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ASME UG-140... Overpressure protection by System Design PDF Print E-mail
Safety Info Posts - Chemical Process Safety (PSM/RMP)
Written by Bryan Haywood   
Sunday, 02 December 2012 00:00

Not many know this dirty little secret about over pressure protection on ASME coded vessels (formerly called "code case 2211"), but I will let the cat out of the bag... Not all ASME pressure vessels have to have a pressure relief device and I am not talking about those very small vessels, but any size vessel. No I am not crazy, it is a fact. Since 1996 ASME had allowed this practice by issuing "Code Case 2211". Code Case 2211 was first approved in 1996 and reaffirmed until it became UG-140 in 2008. I am in NO WAY advocating this practice, but it is an option and is used in certain processes where relief devices can be easily compromised due to relieving events. This article is to inform those who want to know more about this practice and to ensure those that are already utilizing UG-140 have all the required documentation on hand.

In NO WAY does ASME want this to be the first choice of users, but rather an option in some very specific circumstances. But in some applications, the use of pressure relief devices may be impractical. Some situations may include:

  • Chemical reactions so fast the pressure propagation rate could result in loss of containment prior to the relief device opening. Examples are “hot spots,”decompositions, and internal detonation/fires;
  • Chemical reactions so fast, the lowest possible relieving rate yields impractically large vent areas;
  • Exothermic reactions occurring at uncontrollable rates, resulting in a very high propagation rate for the process pressure;
  • Plugging, polymerization, or deposition formed during normal operation, which have historically partially or completely blocked pressure relief devices;
  • Reactive process chemicals relieved into lateral headers with polymerization and thus plugging, rendering the relief device useless; and
  • Multi-phase venting, where actual vent rate is difficult to predict.

The overpressure protection can be provided in lieu of a pressure relief device ONLY under the following conditions:

  1. The vessel is not exclusively in air, water, or steam service.
  2. The decision to utilize overpressure protection of a vessel by system design is the responsibility of the user. The manufacturer is responsible only for verifying that the user has specified overpressure protection by system design, and for listing Code Case 2211 on theData Report.
  3. The user must ensure the MAWP of the vessel is higher than the highest pressure that can reasonably be achieved by the system.
  4. A quantitative or qualitative risk analysis of the proposed system must be made addressing:credible overpressure scenarios, demonstrating the proposed system is independent of the potential causes for overpressure; is as reliable as the pressure relief device it replaces; and is capable of completely mitigating the overpressure event.
  5. The analysis conducted for (c) and (d) must be documented.

Here is UG-140

(a) A pressure vessel does not require a pressure relief device if the pressure is self-limiting (e.g., the maximum discharge pressure of a pump or compressor), and this pressure is less than or equal to the MAWP of the vessel at the coincident temperature and the following conditions are met:

(1) The decision to limit the pressure by system design is the responsibility of the user. The user shall request that the Manufacturer’s data report state that over- pressure protection is provided by system design per UG-140(a).
(2) The user shall conduct a detailed analysis to identify and examine all potential overpressure scenarios. The “Causes of Overpressure” described in ANSI/API Standard 521, Pressure-Relieving and Depressuring Systems, shall be considered. Other standards or recommended practices that are more appropriate to the specific application may also be considered. A multidisciplinary team experienced in methods such as hazards and operability analysis (Hazop); failure modes, effects, and criticality analysis (FMECA); “what-if” analysis; or other equivalent methodology shall establish that there are no sources of pressure that can exceed the MAWP at the coincident temperature.
(3) The results of the analysis shall be documented and signed by the individual in responsible charge of the management of the operation of the vessel.  This documentation shall include as a minimum the following:

    1. detailed process and instrument flow diagrams (P&IDs), showing all pertinent elements of the system associated with the vessel
    2. a description of all operating and upset scenarios, including scenarios involving fire and those that result from operator error, and equipment and/or instrumentation malfunctions
    3. an analysis showing the maximum coincident pressure and temperature that can result from each of the scenarios listed in item UG-140(a)(3)(b) above does not exceed the MAWP at that temperature

(b) If the pressure is not self-limiting, a pressure vessel may be protected from overpressure by system design or by a combination of overpressure by system design and pressure relief devices, if the following conditions are met. The rules below are NOT intended to allow for normal operation above the MAWP at the coincident temperature.

(1) The vessel is not exclusively in air, water, or steam service unless these services are critical to preventing the release of fluids that may result in safety or environmental concerns.

(2) The decision to limit the overpressure by system design is the responsibility of the user. The user shall request that the Manufacturer’s data report state that over- pressure protection is provided by system design per UG-140(b) if no pressure relief device is to be installed. If no pressure relief device is to be installed, acceptance of the jurisdiction may be required.

(3) The user shall conduct a detailed analysis to identify and examine all scenarios that could result in an over- pressure condition and magnitude of the overpressure. The “Causes of Overpressure” as described in ANSI/API Standard 521, Pressure-Relieving and Depressuring Systems, shall be considered. Other standards or recommended practices that are more appropriate to the specific application may also be considered. A multidisciplinary team experienced in methods such as hazards and operability analysis (Hazop); failure modes, effects, and criticality analysis (FMECA); “what-if” analysis; or other equivalent methodology shall conduct the analysis.

