Safe process envelope

A distinction needs to be made between limits observed to maintain process performance and limits observed to avoid a hazardous operating condition. The above paragraph refers, of course, to the latter limits, as depicted in Fig. 19 using the safe processing envelope concept. 

Figure 19 Processing limits for performance and for safety. Processing limits define the perimeter of the operating envelope that results in the range of desired process performance, whereas the safety limits define the safe processing envelope perimeter given the identified hazards that lie heyond. For example, a distillation is to be carried out at 90—100° C, whereas the high-temperature interlock that shuts off the steam is set at 125° C because a significant exotherm initiates at 160°C.

Figure 21 The contoured safe processing envelope. Detailed analysis of hazards in a specific context (specific process and plant designs) usually reveals specific risks, and implementation of the process in that context may require that the original safe processing envelope be contoured (i.e., modified for the said risks). In the figure, the original envelope has been shaped so as to create a distance from the risks that bears proportion to the magnitude of the risk, which is depicted by the size of each circular symbol for the risk.

What are the identifiable risks from the know process hazards Where are the safe processing boundaries (i.e., the safe processing envelope described in Fig. 19 of Chapter 2) ... 

The safe process envelope is defined by several parameters which are examined below ... 

Equipment used in different processes during its lifecycle. Surplus equipment or existing equipment reused for different use. Possibility of equipment being used outside its safe operating envelope. 

Available equip- Procure equipment that can be used in other ment determines processes (current or future) without operating the process chem- close to its operating envelope, istry selected., Provide equipment with comparable pressure Operating close to rating for the entire system the safe operating envelope of the batch sizes to equipment capabilities equipment and the relief capability. ... 

Preventive measures either take automatic remedial action or allow for manual intervention to prevent the conditions for uncontrolled reaction being achieved. They include the use of sensors, trips, alarms, control systems, and other safety features. These measures require a.thorough understanding of the safe operating envelope within which the process must be maintained.. . ... 

At PPG, Class 1 Prooftesting also covers 250 Safety Instrumented System loops in the PSM Safety Systems. A Safety Instrumented System (SIS) is composed of sensors, logic solvers, and final control elements for the purpose of taking the process to a safe state when predetermined conditions are violated. SISs are normally controlled by a PLC with the sole function of monitoring a process to insure operation is maintained within the safe operating envelope. 

The Safe Operating Envelope has been defined as the range of process parameters (such as temperature, pressure, flow, level, composition) that are critical to safe operation. The Safe Operating Envelope upper/lower limits are listed in the Safe Operating Envelope tables maintained under Process Safety Information. Operating beyond these limits will cause a process upset. Key maximum and minimum Emits are set based on manufacturers specifications, theoretical calculations, or operating experience. 

Are adequate safeguards in place to ensure the process is controlled within the safe operating envelope ... 

The safe operation of a reactor requires that the controls keep the system operating within a safe operating envelope (see also Chapter 21). The envelope is based on many of the design and process constraints. These are placed on the unit to ensure the proper operation. Additional controls may be installed to operate additional equipment needed for energy recovery, neutralization, etc. 

Most industrial process plants are safe to be around. Plant management, aided by reliable operators, who are in turn backed up by still-more-reliable automatic controls, does its best to keep operations moving along within the limits usually considered reasonably safe to man and machine in order to have a safe operating envelope. Occasionally, however, there is a whoosh or a bang that is invariably to the detriment of the operation, endangering investment and human life, and radely upsetting the plant s loss expectancy. 

The licensing process should generally include a consideration of the OLCs in the form of controls, limits, conditions, rules and required actions that are formally derived from the safe operating envelope. The safe operating envelope is encompassed by the possible operating states included in the... 

Evacuation Plans. Many emergency events require the evacuation of at least some facility personnel, if only for precautionary reasons. This presents other needs, which must be addressed in the evacuation plan. Personnel who are directed to evacuate their normal work stations need visible wind direction indicators to help them move away from a dangerous gas cloud drifting downwind of the release point. If the building is enveloped in the cloud, however, they will need personal protective equipment in order to evacuate the building safely. When there are processes that must be rapidly shut down and/or isolated as evacuation proceeds, the plan must include written procedures and appropriate means to ensure incident-free action. 

Operation conditions. A specially designed oxygen mixer and the interlock system assure safe operation outside tbe flammability envelope. Simple design and high quality material specifications result in two to three year intervals between major turnarounds and on stream times as high as 98+% over one year. The heat of reaction is recovered as high-pressure steam (18+ barg), which can be used by other parts of tbe VCM process (e.g., pyrolysis). Wastewater does not contain suspended catalyst fines therefore, there is no need to settle them and then dispose as a hazardous waste. Copper levels in fixed-bed effluents are below 0.3 ppm thus, no treatment is required. 

The use of prevention as a basis of safety requires the early identification of process hazards and the conditions under which they can occur. This allows the specification of boundary conditions, or an envelope within which the process must be maintained if it is to be operated safely, and the provision of measures to ensure that the process remains within the envelope. Where possible this should be achieved by the design of the process and plant, but it may also depend on instruments, trips, alarms and control systems. In addition, organizational procedures such as rigorous instruction, strict enforcement of operating conditions and the provision of adequate maintenance are required. 

ANSI/ISA-84.00.01-2004-1 does not require physical separation. However, many owners/operators do use physical separation and diverse logic solvers. It is paramount to remember the primary purpose of many SIS. The SIS is designed to restore the plant to a safe state when the process moves out of the normal operating envelope for a number of causes, including a failure or erroneous operation of the control functions. The need for the SIS to act as a layer of protection in the event of control function failure has resulted in many standards and practices recommending that SIF and control functions be implemented in physically separate and diverse equipment. 

Figure 2.7.4 elaborates on this idea. It adds the notion of a defined operational boxmdary which provides a safety margin before one reaches the edge of the safety envelope. This is usually defined by the organisation, based on procedures. Between this and the edge of the safe envelope is another boundary, which shows how far the person concerned is still in control of the process. Once the situation goes beyond this boxmdary of controllability, it is only external or chance factors which can prevent... 

This topic is very close to the concept of situation awareness which has been widely studied since the work of Endsley, who studied the way in which fighter pilots keep sufficient awareness of their position and situation to be able to function efficiently. This depends on their having mental models which tell them what to pay attention to, which integrate many sources and types of information into an overall picture of where (in the safe envelope) they are, and how the situation will develop. For all traffic controllers and drivers/pilots - air, train or car - and for other supervisors of processes - control room operators - this is a vital skill. 

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