In all petrochemical and refinery processes, various types of thermal equipment, such as heat exchangers and turboexpanders, are used to create the necessary temperatures, and separation equipment, like drums and separation columns, are utilized to separate different components of the feed or to purify the output product. In cases where the process requires ultra-cold (cryogenic) temperatures, some of these thermal and separation equipment are placed inside a cold box based on the needs of the unit.
The pipes of this cryogenic equipment must be kept in vacuum insulation and completely dried using specific methods before operation to prevent damage to the cryogenic equipment.
This paper aims to review the different types of this equipment, the standards and requirements for their maintenance and repair, and the leak detection methods employed.
What is a Cold Box?
A cold box is a cryogenic thermal system enclosed in a carbon steel casing. Typically, only one cryogenic heat exchanger is placed inside the casing (in some cases, like air separation units, other cryogenic equipment, such as a combination of heat exchangers, condensers, and separation columns, are also included). Perlite (used for thermal insulation) is filled inside the casing. To ensure that the perlite remains fresh (as moisture absorption reduces its insulating properties), pure nitrogen gas must always flow inside the steel casing as a purge stream.
Heat Exchangers Used in Cold Boxes
The cryogenic heat exchangers inside the cold box are categorized as follows:
- Plate fin heat exchangers such as Brazed Aluminum (BAHX)
- Stainless Shell & Tube
- Printed Circuit (PCHE)
- Spiral Wound Heat Exchangers (SWHE) such as Coil Wound Heat Exchangers
BAHX is commonly used in most units due to the need for significant heat exchange between numerous streams. However, in LNG units (which are not available in our country), a combination of Spiral Wound and BAHX heat exchangers is used inside the cold box.
Helium Leak Detection in Cold Boxes
Helium leak detection is the most effective method for identifying even the smallest leaks (down to the atomic diameter of helium). In cold boxes, leaks result in pressure drops and temperature changes, reducing the purity of materials and the overall efficiency of the unit.
There are various methods for detecting leaks in cold boxes and turboexpanders, which are beyond the scope of this article. However, the most common issues include:
Cross-stream leaks: These leaks reduce the performance of the cold box and cannot be detected without precise methods like helium testing.
External leaks: These leaks often cause frost formation.
Units with Cold Boxes
Cold boxes are found in units like Olefins, LNG, LPG, ASU (Air Separation Units), and Ethane Recovery (typically found in gas refineries). According to ALPEMA standard (5.6.2.2) and API 668 (7.9.3 B), helium leak detection is mandatory for the repair of BAHX exchangers. Additionally, API 617 mandates helium leak detection for turboexpanders, which are often connected to cold boxes (section 6.3.7.3).
Procedure
Depending on the type of cold box exchanger, the number of streams, and operating conditions, different methods such as the hood test can be employed for leak detection.
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Conclusion
In summary, if there is a reduction in efficiency in air separation units (ASU), such as nitrogen production, or in olefin units and units separating ethane from natural gas and LPG, conducting leak detection in the cold box, as well as the related equipment and pipelines, according to standard requirements, can help restore efficiency to its previous state.