Push Dhillon reports on a heat exchanger designed for maximum uptime in minimum space
Offshore, more than anywhere, uptime is critical. Equipment fouling, failing or scaling can cause expensive downtime, loss of production or even safety- and environmental hazards. Truly robust high-pressure heat exchangers that can fit a large thermal duty into a small footprint but remain readily accessible for all kinds of maintenance have not been available – until now.
When it comes to heat exchanger technology for high-pressure gas cooling, offshore oil and gas producers have thus far been limited to choosing between bulky shell-and-tube heat exchangers and the super compact printed circuit heat exchangers (PCHE), with their micro channels. Both technologies have their advantages, but also clear drawbacks.
Being highly compact, the PCHEs are more prone to fouling, as the fine flow channels are easily blocked. When that happens, efficiency and then output are reduced. In the worst-case scenario, fouling may even cause unplanned production stops. To deal with the fouling and scaling, a carefully monitored maintenance schedule is usually set up, including multiple identical units being rotated and shipped onshore for service, which unfortunately adds unnecessary cost and effort.
No more fouling problems
“Preventing and minimising the effect of fouling and scaling in the flow channels has been one of our top priorities in designing the DuroCore high-pressure offshore compression cooler,” says Magnus Hoffstein, manager Business Unit Gas at Alfa Laval. “To start with, the corrugated plates create turbulent flow, minimising the risk for fouling. Should there be fouling, however, the large open channels in the DuroCore are more forgiving, ensuring long, trouble-free operation without losing efficiency due to scaling on the cooling medium side or fouling on the gas side. The discrete micro channels in a PCHE can become completely plugged, and that’s a risk we completely eliminate.”
With the solution’s large channels, the need for sophisticated filtration will also be less pronounced, and a larger mesh makes maintenance easier.
“This new system is specifically designed to withstand variations in temperature and pressure,” explains Hoffstein. “To accomplish that, we had to move away from the traditional plate-and-shell design with its weak, peak-stressed areas. Contrary to the traditional corrugated plate-and-shell pattern, DuroCore’s ‘roller coaster’ plate pattern is equally strong in all directions. The result is a plate pack that is extremely robust against both thermal and pressure fatigue, even against extreme shocks.”
With shell-and-tubes, there is always a risk for tube vibrations causing leaks. The DuroCore eliminates that risk, as there are no tubes in the heat exchanger and the plate pack is designed not to vibrate.
“The key to the product’s ability to reliably master the challenges of offshore production lies in the design,” says Hoffstein. “By using large channels in the heat exchanger, we have designed away the need for frequent maintenance due to fouling and scaling. Yet the plate technology allows us to keep the unit compact, compared with a shell-and-tube for the same duty, saving precious deck space. We have also addressed the operators’ need for quick and easy maintenance, even offshore, by providing a core that is easily and entirely removable, with all its welds accessible for visual inspection.”
The heart of the cooler is a cylindrical core of corrugated, laser-welded heat-transfer plates in stainless steel or titanium, depending on the cooling medium. The gas enters the plate pack through the side connection, while cooling medium flows in from the bottom via the central connection.
Offshore gas compression is a demanding and process-critical duty where both PCHE and shell-and-tube heat exchangers have their pros and cons. With the DuroCore solution, Alfa Laval has focused on the operators’ need to keep production up and running, without interruptions. Hoffstein concludes: “DuroCore will solve many of the operators’ problems in offshore gas compression cooling. We can help them perform at their best.”
For more information at www.engineerlive.com/iog
Push Dhillon is with Alfa Laval.