All FCC units are designed with slide valves on catalyst transfer lines. In some units these are used continuously for controlling catalyst flow, while in others they are used only in Startup, Shutdown, or Emergency situations. Whatever their
application, these valves must operate reliably in the service for which they are intended. In control applications, it is obviously important that slide valves operate with good response characteristics. In emergency applications, it is usually critical that the slide valve(s) close when required to do so, to permit discontinuing feed charge to the unit, and to put the unit in a safe condition. Situations in which slide valves “stick” result in either unsatisfactory control, or can jeopardize safe and reliable unit operation.
What causes slide valves to stick?
These are some causes:
- Distortion of slide guides
- Distortion of slide disc
- Inadequate clearances between disc and guides
- Distortion of valve body
- Fouling/ binding with foreign matter (e.g. catalyst)
- Inadequate power of valve actuator
These are some solutions:
Distortion of guides or discs can occur over extended periods of operation, particularly if the valves are exposed to severe temperature excursions. During a shutdown or turnaround opportunity, these should be checked to ensure trueness of both the guides and the disc.
Clearances between guides and disc must also be maintained as recommended by the slide valve supplier or local refinery specialists. These may need to be adjusted during a shutdown or turnaround if a particular slide valve has had a history of sticking.
Distortion of valve body can occur if great abnormal stresses are imposed on the valve; for example, due to improper support of large piping attached to the valve. Another reason can be temperature stresses due to incorrect thermal protection, i.e. cold wall valves that are externally insulated, or hot wall valves where the required external insulation is missing.
Fouling between guides and disc can be avoided by the correct placement and rate of purges usually provided in the valve design. The recommendations of supplier and/or local specialists should be followed.
Inadequate actuator power. In some instances, the valve may appear to be sticking, but the fault is actually with an actuator which does not have sufficient power to efficiently control movement of the slide valve. This can be the case with older units which may still have air-motor actuators. In those cases, replacement with modern hydraulic systems should be considered.