Brazed Aluminum Heat Exchangers Operation and Maintenance for Asset Integrity

Introduction

Brazed aluminum heat exchangers (BAHX) serve a critical role in various industrial cryogenic processes, including processing and liquefaction of natural gas, air separation, hydrogen and helium liquefaction, and petrochemical applications around the world. While there is a significant body of literature on how to operate and maintain a BAHX, personnel in mechanical integrity roles often have difficulty locating and implementing this information. This article addresses many of the operation and maintenance issues associated with BAHX units and provides a helpful reference for further reading.

Operation Basics

Since a BAHX typically operates cryogenically, it is usually covered with slab insulation or installed in a cold box and buried in insulation. The insulation shrouds the BAHX, making what is covered by it unfamiliar to those operating the equipment.

There is a simple mantra for properly operating a BAHX: keep it clean, keep it dry, and manage thermal gradients. While a myriad of issues can lead to premature equipment retirement, these three are the main culprits.

Keep It Clean

BAHX units can be cleaned, but it is not easy, and the result may not restore the unit to “like-new” conditions. To avoid this issue, it is best practice to use proper filtration. ALPEMA Standards recommend a 177-micron screen, equivalent to 80 mesh Tyler filters.

Keep It Dry

BAHX units typically operate at temperatures below the freezing point of water. Water expands when it freezes, leading to potentially detrimental effects on the brazed joints and pressure retaining ability. It is important to minimize the time between when the unit is opened, which releases the dry shipping purge, and when it is connected to the plant piping. Covering the unit to limit or eliminate any debris or water ingress is critical. Finally, the unit should be purged with warm, dry gas at -40°F dewpoint to dry out all moisture before start-up.

Manage Thermal Gradients

To avoid the ramifications of thermal fatigue and stress, it’s critical to follow ALPEMA standards on operating a BAHX. Below are some suggested guidelines to minimize thermal fatigue:

1. Limit the temperature difference between the streams at any point along the exchanger length to 50°F (28°C) (See Figure 1).
2. If a stream temperature difference at introduction exceeds 50°F (28°C), introduce the flow slowly (crack the valve) until the stream temperature difference is within 50°F (28°C) and then slowly ramp the flow rate to full flow.
3. For frequent events (what could be labeled steady state flow) limit the cyclic stream temperature fluctuations to 1.8°F (1°C).
4. For infrequent events like startup and shutdown, limit the stream inlet and outlet temperature rates of change to less than 108°F/hour (60°C/hour) with allowance up to 9°F/min (5°C/min).