How to write a good process operation description document
Back to Basics: Describe your process to preserve the process engineer knowledge for the future. To program the process controller, programmable logic controller (PLC), or distributed control system (DCS), follow these steps and methodology, starting with understanding the requirements and documenting in a requirements document, also called a BPO, CFE, an FS, or URS. Online, see five tables and detailed examples.
Describe a process in such a way as to preserve the process engineering knowledge for the future. To program the process controller, programmable logic controller (PLC), or distributed control system (DCS), a few steps must always be followed. Sometimes these steps are done orally, and in other cases, where a controlled environment exists, these are done with written documents following strict procedures. Whatever methodology is used, the first step is always "understand the requirements." When control systems' programmers are asked to write/modify a program to control the process, the first thing they should request is a written document that describes the requirements.
Names vary for requirements documents
This requirements document has different names in various industries. Under good automated manufacturing practice (GAMP), common in the FDA regulated industries, it is called user requirement specification (URS). In batch industries it is called basic process operations (BPO), and in continuous plants it is usually called conceptual functional specifications (CFS) or simply functional specifications (FS).
A BPO document is a first step in creating a control system for controlling the (production) process. The BPO document is a generic document independent of the platform upon which it will be implemented. The BPO document should be written with enough details so a (process) control engineer / programmer would be able to use it to write a detailed functional specification (DFS) document. The DFS is a translation of the BPO document into a specific type/product of control system. Reading the DFS enables the control engineer/programmer to write/build the control software/application for controlling process operation. [subhead]
Two BPO answers
The BPO document should answer two basic questions: 1) How should the process behave under normal operations? and 2) What actions should be taken in case of unforeseen events?
The BPO document is a process state/operations description, and it is mostly implemented in the process controller and not in the operator interface (HMI). One of the most important features is that it documents the process operation as designed and seen through the eyes of the process engineer and maintains the process knowledge in it for future readers and users of the operational unit.
A process operation is a detailed description of multiple transitions between states.
A state is a specific condition of the equipment, usually at rest or steady state condition, and a transition is a shift of the equipment between two states.
For simplification, the following basic states of the equipment will be described:
- IDLE (initial state)
For further details on the use of these states in the batch industry, the reader is referred to the S88 standard.
Each process operation has four operational stages: 1) IDLE, 2) Before the execution, 3) Execution, and 4) Termination. The possible transition between these states is described in Drawing 1 and Table 1.
The BPO document should contain the following chapters:
1. The unit
1.1. P&ID / process sketch
1.2. List of main/relevant equipment and all instruments
2. The process
2.1. General process description (in words)
2.2. Process parameters
2.3. General interlocks and alarms
3. The operation
3.1. Preliminary conditions
3.2. Detailed process description
3.3. Hold execution
3.4. Abort execution
1 The unit
1.1 P&ID / Process sketch
The first thing that we need to do when defining a process is to define its physical limits/boundaries. In this example, refer to the simplified equipment units' arrangement (Drawing 2). It contains a clean water source tank (T-8), a pump (P-160), 1 circulation valve, 3 valves manifold, and 1 recipient tank (T-9). The operation under control will be the transfer of water from T-8 to T-9.
1.2 List of main/relevant equipment and all instruments
As it is our main goal to preserve our knowledge and explain why we are asking for a specific action to be performed, we must be as clear as possible. We need to make sure that we identify all the relevant equipment and instruments that are involved with our process operation. Later in the document, we will be referring to items ONLY from this list.
If the process operation is generic, the tag names that will appear in the table will be generic and there will be a table to convert between the generic name and specific name for each piece of equipment in the following format. See Table 2.
Table 3 lists the equipment for the current process operation.
2 The process
2.1 General process description (in words)
Tank T-8 contains clean water main storage. To keep the water clean, storage tanks are circulated constantly through a filter F-8. Occasionally, as process operations require, we need to transfer clean water to other locations at the site. During a transfer we should not circulate as we want the water to be sent at the shortest time possible to the operational area. When transferring water, only one of the manifold valves can be opened at a time in order to prevent backflow and possible contamination of the manifold and water in the main tank T-8. During the transfer, water does not enter into the clean water main tank T-8.
