Jul-2011

Automating procedural operations for continuous processes

Progress towards a standard for automating operational procedures in 
refinery operations. Process plants are run according to operational procedures.

Maurice Wilkins and Marcus Tennant
Yokogawa Electric Corporation

Article Summary

These procedures consist of a set of tasks that are executed in a consistent manner to achieve a specific objective, such as starting up, shutting down or transitioning a unit as part 
of making a product. The level of detail, purpose and frequency of use of these procedures varies by process, company and site, but they are the basis for how to operate plants.

This article reviews progress and developments in the recently formed ISA106 standards and practices group: Procedural Automation for Continuous Process Operations. It outlines current models 
and best practices to follow in order to improve a plant’s operational procedures. Several examples highlight different requirements and how 
various tools can be applied 
to implement operational procedures. These best practices will enable plant operations to execute procedures efficiently and consistently.

Challenges affecting process operations
As an example of the issues facing the process industries, let us focus on distillation, as it is the most widely used separation process. In the manufacturing sector, distillation uses 24% of the total energy demand. Oil refineries, in particular, rely heavily on distillation, which consumes approximately 40% of all the energy used in refineries.

Consider the distillation 
process from a steady-state perspective, with most attention focused on achieving an optimal steady state to ensure that the process remains within safe operating limits. Those who run and manage continuous process units understand that procedural activities are critical to safe and efficient operations. Often this procedural information is not well documented, not readily available or can reside in an operator’s individual knowledge acquired through years of experience. Procedural activities in a distillation process are most prevalent and critical at certain times of the process operation

Startup
Safe and efficient startup of a distillation unit is critical to its overall operation. Startup frequency can vary, from once every day to once every five years. If startups occur often, they can occur on different shifts, and the time and skill of the operator on duty can determine the efficiency of the startup. If startups occur at longer intervals, companies run the risk of not having 
experienced personnel available to run and oversee the startup. There have been cases where companies have had to bring operators out of retirement to restart a unit after a shutdown.

Shutdown
Orderly and safe shutdown of a distillation unit is just as critical as a startup. One key item in the shutdown procedures is recognising that a shutdown might not be scheduled. System problems or severe weather such as approaching hurricanes can require a shutdown of distillation operations at very short notice. As with startups, on-shift operating personnel might not have the most experience in shutting down a system.

Feedstock and product output transitions
Many chemical processing units regularly operate at more than one optimum steady state. In refineries, for example, many refiners purchase crude oil of different types in tankers. As they run out of one type of oil, they must adjust operations to process correctly the next type of oil. Similarly, in chemical plants, a single plant will often make a variety of products in a campaign fashion by changing operations to meet the new product specification.

Transitions inherently increase the risk of disruptions that can lead to incidents. According to a report by Marsh Ltd, over the period 1975–2009, the five-year loss rate (adjusted for inflation) in the refinery industry continues to rise (see Figure 1). Incidents that occur during startup and shutdown continue to be a major factor.

An additional study by J & H Marsh & McLennan shows that the examination of major incidents by the average loss per incident indicates that operational error represents the largest average dollar loss (see Figure 2).

With distillation operations consuming a large amount of the energy requirements in a refining or petrochemical application, efficient production is a key to meeting product specifications and producing the best possible yields of valuable products. Performing procedural operations in an inefficient or time-consuming manner will have a significant economic impact on the complete operation.

Need for procedural best practices in continuous process operations
A 2008 survey by the ARC Advisory Group indicates that continuous process manufacturers are now seeing effective and repeatable transition management, along with the use of sequence-based operating procedures, as a competitive advantage. Additionally, the safety aspect of automating procedures should be assigned a high tangible value. The cause of some recent industrial accidents has been linked, in part, to a lack of good emergency shutdown procedures during an abnormal condition. This put too much pressure on operators in a crisis, leading to improper procedural operations that resulted in disastrous consequences.

Process industries have used semi-automated and automated procedures in the process industries for years, especially in batch processes. These procedures, when implemented in recipes, use a modular approach in accordance with the ISA-88 batch control standard.

Semi-automated and automated procedures are not widely adopted in continuous processes due to a lack of general industry expectations and standards. Increased operational excellence has put more focus on business drivers that require increased safety, improved throughput, reduced cost and knowledge capture to retain years of operational experience that would soon be lost due to retirement.

Automated procedures for continuous processes in the past have typically been implemented using ad hoc designs and programming techniques, usually resulting in difficult-to-maintain code. While this can provide short-term operational benefits, the total cost of ownership of these procedures is higher than necessary. This is a result of the increased implementation costs of changing and updating successive procedures over time due to a lack of reusable software modules.