Plate-fin heat exchangers, commonly referred to as brazed aluminum heat exchangers (BAHXs), are widely used in cryogenic fractionation and processing compared to other heat exchanger types because of their high thermal efficiency and reliability. Applications include air separation, natural gas liquefaction, nitrogen rejection, natural gas liquids recovery, ethylene production, hydrogen recovery and propane dehydrogenation.
Typical industry expectation for a BAHX life span is approximately 20 years, although experience shows units that have been operated diligently with respect to the operating and maintenance guidelines can double their expected service life. The majority of BAHXs are retired from service due to leaks attributable to plugging, cleanliness issues or thermal stress damage. Therefore, keeping them clean and dry and avoiding thermal gradients is the mantra for best operating practices.
Keeping it clean
A BAHX operates most efficiently when the stream passages are free of obstructions and fouling. It will have the longest life expectancy when corrosive or chemically reactive compounds are prevented from entering the heat exchanger. The best way to keep a BAHX clean is to ensure the incoming streams are appropriately conditioned before they enter the heat exchanger.
Plugging occurs when the passages inside the BAHX become obstructed, typically through particulate matter. In most instances, pipe scale and molecular sieve dust are the culprits, but construction debris can also be found.
Manufacturers recommend installing permanent upstream 80 Mesh Tyler Standard filters for all process streams to prevent debris from entering the BAHX. Establishing a program to clean upstream piping and vessels before commissioning and after construction, turnaround or repair is another recommended avoidance measure.
Keeping it dry
BAHXs typically operate at very cold temperatures; hence water will freeze inside and the resultant expansion can damage the aluminum fins. Hydrates are substances that contain water and can form snow or ice-like crystals at certain temperatures and pressures. CO2 is another substance that can freeze.
Fin ruptures cannot be repaired, so continuous dew-point monitoring of stream inlets and a properly functioning hydration system are essential. Although hydrates and CO2 do not expand, they can impair a heat exchanger's thermal performance. Hydrate formation typically occurs in natural gas feed streams, and prevention is accomplished through good process control and making sure stream pressure and temperature regimes don't overlap with operating conditions. Ensuring the CO2 concentration is below the threshold for ice formation is normally accomplished via an upstream removal system, such as an amine unit.
Avoiding thermal gradients
As well as being the most prevalent cause of BAHX failure, thermal cycling is also the least understood. Thermal gradients are temperature differences within the unit that cause thermal stress. Damage generally accrues slowly through fatigue due to repeated thermal cycling, although it can also manifest through a single, or very few, thermal shock events.
Thermal fatigue will eventually lead to leaks if it is allowed to continue and because BAHXs don't exhibit wear patterns during operation (like brake pads and tires); there's no reliable way of directly measuring damage before a leak occurs. Consequently, in the next issue of BIC, we will cover the subject in detail, including stating the acceptable rates of change, showing how to identify transient conditions and explaining how to avoid them. We will also demonstrate how past operating data can be used to identify specific events of concern and be coupled with distributed control system analysis to produce a health scorecard that aids fatigue failure predictions.
For more information, email info@ ChartLifecycle.com or visit www.ChartLifecycle.com.
