2010 SECTION VIII — DIVISION 1 NONMANDATORY APPENDIX M – INSTALLATION AND OPERATION

M-1 INTRODUCTION

M-2 CORROSION

M-3 MARKING ON THE VESSEL

M-4 PRESSURE RELIEVING DEVICES

M-5 STOP VALVES LOCATED IN THE RELIEF PATH

M-6 INLET PRESSURE DROP FOR HIGH LIFT, TOP GUIDED SAFETY, SAFETY RELIEF, AND PILOT OPERATED PRESSURE RELIEF
VALVES IN COMPRESSIBLE FLUID SERVICE

M-7 DISCHARGE LINES FROM PRESSURE RELIEF DEVICES

M-8 PRESSURE DROP, NONRECLOSING PRESSURE RELIEF DEVICES

M-9 GENERAL ADVISORY INFORMATION ON THE CHARACTERISTICS OF PRESSURE RELIEF DEVICES DISCHARGING INTO A COMMON HEADER

M-10 PRESSURE DIFFERENTIALS FOR PRESSURE RELIEF VALVES

M-11 INSTALLATION OF SAFETY AND SAFETY RELIEF VALVES

M-12 REACTION FORCES AND EXTERNALLY APPLIED LOADS

M-13 SIZING OF PRESSURE RELIEF DEVICES FOR FIRE CONDITIONS

M-14 PRESSURE INDICATING DEVICE

 

M-1 INTRODUCTION

(a) The rules in this Appendix are for general information only, because they pertain to the installation and operation of pressure vessels, which are the prerogative and responsibility of the law enforcement authorities in those states and municipalities which have made provision for the enforcement of Section VIII.

(b) It is permissible to use any departures suggested herein from provisions in the mandatory parts of this Division when granted by the authority having legal jurisdiction over the installation of pressure vessels.

 

M-2 CORROSION

(a) Vessels subject to external corrosion shall be so installed that there is sufficient access to all parts of the exterior to permit proper inspection of the exterior, unless adequate protection against corrosion is provided or unless the vessel is of such size and is so connected that it may readily be removed from its permanent location for inspection.

(b) Vessels having manholes, handholes, or cover plates to permit inspection of the interior shall be so installed that these openings are accessible.

(c) In vertical cylindrical vessels subject to corrosion, to insure complete drainage, the bottom head, if dished, should preferably be concave to pressure.

 

M-3 MARKING ON THE VESSEL

The marking required by this Division shall be so located that it will be accessible after installation and when installed shall not be covered with insulation or other material that is not readily removable [see UG-116(j)].

 

M-4 PRESSURE RELIEVING DEVICES

The general provisions for the installation of pressure relieving devices are fully covered in UG-135. The following paragraphs contain details in arrangement of stop valves for shutoff control of pressure relief devices which are sometimes necessary to the continuous operation of processing equipment of such a complex nature that the shutdown of any part of it is not feasible. There are also rules with regard to the design of inlet and discharge piping to and from pressure relief devices, which can only be general in nature because the design engineer must fit the arrangement and proportions of such a system to the particular requirements in the operation of the equipment involved.

 

M-5 STOP VALVES LOCATED IN THE RELIEF PATH

M-5.1 General

(a) Stop valve(s) located within the relief path is not allowed except as provided for in M-5.5, M-5.6, M-5.7, and M-5.8, and only when specified by the user. The responsibilities of the user are summarized in M-5.3. The specific requirements in M-5.5, M-5.6, M-5.7, and M-5.8 are not intended to allow for normal operation above the maximum allowable working pressure.

(b) The pressure relief path shall be designed such that the pressure in the equipment being protected does not exceed its maximum allowable working pressure before the pressure at the pressure relief device reaches its set pressure and the pressure does not exceed the limits of UG-125(c).

M-5.2 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 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 larger 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 Division.

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 adminstrative 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: consists 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: measure 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:

• mechanical interlocks which are designed to prevent valve operations which could result in the blocking of a pressure relief path before an alternative pressure relief path is put into service. 
• instrumented interlocks which function similar to mechanical interlocks, except that instrument permissives and/or overrides are used instead of mechanical linkages/devices to prevent valve positions that block the pressure relief path.
• three-way valves designed to prevent a flow path from being blocked without another flow path being simultaneously opened.

