|MN471000, Pressure Safety Manual
Sponsor: Michael W. Hazen, 4000
|Revision Date:March 31, 2008
Replaces Document Dated: October 8, 2007
This document is no longer a CPR. This document implements the requirements of Corporate Procedure ESH100.2.PS.1, Control Pressure Safety Hazards.
IMPORTANT NOTICE: A printed copy of this document may not be the document currently in effect. The official version is the online version located on the Sandia Restricted Network (SRN).
Subject Matter Experts: Shane Page, and David Paoletta
Contributor: Pressure Safety Committee
MN471000, Issue S
Revision Date: March 31, 2008; Replaces Document Dated: October 8, 2007
Administrative Changes: June 8, 2010, and May 26, 2011, and January 19, 2012
The Pressure Installer is the key individual in determining the appropriate testing of a pressure system prior to placing it into operation, and in the periodic reevaluation of the system to assess the consequences of mechanical degradation. The level of rigor and the intervals for reevaluating a pressure system are dependent on the complexity of the system; the effects of factors such as materials compatibility, time, and cycles upon the system; and the types of components that can be pressure tested. Pressure Safety SMEs and organizational Pressure Advisors (PAs) can provide assistance in developing a plan for testing pressure systems.
The Pressure Installer should be aware of the following principles for pressure testing:
Sandia can provide many services for pressure testing assistance. Consult Pressure Safety Assistance Index for a listing of qualified individuals. Pressure Safety SMEs are also available for assistance.
Note: Pressure Installers can use one or more of the following tests on pressure components and systems:
The objective of a properly administered overpressure test is to demonstrate that failure should not occur with a vessel at its maximum allowable working pressure (MAWP) and temperature. An overpressure test is not performed on piping systems assembled from supplier-established pressure-rated components.
Of the various types of pressure tests that can be done at SNL, or at SNL discretion, by a qualified outside source, the overpressure test is the most important because it:
Note: Sudden release of pressure in the vessel usually constitutes a greater hazard than release of pressure elsewhere in the system, because the vessel itself stores more of the energy.
Note: More comprehensive testing should be considered for systems containing toxic or radioactive materials.
Note: Nondestructive Testing (NDT) is necessary to establish the existence of flaws or the potential for future flaw growth. NDT includes radiography, ultrasonics, magnetic particle inspection, and acoustic emissions monitoring (see Nondestructive Evaluation (NDE) in this chapter). Supplementary monitoring (acoustic emissions, strain, displacement, etc.) during an overpressure test creates a much stronger safety argument, is useful in future evaluations, and is recommended whenever possible.
Note: Even experts disagree as to adequate holding time at maximum pressure during overpressure test. The reason is that the number of variables may differ widely. If the vessel has not reached general yield, the pressure usually stabilizes quickly at proof level. If general yield is occurring, the pressure may never stabilize, and even long holding times will not matter.
Members of the Workforce may consider the test successful if there is no abnormal drop in pressure after a 3-minute holding time.
Caution: If you observe a drop in pressure, general yielding is the probable cause if there are no system leaks. This condition results in failure to pass the overpressure test.
If leakage is detected during the overpressure test, Members of the Workforce should:
Note: General yield differs from local yield, which is part of the "shakedown" process in a well-balanced design and is usually harmless.
Do not proceed with testing until these persons have decided that the risks are acceptable.
Note: Leak tests may require physical exposure of the operator to a pressure system.
Members of the Workforce may perform an operational leak test at MAWP on a vessel or system in service when reevaluation has determined that a full overpressure test is not necessary.
Section V of the ASME Code relates to the requirements and methods for nondestructive examination. These nondestructive examination methods are intended to detect surface and internal discontinuities in materials, welds, and fabricated parts and components. Examples include radiographic examination, ultrasonic examination, liquid penetrant examination, magnetic particle examination, eddy current examination, visual examination, and leak testing. In the foregoing methods, the skill, experience and integrity of the personnel performing these examinations are essential to obtain meaningful results. Nondestructive examination personnel shall be certified in accordance with ASNT-TC-1A to at least Level II. The following sections discuss NDE procedures that are useful in pressure system evaluations and describe the type of data the user will obtain from these inspections.
See Pressure Safety Assistance Index for assistance in conducting NDE.
Caution: Tests such as the radiography, ultrasonics, liquid-penetrant, magnetic particle, and acoustic emission tests do not jeopardize the vessel, but they may identify the need for repair of detected defects. Any repairs would invalidate an overpressure test.
Persons who interpret radiographic films of welds should have not only a knowledge of welding and welding discontinuities, but also be able to exercise good judgment as to whether the discontinuities are actually defects.
