Search

Frank Tracy (ConocoPhillips)

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. These can be seen in the photograph.

The largest stresses typically occur during the water quench portion of the drum cycle. Strain gauges allow a refiner to minimize stresses during this critical portion of the drum cycle by adjusting quench water rates. However, this is a longer-term project as you must evaluate enough cycles to have representative data for evaluation before making an operating change and then collect enough data afterwards to make a meaningful comparison.

Selecting the locations for strain gauge installation is something that requires some careful consideration. Strain gauges can be retrofit onto existing drums and are recommended as part of new coke drum installations. Typically, it is only necessary to fit strain gauges on one drum in each set of identical drums receiving the same feed since all drums will behave similarly.

Skin temperatures alone do not provide the same information as strain gauges. They can be used to help ensuring adequate back warm. They may be useful, if strategically deployed, to possibly identify uneven quenches.

We have used or are currently using strain gauges at several of our cokers and plan to use them at some additional locations.

Eric Thraen (Flint Hills Resources)

We have conducted several coke drum stress studies using outside consultants. Coke drum skin temperatures and strain gauges are included in the stress studies. The strain gauges are used primarily for these engineering studies, while the temperature indicators are used by the operating personnel, along with quench water flowrates and coke drum pressure rate of change during the drum cooling step to manage the rate of cooling so as to meet the targeted coke drum lifecycle. Quench water flow and coke drum pressure are included in the coke drum cooling program as added checks to ensure against too rapid quenching during the cooling cycle. The minimum coke drum preheat prior to switching feed into a drum is based on strain studies by the outside consultants. Although the stresses during water-cooling are generally highest, a minimum drum preheat is also established to prevent excessive stresses during the drum heating cycle.

Eberhard Lucke (Commonwealth E&C)

Drum skin temperatures are mainly monitored in the critical areas around the drum skirt attachments and the first 1-2 circumferential welds from the bottom in the cylindrical portion of the drum. The data from these skin temperature elements can be used in the DCS to monitor the change in temperature over time during the critical stages of the drum cycle, the drum quench and the drum warm-up/switch. Keeping the dT/dt in a certain range will minimize the thermal stress on welds and drum wall and will increase the lifetime of the coke drums. These dt/dt values can be integrated into the control scheme for the quench water ramp function, the warm-up controls and the drum switch control, depending on the degree of automation you have in your unit.

Frank Tracy (ConocoPhillips)

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.

Within ConocoPhillips we have multiple cokers that have retrofitted valve-type unheading devices onto bottom heads and use single side feed entry. These drums have not had any significant operational or mechanical issues resulting from the retrofits to date. We also have one coker, which still has manual bottom unheading, that has had dual side entry for more than fifty years. We have one new coker that is in construction that will utilize dual side feed entry.

One ConocoPhillips Coking Technology Licensee installed valve-type unheading with single-side entry. Upon converting to single side entry, they experienced significant temperature gradients in the drum resulting in drum deflection and a permanent “banana” shape as well as an increase in blow outs. They are converting to dual side entry and early reports are that this has improved their operation back to where they were with original bottom center upflow arrangement.

Because there are so many factors to consider; including feed properties, operating conditions, and drum geometry; we are using CFD modeling to aid in assessing the number, location, and entry angle for feed nozzle(s). CFD modeling in this application is still a work in progress for us, but we believe it is a useful tool. In some cases,

the additional complexity and cost of dual-side feed entry is warranted while in others it is not.

The next three CFD model snapshots illustrate: 1) bottom dead center feed, 2) single side entry for a case where single side entry might be selected, and 3) a case where single side entry as configured might not be recommended. These are different drums with different feeds, conditions, and geometries. ConocoPhillips have CFD modeled several variations around single vs. dual side entry, location of feed, and entry angle.

For dual-side entry there must also be a reliable piping system that is designed to support the nozzle configuration.

ConocoPhillips have licensed four retrofits to dual feed nozzle systems and can provide Licensing assistance in this area.

Finally, DeltaValve have a prototype for a retractable feed device that attempts to simulate bottom dead center feed entry. This is reported to be going into trial sometime in the fall of 2010.

Jim Johnson (Marathon Petroleum)

It is our understanding that industry is leaning toward dual feed nozzles at a 45-degree angle to the bottom cone when installing slide valves. Marathon’s refineries have nine drums with single side entry and have no issues. We did not see our hot drum occurrence increase when we converted from a bottom entry to a side entry when installing the slide valves.

Eberhard Lucke (Commonwealth E&C)

I have seen no major problems with the single inlet nozzle on the side of the drum cone, if designed properly. I also haven’t seen any significant advantage in using multiple feed points around the cone. The single feed injection point should be angled slightly upwards. I like the concept of going back to the old “bottom inlet” flow pattern with the new slide valve designs. I would have to see that one in operation though before I would recommend it.