wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

25-4 Industrial Communication Systems
25.2.1 Process Model
A batch process can be segmented hierarchically according to the process model of the standard into
process, process stage, process operation, and process action.
• A process represents the sequence of chemical, physical, or biological activities for the transformation,
transport, or storage of material or energy. IEC 61512 presents the production of polyvinyl
chloride (PVC) as a batch process example.
• A process consists of one or more process stages that usually run independently from each other
in a serial or parallel arbitration to conduct a scheduled sequence of transformations of the processed
material. Regarding the PVC process, typical process stages could be the polymerization of
vinyl chloride monomer to polyvinyl chloride or the drying of PVC powder.
• A process stage contains a number of process operations. Each process operation refers to a larger
process activity, which in fact means usually one chemical or physical transformation of the processed
material. Typical process operations of the PVC process could be the preparation of the
reactor or the reaction itself.
• Process actions represent smaller process activities that are combined to a process operation.
The process operation “reaction” could consist of the following process actions:
1. Fill a certain amount of catalyst into the reactor.
2. Fill a certain amount of vinyl chloride monomer into the reactor.
3. Heat up the reactor to a temperature between 55°C and 60°C.
4. Hold the reactor temperature between 55°C and 60°C until the reactor pressure drops.
25.2.2 Physical Model
The physical model abstracts the physical organization of a batch manufacturing company into a hierarchical
structure. Usually, groupings of lower levels are merged to form a higher level but ISA/ANSI S88 treats the
upper three levels—enterprise, site, and area—of the physical model only marginally since these levels are
connected closer to entrepreneurial than technical regards. The lower four levels—process cell, unit, equipment
module, and control module—represent delimited groupings of equipment. Combining equipment
into functional groupings eases operational tasks since each of these groupings can be treated as a larger
single device.
• An enterprise is an organization that coordinates the operation of one or more sites. Product
decisions are made on this level as well as determinations about the manufacturing location and
production methods.
• A site represents a structurally, geographically, or logically limited part of a batch producing
enterprise. Generally, these limitations are based rather on organizational as well as entrepreneurial
than on technical criteria.
• An area refers to a structurally, geographically, or logically limited part of a batch producing
site. Likewise, regarding sites, these limitations are based on organizational and entrepreneurial
criteria.
• A process cell is a logical grouping of equipment to process one or more batches. Logical control
possibilities are defined according to the structure of process cells within an area to develop control
strategies (e.g., in the case of emergency situations).
• A unit contains control modules and/or equipment modules to conduct larger process activities
of one batch (e.g., reaction or crystallization of a dilution). A unit combines all necessary process
devices to conduct these activities as an independent equipment grouping and may allocate also
common resources for its tasks. Generally, its elements are arranged around a processing installation
(e.g., a reactor tank). According to ISA/ANSI S88, a unit processes not more than a single
batch or one part of it at any point in time.
© 2011 by Taylor and Francis Group, LLC