Expander Butterfly Valves

Whenever flue gas enters the expander lines from the catalyst regenerator of the FCC unit, it passes through one of the following Butterfly Valve types:

These valves are designed for minimum pressure drop since the main pressure drop should occur across the expander. The size of control and shut-off butterfly valves is determined by the flue gas volume.

Under normal operation the high flow by-pass valve is closed and the low flow by-pass is controlling, while the shut-off valve is fully open.

Low flow by-pass valves are active at all times. The relatively small size of the valve is due to to the fact that they take the full gas pressure drop.

High flow by-pass valves are normally closed. They are designed to direct the flue gas flow directly to the boiler burners. In order to prevent any large back pressure peak in the regeneration in the event of an emergency shutdown of the expander, this valve must be able to open at the same rate of speed.

Control valves are designed to regulate the flow to the expander and the expander speed. They are able to close as rapidly as a safety shut-off valve.

The shut-off valve is the main protection of the expander and in order to do so it needs to be tightly sealed and to have the capability to close at the required speed. This valve is activated by a signal from the expander control.

Regardless of the type, the main components of a butterfly valve are body, shaft, disc, springs, bushings and stuffing boxes.

All body internal surfaces are hard-faced to minimize erosion caused by catalyst particles and the extension and shape of hard-faced areas are calculated with the aid of Computational Flow Dynamics analysis.

To provide the required mechanical strength at the design temperature, minimize its size and consequently maximize the flow coefficient Cv, the shaft is made of INCONEL X-750.

In order to minimize friction losses and torque unbalancing the disc shape too is determined with the aid of Computational Flow Dynamics analysis.

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