M-5.3 Responsibilites

The User has the responsibility to establish and maintain a management system that ensures a vessel is not operated without overpressure protection. These responsibilities include, but are not limited to, the following:

1. Deciding and specifying if the overpressure protection system will allow the use of stop valve(s) located in the relief path. 
2. Establishing the pressure relief philosophy and the administrative controls requirements. 
3. Establishing the required level of reliability, redundancy, and maintenance of instrumented interlocks, if used. 
4. Establishing procedures to ensure that the equipment is adequately protected against overpressure.
5. Ensuring that authorization to operate identified valves is clear and that personnel are adequately trained for this task.
6. Establishing management systems to ensure that administrative controls are effective.
7. Establishing the analysis procedures and basis to be used in determining the potential levels of pressure if the stop valve(s) were closed.
8. Ensuring that the analysis described in M-5.3(g) is conducted by personnel who are qualified and experienced with the analysis procedure.
9. Ensuring that the other system components are acceptable for the potential levels of pressure established in M-5.3(g).
10. Ensuring that the results of the analysis described in M-5.3(g) are documented and are reviewed and accepted in writing by the individual responsible for operation of the vessel and valves.
11. Ensuring that the administrative controls are reviewed and accepted in writing by the individual responsible for operation of the vessel and valves.

NOTE: The procedures contained in ISA S-84, “Application of Safety Instrumented Systems for the Process Industries,” or IEC 61508, “Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems,” may be used for this purpose and analysis.

M-5.4 Requirements of Procedures/Management System

(a) 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.
(b) 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.

M-5.5 Stop Valves Provided in Systems for Which the Pressure Originates Exclusively From an Outside Source

A vessel or system [see UG-133(c)] for which the pressure originates from an outside source exclusively may have individual pressure relieving devices on each vessel, or connected to any point on the connecting piping, or on any one of the vessels to be protected. Under such an arrangement, there may be stop valve(s) between any vessel and the pressure relieving devices, and these stop valve(s) need not have any administrative controls, valve operation controls, or valve failure controls, provided that the stop valves also isolate the vessel from the source of pressure.

M-5.6 Stop Valve(s) Provided Upstream or Downstream of the Pressure Relief Device Exclusively for Maintenance of That Device

Full area stop valve(s) may be provided upstream and/or downstream of the pressure relieving device for the purpose of inspection, testing, and repair of the pressure relieving device or discharge header isolation, provided that, as a minimum, the following requirements are complied with: 

1. Administrative controls are provided to prevent unauthorized valve operation.
2. Valves are provided with mechanical locking elements.
3. Valve failure controls are provided to prevent accidental valve closure due to mechanical failure.
4. Procedures are in place to provide pressure relief protection during the time 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 pressure conditions of the vessel and shall be capable of responding promptly with documented, pre-defined 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.
5. The system shall be isolated from its pressure relief path only for the time required to test, repair, and or replace the pressure relief device.

M-5.7 Stop Valve(s) Provided in the Pressure Relief Path Where There Is Normally Process Flow

Stop valve(s), excluding remotely operated valves, may be provided in the relief path where there is normally a process flow, provided the requirements in M-5.7(a) and (b), as a minimum, are complied with. These requirements are based on the potential overpressure scenarios involving accidental closure of a single stop valve within the relief path [see M-5.3(g)]. The accidental closure of these stop valve(s) in the pressure relief system need not be considered in setting the design pressure per UG-21.

(a) The flow resistance of the valve in the full open position does not reduce the relieving capacity below that required by the rules of this Division.

(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, and (1) if the pressure due to closure of the valve can not exceed 116% of MAWP, then no administrative controls, mechanical locking elements, valve operation controls, or valve failure controls are required, or (2) if the pressure due to closure of the valve can not exceed the following:

1. the documented test pressure, multiplied by the ratio of the stress value at the design temperature to the stress value at the test temperature, or  
2. if the test pressure is calculated per UG-99(c) in addition to the ratio in M-5.7(b)(2)(a), the test pressure shall also be multiplied by the ratio of the nominal thickness minus the corrosion allowance to the nominal thickness then, as a minimum, administrative controls and mechanical locking elements are required, or
3. if the pressure due to closure of the valve could exceed the pressure in M-5.7(b)(2), then the user shall either  

• • eliminate the stop valve, or 
  • apply administrative controls, mechanical locking elements, valve failure controls, and valve operation controls, or
  • provide a pressure relief device to protect the equipment that could be overpressured due to closure of the stop valve

M-5.8 Stop Valves Provided in the Relief Path of Equipment Where There Is Normally Process Flow and Where Fire Is the Only Potential Source of Overpressure

Full area stop valve(s) 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 depressured and free of liquids.

