1. 36 • September 2013 Chemical Engineering World
Features CEW
Application of UG-140 for Overpressure Protection
It’s a common understanding that for a pressure vessel, pressure relief device is required to protect against overpressure scenario. In the event of credible overpressure scenario, the device is sized for the scenario.
However, if there are no credible overpressure scenarios, a practice of providing a “token valve” sized arbitrarily is generally followed. This article addresses the question on whether the relief device is required when there are
no credible scenario and how to meet the code requirements even without providing the pressure relief device.
Pressure vessel is defined as the vessel designed above 15 psig pressure. When the vessel is designed per code ASME Section-VIII Division-1 (henceforth referred as code), the requirements of the code are followed for design, fabrication, and inspection & testing. ASME coded vessels are marked with ASME “U” stamp on nameplate. One of the basic requirements of the code is to provide overpressure protection for the vessel covered by ASME Section-VIII Division-1, UG-125 through UG-140.
Overpressure protection for pressure vessel can be achieved by either or combination of following three options:
1. Relief Device: Requirements of relief device for pressure vessel are covered by UG-125 through UG-
138. When the findings of detailed evaluation of various overpressure scenarios per API Standard-521 state that there are credible cases for overpressure, pressure relief devices are provided and sized per applicable equations given in API Standard-521.
2. System Design (Inherently Safe Design): This option overcomes the requirement of a relief device to protect pressure vessel. The documentation and other specific requirements of this option are covered by UG-140(a).
3. Safety Instrumented or High Integrity Protection System (HIPS): This option can overcome the requirement of a relief device in specific cases. The documentation and other specific requirements of this option are covered by UG-140(b).
The ASME UG-140 requirements and procedures are commonly known as “Code Case 2211”. In 2008, ASME incorporated “Code Case 2211” into the body of Section-VIII Division-1 as Section UG-
140. All overpressure protection systems addressed by second or third option or a combination of second and third require compliance of ASME UG-140 including all approvals, responsibilities, analysis and reviews including documentation and
sign-offs.
Detail Requirements of UG-140 (a)
and (b)
Per UG-140, pressure relief device is not required to protect a pressure vessel if the pressure is self-limiting and this pressure is less than equal to MAWP of the vessel at coincident temperature.
The major requirements of UG-140 are Detail analysis of various overpressure scenarios; Multidisciplinary team review; Risk assessment in the event of credible scenarios; Manufacturer data report to state the system protected by design; and Documentations & signoffs.
1. Detail analysis of various credible scenarios: API Standard 521, Pressure-Relieving and Depressuring Systems, describes “Causes of Overpressure”. Other standards or recommended practices that are more appropriate to the specific application may also be considered. The typical overpressure scenarios that need to be analyzed are Loss of cooling water or cooling medium, Loss of power, Loss of steam, Loss of instrument air or electric supply failure, Individual control valve failure, Inadvertent valve operation, Blocked outlet, Individual equipment failure, Loss of air cooling, Thermal expansion, Heat exchanger tube rupture, Fire case, Liquid overfill, Gas Blow by Reactive hazard
All the scenarios shall be evaluated with detail explanation to identify whether the scenario is applicable and if it is applicable how the pressure reached in the scenario is less than MAWP of pressure vessel at co-incident temperature. All the relevant documents such as Piping and Instrumentation Diagrams (P&IDs), Process Flow Diagrams (PFDs), Equipment General Arrangement (GA) Drawings, Equipment Data Sheets, Plot Plan, Equipment Elevation Drawings, Paving and Drainage Plans, Process Control System Documentations, Instrument Data Sheets and Material Safety Data Sheets (MSDS) of chemicals involved etc. shall be used to evaluate credibility of overpressure scenarios. All the relevant documents shall be attached as a part of documentation.
2. Multidisciplinary team review: The documentations shall be reviewed and evaluated by multi-disciplinary team of people including Process Safety engineer, Process Technology Leader, Manufacturing Representatives, Process Engineers and Relief Design Subject Matter Experts (SME).
