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Pressure and Vacuum Systems Safety Supplement Part
4: Overpressure Protection |
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This section describes the requirements for overpressure protection for pressure systems and components. In this section, the term “relief device” shall apply only to those relief devices providing overpressure protection. The requirements of this section do not apply to devices serving other functions. The system design must account for all reasonable sources of overpressure and mitigate them through the use of relief devices, system design, or other suitable method. Excellent guidance for this is given in API 520 and API 521. The DA is strongly encouraged to consult these references when designing system overpressure protection.
For systems and components with design pressures (or MAWP) higher than 15 psi, the DA shall determine by calculation and analysis all credible causes of overpressure (see Part 4: Section 4.2 Potential Causes of Overpressure) by performing the following:
· Analysis of possible failure modes and their effects
o This can be a simple analysis or a more formalized analysis such as a Failure Mode, Effects, and Criticality Analysis (FMECA) or Fault Tree Analysis. The DA shall determine the extent of analysis required.
· Calculations determining the justification for the relief capacity required for each failure mode
o The capacity shall be adequate to ensure the accumulated pressure during a relief event does not exceed that which is allowable by the applicable code(s).
· Consider discharge effects (if applicable) such as
o Reaction forces from relief loads
o Venting to safe locations
o Environmental factors and fluid properties (e.g. relief of toxic fluids at ground level)
For Excluded Elements and Vessels, with design pressures (MAWP) higher than 15 psi, overpressure protection requirements shall be as given in ASME BPVC VIII D1.
The overpressure protection analysis shall undergo a Technical Review as described in Part 2: Section 5 Reviews. It shall also be maintained in the appropriate folder in the Pressure Systems database.
Relief devices protecting vacuum components from pressurization above 15 psi (Category 2 and 3) shall meet the requirements of Part 7: Section 7.5.1 Pressure Relief and this section.
This section describes some of the principle causes of overpressure in pressure systems (piping and vessels). Overpressure results from a disruption or unbalance of the flow of fluid or energy that causes the buildup of fluid, energy, or both in all or part of a system. Below is a partial list of potential causes of overpressure.
· Operator Error: Operator error is the most common cause of system overpressure
· Closed outlets on vessels
· Inadvertently closed or opened block valves
· Check valve leakage:
· Utility failure: power or water failure
· Electrical or mechanical failure
· Loss of cooling or heating
· Heat exchanger tube failure
· Transient pressure surges: surges such as water hammer and pressure waves resulting from fast acting valves.
· Combustion or explosion of system fluid
· External fire
· Loss of insulting vacuum on vessels or piping containing cryogenic fluids
· Blocked or partially obstructed vent lines
For relief devices installed in exterior environments, the following shall be considered:
· It is recommended that all relief devices installed outdoors be equipped with bug screens.
· The exhaust of relief devices shall be protected from rain, snow, and ice
· Relief devices exposed to rain or snow should be made from materials compatible with water exposure
The DA shall determine the largest credible relief capacity required and size relief devices appropriately. The maximum allowable accumulation for a pressure component shall be determined by the most applicable Code. For Excluded Elements and Vessels, with design pressures (MAWP) higher than 15 psi, overpressure protection requirements shall be as given in ASME BPVC VIII D1. Relief devices shall be sized such that the total capacity of all relief devices is sufficient for the calculated mass flow. The relief path shall also be analyzed for adequate capacity and pressure drop.
Exposure of a pressure component to external fire can result in extremely large relief loads. It is further expected that prolonged exposure to a fire will result in vessel or piping failure regardless of any reasonable protective measure. Excellent guidance for exposure of pressure equipment to liquid hydrocarbon pool fires is given in API 521. This type of fire represents the highest credible heat flux for fires of any type (with the exception of metallic combustion). Sizing of reliefs for this type of fire shall be considered acceptable practice for any fire exposure. The Jefferson Lab Fire Protection Engineer shall provide guidance for any design where the hydrocarbon pool fire is considered inappropriate or excessive.
Fire mitigation may also be provided by preventative measures. These measures may include the following:
· Adequate drainage, dikes, etc. to prevent hydrocarbon fuels from pooling near or under vessels and piping
· Removal of brush from areas near vessels and piping
· Elimination or limitation of flammable material near vessels or piping
· Installation of fire suppression systems
· Insulation and fireproofing installed on vessels and piping
Such measures may obviate sizing of reliefs for fire conditions.
It is Jefferson Lab policy that piping of size NPS 6 or less need not be protected from overpressure due to fire if the system fluid is not Category M.