 

M-6 INLET PRESSURE DROP FOR HIGH LIFT, TOP GUIDED SAFETY, SAFETY RELIEF, AND PILOT OPERATED PRESSURE RELIEF
VALVES IN COMPRESSIBLE FLUID SERVICE

(a) The nominal pipe size of all piping, valves and fittings, and vessel components between a pressure vessel and its safety, safety relief, or pilot operated pressure relief valves shall be at least as large as the nominal size of the device inlet, and the flow characteristics of the upstream system shall be such that the cumulative total of all nonrecoverable inlet losses shall not exceed 3% of the valve set pressure. The inlet pressure losses will be based on the valve nameplate capacity corrected for the characteristics of the flowing fluid.

(b) When two or more required safety, safety relief, or pilot operated pressure relief valves are placed on one connection, the inlet internal cross-sectional area of this connection shall be either sized to avoid restricting flow to the pressure relief valves or made at least equal to the combined inlet areas of the safety valves connected to it. The flow characteristics of the upstream system shall meet the requirements of (a) above with all valves relieving simultaneously.

 

M-7 DISCHARGE LINES FROM PRESSURE RELIEF DEVICES

(a) Where it is feasible, the use of a short discharge pipe or vertical riser, connected through long-radius elbows from each individual device, blowing directly to the atmosphere, is recommended. Such discharge pipes shall be at least of the same size as the valve outlet. Where the nature of the discharge permits, telescopic (sometimes called “broken”) discharge lines, whereby condensed vapor in the discharge line, or rain, is collected in a drip pan and piped to a drain, are recommended.1

(b) When discharge lines are long, or where outlets of two or more devices having set pressures within a comparable range are connected into a common line, the effect of the back pressure that may be developed therein when certain devices operate must be considered [see UG-135(f)]. The sizing of any section of a common-discharge header downstream from each of the two or more pressure relieving devices that may reasonably be expected to discharge simultaneously shall be based on the total of their outlet areas, with due allowance for the pressure drop in all downstream sections. Use of specially designed devices suitable for use on high or variable back pressure service should be considered.

(c) The flow characteristics of the discharge system of high lift,top guided safety, safety relief, or pilot operated pressure relief valves in compressible fluid service shall be such that the static pressure developed at the discharge flange of a conventional direct spring loaded valve will not exceed 10% of the set pressure when flowing at stamp capacity. Other valve types exhibit various degrees of tolerance to back pressure and the manufacturer’s recommendation should be followed.

(d) All discharge lines shall be run as direct as is practicable to the point of final release for disposal. For the longer lines, due consideration shall be given to the advantage of long-radius elbows, avoidance of closeup fittings, and the minimizing of excessive line strains by expansion joints and well-known means of support to minimize line-sway and vibration under operating conditions.

(e) Provisions should be made in all cases for adequate drainage of discharge lines.

NOTE: It is recognized that no simple rule can be applied generally to fit the many installation requirements, which vary from simple short lines that discharge directly to the atmosphere to the extensive manifold discharge piping systems where the quantity and rate of the product to be disposed of requires piping to a distant safe place.

 

M-8 PRESSURE DROP, NONRECLOSING PRESSURE RELIEF DEVICES
Piping, valves and fittings, and vessel components comprising part of a nonreclosing device pressure relieving system shall be sized to prevent the vessel pressure from rising above the allowable overpressure.

 

M-9 GENERAL ADVISORY INFORMATION ON THE CHARACTERISTICS OF PRESSURE RELIEF DEVICES DISCHARGING INTO A COMMON HEADER
Because of the wide variety of types and kinds of pressure relief devices, it is not considered advisable to attempt a description in this Appendix of the effects produced by discharging them into a common header. Several different types of pressure relief devices may conceivably be connected into the same discharge header and the effect of back pressure on each type may be radically different. Data compiled by the manufacturers of each type of pressure relief device used should be consulted for information relative to its performance under the conditions anticipated.

