Background
The explosion and fire involving a High-Pressure Breech Lock Heat Exchanger in Japan in 1992 and subsequent incidents of failure reported from several other parts of the world resulted in the diminishing acceptability of Breech Lock design. TEMA India conducted extensive research to make this design safe and free of maintenance problems.
Identifying the problem
FEA analysis carried out on standard units revealed that undue stresses develop on account of uneven thermal expansions. This results in bell-mouthing in the threaded region, reducing the shear area of the threads. In extreme circumstances, this can further lead to the plug being expelled, thus causing an explosion.
The other condition often experienced is known as thread galling or seizing. Given the large amount of clearance in the threads, there is an increased chance of the plug being displaced, resulting in thread galling. It occurs when surfaces of mating parts are of the same metallurgy and there is loss in lubrication due to displacement of the plug. Thread galling can become severe enough for the mating parts to seize, making it virtually impossible to separate them and when tried to pry open, a seal lock is created.
Finding a solution
The magnitude of bending stress can amount to three times the average longitudinal stress in the undercut area. The combined bending and longitudinal stresses on the undercut surface of the threads produce a wedging action that results in radial displacement, causing dilation of the channel. This dilation – a cause of reduction in the shear area – has to be restricted by increasing wall thickness in the threaded region.
Extensive structural analysis established the need for added steel on the threaded portion of the enclosure. Destructive testing and other experiments on prototypes revealed that the added steel restricts distortion by creating positive stresses and results in easy removal of the plug.
Patented shrink-ring technology
TEMA India adopted its shrink-ring technology of added steel on the threaded area of the closure to overcome the phenomena of bell mouthing and thread galling (for hi-hi and hi-low applications) and was awarded an Indian patent for it in 2009.
The prototype of the modified design has been hydro-tested several times under several pressure and temperature conditions. These experiments were conducted to verify and adopt this thicker section on threaded area of the closure technology and were witnessed by representatives of a number of global companies from time to time.
Two more patent applications for Internal Cylinders for Screw Plug Heat Exchanger and In-situ modification of Screw Plug Heat Exchanger provisionally accepted by the Patents Registry authority await final issuance.
In addition, there have been innovations such as Recordable Ultrasonic Testing, Tube Bundle Insertion Machine, In-house Furnace for Post-weld Heat Treatment, procedure for Electro Slag Strip Cladding with 90 mm strip, and Double Operator Technique using GTAW process for Stainless Steel.
Presentation of Technical Papers at the ASME PVP Conference
TEMA India Managing Director Haresh Sippy became the first Indian to present a paper on screw-plug design at an ASME PVP Conference when his paper An Improved Design of Threaded Closures for Screw Plug (Breech Lock) Heat Exchangers was accepted and presented at the 50th ASME PVP (Pressure Vessels & Piping) Conference in Vancouver, Canada on 20 July 2016. It was well received as it showcased proven solutions to the problems refineries face the world over. The copyrights have been assigned to ASME. The paper is available for download at http://proceedings.asmedigitalcollection.asme.org/proceeding.aspx?articleid=2590185
The paper addresses the two critical problems that arise while deploying Screw Plug (Breech Lock type) Heat Exchangers, these being the unscrewing of the plug during maintenance and the intermixing of fluids contained in the shell and the tube.
The findings have been assimilated into the technology used to design heat exchangers for refineries across the globe. The analysis and subsequent findings have helped refiners retrofit and refurbish exchangers in situ during routine turnarounds. The technology also plays a vital role in the upgradation of refineries to comply with US EPA Tier 4/ EURO VI emission standards.
In continuation with the paper presented in 2016, ASME accepted another paper titled Design of Threaded Closures for High-Pressure Screw Plug Heat Exchangers Designed to ASME Section VIII Division 2 which was presented at the ASME PVP Conference in Waikoloa, Hawaii on 20 July 2017. This paper will be available for download from the ASME website by December 2017.
Application of patented technology
In India, refineries have expressed concern over leakage in high-pressure exchangers as it hinders their ability to produce clean fuels following the stringent norms of BS-VI. With the Indian government advancing the implementation of the BS-VI standard to 2020, TEMA India’s patented Screw Plug (Breech Lock type) Heat Exchangers will play a crucial role in refinery upgradation projects and new refinery installations.
Using its patented technology, TEMA India has manufactured and installed over 160 such Screw Plug (Breech Lock type) Heat Exchangers at refineries across the globe. All of them continue to operate at full efficiency, making TEMA India arguably the only manufacturer in the world to be able to make this claim. The technology also augments the company’s contribution to the Make In India program of nation-building.