This BPO will deal with water transfer from T-8 to T-9.
2.2 Process parameters
All parameters that are relevant for the current process operations are listed in this chapter. There are three kinds of parameters.
1. The first is an external parameter. It is related to the process operation's functionality and serves as the interface between the equipment and the entity that manages it, whether it is an operator or a higher-level automated process operation. Each external parameter has two values that should be kept in historical records. The target value (SP) and the current value (PV), which is dynamic and changes as the process operation is executed.
2. The second is an internal parameter that is related only to the process operation setup. It contains information about the equipment itself.
3. The third parameter is an internal parameter that is used only during the process execution and is not saved or monitored after the execution.
A parameter is not changeable after the process operation has started its execution.
The parameters for our process are detailed in Table 4.
2.3 General interlocks and alarms
This section lists the interlocks and alarms that are active all the time. An interlock/alarm is being activated after an issue related to the core operation of the process operation had been identified. Every interlock that is being activated during the execution of a specific phase/step indicates a process operation problem, and it will automatically cause the process execution to switch to a HOLD state. Every alarm being generated indicates a possible problem developing, and therefore, in each case, we must define if the alarm should automatically switch the process operation execution to a HOLD state.
Each interlock/alarm must be defined using the following items:
- Purpose of the interlock/alarm
- How this state is identified
- Corrective action.
The methodology of defining an interlock or alarm is as follows:
[Level ] Verbal interlock/alarm title
Alarm — corrective action by a person
 To prevent (Describe the situation that may happen),  which is identified by (describe the measurement), (Tagname), in state (PV) (for a duration longer than xx seconds),  generate alarm at [level].  Define operator action. ( Switch to a HOLD state.)
Interlock - corrective action by an automated control system
 To prevent (Describe the situation that may happen),  which is identified by (describe the measurement), (Tagname), in state (PV) (for a duration longer than xx seconds),  set (description of equipment), (Tagname), to SP.
126.96.36.199[Level 2 - Medium] Empty water tank T-8
 To prevent a mechanical failure of the water transfer and circulation pump P-160,  when the water level in T-8 tank, identified by LI-81, is below 10%,  Set the command to the Water Transfer and Circulation pump P-160 to STOP.
188.8.131.52 [Level 2 - Medium] Clogged clean water filter F-8
 To prevent a mechanical failure of the water transfer and circulation pump P-160,  when the pressure drop on F-8 filter, identified by PDI-85 is above 2.5Bar for more than 60 seconds,  Set the command to the water transfer and circulation pump P-160 to STOP.
184.108.40.206 [Level 2 - Medium] All valves closed
 To prevent a mechanical failure of the water transfer and circulation pump P-160,  when all exit valves are closed, identified by XV-800, XV-900, XV-901, and XV-902 states are all CLOSED for more than 60 seconds,  Set the command to the water transfer and circulation pump P-160 to STOP.
220.127.116.11 [Level 3 - Low] Receipt tank, T-9, full
 To prevent an overfill of the receipt tank T-9,  when the tank level is high, identified by LI-91 is above 90%, for more than 10 seconds,  Set the command to the water filling valve, XV-902, to CLOSE.
18.104.22.168 [Level 2 - Medium] Clogged clean water filter F-8
 To prevent a dirt accumulation in the clean water main tank, T-8,  when the pressure drop on F-8 filter, identified by PDI-85 is above 1.5Bar for more than 60 seconds,  generate an alarm level 2.  Operator action: The operator should verify why the filter is clogging and monitor its operation in the next 3 hours to verify that there is no problem with dirt arriving and accumulating in the T-8 tank.
22.214.171.124 [Level 2 - Medium] Receipt tank, T-9, is nearly empty
 To prevent the operation from being stuck without water,  when the water level in T-9, identified by LI-91 is below 50%,  Generate an alarm level 2. Operator action: The operator should verify that there is enough water in the clean water main tank, T-8, and that the process is ready to perform a transfer of the water to the clean water operational tank T-9 within the next 30 minutes.
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