Michael Herbstritt reports on valve automation carried out during extension to an Indian refinery.
The Mahul refinery near MumbaiIndiais the third largest site of its kind in India. It is operated by BPCLBharat Petroleum Corporation Ltd. During the past few monthsthe refinery was extended. A plant for manufacturing motor lubricants was builttogether with a tank farm for the products (LOBS = Lube Oil Base Stock). In April 2006 the new plant sections were put into operation. This article deals with the valve automation for this project.
Actuators are geared motors which have been specially designed for the requirements in valve automation. This includesself-locking gears for examplehigh starting torques or self-locking. Actuators are equipped with sensors for measuring the travel and the torque present at the valve.
Modern actuators have integral controls which automatically disconnect the actuator when switch-off criteria occurbasically when a valve end position is reached or an excessive torque is determined.
Electric actuators are always equipped with a handwheel for emergency operation. Compared with pneumatic actuators their advantages lie in the simple energy supply with electricity and that even valves with large strokes can be easily automated. Contrary to the pneumatic actuatorsthe enhancement of the basic version eg position controllingno additional components have to be mounted to the actuator. Everything is located in a single housing in high enclosure protection IP67 or IP68.
The tanks have to be filledemptied and in a mixing plantthe various products with different viscosity classes can be mixed. Thirty-nine valves equipped with electric actuators are required for this. The tender contained the requirement for an integral solutionie not only the supply of actuators but a system solution for the automation of the valves situated in the tank farm. Therefore this plant section can be installed and commissioned without influencing the operation of the existing plant sections. The following points had to be considered:
- Relief of the control system by an intermediate master. From the perspective of the control systemall 39 actuators are to be represented in one single interface.
- Redundancy of the data channels.
- Master redundancy
- Usage of open fieldbus protocols (Modbus-RTU)
- Short reaction and cycle times
- Actuators in explosion-proof version
- Setting of the actuatorswithout disabling the explosion protection of the devices.
The task was solved as follows (refer to plant layout Fig.4):
- Two master stations control the communication between the control system and the actuators. One of the two masters operates in stand-by mode and synchronises itself permanently with the active master. If requiredthe communication processing is automatically resumed by the stand-by master.
- The master stations monitor and control the multitude of automated valves and concentrate all process-relevant data in one single interface to the control system. Basicallythese are the signals of the valve end position and the valve position. All other events and data are collected in the master stations and if necessarycan be accessed from the control system eg for fault diagnosis.
- Each master continuously monitors the activeas well as the redundant Modbus channel to the actuators. Therefore it is guaranteed that the redundant channel is ready for operation in case of a communication fault.
- The 39 explosion-proof actuators can be set up non-intrusively. The complete programming of the devices can be performedwithout opening the housing or without using any tools. The parameters can easily be changed by using the push buttons on the local controlsguided via a menu.
Valve automation in process
An emergency shut down (ESD) plays an essential role in the safety concept for this plant section. In an emergencyeg firethis function can be activated by several switches distributed throughout the plant. Via an emergency input the actuators are integrated in this function (refer to plant layout Fig.4). If it is activatedthe actuator performs a pre-defined actioneg closing the valve. In such an emergencythe overload protection mechanism in the actuator is ignored.
A significant element of this emergency concept is the fireproofing of the actuatorsie that an actuator in case of fire remains operable for a certain timeto be able to perform the actions according to the ESD concept. This is achieved by a special patented coating of the devices. The actuators remain fully operational for at least 30minutes if exposed to a temperature of up to 1100°C.
The manufacturers of sensors and actuators have to be prepared to adopt the role of a system provider. Flexibility is in demandon the one hand regarding service conditions(explosion protectionfireproofing)but also with regards to the integration of the devices into the control system. The depicted example shows clearly that even within one plantdifferent automation concepts can be pursued at the same time. Those who are able to fulfil the variety of requirements have good chances to win such and similar interesting and sophisticated projects.
Michael Herbstritt is an electronic engineerworking as technical writer for AUMA Riester GmbH & Co KGMüllheimGermany. www.auma.com"