The principal advantages of ultrasonic inspection are that it can determine, to a degree, the position, size, nature and orientation of flaws (e.g., internal cracks) located deep within the component. In addition, it provides instantaneous indications of flaws and its high sensitivity permits the detection of extremely small flaws. All examinations performed shall be done according to a written procedure.
Members of the Workforce should use the magnetic particle method only on ferromagnetic materials to reveal surface discontinuities and, to a limited degree, those located below the surface. The sensitivity of this method decreases rapidly with depth below the surface being examined; therefore, it is used primarily to examine for surface discontinuities.
Note: The acoustic waves propagate throughout the vessel from the site of the generating flaw. A single sensor can detect flaw growth in a vessel, while an array of sensors can locate the site of the growing flaw. AE performs a 100% volumetric inspection of a vessel without scanning. The only flaws that it detects are flaws that are activated by the pressurization; flaws that are not affected by the pressurization, and are therefore benign, are not detected by AE. The sensitivity is such that the fracture of a single crystallite in a metal can be detected.
Note: AE monitoring is done in real time so that a measure of the flaw growth as a function of time or pressure is always available to the operator. AE is capable of determining the severity of the flaw with increase in pressure but it cannot determine the actual size of the flaw. Once AE has located a flaw, sizing should be done by ultrasonics or radiography.
Pressure Installers shall establish any applicable criteria upon which reevaluations are based and document such in the Data Package.
Note: The usage history of the system may indicate revised reevaluation criteria. The current applicable criteria should be noted within the system TWD.Pressure Installers should consider the following information when establishing intervals for reevaluation, testing, inspection, and maintenance of pressure systems:
Note: The intervals and procedures should be monitored by the Pressure Installer to ensure that the appropriate functions are performed at the specified intervals. This could be controlled by the Data Package, an operator's service log, start-up/shut-down procedures, operating procedure or division recall system (e.g., computer or index tickler file).
Pressure Installers should be aware that pressure systems deteriorate over time, just as does any piece of equipment. The amount of attention placed on a system is based on an evaluation of the degree of hazard associated with the failure of any component. Reevaluation criteria, such as system design, maintenance, component replacement, inspection, de-rating, re-test, or removal from service, should be assessed prior to placing the system into operation. The system user is in the best position to make these determinations, using the guidance provided and with consultation as necessary.
Pressure Installers should reevaluate pressure systems for the following reasons:
Pressure installers should perform the following steps as part of the pressure system reevaluation:
Pressure Installers should be aware that environmental effects are among the primary causes of time-dependent degradation and are important considerations for pressure system evaluations. Stress corrosion cracking, for example, is one of the most common causes of pressure vessel failure. When discussing fatigue, it is the environmentally assisted failures that are the most common. When incompatible materials are used together with cyclic loading, corrosion fatigue becomes a primary failure mode. Thermal cycles should be factored into fatigue considerations. In these situations, crack initiation and/or growth can be assessed by NDE techniques.
Pressure Installers should develop a reevaluation program for all pressure systems when they are installed. Since life evaluation considerations are system dependent, the users should consult their PA and seek assistance from appropriate personnel in Pressure Safety Assistance Index. This will help the user establish reevaluation intervals, as well as the type of inspections and tests that should be performed at that time. The reevaluation program should take into account factors such as cracking, corrosion, effects of other adverse environments, the type and likelihood of failure (are there welds that require periodic inspection?) and the consequences of failure. Finally, the reevaluation program should be documented and placed in the system Data Package so that the proper inspection intervals can be maintained.
Intervals for systems that have a corrosive working medium should be more frequent. Because all pressure systems are unique, it is not possible to give a specific interval that would apply to all systems. Inspection and reevaluation intervals are determined by each owning organization, using the guidance provided in this manual along with assistance from the PA and other experts listed in Pressure Safety Assistance Index.
Overpressure testing: The Pressure Installer should have overpressure testing done as part of the reevaluation process if either of the following conditions is met:
Some form of NDE should also be employed during the overpressure test. The process consists of more than simply observing that failure did not occur.
Note: Do not automatically perform an overpressure test when reevaluation is due. Unless the reevaluation criteria for the vessel established a requirement for an overpressure test at a specified interval, or it is determined that one is necessary, an overpressure test may not be beneficial to the vessel, especially without other NDE techniques included to provide amplifying information. See "Limitations of Overpressure Test" in this chapter.
Note: Overpressure testing cannot by itself qualify a vessel, but is only one of the three criteria discussed in Chapter 4. The Data Package should contain confirmation that these criteria have been met.
Note: This assistance is especially important in instances, in which non-code (e.g., in-house) designs have been carried out with minimal analysis, certification and documentation. In these cases, a more in-depth reevaluation process is needed to prove a continued safe operation.
Shane Page, firstname.lastname@example.org
Al Bendure, email@example.com
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