3. Risk assessment in case of credible overpressure scenario: For a pressure vessel, if a scenario is found to be credible for overpressure, then risk associated with the event shall be judged to decide whether or not the relief device shall be provided. The design shall comply with local regulations and owner’s risk tolerance criteria. This approach is scenario specific and if new Daelim.indd 36 01-10-2013 21:37:09

2. 38 • September 2013 Chemical Engineering World
Features CEW
it can be justified that having a relief device does not benefit then one can go without having a relief device even when the case is credible for overpressure. To illustrate, examples can be as follows:
• If fire scenario is applicable to a pressure vessel operating in gas service, pressure relief device is of no help in protecting the vessel as the vessel ruptures due to overheating before the pressure reaches the set pressure of relief device.
• The pressure vessel is empty and fire case is applicable, relief device is of little help for the same reasons.
• Pump Seal pots: Consider 5 gallon U stamped seal pot is provided for API pump and fire case is applicable. During a fire, the seal pot will likely fail, regardless of whether it has a PSV, and regardless of that PSV’s size. For vessel size being that small, a fire-sized PSV will quickly result in the seal pot being emptied of liquid. Once the liquid is gone the vessel will fail due to excessive heating (excessive wall temperature). Thus, a PSV can’t adequately prevent this vessel from failing during a sustained pool fire.
One can go with UG-140 documentation for such cases.
4. Documentation and Sign off: All the involved parties shall sign off the document to conclude and certify that the vessel is protected by system design and in no credible event the pressure will exceed MAWP at coincident temperature.
5. Manufacturer’s data report to state the system protected by design:
Annexure-W of ASME Section-
VIII Division-1 provides details of Manufacturing Data reports. UG-140 requires that manufacturing data report shall mention that the vessel is protected by system design. Typically, vessel manufacture provides data report and this requirement is mentioned in the notes section of the report. To the data report, a U-1 or U-1A form is enclosed. Typically, U-1 or U-1A form should say “Overpressure protection is provided by others per UG-140 (a) or (b). OSHA inspector is likely to see this form to check for overpressure protection of the vessel when the ASME code stamped vessel is not provided with a pressure relief device.
High Integrity Pressure Protection System (HIPS)
Industrial use of HIPS is to reduce the pressure relief device’s size for specific scenarios. It is not intended to drive the complete removal of pressure relief devices. However, in specific cases, use of HIPS can lead to elimination of relief device. The documentation and other specific requirements of this option are covered by UG-140(b).
HIPS typically involve an arrangement of instruments, final control elements (e.g. valves, switches, etc.), and logic solvers configured in a manner designed to avoid overpressure incidents by removing the source of overpressure or by reducing the probability of an overpressure contingency to such a low level that it is no longer considered to be a credible case. With appropriate levels of redundancy, a HIPS can be designed to achieve a level of availability equal to or greater than a mechanical relief device.
A detailed description of any safety critical instrumentation (HIPS-High Integrity Protection System) used to limit the system pressure, including the identification of all truly independent redundancies and a reliability evaluation (qualitative and quantitative) of the overall safety system shall be provided as part of UG-140 documentation. HIPS finds its use in complex process applications. For example, the application of HIPS is for a reactor o perating at high temperature and pressure conditions where coolant is circulated to remove exothermicity of reaction and loss of coolant would lead to high temperature rise leading to runaway reaction or boil up of coolant. In such application, HIPS is used to cut off the feed once coolant flow is stopped.
The table (on the next page) can be used as reference when making a choice between the options to be followed for overpressure protection of pressure vessel while meeting code requirements.
