Taper Lok for heat Exchangers
Success in a High-Pressure Refinery Heat Exchanger Diaphragm Removal/Retrofit with Taper-Lok. This article describes the successful removal of a tube side cover plate diaphragm (gasket) from a high-pressure heat exchanger. The diaphragm is replaced with a metal pressure-energised Taper-Lok seal ring. While the technology has been utilised in piping and offshore applications for many years, this retrofit was its first known application on a refinery heat exchanger of this magnitude. Modification of the cover plate to accept the pressure-energised seal ring eliminated the need to reinstall the metal diaphragm gasket, thereby saving 75% of the reassembly cost of the exchanger.
Issue;
Utilised in conjunction with mechanical multi bolt tensioners, this innovative retrofit has eliminated a recurring, costly problem (in both maintenance cost and loss of opportunity). This additional reliability coupled with the significant future cost savings from less downtime justified the retrofit—not including the cost savings foreseen by preventing unplanned outages from the exchanger. This modification eventually could change the way that refiners will specify how high-pressure heat exchangers in hydrocracking services are designed, constructed and has is very successful. Issues.
It is not uncommon for diaphragm plates in high-pressure heat exchangers to develop cracks in their seal welds. The diaphragm, which is generally a thin plate of alloy steel, serves as the gasket and corrosion resistant liner for the channel cover (see above). This arrangement is common for heat exchangers in hydrogen services at operating pressures above 1600 psi in our refinery’s Gas Oil Hydrotreater (GOHT) unit. There have been multiple diaphragm leaks in refineries over the past several years on a global scale.
The repair process had been the same for some time and it entailed removing the diaphragm, machining the channel face, welding and re-machining a nickel layer on to the channel face and finally welding on a new diaphragm under controlled heat. This repair would suffice for a time, until some process upset or other anomaly would create another cracking “event.” This procedure had become the standard operation of repair and, in turn, our “insanity clause,” as sites continued to perform the same repair steps repeatedly, and yet, after returning the exchanger to service, would expect a different result, however after a short time leaks would reappear.
Seeking a solution The Reliability and Maintenance groups & site owners designed or investigated a handful of possible solutions, but none of them received any great recommendations. After several months of study, an innovative, alternate solution was identified. It involved eliminating the diaphragm plate entirely and replacing it with the pressure energised seal ring (see below). The solution from Taper-Lok? was simple, effective and quite field compatible within the timeframe of a shutdown.
Seal concept;
The Taper-Lok metal pressure-energised seal ring was designed to use on piping applications for topsides of offshore platforms, flow lines, production risers, manifolds, chemical plants, refineries, power generation, supercritical wet oxidation and numerous other practical applications. Most assemblies consist of a male flange, female flange, seal ring, and a set of studs and nuts. The pressure-energised seal ring seats into a pocket in the female flange and is wedged and seated by a male nose located on the male flange.
Utilising this concept, the exchangers channel cylinder would contain the female pocket, while the channel cover would have the male nose geometry, this also gave not just a great sealing area but also a capability of any misalignment.
In the pre-bolted condition, the Taper-Lok seal ring lip stands off of the face of the channel. The converging seal surfaces are brought together like a wedge during bolt up. This wedging motion forces the seal ring onto the male nose and into the female pocket forcing a compressive hoop stress. Minimal bolt load is required to achieve the required contact stress on the seal surfaces. The converging angles of the seal ring create a wedge or “doorstop” effect. As the equipment internal pressures increases, the seal seats tighter into this sealing wedge.
Taper-Lok seals are made from the same material as the process equipment (exchanger channel and cover) to ensure that thermal expansions are consistent across all components. The effects of bi-metallic (galvanic) corrosion are eliminated. A baked-on moly coating is applied to the seal to prevent galling.
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These seals also have performed well in temperatures from cryogenic to 1600 F and at pressures to 40,000 psi. Applications of note included:
Subsea Applications
The Taper-Lok Compact Engineered flanges are able to handle high fatigue and high bending moments. We can optimize the design of the connector as needed to meet project requirements. The design usually has less weight and a smaller outside diameter than ANSI or API flanges. In addition, our connectors typically use smaller bolt sizes and less bolts that allow for easier handling and quicker make up times. Even faster make-up times can be realized when using our Taper-Lok spherical nose feature that allows for 2-3 degrees of misalignment. Applications include pipelines, flowlines, risers, spools, tapered stress joints and flex joints. Taper-Lok Connectors with a male/female self-aligning flange configuration installs up to 75% faster and easier than ANSI or API flanges.
CNOOC PL 19-3 Phase II project was installed 20 days early and saved the client $3 million due to the ease of installation. Weld neck and swivel flange configurations are available and have been used extensively on subsea projects around the world.
Design codes;
The Taper-lok is designed under ASME B31.3, ASME VIII Div 1 & 2 and DNV Rules DNV OSF 101.
For further information on Taper-Lok please visits the AFGlobal website or please contact Mark Lamyman at AFGlobal Corporation.
Published by
Mark Lamyman [email protected]
International Business Development Manager at AFGlobal UK Ltd an AFGHoldings Company