Process Factors

Factors inherent to the process that impact the nature of abnormal situations include:

  • Type of manufacturing (batch vs. continuous),
  • State of operation,
  • Type of materials being processed, and
  • Process complexity

Identification of these factors may be a crucial first step in applying any solution concepts to manage abnormal situations. In the following, we describe how each of these factors cause different kinds of abnormal situations. Wherever possible, examples are provided to illustrate the differences.

Type of Manufacturing - Batch vs. Continuous

Chemical manufacturing can be broadly classified as batch processes, continuous processes, or some combination of batch and continuous. Batch processes offer some of the greatest challenges for abnormal situation management due to the following reasons (Fisher, 1990):

  • Quantity of material produced & flexibility—Typically batch processes are used to manufacture a large number of products. Within each product there are often a number of grades with minor differences. Since frequent product and process changes are a way of life in batch processes, they must be flexible and allow relatively easy process adjustments. Continuous processes, on the other hand, are more economical when large quantities of material are to be processed and the same product will be made for a long period of time. A continuous process may operate under different steady state conditions at different times depending on the nature of products being manufactured. For example, a refinery may produce a greater amount of the gasoline cut during one season compared to another, resulting in different operating steady states in different seasons.

  • Steady state vs. transient operations—A batch process is transient under normal conditions. It involves a sequence of phases carried out on a discrete quantity of material within a piece of operating equipment. On the other hand, continuous processes normally operate at or close to steady state. The values of process variables should be essentially static at any point in the process. However, specific portions of a continuous process may be executed in a batch manner. While the overall process (from an input/output perspective) is still continuous, one or more steps are carried out in a batch manner due to changes in the state of materials being used (e.g. change from liquid to solid state where the solids have to be manually taken out of the reactor, or fouling of fixed beds of catalyst/adsorbent which have to be regenerated off-line).

  • Regulatory control vs. sequential logic control—In batch processes, due to the transient nature of processing, control loops are used for a period of time in one mode, and then the mode is changed. Large quantities of two-state devices, such as automatic block valves, are used. Batch processes generally require more sequential logic control as part of the normal operating mode. Control systems for continuous processes work to minimize fluctuations in these process variables caused by external disturbances. Disturbances include events such as changes in process variables of the raw materials (e.g. flow rate, composition, temperature) and changes in equipment performance parameters. Although sequential logic is sometimes used with continuous processes, it is typically used only during the start-up or shutdown of the system.

  • Processing problems—The on/off control requirements of batch processes contribute to the difficulty and complexity of the control system application, since many on/off functions must be performed during each phase of the process. Each phase and step of a batch process usually has a specified set of criteria that establish the success/failure of that phase or step. Procedures are often available that describe re-processing of phases and steps that can fail; therefore, special failure routines are included. When a failure occurs in a continuous process, that part of the process becomes non-functional resulting in production or product quality loss. In an extreme case, this can cause a shutdown of a unit or a plant. Continuous processes usually have problems of incorrect processing conditions – i.e. incorrect temperature, pressure, flow, level or composition – that are generally caused by process disturbances which cannot be handled by the regulatory control system. Since the plant is designed (in terms of equipment size and connectivity) to take care of the correct sequence of processing and the right duration of processing these problems are relatively infrequent in continuous processes.

State of Operation

The operational state of a plant has a significant influence on the types of abnormal situations that occur and their consequences. The following four states of operation are the most commonly encountered under regular process operations of petrochemical plants:

  • Steady State
  • Startup
  • Shutdown
  • Transitions (not including startup / shutdown)

Rasmussen and others have also identified a similar taxonomy of plant states that needs to be considered while analyzing abnormal situations in a nuclear power plant. While the above states are included in their taxonomy, they include some other specific states of operation for nuclear power plant operation.