Example of Pressure Protection of a
Pressure Vessel Operating in Chlorine
Service by System Design (UG-140 (a)) (See
Figure 2 below)
Figure1: Example of System Architecture for HIPS
Figure 2: D-101 Schematic
new Daelim.indd 38 01-10-2013 21:37:09

3. 40 • September 2013 Chemical Engineering World
Features CEW
Pros and Cons of Relief Device, Safe Design and HIPS UG-140 (b)
Relief Device Inherently safe design UG-140 (a)
HIPS
UG-140(b)
Pros
1. Commonly used
2. Highly reliable
3. Designer feels confident with functionality
4.Standalone Mechanical device
5.Can handle runaway reactions
Pros
1. Safest way of design
2. Eliminate relief device a n d associated disposal system
3. Preferred option for toxic services
Pros
1. Availability / Reliability
2. Eliminate relief device and associated disposal system
3. Suitable for toxic services
4. Can eliminate overpressure scenarios such as runaway reactions
5. Can provide protection
against excessive
temperature due to process
upsets (*)
6. Useful when environment restrictions and safety constraints limit the venting
Cons
1. Requires maintenance
2. Can plug in heavy service
3. Requires disposal system downstream for safe handling
4. When not used in combination of HIPS, may require bigger disposal system such as large flare headers and scrubbers
5. Not desirable in toxic services
6. Cannot provide protection against excessive temperature
Cons
1. Higher design pressures
2. HIPS needs to be used in combination to handle runaway reactions and protection against excessive temperature
3. Designer needs to
meet requirement of
UG-140 by way of documentations, reviews
and sign-offs
Cons
1. Maintenance, testing and inspection
2. Complex systems
3. Expensive systems
System Information
1.1 Protected Equipment : D-101
1.2 Fluid HandleD : Chlorine
1.3 Operating Pressure / Temperature
: 6.5 Barg / 50°C
Description of Overpressure Scenario
1. Loss of Cooling Water or Other Cooling Fluid: No cooling is supplied to D-101. Hence, the case is
not applicable.
2. Loss of Power: There are no power operated equipment associated with D-101. Hence, the case is not applicable.
3. Steam Failure: No steam is associated with D-101. Hence, the case is not applicable.
4. Instrument Air / Electrical Supply Failure: In the event of instrument air failure, following valves shall close as failure position being Fail Closed. [Inlet of D-101 - 1. EBV-01: Closure of the valve shall lead to loss of chlorine gas flow to D-101. Hence, there shall be no overpressure. Outlet of D-101 - 1. FCV-01 / EBV-02: Closure of the valves shall not lead to overpressure as chlorine gas flow to D-101 shall be lost due to closure of EBV-01.]
5. Individual Control Valve Failure: Failure Opening of FCV-01: The valve is located on vent outlet of D-101. The case of failure opening of control valve shall result in no overpressure to D-101 as the valve shall provide open path to downstream system.
6. Inadvertent Valve Operation: Failure closure of FCV-01: Failure closure of the valve shall not result in overpressure as MAWP of D-101 is higher than the maximum pressure of chlorine gas at battery limit which is 9 barg.
7. Individual Equipment Failure: There are no mechanical equipment associated with D-101. Hence, the case is not applicable.
8. Loss of Air Cooling :No air cooling is provided for inlet and outlet streams of D-101. Hence, the case is not applicable.
9. Blocked Outlet: Refer to Case 6. No overpressure.
10. Gas Blow Through:The case is not applicable.
11. External Fire: Per Equipment GA drawing, the equipment is located above fire height of 7.6m. Hence, overpressure is not credible.
12. Exchanger Tube Rupture: D-101 is not connected to any heat exchanger. Hence, the case is not applicable.
13. Liquid Overfill: D-101 is operating in gas service. Hence, the case is not applicable.
14. Thermal Expansion: D-101 is operating in gas service. Hence, the case is not applicable.
15. Vacuum: D-101 is designed for full vacuum. Hence, the case is not deemed to be credible.
16. Runaway Reactions (Reactive Hazards): No reactive hazard is associated with D-101. Hence, the case is not applicable.
17. Any Other: Low pressure nitrogen can be hooked up to D-101 during maintenance activities. However, maximum operating pressure of LP Nitrogen is 7 barg which is less than MAWP of D-101. Hence, overpressure is not credible.
