| American Fuel & Petrochemical Manufacturers

Question 56: Some crude tower overhead deposition appears to be linked to corrosion treatment programs (i.e. filming corrosion inhibitors and neutralizers). Have you confirmed this and what are the potential mechanisms that can lead to this deposition?

Corrosion inhibitors (filmers) have been known to cause deposition in several different ways. Generally, the cause is injection into an overhead that is too hot, flashing off the carrier, or injection of neat chemical, flashing off its solvent.

Question 55: What is considered the best-in-class design for coker main fractionator wash zones? Have refiners seen good performance and target run lengths using grids in this service?

Cokers today are normally designed to operate to maximize liquid yield from the unit. This will require the coker to operate at as low a recycle as possible while still maintaining the required product specifications for the heavy coker gas oil (carbon content, metals, asphaltenes, etc.). For a low recycle operation a spray chamber is the best-in-class.

Question 54: What is the current best practice for number of feed nozzles, angle, and location on coke drums considering the use of slide valves for the bottom unheading device?

There is not a one-size-fits-all answer to this question. However, we believe that an arrangement that closely matches the conventional bottom center upflow arrangement will provide more uniform thermal stresses on the drum and minimize operational impact.

Question 53: During the coking cycle, how are drum skin temperatures used to monitor drum wall condition?

Monitoring drum wall conditions during the drum cycle is best performed using strain gauges. Each strain gauge location will typically be fitted with gauges in both the axial and hoop directions and have an associated skin thermocouple.

Question 52: What areas of a delayed coker are susceptible to naphthenic acid corrosion? How do you determine the maximum allowable TAN for these areas?

The primary areas of concern include: •Coker feed circuit above 450 °F •Bottom section of the fractionator, including internals •Heater charge pumps and associated piping •Heater tubes.

Question 51: In your experience, what are the implications on coker heater run length and coke drum operations with the following feedstock quality: Contaminants (Na, Ca), low saturates or high asphaltenes, crude compatibility, solvent deasphalt (SDA) pitch, low asphaltenes and high saturates?

There are at least three main mechanisms under which the heater tubes become fouled: •Inorganic material deposition or precipitation, •Rapid asphaltene precipitation, •Coke formation

Question 50: Chemical cleaning of towers and vessels prior to entry is being used to reduce time to entry. What practices are you employing and how much time is saved?

Our chemical cleaning practices for Crude/Vacuum and Coking units have evolved over many years. The starting point in most cases, after feed is out and the unit has circulated down, cooled and pumped out, is a flush with diesel or LCO followed by a good steamout.

Question 49: What testing procedures do you use for emergency shutdown valves? What are the parameters you measure and what are acceptable values?

The appropriate required testing of emergency shutdown valves is included in each Marathon refinery’s Mechanical Integrity (MI) program, complying with OSHA’s Process Safety Management regulation and EPA’s Risk Management Plan regulation.

Question 48: In your experience, what is the preferred online (non-destructive) method to identify risk of HIC (hydrogen induced cracking) in gasoline processing units?

The preferred method of identifying the risk of hydrogen induced cracking is to measure the permeation or flux of hydrogen on the outside surface of the equipment and correlate this to the corrosion rate on the inside of the pipe.

Question 47: What are the best practices for corrosion probe selection, installation and reliability, especially in high temperature and/or high H2S environments?

Some things to consider when selecting and installing corrosion probes are to match the metallurgy of the probe to the pipe.