspections to be performed.
All other fabrication of the system may be overseen by a qualified vacuum technician.
In addition to any leak testing to demonstrate performance, Category 2 systems shall require an evacuation test (i.e. negative pressure test) which shall be supervised and witnessed by a Responsible Vacuum Engineer. For ordinary vacuum systems, the test pressure shall be full atmospheric pressure differential. For vacuum systems not intended to be pumped out to the full atmospheric pressure differential, the test pressure shall be 110% of the maximum allowable external differential pressure, but not more than full atmospheric pressure. The process steps for completing this test shall be as given in Part 6: Section 6.1.3 Process Steps, with the exception that Steps 5 and 6 may be omitted.
For a vacuum system within a pressure vessel, the test differential pressure shall be 110% of the maximum allowed working pressure differential. Thin windows and other delicate equipment may be removed while testing the vacuum system.
Documentation for Category 2 systems (design calculations, material certifications, relief device calibration data, etc.) shall be maintained by the Responsible Vacuum Engineer. A Vacuum System folder within the Pressure Systems webpage is available for documentation storage at the discretion of the Responsible Vacuum Engineer.
Category 3 vacuum systems are considered pressure systems with a design pressure (or MAWP) exceeding 15 psi. The DA shall ensure that all work on Category 3 vacuum systems including system design, fabrication, testing, alteration, and repair meets the requirements of the ES&H Manual Chapter 6151 Pressure and Vacuum Systems Safety Program and all sections of this Supplement. A PS number and folder structure in the Pressure Systems database are required for Category 3 vacuum systems. The System Owner of a Category 3 vacuum system must follow the operation, maintenance and in-service inspection requirements described in Parts 8: Operation and Maintenance and Part 9: Pressure Equipment in-Service Inspection Program of this Supplement.
Thin windows are common at Jefferson Lab. These components
are often installed on spectrometer vacuum systems, scattering chambers, beam
dumps, and detectors. They are required to reduce background interactions and
multiple scattering of the main electron beam and of recoil particles impinging
on detectors. These components are crucial to the mission at Jefferson Lab.
This section details the requirements for thin window design for Category 1 and
2 vacuum system components with sections where the wall thickness is less than
0.02 inches larger than 33 in2 in area or when installed in critical
applications. Windows installed on
Category 3 vacuum systems are subject to the ASME codes or equivalent measures.
The rules defined below are not equivalent measures to the codes and therefore
shall not be used for Category 3 systems.
Category 1 and 2: The safety of the window design shall be assured by a Jefferson Lab DA experienced in thin window design.
The design of any thin vacuum window shall consider the following:
· Material compatibility
· Life cycle and fatigue.
· Effects from radiation or corrosion
· Possible accidental damage from puncture etc.
· Magnitude of deformation
· Crack and tear propagation
Design calculations (see Part 7: Section 7.7.2.2 Stress and Deformation Calculations) or proof test (see Part 7: Section 7.2.3 Proof Test) shall be performed to ensure that the stresses in the thin window are acceptable
The stress in the window shall be determined after all fabrication steps have been completed (i.e. hydro-forming). The calculated stress in the window shall be less than the allowable stress. The allowable stress in tension for any thin vacuum window shall be the lesser of:
· Sa = 0.5 Sut (allowable stress is ½ ultimate tensile)
· Sa = 0.9 Sy (allowable stress is 9/10 yield)
Alternatively, a proof test may be performed. The design pressure (or MAWP) of the window shall be given by:
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Pdesign < |
Ptest |
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Slist |
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2 |
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Stest |
Where:
Pdesign = design pressure
Ptest = the test pressure
Slist = the specified minimum ultimate tensile strength
Stest = the ultimate tensile strength of the material used in the test
If it is known that only material from one batch, heat or lot is to be used, then the stress ratio in the above equation may be assumed to be 1. If the window material is not listed and its properties are not known, a sample from each batch, heat or lot shall be proof tested using the above equation where the stress ratio is assumed to be 1. A different maximum design pressure shall be determined for each material heat, batch, or lot.
Documentation for thin windows (design calculations, material certifications, test data, etc.) shall be maintained by the Responsible Vacuum Engineer. A Vacuum System folder within the Pressure Systems webpage is available for documentation storage.
Windows that are shaped by hydro-forming or some other process can often be safely made from much thinner material than a corresponding flat window of the same diameter. Typically, a thin flat disc of Aluminum is hydro-formed into a predetermined spherical shape. A typical hydro-forming pressure is two to three times the usual operating pressure (14.7 psi). This pressure is necessary to yield the material into the desired shape and has the benefit of an inherent overpressure test. Many hydro-formed windows exist at Jefferson Lab - typically made from Al 2024 Alclad which is available in a half soft state and has a large elongation and a moderately high yield and ultimate strength. It is recommended that only 30% of the available elongation be used so that adequate reserve remains in the window to provide safety against foreign object penetration. The thin window stock or precut blanks shall be carefully inspected prior to use to ensure that there are no defects, deep scratches or wrinkles that could easily compromise the strength of these thin materials.
Windows that are formed in situ are simply thin pre-tested materials that are highly deformed during the initial evacuation. Examples of thin windows of this type are the spectrometer entrance and exit windows that are made from Mylar-Kevlar-Mylar laminates and the Aluminum (5054) or Kapton scattering chamber windows. These windows shall be tested to determine that, after the initial deformation, an adequate safety margin exists. Alternatively an Elastic-Plastic analysis may be performed, instead of a test, using an accurate model of the material. Once these windows are tested, replacement windows may be made from the same heat, batch, or lot of material used to make the test window. All new material stocks without a certified material test report (CMTR) shall be tested to verify that the material meets all design requirements. A proof test, as described in Part 7: Section 7.7.2.3 Proof Test, meets this requirement. Experience at Jefferson Lab shows that laminated window materials can vary substantially with each batch while metal window materials have always been consistent within the normal range of material specifications.
With the exception of formed in situ windows, each thin window shall be tested either during a forming process or during a dedicated pressure test. The test pressure shall be greater than 110% of the design pressure (or MAWP). Windows made from material where tear propagation may be a factor should be puncture tested to determine the extent of this affect.
Materials, supplied in lots, batches or heats, to be used for formed windows shall be tested to verify the ultimate strength; a CMTR supplied by the material manufacturer is acceptable. These windows are often formed with pressures exceeding the required test pressure. If such is the case, this forming may be substituted for the final pressure test of the window.
Reasonable efforts to protect thin windows from accidental damage shall be performed. Such efforts include both engineering and administrative controls. Examples include covers for thin windows that are temporarily installed on scattering chamber windows and removed just prior to the start of a run period. Some thin windows installed on vessels with a high stored energy and having large aperture may require remotely operated covers that are moved into position prior to personnel access into a given area where the thin window resides. Examples of this type of control include the HMS exit vacuum window. Proper PPE such as safety glasses and hearing protection may be required when working near thin windows covering large volumes.
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ISSUING AUTHORITY |
SUPPLEMENT AUTHOR |
APPROVAL DATE |
REVIEW DATE |
REV. |
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QA/CI Dept. |
11/06/15 |
11/06/20 |
1.0 |
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