For decades, we used Appendix M-5 in Section VIII as our design basis for having intervening/stop valves before and/or after a Pressure Relief Device (PRD). But in 2021 (amid the global pandemic), ASME published its new Section XIII, Rules for Overpressure Protection. We now use Appendix B of this new ASME section when we have a design basis using “stop valves” (or sometimes called “intervening valves”) either before or after the PRD.
This article is a breakdown of the new Appendix and any new requirements from the old Appendix M-5.
APPENDIX B of ASME Section XIII – OVERPRESSURE PROTECTION guides the installation and use of stop valves in pressure relief systems. The equipment’s code or standard determines the use of stop valves in pressure relief systems. Where their use is permitted, stop valves shall be installed per the equipment’s code or standard unless the code or standard has also adopted by reference Appendix B. In addition, the use of stop valves may involve jurisdictional approval. Where a code, standard, or jurisdictional authority does NOT address the use of stop valves, the guidance in this Appendix may be used.
STOP VALVES LOCATED IN THE RELIEF PATH
Stop valves may be located within the relief path provided for in B-6 through B-9:
- B-6 STOP VALVES IN SYSTEMS WITH PRESSURE FROM AN OUTSIDE SOURCE
- B-7 STOP VALVES UPSTREAM OR DOWNSTREAM OF THE PRESSURE RELIEF DEVICE
- B-8 STOP VALVES IN THE PRESSURE RELIEF PATHS HANDLING PROCESS FLOW
- B-9 STOP VALVES IN THE RELIEF PATHS WHERE FIRE IS SOURCE OF OVERPRESSURE
but ONLY when specified by the user. The user’s responsibilities are summarized in the RESPONSIBILITIES section below. The specific requirements in B-6 through B-9 are NOT intended to allow normal operation above the pressurized equipment’s maximum allowable working pressure (MAWP).
The pressure relief path shall be designed so that the pressure in the protected equipment does NOT exceed its MAWP before the pressure at the pressure relief device reaches its set pressure and the pressure does not exceed the pressurized equipment’s maximum permissible relief pressure of the governing code.
DEFINITIONS
Administrative controls: procedures that, in combination with mechanical locking elements, are intended to ensure that personnel actions do not compromise the overpressure protection of the equipment. They include, as a minimum, documented operation and maintenance procedures, and the training of operator and maintenance personnel in these procedures.
Full-area stop valve: a valve in which the flow area of the valve is equal to or greater than the inlet flow area of the pressure relief device.
Management system: the collective application of administrative controls, valve operation controls, and valve failure controls in accordance with the applicable requirements of this Section.
Mechanical locking elements: elements that when installed on a stop valve provide a physical barrier to the operation of the stop valve such that the stop valve is not capable of being operated unless a deliberate action is taken to remove or disable the element. Such elements, when used in combination with administrative controls, ensure that the equipment overpressure protection is not compromised by personnel actions. Examples of mechanical locking elements include locks (with or without chains) on the stop valve handwheels, levers, or actuators, and plastic or metal straps (car seals) that are secured to the valve in such a way that the strap must be broken to operate the stop valve.
Pressure relief path: path consisting of all equipment, pipe, fittings, and valves in the flow path between any protected equipment and its pressure-relieving device, and between the pressure-relieving device and the discharge point of the relieving stream. Stop valves within a pressure relief path include, but are not limited to, those located directly upstream and downstream of the pressure relief device (PRD) that may be provided exclusively for PRD maintenance.
Valve failure controls: measures taken in valve design, configuration, and/or orientation for the purpose of preventing an internal failure of a stop valve from closing and blocking the pressure relief path. An example of valve failure controls is the installation of gate valves with the valve stem oriented at or below the horizontal position.
Valve operation controls: devices used to ensure that stop valves within the pressure relief path are in their proper (open/closed) position. They include the following:
- (a) mechanical interlocks designed to prevent valve operations that could result in the blocking of a pressure relief path before an alternative pressure relief path is put into service. Mechanical interlocks include physical linkages such as shafts or levers between stop valves and key-based interlocking systems.
- (b) instrumented interlocks that function in a way similar to mechanical interlocks, except that instrument permissives and/or overrides are used instead of mechanical linkages or devices to prevent valve positions that block the pressure relief path.
- (c) three-way valves designed to prevent a flow path from being blocked unless another flow path is simultaneously opened.
RESPONSIBILITIES
The user is responsible for establishing and maintaining a management system that ensures a vessel is NOT operated without overpressure protection. These responsibilities include, but are not limited to, the following:
- (a) deciding and specifying whether the overpressure protection system will allow the use of stop valve(s) located in the relief path
- (b) establishing the pressure relief philosophy and the administrative control requirements
- (c) establishing the required level of reliability, redundancy, and maintenance of instrumented interlocks, if used
- NOTE: The procedures contained in IEC 61508 or ISA S-84 may be used for the purpose and analysis described in (c).