Safety Critical Instrumentation And Reliability Evaluation
For D-101, no safety critical instrumentation is used for purpose of overpressure protection. Hence, this section is not applicable. As detailed evaluation of possible overpressure scenarios suggests that in no case the pressure can exceed MAWP of D-101, it can be concluded that pressure is self-
limiting and D-101 is protected by system design. Hence, no relief device is required for overpressure protection.
Conclusion
In certain services, having Pressure relief devices adds to issues related to leaking and maintenance due to plugging. If it can be justified that the pressure vessel Table1: Comparison of Options for Overpressure Protection , (*) Fire case is not considered as a process upset scenario.
Equipment / System Protected
D-101 (Chlorine gas knock out drum)
MAWP @ Coincident Temperature
21.5 barg / FV @ 190 °C
new Daelim.indd 40 01-10-2013 21:37:09

4. 42 • September 2013 Chemical Engineering World
Features CEW
is designed to withstand all credible scenarios, UG-140 comes handy to meet the requirements of the code. In toxic services such as phosgene, it is desirable to protect the vessel by system design and/or by employing HIPS system. Pressure relief devices operating in phosgene service require proper disposal system and may create operational hazards. These devices also require critical procedures in place for inspection and maintenance.
HIPS typically finds its application in critical process operations handling hazardous services. On the other hand, having a relief device on the vessel provides a kind of insurance for the vessel. Decision to have a relief device to protect the vessel depends upon number of factors as service it is operated with, type of overpressure scenario and experience of engineers working with the technology. As there are number of factors which influence the decision making to protect the vessel and the decision being scenario specific, its users responsibility to decide the option to be followed for overpressure protection keeping in mind that the code permits not to have a relief device by way of UG-140 provided the detail evaluation is performed and analysis is documented to ensure no hazardous situation would arise due to overpressure that may lead to loss of containment or risk to operating personnel.
References
1. Pressure-relieving and Depressuring Systems, ANSI/API Standard 521, Fifth Edition, January 2007, Addendum May 2008.
2. ASME Boiler and Pressure Vessel Code VIII, Division 1, Rules for Construction of Pressure Vessels, Edition 2010
Definition of Terms
1. Design pressure: Pressure, together with the design temperature, used to determine the minimum permissible thickness or physical
characteristic of each component, as determined by the design rules
of the pressure-design code
2. MAWP: Maximum gauge pressure permissible at the top of a completed vessel in its normal operating position at the designated
coincident temperature specified for that pressure
3. Overpressure: Pressure increase at the relief valve inlet above the set pressure, when the relief valve is relieving (Overpressure is
expressed in pressure units or as a percentage of the set pressure.)
4. Relief device: Device used to relieve excess pressure and/or vacuum that has developed in a tank
5. Set pressure: The inlet gauge pressure at which pressure relief valve is set to open under service conditions
Author’s Details
Sushant G Labhasetwar (P.E)
Assistant Manager - Process Engineering Team
Daelim Industrial Corporation, Seoul Korea
Email: sushant@daelim.co.kr
Dear Readers,
Chemical Engineering World (CEW), the
Official Organ of CHEMTECH Foundaon, is
India's premier technology magazine for the
chemical process industry professionals. This
highly reputed monthly publicaon provides
novel insights on the dynamics of Indian and
global process industries. CEW, a niche
publicaon with proven track record, has
been disseminang authenc informaon on
process industry innovaons for more than
four decades.
You can contribute in the magazine with
technical arcles, case studies, and product
write-ups. The length of the arcle should
not exceed 1500 words with maximum three
illustraons, images, graphs, charts, etc. All
the images should be high-resoluon (300
DPI) and a2ached separately in JPEG or JPG
format. Have a look at our editorial calendar
on the website – www.cewindia.com.
To know more about Chemtech Foundaon,
Jasubhai Media and other publicaon and
events, please visit our website –
www.chemtech-online.com
Thank you,
Regards,
Mi$ravinda Ranjan
Editor
Jasubhai Media Pvt Ltd
Tel: +91 22 4037 3636 (Dir: 4037 3615)
E-mail: mi2ra_ranjan@jasubhai.com
Contribute in CEW
new Daelim.indd 42 01-10-2013 21:37:14