Valve actuators introduce intelligent control

Rotork valve actuators introduce intelligent control for crucial ethylene production process.

Rotork has introduced intelligent valve actuation and two-wire sequential control as an alternative to the traditional mechanical linkages on a crucially important part of the manufacturing process for the petrochemical ethylene.

The Rotork package assures the accurate operation of the process and removes the mechanical constraints imposed on the construction of the plant pipework by a fixed linkage system, which dictates that the two valves involved must be positioned next to each other.

The Rotork package controls the heater de-coking operation, which is essential to prevent the build-up of carbon granules in pipework and valves that can lead to problems with the vital supply of heat to the ethylene production process.

De-coking is performed by two valves, the transfer line valve and the de-coke valve, whose positions must be reversed in a synchronised operation for de-coking to be performed.

Synchronised operation is critically important as ethylene is a volatile substance with a low flash point, imposing a fire and explosion risk if it is vented to atmosphere.

The control system must operate the valves without imposing a high differential pressure on the transfer valve whilst maintaining a positive pressure between the heater and the process.

The Rotork equipment consists of hazardous area approved IQ intelligent valve actuators, controlled remotely over a 2-wire link by a Rotork Pakscan llS sequencer master station.

A local control panel is provided to show valve position information and the status of the control sequences, and from which the de-coke operation can be initiated.

The master station is programmed to move the actuators in accordance with pre-set profiles provided by the valvemaker to ensure that the differential pressures are maintained in the correct ratio to prevent any backflow of the medium.

Each IQ actuator transmits a highly accurate position measurement signal to report and confirm the valves' positions.

This data is collected by the Pakscan master station and communicated over a serial link to the plant DCS or other such supervisory system.

Sequence interlocks programmed into the master station ensure that neither valve can be moved if there is a fault, which is indicated by warning lights on the local panel.

The system is also able to cater for different flow rates from the heater.

In addition, because each Pakscan llS master station can support up to 32 field units for data collection and control, extra information such as differential pressure readings from critical plant areas can be used in the control scheme.

The Rotork system has been adopted by Zimmermann and Jansen, a major manufacturer of the valves used in this task, and has recently been installed in ethylene production plants in India and China.

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