- (d) establishing procedures to ensure that the equipment is adequately protected against overpressure
- (e) ensuring that authorization to operate identified valves is clear and that personnel are adequately trained for this task
- (f) establishing management systems to ensure that administrative controls are effective
- (g) establishing the analysis procedures and basis to be used in determining the potential levels of pressure if the stop valves were closed
- (h) ensuring that the analysis described in (g) is conducted by personnel who are qualified and experienced with the analysis procedure
- (i) ensuring that the other system components are acceptable for the potential levels of pressure established in (g)
- (j) ensuring that the results of the analysis described in (g) are documented and are reviewed, and accepted in writing by the individual responsible for the operation of the vessel and valves
- (k) ensuring that the administrative controls are reviewed and accepted in writing by the individual responsible for the operation of the vessel and valves
PROCEDURES AND MANAGEMENT SYSTEMS
Procedures shall specify that valves requiring mechanical locking elements and/or valve operation controls and/or valve failure controls shall be documented and clearly identified as such. The management system shall document:
- the administrative controls (training and procedures),
- the valve controls, and
- the performance of the administrative controls in an auditable form for management review.
STOP VALVES UPSTREAM OR DOWNSTREAM OF THE PRESSURE RELIEF DEVICE
Full-area stop valves may be provided upstream AND/OR downstream of the pressure relieving device (PRD) exclusively for inspection, testing, and repair of the PRD or discharge header isolation, provided that, as a minimum, the following requirements are met:
- (a) Administrative controls are provided to prevent unauthorized valve operation.
- (b) Valves are provided with mechanical locking elements.
- (c) Valve failure controls are provided to prevent accidental valve closure due to mechanical failure.
- (d) Procedures are in place to provide pressure relief protection when the system is isolated from its pressure relief path. These procedures shall ensure that when the system is isolated from its pressure relief path
- an authorized person shall continuously monitor the vessel’s pressure conditions and
- shall be capable of responding promptly with documented, predefined actions, either stopping the source of overpressure or opening alternative means of pressure relief.
This authorized person shall be dedicated to this task and shall have NO OTHER DUTIES WHEN PERFORMING THIS TASK.
- (e) The system shall be isolated from its pressure relief path ONLY for the time required to
- test,
- repair, and/or
- replace the PRD
STOP VALVES IN THE PRESSURE RELIEF PATHS HANDLING PROCESS FLOW
Stop valves, excluding remotely operated valves and process control valves, may be provided in the relief path where there is normally a process flow, provided the requirements in (a) through (c), as a minimum, are met.
These requirements are based on the potential over-pressure scenarios involving accidental closure of a single stop valve within the relief path. The accidental closure of these stop valves in the pressure relief system need not be considered as a condition for establishing the pressurized equipment’s design pressure in accordance with the referencing Code or Standard.
- (a) The flow resistance of the valve in the full open position does not reduce the relieving capacity below that required by the requirements of the referencing Code or Standard.
- (b) The closure of the valve will be readily apparent to the operators such that corrective action, in accordance with documented operating procedures, is required.
- (c) One of the following conditions shall be met:
- (1) If the pressure due to closure of the valve cannot exceed the multiple relief device maximum allowed relief pressure, such as 116% of MAWP for Section VIII, Division 1 or Division 2 vessels, then NO administrative controls, mechanical locking elements, valve operation controls, or valve failure controls are required.
- (2) If the pressure due to closure of the valve cannot exceed the value calculated in (-a) or (-b), then administrative controls and mechanical locking elements are required at a minimum.
- (-a) the documented vessel test pressure multiplied by the ratio of the stress value at the design temperature to the stress value at the test temperature
- (-b) if the governing code or standard permits the vessel test pressure to be calculated on a value greater than the vessel design pressure, then, in addition to being multiplied by the ratio in (-a), the test pressure shall be multiplied by the ratio of nominal thickness minus the corrosion allowance to the nominal thickness
- (3) If the pressure due to closure of the valve could exceed the pressure indicated in (2), then the user shall do one of the following:
- (-a) Eliminate the stop valve.
- (-b) Apply administrative controls, mechanical locking elements, valve failure controls, and valve operation controls.
- (-c) Provide a pressure relief device to protect the equipment that could be over pressured due to the closure of the stop valve.
STOP VALVES IN THE RELIEF PATHS WHERE FIRE IS SOURCE OF OVERPRESSURE
Full-area stop valves located in the relief path of equipment where there is normally process flow and where fire is the ONLY potential source of overpressure do not require physical elements such as locks or car seals, valve operation controls, or valve failure controls, provided the user has documented operating procedures requiring that equipment isolated from its pressure relief path is depressurized and free of liquids.