News | July 7, 1999

Pressure-Relief Systems: Your Work Isn't Done Yet

By Patrick Berwanger, P.E., Robert Kreder and Wai-Shan Lee, Berwanger, Inc.

Contents


  • Statistical Analysis

  • Despite increased attention, 40% of installations have noncompliant features


    Nearly half of the equipment in the oil, gas, and chemical industries lack adequate overpressure protection as defined by recognized and generally accepted good engineering practice. This analysis was based on data collected from a large number of government mandated (per OSHA 1910.119) pressure-relief system design audits performed by an independent contractor. The vast majority of these deviations from good practice were not identified during conventional process hazard analyses (PHA) performed on these same facilities. Most of the units had also been designed by reputable design firms.

    We believe that, therefore, conventional PHA methods are ineffective tools for evaluating pressure relief systems. An equipment-based approach could improve the process.

    The OSHA process safety management (PSM) regulation (29CFR1910.119) has lead many oil, gas, and petrochemical operating companies to the conclusion that they should take a close look at the adequacy of their pressure relief systems. This conclusion generally results from the PSM regulation's requirement that process hazard analysis (PHA) teams and other employees have up-to-date pressure-relief system design and design basis information available to them.

    Audit Results (back to top)
    Now since quite a few pressure relief systems have been evaluated in some level of detail, it is informative to look at the data that has been collected to better inform future decision making. Given that roughly 30% of process industry losses can be at least partially attributed to deficient pressure relief systems [1], this matter is of importance. To this end, audits were performed at more than 250 operating units by an independent contractor. Based on the analysis of this information, we can draw the following conclusions:

    1. About 40% of equipment have at least one pressure-relief system deficiency
    Nearly half (more than 40%) of the equipment in the oil, gas, and chemical industries had some pressure relief system deficiency as compared to widely accepted engineering practices. The types of deficiencies are roughly split equally between absent and/or undersized pressure relief devices and improperly installed pressure relief devices.

    2. Current PHA methodologies do not capture most deficiencies
    Of the more than 250 operating units in the sample pool, essentially all had already undergone process hazard analyses (PHAs). It is the opinion of the authors that most deficiencies are not identified in PHAs because of time constraints and a general lack of pressure-relief-system expertise on PHA teams.

    3. Contractor design methods can be improved by adopting an equipment-based approach
    Perhaps surprisingly, most of the more than 250 operating units in which deficiencies were found had been designed by reputable design contractors. The authors believe that the main reason so many deficiencies slip through the design process is that a vigorously enforced, equipment-based approach to pressure relief system design is not used.

    4. Current information management techniques have not worked
    In an effort to comply with the PSM regulation, many operators have recreated their pressure relief system design and design basis information essentially from scratch. This is because over the years, most operating companies have relied on pressure-relief device datasheets as their primary repository for their pressure-relief system design basis information—a purpose for which they were never intended.

    Statistical Analysis (back to top)
    A total of 272 processing units, 31,509 pieces of equipment, and 14,873 pressure relief devices were surveyed in this analysis. Each equipment item was categorized as a vessel, heat exchanger, air cooler, compressor, pump, filter, or other. Due to the fact that centrifugal pumps normally do not require overpressure protection, these items (7,206) were excluded from the original total of 31,509 leaving a total of 24,303 pieces of equipment. The chart below shows the percentage of each equipment category relative to the total after the centrifugal pumps are excluded.

    All deviations from recognized and generally accepted good engineering practices ("Ragagep") were cataloged and categorized in a database. A total of approximately 10,000 such deficiencies were identified during the review of the 272 processing units. Most deficiencies fell in one of the three following categories:

    100 Series: No relief device present on equipment with one or more potential overpressure scenarios.
    200 Series: Undersized relief device present on equipment with one or more potential overpressure scenarios.
    300 Series: Improperly installed relief device.

    Within each series, categories were defined to further describe the types of deficiencies that were encountered. A summary table and/or chart is also presented for each series. The deficiencies presented in this paper deal primarily with fluid hydraulic issues. Other types of deficiencies did not fall into any of the three series above, and are not included in the statistical analyses presented below. These deficiencies included concerns about items such as excessive flare radiation levels, inadequate knockout drums, poorly designed quench systems, discharge of toxic fluids to atmosphere, discharge of combustible liquids to atmosphere, and a general lack of process safety information upon which to base a safe pressure-relief system design. Thus, we may actually be understating the actual overall industry deficiency rate.

