Some creative thinking and willingness to take the initiative helped resolve a significant recurring reliability issue, all during a quick turnaround window.

This article describes the successful removal of a tube side cover plate diaphragm (gasket) from a high-pressure heat exchanger. The diaphragm was replaced with a metal pressure-energized seal ring. While the technology has been utilized in piping and offshore applications for several 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-energized seal ring eliminated the need to reinstall the metal diaphragm gasket, thereby saving 75% of the reassembly cost of the exchanger.

Utilized in conjunction with mechanical multi-jack 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.

It is anticipated that this modification eventually could change the way that refiners will specify how high-pressure heat exchangers in hydrocracking services are designed and constructed.

The problem
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 (Fig. 1). This arrangement is common for heat exchangers in hydrogen services at operating pressures above 1600 psig in our refinery’s Gas Oil Hydrotreater (GOHT) unit. We have had multiple diaphragm leaks over the past several years.

Until recently, the repair process had been the same. It entailed removing the diaphragm, machining the channel face, welding and re-machining a nickel “butter-coat” layer on

This procedure had become the standard operation of repair and, in turn, our “insanity clause,” as we continued to perform the same repair steps repeatedly, and yet, after returning the exchanger to service, would expect a different result.

Criticality of process [1]
Hydrotreaters process feedstock for fluid catalytic cracking units (FCCU) and hydrocrackers. The economic impact of these conversion units are crucial to a refinery’s profit and loss...(Read whole article)