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Let us briefly discuss the concepts of steady state, equilibrium, dynamic and transient processes citing some examples.
Steady state process is a process that does not manifest changes with time. A plant at a steady state process has the same mass flow, temperature, and pressure through time in absolute terms or at least does not show significant fluctuations in mass flow, temperature, and pressure within a certain acceptable difference. This differences could be real differences within acceptable margin or due to marginal and acceptable instrumentation errors. Most plant simulations have been based on steady state assumptions. Such simulations and analysis help to study plant responses at different steady state inputs and help to design plants and equipment for a certain minimum and maximum acceptable range of flows, temperatures and pressures.
Equilibrium processes take place in various physical phenomena such as in chemistry-concentrations of reactants and products not changing in time, in physics-balance of force or momentum and static condition of objects, in distillation columns- Thermodynamic VLE, where temperature and pressure of vapor and liquid phase on a tray are the same or vapor and liquid coming out of a plant vessel at a saturated condition have the same liquid and vapor temperature and pressure. Equilibrium conditions are in effect a special aspect of steady state processes.
On the other hand, dynamic processes show change with time. In reality, there is no plant phenomenon such as flow, temperature and pressure that does not change with time. Not only plant processes, but life by itself in essence is all dynamic. We see changes through time and nothing remains the same.
To reflect real plant processes, we have dynamic simulations that try to reflect what happens in the plant. Such plant dynamic simulations include inputs on plant equipment sizes and capacities (volumes), control valve designs, and integrate inputs of process control systems to reflect real plant responses to changes.
A plant basic control system includes the controller (usually on DCS), final control elements such as control valves that receive the output of the controller, the plant process such as feed flow changes, and the sensors/transducers such as flow measurement elements that send process inputs to the controller. Based on the plant measured inputs, for a basic controller, the set points are usually given by the operator. The controller then compares the plant input and set point provided by the operator and sends an appropriate output to the final control element using defined algorithms. For the most part, the way controllers deal with deviations between plant inputs and set points is based on the well-established PID system: Proportional, Integral, and Derivative algorithms or different combinations of them.
Dynamic plant processes are not only controlled using basic regulatory control systems.
There are also Advanced Process Control/Multi Variable Control systems that integrate various plant target (control), manipulated and disturbance variables enabling to do what the operator does by changing set points to get the plant or system on the right track.
Representing what a plant process does through time and predicting process variables and controlling plant dynamic process have been extensive sources of process control research and have resulted in a number of simple or basic and advanced process control systems available in literature and in the market. This area of study remains an endless source of continuous improvement.
Another aspect of a dynamic process is a transient process: A transient process usually occurs during start-ups, shut downs, plant upsets, pump or control valve failures. This phenomenon usually refers to the impact of drastic changes on pipe lines (pumps suddenly shutting down, control valves suddenly opening widely or shutting off and creating pressure waves through pipe lines), water hammer phenomena created due to sudden pressure changes and expansions in vessels including storage tanks or lines (sudden mixing of a higher temperature fluid with a saturated or sub cooled fluid creating a high volume of vapor and as a consequence significant pressure). Transient processes are dynamic since they show change in time. However, they are at the edge of dynamic processes as they take place instantly or quickly, usually measured in seconds or very few minutes. Their impact and response is so fast and can create significant damage to plants and equipment. That is why process designers need to take them into account and put in place mitigating measures to prevent them. Some of the measures to mitigate impact of transient upset processes involve the proper design of the following:
- Line sizes, metallurgy, their thickness, support structures, and safety devices
- Vessels for worst upset conditions including the possibilities of high pressures and full vacuum with their safety devices
- Pumps and their power supplies as well as their safety devices
- Control valves
- De-pressuring systems
- Redirected flow hold up capacities
- Uninterrupted Power Supply system (UPS) to avoid sudden failure of especially critical plant systemsRobust process control and alarm system that alerts about any transient possibilities and responds reliably to such upset conditions
- Correct Management of Change is also a critical part of these mitigating factors. Missing the impact of capacity, metallurgy changes or any plant parameter change can lead to catastrophic failures in transient processes
- Updating plant process changes accurately on P&IDs and on instrument data sheets as part of management of change and making this information easily accessible to design engineers, technicians, operators and to all who deal with plant processes is also a critical part of mitigating factors
- Analysis of the potential impact of transient conditions using available simulation tools is also a vital element.
The above points are just few of the many mitigating solutions that one can come up with and they are not in any way exhaustive or prescriptive.
Steady state, equilibrium, dynamic, and transient processes are key concepts that process engineering designers deal with in their day to day work. Research in these areas and the discovery of tools to deal with issues that arise in plants due to these phenomena are fascinating and provide endless possibilities to those who are interested.
Asking the following questions before any design/specification work would be valuable: understanding the question is half of the answer.
- What is the objective of having or installing this equipment or facility?
- Is the design for a new system (Green field) or for an existing system (Brown field)?
- How we question and understand system interactions for a totally new facility and an existing facility, although equally required in all cases, are different. An existing facility that is live is very sensitive to any changes and needs the utmost care and analysis before design changes are made. A new facility that is not live and is yet to be built gives the opportunity to run quite few simulations and analysis with a number of iterations to get interactions and the integrity of the system right. Same principle can also be applied to a live system to ensure that changes are not implemented without knowing their impact on other live systems. In this regard, change management systems are also critical to catch any loop holes that might be missed
- Is the equipment or facility new or existing within a system?
- What are the safety and reliability questions that we have to ask in this process?
- Is there any impact on the environment, what can be done to mitigate or minimize impact, what are the environmental regulations that should be met?
- Can the existing facility or plant do its job without this new addition or modification?
- If the design is to modify existing equipment or facility, ask why can it not do its job?
- What is the relationship of this equipment or facility with others?
- What could go wrong or what could be improved if this change is made?
- Is it possible to pin point the root cause and resolve the issue without investing too much to add new equipment and facility?
- What are the real estate issues associated with this design?
- Do we meet all required spacing specifications for safety such as fire protection, access and egress, maintenance and turn around requirements?
- Do we understand the process including associated instruments and control system in the facility or plant?
Once we understand the background of the process and all other above information, we need to ask about what tools are required to complete the analysis or required calculations
- What are the assumptions that we need to state to complete the analysis?
- What are known facts that serve as design basis?
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