It is common practice to insulate piping and vessels containing cryogenic fluids with a combination of vacuum and single and multi-layer insulation (MLI or super-insulation). This is achieved by separating an inner fluid space and a surrounding vacuum insulation space by an inner pressure boundary. Loss of vacuum in the insulation space, due to a failure of a component, operator error, etc., will subject the inner pressure boundary to a high temperature gradient between the fluid space and the vacuum space. This high temperature gradient can cause rapid boil-off of the cryogenic fluid resulting in a rapid pressure rise in the inner fluid space. Failure of the vacuum space shall be considered for piping and vessels containing cryogenic fluids insulated in this manner.
The DA shall ensure that adequate relief capacity is installed on the system to prevent components from overpressure beyond Code limits (see ASME BPVC UG-125 (c)(3)). Determination of the adequacy of this relief depends on many factors (see references below):
· Fluid properties and chemistry
o Boiling temperature and pressure
o Heat transfer within the fluid and thermal conductivity
o Latent heat
o Densities etc.
· Geometry of piping and or vessels
· Insulation
o Type of insulation
o Thickness of insulation
· Temperature gradient across insulation and internal pressure boundary
The determination of the heat transfer rate is critical to calculating the required relief capacity. Heat transfer rates between 25 and 40 kW/m2 have been used for bare metal surfaces. Rates for insulated surfaces range from 1 to 7 kW/m2. The DA shall employ sound engineering judgement to determine an appropriate heat transfer model. The DA is encouraged to collaborate with other DAs experienced on this subject.
The insulating vacuum space must also be relieved. This may be accomplished by commercial or Jefferson Lab fabricated devices. Guidance for the relief capacity required for insulating vacuum spaces is given in the Compressed Gas Association CGA S-1.3.
Loss of insulating vacuum on high temperature surfaces (e.g. vacuum induction furnaces) may result in irreversible catastrophic system failure. Due to the unique nature of these systems, they shall only be operated under an OSP/TOSP by personnel trained explicitly on the individual system.
Barron: Cryogenic Heat Transfer (1999)
Collier: Convective Boiling and Condensation (1972)
Harrison: Loss of vacuum experiment on superfluid helium vessel. (2001)
Johnson: Thermal performance of cryogenic insulation (2007)
Lehman: Safety Aspects for the LHE Cryostats and LHE Transport Containers (1978)
Lienhard and Lienhard: A Heat Transfer Textbook.
Weisend: Cryogenic Engineering (1998)
Wiseman: Loss of cavity vacuum experiment at CEBAF. (1991)
Relief devices generally have two forms: reclosing (such as a spring loaded safety relief valve) and non-reclosing (e.g. rupture disk). Requirements for relief devices depend on the date of installation of the device and the set pressure of the device.
Relief devices installed (either for new construction or replacement of an existing valve) on or after 1 March 2008 with a set pressure at or above 15 psi shall comply with the following:
· All relief devices providing overpressure protection shall comply with the most applicable section of the ASME BPVC unless approved otherwise by the Pressure Systems Committee Chair. For systems where no section of the ASME BPVC is applicable, relief devices shall comply with ASME BPVC VIII D1.
· Relief devices providing overpressure protection for pressure vessels (ASME or otherwise), Category M fluid systems (regardless of application), and piping larger than 6 NPS shall be registered as described in Part 4: Section 4.7 Relief Device Registration.
Relief devices that were installed or in the process of being installed on unaltered systems prior to 1 March 2008 with a set pressure at or above 15 psi shall comply with the following:
· All devices providing overpressure protection for ASME Section I and IV pressure vessels and on all vessels servicing Building Heating Water, Building Chilled Water, Compressed Air, and Instrument (Control) Air comply with the appropriate section of the ASME BPVC edition in effect at the time of vessel installation or edition in effect at the time of the device replacement.
· All devices providing overpressure protection for ASME Section VIII vessels, not considered boilers, are either CE/PED or ASME stamped and installed in accordance with the ASME BPVC code edition in effect at the time of vessel installation. If an existing relief device is replaced, the replacement shall comply with the edition of the ASME BPVC VIII in effect at the time of vessel installation or the edition in effect at the time of device replacement.
· All devices providing overpressure protection for pressure system piping are of reputable manufacture and are either ASME stamped, CE/PED certified, or non-certified. If an existing relief device is replaced, the replacement shall comply with the edition of the ASME BPVC VIII in effect at the time of device replacement.
· All devices providing overpressure protection for Excluded Vessels are of reputable manufacture and are either ASME stamped, CE/PED certified, or non-certified. If an existing relief device is replaced, the replacement shall comply with the edition of the ASME BPVC VIII in effect at the time of device replacement.