 

M-10 PRESSURE DIFFERENTIALS FOR PRESSURE RELIEF VALVES
Due to the variety of service conditions and the various designs of safety and safety relief valves, only general guidance can be given regarding the differential between the set pressure of the valve (see UG-134) and the operating pressure of the vessel. Operating difficulty will be minimized by providing an adequate differential for the application. The following is general advisory information on the characteristics of the intended service and of the safety or safety relief valves that may bear on the proper pressure differential selection for a given application. These considerations should be reviewed early in the system design since they may dictate the MAWP of the system.

(a) Consideration of the Process Characteristics in the Establishment of the Operating Margin to Be Provided.  To minimize operational problems, it is imperative that the user consider not only normal operating conditions of fluids, pressures, and temperatures, but also start-up and shutdown conditions, process upsets, anticipated ambient conditions, instrument response times, pressure surges due to quick closing valves, etc. When such conditions are not considered, the pressure relieving device may become, in effect, a pressure controller, a duty for which it is not designed. Additional consideration should be given to hazard and pollution associated with the release of the fluid. Larger differentials may be appropriate for fluids which are toxic, corrosive, or exceptionally valuable.

(b) Consideration of Safety Relief Valve Characteristics. The blowdown characteristic and capability is the first consideration in selecting a compatible valve and operating margin. After a self-actuated release of pressure, the valve must be capable of reclosing above the normal operating pressure. For example, if the valve is set at 100 psig (700 kPa) with a 7% blowdown, it will close at 93 psig (641 kPa). The operating pressure must be maintained below 93 psig (641 kPa) in order to prevent leakage or flow from a partially open valve. Users should exercise caution regarding the blowdown adjustment of large spring-loaded valves. Test facilities, whether owned by Manufacturers, repair houses, or users, may not have sufficient capacity to accurately verify the blowdown setting. The settings cannot be considered accurate unless made in the field on the actual installation.  Pilot-operated valves represent a special case from the standpoints of both blowdown and tightness. The pilot portion of some pilot-operating valves can be set at blowdowns as short as 2%. This characteristic is not, however, reflected in the operation of the main valve in all cases. The main valve can vary considerably from the pilot depending on the location of the two components in the system. If the pilot is installed remotely from the main valve, significant time and pressure lags can occur, but reseating of the pilot assures reseating of the main valve. The pressure drop in the connecting piping between the pilot and the main valve must not be excessive; otherwise, the operation of the main valve will be adversely affected. The tightness of the main valve portion of these combinations is considerably improved above that of conventional valves by pressure loading the main disk or by the use of soft seats or both. Despite the apparent advantages of pilot-operated valves, users should be aware that they should not be employed in abrasive or dirty service, in applications where coking, polymerization, or corrosion of the wetted pilot parts can occur, or where freezing or condensation of the lading fluid at ambient temperatures is possible. For all applications the valve Manufacturer should be consulted prior to selecting a valve of this type. Tightness capability is another factor affecting valve selection, whether spring loaded or pilot operated. It varies somewhat depending on whether metal or resilient seats are specified, and also on such factors as corrosion or temperature. The required tightness and test method should be specified to comply at a pressure no lower than the normal operating pressure of the process. A recommended procedure and acceptance standard is given in API 527. It should also be remembered that any degree of tightness obtained should not be considered permanent. Service operation of a valve almost invariably reduces the degree of tightness. Application of special designs such as O-rings or resilient seats should be reviewed with the valve Manufacturer. The anticipated behavior of the valves includes allowance for a plus-or-minus tolerance on set pressure which varies with the pressure level. Installation conditions, such as back pressure, variations, and vibrations, influence selection of special types and an increase in differential pressure. 

(c) General Recommendations. The following pressure differentials are recommended unless the safety or safety relief valve has been designed or tested in a specific or similar service and a smaller differential has been recommended by the Manufacturer. A minimum difference of 5 psi (35 kPa) is recommended for set pressures to 70 psi (485 kPa). In this category, the set pressure tolerance is ±2 psi (±13.8 kPa) [UG-134(d)(1)], and the differential to the leak test pressure is 10% or 5 psi (35 kPa), whichever is greater. A minimum differential of 10% is recommended for set pressures from 71 psi to 1,000 psi (490 kPa to 6.9 MPa). In this category, the set pressure tolerance is ±3% and the differential to the leak test pressure is 10%. A minimum differential of 7% is recommended for set pressures above 1,000 psi (6.9 MPa). In this category, the set pressure tolerance is ±3% and the differential to the leak test pressure should be 5%. Valves having small seat sizes will require additional maintenance when the pressure differential approaches these recommendations.