  • Steady state vs. transitions—As explained in the previous subsection, steady state operation is the normal mode of operation for continuous processes. Degree of deviation from steady state provides an indication of the severity of the consequences of the abnormal situation. In continuous processes, several abnormal situations arise during plant transitions from one operating state to another. Transitions occur as a result of changes in raw material properties, changes in resources (equipment or raw material) or changes in demand for products. Such transitions occur rather infrequently and therefore their consequences are not well understood or well documented in plant procedures. For example, change in gas molecular weight can cause a compressor to surge. Usually changes in gas molecular weight are neither measured nor are they readily apparent. For batch processes, transitions are the normal mode of operation. These transitions are typically guided by well-defined recipes. However, due to the lack of wide-spread automatic control techniques for batch processes, many of these transitions are carried out manually. This can cause significant variations from operator to operator resulting in various abnormal situations.

  • Shutdowns—Shutdowns are the least frequent type of transition. Equipment, units or plants may be shutdown for scheduled maintenance or under emergency conditions. Emergency shutdowns occur due to an imminent danger to plant equipment or personnel. Typically these shutdowns occur over a very short time period and there is little time to reason about the root cause of the problem. In many plants, emergency shutdown procedures are automated. Scheduled shutdowns, on the other hand, do not usually pose an imminent danger. These are guided by standard procedures that an operator can follow over a relatively longer period of time. Even during such scheduled shutdowns abnormal situations arise because different operators execute procedures (slightly) differently which can result in different process reactions. It is not uncommon for an operator to get 2 alarms per minute, even during a scheduled shutdown that extends over several hours. Moreover, many of these alarms are expected because process parameter values must go outside of their normal run alarm limits when equipment is turned off.

  • Startups—Startups are also relatively infrequent transitions. While there is documentation available to assist operators in startup, there is usually a greater amount of uncertainty associated with bringing a plant to steady state. Especially when the startup involves an entire unit of a plant, the amount of interaction between various equipment items can lead to a significant number of upset situations. A startup occurring after a turnaround (i.e. a scheduled shutdown for maintenance or upgrading activities) is particularly challenging because the exact response of new (or repaired) equipment is not known in advance. Thus, startup conditions are not well understood resulting in greater probabilities of abnormal situations.

Type of Materials in Process

The consequences of an abnormal situation in a chemical plant often depend on the nature of the materials being processed. A preliminary classification of materials handled is as follows:

  • Hazardous vs. Non-hazardous chemicals
  • State of the chemicals - solids/liquids/gases
  • Flammable vs. Non-flammable substances

The corrective actions to be taken in case of abnormal situations involving different kinds of materials can be significantly different, both in terms of the required response time as well as the procedures followed. For example, the pulp and paper industries typically use chlorine for bleaching purposes. An abnormal situation resulting in a chlorine leak calls for rapid corrective actions to contain the leak and neutralize its effects. Petroleum and petrochemical industries, on the other hand, deal with highly flammable materials. Hence abnormal situations may result in potential fire hazards which is an additional dimension to be considered for abnormal situation management. The physical state of the chemicals - solid, liquid or gas - can also affect the nature of abnormal situations. Each of these presents unique problems in the management of abnormal situations.

Process Complexity

While it is difficult to define process complexity, most analysts agree that the greater the perceived complexity of a process, the more difficult it is to manage an abnormal situation. Complexity of a process decreases as humans gather greater experience with the process. A new installation is likely to be viewed as more complex than one that has been around for a while. Similarly, relatively inexperienced operators tend to view the process as a fairly complex unit. Inadequate information about process state or behaviors, lack of understanding of a process or an excessive amount of interactions between different parts of the process can also result in a process being viewed as a complicated one. There are several such metrics that have been proposed to characterize process complexity (Weir,1991).

Processes become complex largely due to material and energy integration. For example, due to energy integration, several heat exchangers in a refinery are networked. Consequently, if there is a loss of heating capability, it may become difficult to isolate the root cause of the problem. Similarly, relief valves of several units discharge to the same flare. When the flare is found to burn at a higher than normal rate, it may become difficult to identify which specific unit is operating abnormally. When there are multiple trains of a process, it may become difficult for the operators to isolate the root cause to a particular train. In general, complex interactions between multiple process units can often result in difficulty in resolving the abnormal situation.