Relief devices with a set pressure below 15 psi are not required to comply with the full requirements of the ASME BPVC. These devices may be procured or manufactured whole or in part on or off site. Relief devices, with set pressures less than 30 psi, protecting vacuum jackets for cryogenic piping less than or equal to 6 NPS are also not required to comply with the full requirements of the ASME BPVC.
Devices shall be procured from reputable manufacturers. The capacity of the device shall be determined by either the manufacturer or the DA through suitable calculations and/or testing. The set pressure of any reclosing device shall be verified by test (see Part 9: Section 9.3 Procedures for Operational Inspection and Testing of Relief Devices). Devices protecting insulating vacuum spaces from pressurization from condensed and frozen gasses (i.e. CVI vacuum pump outs etc.) are exempt from this requirement if the DA is assured of their effectiveness.
Design and fabrication of these devices shall comply with the requirements of this Supplement. The capacity of the relief device shall be determined by calculations and/or suitable tests that comply with ASME BPVC VIII D1 UG-131, Appendix 11, ASME PTC 25, API 520, API 521, or other applicable method. The set point of any reclosing device shall be determined by pressure test (see Part 9: Section 9.3 Procedures for Operational Inspection and Testing of Relief Devices).
Unless measured by suitable technique, the maximum coefficient of discharge (as defined in ASME BPVC VIII D1 UG-131) shall be no greater than Kd = 0.62 for parallel plate relief valves.
Relief devices shall be installed by qualified and experienced technicians. Block or stop valves may be installed upstream of a relief device provided the requirements of ASME BPVC VIII D1 Appendix M (or equivalent) are met. Such requirements include operating procedures and the installation locks or anti-tampering devices on the valves. In all cases, direction of discharge shall be considered in regards to safety to personnel and equipment.
A Pressure Relief Device Data Sheet (Form PS-5) shall be completed by the DA, and sent to the Vessel Inspection Coordinator, for any relief device providing overpressure protection for any of the following components or systems
· Pressure vessels (ASME Boilers, ASME Pressure Vessels, and Excluded Vessels)
· All Category M fluid components
· Piping larger than NPS 6, installed on or after 1 March 2008.
Exceptions to this requirement are:
· Relief devices with set points less than 15 psi.
· Relief devices on any system where all of the following criteria are met
o The maximum system pressure cannot exceed 15 psid (pounds per square inch differential) at any time including all credible failure modes
o The system fluids are nonflammable, nontoxic, and not harmful to human tissue (as described in ASME B31.3 Para 300.2)
o The system design temperature is greater than -20F and less than 366F
The forms for all relief devices installed on a pressure system shall be maintained in the "Overpressure Protection" folder in the system documentation folder.
Overpressure protection by system design (see ASME BPVC VIII D1 UG-140) is recommended in situations where release of the system fluid may be hazardous to humans or the environment, where there are no credible sources of overpressure, or when traditional relief devices are not effective. If the system fluid is water with a maximum design temperature less than 130 oF and the system does not include a pressure vessel, there are no further requirements and a report is not necessary. In cases where overpressure protection by system design is chosen by the DA for new pressure equipment with a design initiated on or after January 4, 2016, the following requirements shall be met.
A formal report shall be written addressing the following:
· The reason for using overprotection by system design.
· A detailed failure analysis (such as Failure Modes Effects and Criticality Analysis (FMECA) or Fault Tree Analysis) shall be performed by a multi-disciplinary team, to determine all credible failure modes.
· A detailed analysis to determine the maximum credible pressure that must be contained by the system shall be documented in the report.
· The requirements for periodic inspections and testing of controls (both hardware and soft or firm ware), procedures, and instrumentation used for overpressure protection
Form PS-2 shall be completed by the DA, submitted for approval by the Pressure Systems Committee Chair as provided by Part 4: Section 4.8.2 Approvals below and filed in the Overpressure Protection folder for the pressure system.
For all new ASME Pressure Vessels, the manufacturer data report (Form U-1 or U-1A for Div 1 vessels) shall indicate that overpressure protection is by system design. Existing pressure vessels may be used in systems where overpressure protection is by design provided written approval from the Pressure System Committee Chair is obtained. All other requirements of the code of construction shall be met (i.e. ASME BPVC VIII D1 UG-140). This documentation shall be maintained in the Overpressure Protection folder for the pressure system.
All systems, unless excepted below, where overpressure protection is by system design shall require formal written approval of the Pressure Systems Committee Chair prior to operation. Form PS-2 shall be reviewed and signed by the Pressure Systems Committee Chair. The following systems are excepted from this approval requirement:
· ASME B31.3 Category D Service Piping
· ASME B31.5 Piping
· ASME B31.9 Piping
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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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