    Series 100: No relief device present on equipment with one or more potential overpressure scenarios
    The most common problem encountered in this series is a potential overpressure caused by an external fire. API RP 521 Section 3.15 contains extensive guidance on this. To paraphrase, a pressure relief device adequate for the external fire scenario should be installed for all vessels, heat exchangers, and filters with a liquid inventory that are located within a potential fire zone. The guidelines are less clear on the treatment of vapor-filled equipment that may be subject to thermal failure prior to overpressure. In these cases, other preventative measures such as depressuring systems may be more appropriate. This deficiency affected 5.8% of the installations.

    Other problems are overpressure due to blocked outlets (an API RP 521 Section 3.5 [2] misapplication); overpressure due to a failed control valve (API RP 521 Section 3.10.3); potential for overpressure due to heat-exchanger tube failure (ASME VIII Division 1, Paragraph UG-133(d) [3] and API RP 521 Section 3.18 ) after which high-pressure fluid flows to the low-pressure side; overpressure due to thermal expansion of piping or vessels (API RP 521 Section 3.14); a "multiple scenarios" situation in which a piece of equipment had more than one potential overpressure scenario; and an "other" scenario (based on API RP 521 Section 3.1 Table 1). The overall deficiency frequency for Series 100 was 15.1%.

    In all these cases, the main deficiency is that there is no pressure relief device present. Adding additional pressure relief, or re-evaluating the design and function of the process equipment, would address the problem

    Series 200: Undersized relief device present on equipment with one or more potential overpressure scenarios
    Series 200 deficiencies pertain to undersized pressure relief. The most common deficiency is attributable to the potential for overpressure due to tube failure (API RP 521 Sec. 3.18). There is also a "two-thirds rule" that stipulates that protection is warranted if overpressure were encountered when two-thirds of the tubes failed. API 521 further stipulates that it is conservative to treat a tube rupture as two sharp-edged orifices having the same cross-sectional area as a tube. The maximum expected pressure upstream of the high pressure side of the heat exchanger and the relieving pressure at the low pressure side were used to determine the pressure drop across the orifice. A failure could be estimated for 4.6% of the total number of equipment.

    Other deficiencies noted are blocked outlets; inlet control-valve failure (particularly "gas blow-by" when pressurized liquid vents through a leak); overpressure due to loss of overhead condensing or reflux failure; external fire (as presented in Appendix D of API RP 520, and calculations of the adequacy of firefighting equipment); and various miscellaneous deficiencies. Overall, 8.6% of the devices surveyed had a Series 200-type problem.

    Series 300: Improperly installed pressure relief device
    The single most common installation problem, affecting as many pieces of equipment as all the others in this category combined, is that the relief pressure is set too high. Both API RP 520 [4] and ASME VIII Division 1 state that the set or burst pressure of at least one relief device shall not exceed the MAWP of the associated equipment. In the case of multiple relief-device installations, additional relief devices may be set at 105% or 110% of the MAWP depending on the scenarios under consideration. A common solution to this problem is to re-rate the equipment, or re-set the relief device.

    Other problems encountered with improper installation include the potetnial to block a relief pathway (with isolation valves or other devices)—an API RP 520 Part II Sec. IV deficiency; setting the relief pressure too high (above 3% of set pressure, per API RP 520, Part II, Sec. 2.2.2); and some miscellaneous deficiencies. The overall deficiency frequency for Series 300 was 22%. It is worth noting that API is currently commissioning a study on the dynamics of relief valve behavior, and that there is considerable disagreement among experts on the causes and effects of valve dynamic performance.

    It is clear from the foregoing that pressure relief system design and information management practices can, and need to be, improved. Fortunately, design and information management methods are available5 that can address this situation.

    REFERENCES

    1 AIChE (1995), "Emergency Relief System (ERS) Design Using DIERs Technology", American Institute of Chemical Engineers, New York, NY.

    2 API (March 1997), "Guide for Pressure-Relieving and Depressuring Systems", API Recommended Practice 521 Fourth Edition, The American Petroleum Institute, Washington, DC.

    3 ASME (1992), "ASME Boiler and Pressure Vessel Code", Section VIII, Divisions 1 and 2, The American Society of Mechanical Engineers, New York, NY.

    4 API (March 1993), "Sizing, Selection, and Installation of Pressure-Relieving Devices in Refineries", API Recommended Practice 520 Sixth Edition, The American Petroleum Institute, Washington, DC.

    5 Berwanger and Kreder (April 1995), "Making Safety Data 'Safe'", Chemical Engineering, McGraw Hill Inc., New York, NY.

    About the authors: (back to top)
    Patrick C. Berwanger is founder and president, Berwanger Inc. The co-authors are on the staff of the company. Based on a presentation at the 1999 Spring National Meeting, AIChE., Houston, TX, March 14-18.

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