Automated Coke Drum Switching and Safety Interlocks

How the System Works

Figure 1

Figure 1 is a drawing of a pair of coke drums and piping systems with automated valves. This drawing can be used, throughout this section, as a reference.

Once the decision has been made to automate and interlock the drum switch, the designer needs to follow the existing coke drum switch procedure as close as possible when building the interlock matrix. The reason for this is to keep changes, to what the operators have been doing for years, to a minimum. When reading through this procedure, remember that some valve movements have probably occurred simultaneously. These valves must be opened and closed in a sequence if they are incorporated into the interlock system. In an interlock system, certain conditions must be met for an interlocked valve to be opened or closed.

Most refiners keep a log of near miss reports or incident reports that have occurred during coke drum switching activities. This is a good starting point for deciding what type of incidents the interlock will prevent from occurring and the magnitude of automation and interlock to be installed.

Basically, to build an interlock matrix, one must walk through the coke drum switch sequence. Starting with the air free of the coke drum, pressure test, warm-up, switch, steamout, drum cooling, to the coke drum vent and drain. Each valve movement through the switch sequence should be looked at as far as a “what can happen” scenario.

Going through the sequence for a typical drum switch starting with air freeing the coke drum, the first valve to open is the steam to the drum at the utility manifold. Normally there are no conditions that need to be met to make this valve movement.

The next step would be to pressure test the drum. The coke drum vent valve must be closed to do this step. If the coke drum has some type of eductor system, for top head removal, it would be necessary for the interlock to verify that the valve to the eductor has been closed to prevent pulling a vacuum on the coke drum. Some refiners block the coke drum safety valve on the discharge when the drum is taken out of service for decoking. If this is the case then the system needs to verify that this valve is opened to prevent over pressuring the coke drum during the pressure test and to ensure that the safety is lined up when the drum is switched into.

The next sequence is the coke drum warm-up. The warm-up/utility isolation valve and the warm-up valves will be opened along with the coke drum overhead vapor valves. When the warm-up/utility and warm-up valves are opened the system needs to verify that the coke drum inlet isolation valve is closed to ensure isolation from the hot feed, and the utility isolation valve is closed to ensure isolation from the condensate generated from the warm-up step. To complete the valve movements for the warm-up step, the coke overhead vapor valves must be opened. The system needs to verify that the coke drum atmospheric vent valve is closed to prevent the release of hydrocarbon vapor to the atmosphere. The coke drum blowdown valves can be interlocked closed at this point to prevent depressuring the fractionator to the blowdown system. This particular interlock will prevent operational upsets. The blowdown valves are already in the interlock system for another situation, so the system can act as a monitor of valve positions associated in the interlock system to prevent not only safety incidents but also operational upsets.

Prior to switching the coke drum many refiners dryout the steam at the utility manifold, of the full drum, in anticipation of the switch. Several valve movements occur with the coke drum switch that are included in the interlock. The first valve is opening the inlet isolation valve on the empty drum or drum being switched into. The warm-up/utility isolation valve should be closed. This step ensures isolation of the hot feed from the warm-up and utility systems. Move the switch valve from the full coke drum to the empty coke drum. The system verifies that the inlet isolation valve on the empty drum is open to prevent blockage of process flow through the heater. The system also verifies that the coke drum overhead vapor valves are open to ensure a pathway and to prevent the coke drum safeties from lifting. Again the system can act as a watch-dog by verifying that the warm-up/utility isolation is closed and the coke drum atmospheric vent valve is closed to prevent the release of hydrocarbons to the atmosphere.

The next step in the sequence is to inject steam into the full coke drum or the drum that was switched out of. For this step to occur, the utility isolation valve and warm-up utility isolation valve must be opened. When the utility isolation valve is opened the system verifies that the warm-up valve is closed to ensure isolation from that system. The warm-up/utility isolation valve will be opened next to allow steam to enter the full coke drum. Again the system verifies the warm-up valve is in the closed position. At this point the system verifies the position of the switch valve making sure that the valve is set filling the opposite coke drum. The next valve to close is the feed inlet isolation valve on the drum that has just come off-line or is steaming.

The next step in the sequence is to put the off-line coke drum overhead vapors into the blowdown system. The valve movements in this sequence is opening the blowdown valves and closing the overhead vapor valves. At this point one of the blowdown valves can be a ball valve with throttling capability along with one of the overhead vapor valves. Installing throttling capability will try and minimize the fractionator upsets when swinging the overhead vapors from the fractionator to the blowdown system. When the blowdown valve is opened the system verifies that the feed inlet isolation valve is closed to ensure the coke drum is off-line. When the overhead vapor valves are closed the system verifies the position of the switch valve being into the opposite drum or in the circulation position. This will prevent process flow blockage through the heater.

Once the on-line drum overhead vapors have reached operating temperature then the coke drum overhead line flushing oil is opened to the on-line drum. The flushing oil is injected to minimize coking in the coke drum overhead vapor system. The interlock system will again verify that the overhead vapor valves are open ensuring this drum is on-line and hot flushing oil is not opened up into an off-line drum.

Once the steaming is complete, water is opened to the drum for cooling. This step doesn’t require an interlock. When drum reaches a satisfactory water level, usually the top level detector, and overhead temperature and pressure is low enough, the coke drum vent and drain can be opened. The interlock will verify that the feed inlet isolation valve is closed to ensure the drum is off-line. Also the system will verify that the overhead vapor, blowdown, flushing oil, and antifoam injection valves are closed to prevent a release of hydrocarbons to the atmosphere.

The interlock system can be as complex as what refiners want to make it. Whatever decision is made and the complexity of the system, this technology exists and refiners are using it. Refiners must commit to maintaining the system, as is for any new technology or equipment installed in their plants.