(4) The overpressure scenario shall be readily apparent so that operators or protective instrumentation will take corrective action to prevent operation above the MAWP at the coincident temperature.

(5) There shall be no credible overpressure scenario in which the pressure exceeds 116% of the MAWP times the ratio of the allowable stress value at the temperature of the overpressure scenario to the allowable stress value at the design temperature. The overpressure limit shall not exceed the test pressure. Credible events or scenario analysis as described in WRC Bulletin 498 “Guidance on the Application of Code Case 2211 — Overpressure Protection by Systems Design” shall be considered.

(6) The results of the analysis shall be documented and signed by the individual in responsible charge of the management of the operation of the vessel. This documentation shall include as a minimum the following:

(a) detailed process and instrument flow diagrams (P&IDs), showing all pertinent elements of the system associated with the vessel

(b) a description of all operating and upset scenarios, including those involving fire and those that result from operator error, and equipment and/or instrumentation malfunctions

(c) a detailed description of any safety critical instrumentation used to limit the system pressure, including the identification of all truly independent redundancies and a reliability evaluation (qualitative or quantitative) of the overall safety system

(d) an analysis showing the maximum pressure that can result from each of the scenarios

 


 

Code Case 2211-1

Pressure Vessels With Overpressure Protection by System Design

Section VIII, Divisions 1 and 2

Inquiry: Under what conditions may a pressure vessel be provided with overpressure protection by system design in lieu of a pressure relief device as required by Section VIII, Division 1, para. UG-125(a) and by Section VIII, Division 2, para. AR-100?

Reply: It is the opinion of the Committee that a pressure vessel may be provided with overpressure protection by system design in lieu of a pressure relief device as required by Section VIII, Division 1, para. UG-125(a) and by Section VIII, Division 2, para. AR-100 under the following conditions: (a) The vessel is not exclusively in air, water, or steam service unless these services are critical to preventing the release of fluids that may result in safety or environmental hazards. (b) The decision to provide a vessel with overpressure protection by system design is the responsibility of the User. The User shall specify overpressure protection by system design and reference this Code Case in writing in the purchase documents (per Section VIII, Division 1, para. U-2) for a Division 1 vessel, or in the User’s Design Specification (per Section VIII, Division 2, para. AG-301) for a Division 2 vessel. The Manufacturer is responsible only for verifying that the User has specified overpressure protection by system design, and for listing this Code Case on the Data Report. (c) The User shall ensure that the MAWP (see Section VIII, Division 1, para. UG-98) of the vessel is greater than the highest pressure that can reasonably be expected to be achieved by the system. The User shall conduct a detailed analysis that examines all credible scenarios that could result in an overpressure condition. The “Causes of Overpressure” described in Section 2 of API Recommended Practice 521 “Guide for Pressure-Relieving and Depressuring Systems” shall be considered. An organized, systematic approach by a multidisciplinary team employing one or more of the following methodologies shall be used: Hazards and Operability Analysis (HazOp), Failure Modes, Effects, and Criticality Analysis (FMECA), Fault Tree Analysis, Event Tree Analysis, or “What-If” Analysis. In all cases, the User shall determine the potential for overpressure due to all credible operating and upset conditions, including equipment and instrumentation malfunctions. (d) The analysis described in (c) shall be conducted by an engineer(s) experienced in the applicable analysis methodology. Any overpressure concerns that are identified shall be evaluated by an engineer(s) experienced in pressure vessel design and analysis. The results of the analysis shall be documented and signed by the individual in charge of the operation of the vessel. This documentation shall include as a minimum: (1) Detailed Process and Instrument Flow Diagrams (P&IDs), showing all pertinent elements of the system associated with the vessel. (2) A description of all credible operating and upset scenarios, including scenarios involving fire, and those that result from equipment and instrumentation malfunctions. (3) An analysis showing the maximum pressure that can result from each of the scenarios examined in (2) above. (4) A detailed description of any instrumentation and control system that is used to limit the system pressure, including the identification of all truly independent redundancies and a reliability evaluation (qualitative or quantitative) of the overall safety system. Prior to initial operation, the documentation shall be made available to the regulatory and enforcement authorities having jurisdiction at the site where the vessel will be installed. The User of this Code Case is cautioned that prior jurisdictional acceptance may be required. (e) This Case number shall be shown on the Manufacturer’s Data Report for pressure vessels that will be provided with overpressure protection by system design, and it shall be noted on the Data Report that prior jurisdictional acceptance may be required.

The Committee’s function is to establish rules of safety, relating only to pressure integrity, governing the construction of boilers, pressure vessels, transport tanks and nuclear components, and inservice inspection for pressure integrity of nuclear components and transport tanks, and to interpret these rules when questions arise regarding their intent. This Code does not address other safety issues relating to the construction of boilers, pressure vessels, transport tanks and nuclear components, and the inservice inspection of nuclear components and transport tanks. The user of the Code should refer to other pertinent codes, standards, laws, regulations or other relevant documents.

 
 

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