 

M-11 INSTALLATION OF SAFETY AND SAFETY RELIEF VALVES
Spring loaded safety and safety relief valves normally should be installed in the upright position with the spindle vertical. Where space or piping configuration preclude such an installation, the valve may be installed in other than the vertical position provided that:

• the valve design is satisfactory for such position;
• the media is such that material will not accumulate at the inlet of the valve; and
• drainage of the discharge side of the valve body and discharge piping is adequate.

 

M-12 REACTION FORCES AND EXTERNALLY APPLIED LOADS

(a) Reaction Thrust. The discharge of a pressure relief device imposes reactive flow forces on the device and associated piping. The design of the installation may require computation of the bending moments and stresses in the piping and vessel nozzle. There are momentum effects and pressure effects at steady state flow as well as transient dynamic loads caused by opening.

(b) External Loads. Mechanical forces may be applied to the pressure relief device by discharge piping as a result of thermal expansion, movement away from anchors, and weight of any unsupported piping. The resultant bending moments on a closed pressure relief device may cause device leakage, device damage, and excessive stress in inlet piping. The design of the installation should consider these possibilities.

 

M-13 SIZING OF PRESSURE RELIEF DEVICES FOR FIRE CONDITIONS
(a) Excessive pressure may develop in pressure vessels by vaporization of the liquid contents and/or expansion of vapor content due to heat influx from the surroundings, particularly from a fire. Pressure relief systems for fire conditions are usually intended to release only the quantity of product necessary to lower the pressure to a predetermined safe level, without releasing an excessive quantity. This control is especially important in situations where release of the contents generates a hazard because of flammability or toxicity. Under fire conditions, consideration must also be given to the possibility that the safe pressure level for the vessel will be reduced due to heating of the vessel material, with a corresponding loss of strength. For some fire situations, there may be an insufficient rise in pressure to activate a pressure relief device. The user should consult other references, which provide guidelines for protecting vessels from the effects of fire.

(b) Several formulas have evolved over the years for calculating the pressure relief capacity required under fire conditions. The major differences involve heat flux rates. There is no single formula yet developed which takes into account all of the many factors which could be considered in making this determination. When fire conditions are a consideration in the design of a pressure vessel, the following references which provide recommendations for specific installations may be used:

• API Recommended Practice 520, Sizing, Selection, and Installation of Pressure-Relieving Systems in Refineries, Part I — Sizing and Selection, Seventh Edition, Jaunary 2000, American Petroleum Institute, Washington, DC 
• API Recommended Practice 521, Guide for Pressure-Relieving and Depressuring Systems, Fourth Edition, March 1997, American Petroleum Institute, Washington, DC 
• API Standard 2000, Venting Atmospheric and Low-Pressure Storage Tanks (Nonrefrigerated and Refrigerated) Fifth Edition, April 1998, American Petroleum Institute, Washington, DC 
• AAR Standard M-1002, Specifications for Tank Cars, 1978, Association of American Railroads, Washington, DC 
• Safety Relief Device Standards: S-1.1, Cylinders for Compressed Gases; S-1.2, Cargo and Portable Tanks; and S-1.3, Compressed Gas Storage Containers, Compressed Gas Association, Arlington, VA 
• NFPA Code Nos. 30, 58, 59, and 59A, National Fire Protection Association, 1 Batterymarch Park, Quincy, MA 02169-7471 
• Pressure-Relieving Systems for Marine Cargo Bulk Liquid Containers, 1973, National Academy of Sciences, Washington, DC 
• Bulletin E-2, How to Size Safety Relief Devices, Phillips Petroleum Company, Bartlesville, OK 
• A Study of Available Fire Test Data as Related to Tank Car Safety Device Relieving Capacity Formulas, 1971, Phillips Petroleum Company, Bartlesville, OK

M-14 PRESSURE INDICATING DEVICE
If a pressure indicating device is provided to determine the vessel pressure at or near the set pressure of the relief device, one should be selected that spans the set pressure of the relief device and is graduated with an upper limit that is neither less than 1.25 times the set pressure of the relief device nor more than twice the maximum allowable working pressure of the vessel. Additional devices may be installed if desired.