UNDER PRESSURE

This month we’ll continue our discussion of emergency procedures for hydroprocessing units with a discussion of hazards and procedures for minor/major loss of containment incidents.

Incidents related to emergency depressuring valves in hydroprocessing units

Incident 1

A VGO hydrotreater suffered a backflow incident on the wash water piping resulting in the overpressure and rupture of the wash water surge drum. One result of the surge drum rupture was the destruction of the emergency depressuring valve for the unit. Since the unit could not be depressured through the emergency depressuring valve, operators had to stand by and allow the unit to depressure through the subsequent fire before securing the unit. It is believed that the inability to depressure the unit caused substantial added damage that could have been avoided

Incident 2

A severe fire occurred in a crude unit which was located next to a hydrocracking unit and also next to the section control room.  Fear of the fire spreading prompted the operators to execute an emergency shutdown on the hydrocracker using the emergency depressuring valve. Shortly after depressuring the unit, operators had to evacuate the control room. It was fortunate that the hydrocracker was secured by depressuring prior to the evacuation.

Incident 3

The feed pump on a coker hydrotreater suffered a catastrophic mechanical failure allowing the piston to become uncoupled and ejected from the reciprocating feed pump. The contents of the high-pressure loop flowed backwards out the open cylinder causing a severe fire. The fire burned up the control cables in a nearby rack disabling all control functions in the unit. Operators were unable to activate the emergency depressuring valve and the fire burned until the unit was fully depressured. Some of the unit damage might have been avoided if the operators were able to depressure the unit to flare rather than being forced to allow the unit to depressure through the ruptured feed pump and continue to feed the severe uncontrolled fire. .

Incident 4

A cat feed hydrotreater suffered a seal failure on the recycle compressor leading to a substantial fire. The emergency depressuring valve on the unit was located just above and near the recycle compressor. The fire disabled the operation of the emergency depressuring valve. An operator was able to reach and open a manual valve to depressure the unit at substantial personal risk. In this case, the emergency depressuring valve was not a fail-open valve.

Lessons from Loss of Containment Events

1. In the incidents described above involving a fire in the hydroprocessing unit, the emergency depressuring valves were not designed and installed correctly. Emergency depressuring valves should fail open on loss of air, control signal, power, or any other possible failure mechanism.
2. Emergency depressuring valves are valuable for stopping a runaway reaction, but they are also critically important for getting the pressure (energy) out of a unit in case of a leak or fire.

Emergency Depressuring System Requirements

A leak or fire in a unit is the primary reason to have an emergency depressuring system in all hydrotreating and hydrocracking units. Hydrocrackers should have the ability to manually trigger the automatic depressuring system. Although hydrotreaters don’t normally have runaway events, a fire or leak in the unit is a good reason to have a manually activated emergency depressuring system.

API 521 requires that all units (especially hydroprocessing units) be able to lower the pressure to 50% of normal operating pressure within 15 minutes. Additional criteria for the design of depressuring systems include:

1. Capacity to depressure the unit entirely within 30 minutes
2. Emergency depressuring valves should be tight sealing. They should never be blocked in by operations to prevent leakage to flare.
3. Emergency depressuring valves should fail open on loss of air, signal, power, or other failure mechanism as the final fail safe to put the unit into a safe posture during an emergency. Fireproofing insulation may be required.
4. Emergency depressuring systems should have a manual push button (protected against inadvertent activation) at the console and in the field to activate the system. Having an activation button hidden among all the control screens will be extra frustrating when the systems must be activated immediately.
5. The valve actuator must be capable of opening the emergency depressuring valve against the full system pressure on one side of the valve.

Emergency Response Procedures

Here is a rough outline of the recommended procedures for loss of containment:

• Large leak, rupture, or furnace tube failure:
  1. Evacuate all personnel from the unit
  2. Activate emergency depressuring system – channel the energy to a safe outlet in the flare system
  3. Chop furnace fires
     • If failure is not a furnace tube
     • Keep minimum fires for a furnace tube failure to burn the toxics and avoid creating a vapor cloud
  4. Depressure unit completely and put it under N₂ to protect catalyst

• Small leak:
  1. Use best judgement on course of action and potential shutdown
  2. Beware of building up a vapor cloud and possible detonation

Application in your unit(s)

1. Does your unit have emergency procedures for both minor and major leaks and loss of containment?
2. Does your unit have an emergency depressuring system as described in this article?
3. Is the emergency depressuring system tested regularly?
4. Are your operators trained in the emergency procedures to safely shut down the unit?
5. Do your operators regularly have drills on the emergency procedures?

As always, we welcome and appreciate feedback, questions, comments, and suggestions on this topic and other topics.