Deviation process

Real-time clocks (RTCs). Real-time systems are required to respond to events, as they occur, in a timely manner. This is especially crucial in process control systems where control actions applied at the wrong time may amplify process deviations or destabilize the processes. The nodes in the systems are interrupted periodically by the real-time clocks to maintain the ac tual elapsed times. 

Hazard and Operability Study (HAZOP) A systematic qualitative technique to identify process hazards and potential operating problems using a series of guide words to study process deviations. 

If operating procedures need to be developed from scratch, this could account for a considerable portion of the pre-startup work. If current, valid standard operating procedures exist, and the toll is only introducing a new set of batch instructions, the task becomes simpler. However, it is recommended that the new batch instructions are reviewed simultaneously with the existing startup, shutdown, normal and emergency operating procedures for the equipment to help ensure potential process deviations are examined and addressed. 

Intershift communication and records System for reporting and follow-up of Plant defects Process deviations... 

Active—Using controls, safety interlocks, and emergency shutdown systems to detect and correct process deviations e.g., a pump that is shut off by a high level switch in the downstream tank when the tank is 90% full. These systems are commonly referred to as engineering controls. 

The completeness of the information described in Steps 1 to 9 prior to the review will determine the quality of the inherent safety review. The chemist needs to define the desired reactions, and to develop an understanding of potential side reactions. Effects on reaction chemistry need to be developed for mischarges or process deviations. These information requirements on process chemistry are discussed in Section 4.2. 

I iach study node is examined for potentially hazardous process deviations. First, i he design inte-iit of the equipment and the process parameters is determined and recorded. Process de iatiuns from the design are determined by associating guide words with important process parameters. (iiiidt words for a HAZOP analysis are shown in Table 3.3.4--1 process parameters and dt. i itions are shown in 1 able, T3.4-2. 

By combining elements of the first column of the guide words with elements of columas 2 or 4 of the process parameters, process deviations may be found. For example, combining the first guide word, "No" from Table 3.3.4-1 with the first process parameter, Flow rate from Table >.3 4-4, the deviation No flow rate" is found. Other deviations may be created similarly... 

A HAZOP study is a systematic, tabular document of process deviations. The study gives the noimal operating conditions and analysis boundary conditions for each item and lists action items for further evaluation. Tables 3.34-3 and 3.3.4-4 are examples of HAZOP analysis of the Dock 8 HF Supply System and the Cooling Tower Chlorination System, respectively (for a more complete study see DOE, 1973). These example show the format for HAZOP tables. A typical HAZOP study... 

The HAZOP team consisted of a HAZOP leader, scribe, process engineer, senior operator, and afety engineer. Previously, the HAZOP leader collected information on the process design (e.g P lDs, operating procedures, emergency procedures and incident reports) for distribution to each team men The leader sectioned the process and identified process deviations prior to the HAZOP. 

As discussed earlier, the successful diagnosis of faults in automated control systems is highly dependent on the mental model the worker has built up of the current state of the plant processes. Such a model takes time to construct. An individual who has to act quickly may not be able to make the necessary diagnoses without time to build up and consult his or her mental model. Even in a highly automated plant, provision needs to be made to display major process deviations quickly. 

Additionally, the shape of the deviation will prompt the operator to search for more detailed information upon which to act. An example of a process deviation as represented by an asymmetrical display might be the low flow of crude through the coils due to a blockage. This may be represented by a decrease in crude supply and fuel supply and an increase in inlet temperature. This type of overview display has the following advantages ... 

Supports the Early Detection of Abnormal Process States Although some diagnostic information is available in the activated alarms, the workers still need to know the size of any deviations from the target states and require a concise picture of those critical parameters which are in alarm and those which are nearing it. This information is available on the overview display and facilitates the early detection of process deviations. 

When the mass transfer process deviates significantly from equimolecular counterdiffusion, allowance must be made for the fact that there may be a very large difference in the molar rates of transfer of the two components. Thus, in a gas absorption process, there will be no transfer of the insoluble component B across the interface and only the soluble component A will be transferred. This problem will now be considered in relation to the Reynolds Analogy. However, it gives manageable results only if physical properties such as density are taken as constant and therefore results should be applied with care. 

Stopping excursions from normal operation conditions before they become hazardous, by correction of process control variables when dangerous situations are detected on-line detection of process deviations associated with trip systems for corrective actions. 

Quality control of intermediate and end-products (plant support process deviations)... 

The HAZOP team examines each study node for potentially hazardous process deviations. First, the design intent is defined to delineate the purpose of the equipment and the process parameters. Process deviations are determined by combining guide words with the important process parameters. The established set of guide words is shown in Table 4.14. 

Process safety information is compiled and made available to ah employees to facilitate the understanding and identification of hazards. This information includes block flow diagrams or process flow diagrams, process chemistry, and process limitations, such as temperatures, pressures, flows, and compositions. Consequences of process deviations are also required. This... 

This section discusses how a runaway reaction occurs and lists some of the process deviations that can lead to such a runaway. Equipment for identifying potentially hazardous process steps is reviewed, and general principles for inherently safe process design are given. 

A part of the test plan must include testing for the consequences of equipment malfunction, deviations in process conditions, and human error. Bench-scale equipment, for example, the RC1, is quite suitable for such experiments. By analysis of the process, critical conditions can be defined, which then need to be tested in order to be able to proceed safely from the laboratory to pilot plant studies. In testing abnormal conditions or process deviations, caution is required to assure that no uncontrollable hazard is created in the laboratory. Typical deviations, including impact on the process, are discussed in the following paragraph. 

In all three operational processes, deviations from the normal way of working occur due to external and internal changes. Such deviations can be identified by technical means, i.e. automatic registration devices on equipment, and human means, from observations of peoples. Both means are used to construct an overall view of the daily problems present in each operational process. 

Mosley et al. (2000) describe a "chemistry hazard analysis" approach, similar to a hazard and operability (HAZOP) study method applied at the early development stages of a new process. Deviations from an intended chemical reaction are identified using typical HAZOP guidewords. Examples of deviations and consequences developed using this approach are shown in Table 4.10. Analyzing the basic chemistry of a process, where chemical reactions are intended to occur, can help ensure the consequences of deviating from the intended reaction are understood. 

Active controls use engineering controls, safety interlocks and emergency shutdown systems to detect process deviations and take appropriate corrective or remedial action. Their effectiveness depends on proper selection, installation, testing, and maintenance. 

With relevant criteria, the highly reactive substance classification would cover the most likely process deviations and inadvertent mixing scenarios leading to injury however, it may not take into account all process-specific conditions, such as inadvertent mixing of unexpected chemicals or addition of an unexpected catalyzing agent. 

---