Petrochemical processing plants have historically used DCSs; however, in the past several years there has been an increasing tendency in the industry to install scada systems instead. To understand what is driving this trend we must first look at the difference between a DCS and scada system.
DCS (distributed control system) and scada (supervisory control and data acquisition) systems both emerged independently within the controls industry and by the early to mid 1970s they both had a strong following. DCS was a term coined by two major control vendors providing plant automation systems in the early 1970s while scada first appeared in print in 1973 as part of the Bonneville Power Administration Study into system automation started in the late 1960s. Although these systems have much in common, the goals of DCS and scada systems are quite different.
DCSs are process centric and have a direct connection to the data source. They are primarily focused on realtime states with past and present process variables the main focus. DCSs are most often directly connected to the hardware device I/O, and therefore require a very reliable network connection to equipment. As a process state driven system, a DCS is sequential in nature with alarms generated not when a point changes but when a process is run; trends primarily focused on data states past and present. In many instances DCSs are not at all aware of a change in state, but simply report the current realtime state at the instant the hardware is polled. In a DCS, events and alarms (both central concepts in scada) are secondary to process displays. Therefore, it is often necessary to set up alarms by hand on each point, and many DCSs have limited alarm filtering. Finally, DCSs are deployed only when the system has a very small geographical footprint, such as a factory or plant, due to the fact that they must have a continuous connection to the hardware.
Scada systems are data centric and have a database driven architecture. Scada systems have historically focused on control of remote equipment where telecommunications may be regularly interrupted. To overcome the unreliable telecommunications to remote field devices, scada systems are indirectly connected via a database to the field equipment and can continue operating even when telecommunications are temporarily lost. In addition, more complex polling capabilities are typically provided by scada systems to help address telecommunication restrictions. This allows scada systems to deal with controlling assets over a very large geographical area, making it suitable for electric grids, and oil and gas pipelines.
Unlike DCSs that are state driven, scada systems are event driven, with change in state driving the functionality of the system as well as being the main criteria in data gathering and presentation. Not surprisingly, this focus on changing events focuses scada functionality on alarms, which are typically automatically set on all points and event logs. Changes to the state of the system triggers alarms, events and database. In addition, functionality such as complex filtering for alarms and event logs, comprise a large part of a scada system's functionality.
Scada systems were designed to be used on large scale systems with remote assets over a very large geographical area. In addition, scada systems are designed to allow a smaller number of operators to control a large number of individual assets. However, the benefits of a scada system have recently become recognised and many areas of automated control that were historically the domain of DCSs are beginning to install full-featured scada systems.
It was often perceived that being simpler in nature, DCSs would be less expensive. Therefore, if the process was geographically limited in scope with reliable telecommunications DCSs were typically chosen. However, recently companies have begun to look at the overall lifecycle cost of the systems and have begun to find that in the long run scada systems, with their more powerful functionality, are less expensive to operate and provide more powerful tools for optimising operations. In addition, abstracting control allows operators to concentrate on the overall goals of the operation and relieves operators from having to concentrate on the minutia of running the system.
While it is possible for a DCS to perform scada functionality, they were not designed to do so and therefore there may have to be considerable customisation required offsetting any savings provided by the use of the DCS as a starting point. In addition, scada systems have begun to incorporate many features designed to allow simple integration with PLCs, allowing these systems to be set up much more cost effectively.
How does one decide what system is best for a particular application? Here are some basic questions to ask:
How many PLCs or pieces of equipment are being controlled? The larger the number of pieces of equipment, the more value can be received by installing a scada system. Scada systems are excellent at abstracting complexity allowing fewer operators to control more processes.
How complex is the process? Complex processes utilising multiple pieces of machinery can benefit from scada systems.
Are there other uses for the data collected? Scada systems are database driven and typically integrate into enterprise applications more seamlessly than DCSs do.
Are changes in state important in the process? If alarming is of value to a process, or if operators are going to be spending a great deal of time waiting for a particular event to occur, scada systems can provide value in these situations over DCSs.
Is continuous improvement (lean practices) important to the operation? Scada systems can more easily be adapted to provide key metrics to improve efficiency in operations.
What does the future offer? DCSs and scada systems have been moving closer together with DCS vendors claiming to provide scada features and scada vendors providing tools to simplify implementation and connection to PLCs. Having said this, it is still more difficult to configure a DCS to provide higher level functionality than it is to implement a scada system and connect it to the end devices. Therefore, many of the traditional DCS vendors have moved away from a state driven architecture towards an event driven scada-like infrastructure. The issue here is that these DCSs have evolved and therefore often have legacy ties that make them less effective and more difficult to implement.
One consideration is to use a DCS when dealing with very simple processes on a limited number of pieces of equipment. It is difficult to expand a DCS once it is in place and customers often find they have to settle for limited functionality. However, the recent advances in scada systems have made them much simpler to install and configure; often it is nearly as easy to install a basic scada system as it is a DCS. Once installed, it is much simpler to expand the functionality of the scada system providing much more value.
Lastly and perhaps the most important consideration is that a DCS is typically a single source system. While great at handling very complex environments, it can be very difficult to change hardware down the road. Open-platform scada systems, on the other hand, allow easier connection to multiple platforms via drivers. This can be an important cost consideration as there are numerous PLC providers, therefore pricing is very competitive.
This observation is supported by the trend towards a scada system by operations that were historically the sole domain of a DCS. Many refineries, gas plants, and factories are choosing scada over a DCS to get access to the increased functionality and lower overall costs provided by a scada system. Petrochemical processing plants, complex in nature, are ideally suited to take advantage of the features offered by scada systems.
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