Hi
Segregation between process control and process shutdown is one of the most important aspects a process engineer should consider when protecting the piping, system, or equipment from an upset condition.
This is done by analyzing the layers of protection to ensure that the plant is well protected.
The plant protection is made through having separate instruments, separate shutdown system, and separate shutdown valve to protect the system. This is a shutdown loop that is totally separate from the control loop.
For example, we can see in this system that there is a level control on V-01. If the level in V-02 startts to decrease, the control valve shall throttle in order to retain the liquid level in V-01.
However, what shall be the case if the control valve failed in the open position? Or what if the level controller failed? What if LT-01 gave some false readings?
In all the above scenarios, this may lead to liquid getting evacuated from V-01. If all the liquid was evacuated, gas shall start to go through to V-02. This means that V-02 which is designed to handle 3 barg, shall be vulnerable to a pressure of 35 barg, which shall lead to overpressure in the system and catastrophic failure.
That's why the shutdown system should be segregated and more reliable than the normal control system. Here we are not talking about adjusting normal operation, but this is a major safety issue.
So in this case, we are considering an emergency shutdown (ESD) system other than DCS for controlling normal operation, another level instrument which is LT-02 that gives a reading of very low liquid level, through this reading the ESD shall act and send a signal to the shutdown valve to stop the flow.
The ESD system, LT-02 and shutdown valve and even the type of signal to send the reading and the action are much more reliable than the ones used for the normal control system.
Analyze your Protection with HAZOP & SIL Sessions
In addition to the above, we should ensure the reliability of the protection of the plant against overpressure. This is commonly done through HAZOP and SIL sessions.
Through this session, the project team from different backgrounds sit with each other, identify the risks and project deviations in each portion of the plant, check out if the design addresses the above scenarios with proper safeguards, and if it doesn’t, then it adds applicable recommendations.
For even more critical points, we may need to conduct a further study against the current layers of protection, check their reliability against the severity of the deviations they are addressing, and based on the result of this study, we end up determining the SIL rating of the shutdown protection system, instruments, and shutdown valves.
In our P&ID course, this was analyzed with several examples. You can check them out whether you are enrolled in the P&ID course, or in the Process Engineering Masterclass course.
If you'd like to enroll in any of these, you can access them through a limited time discount. Just use the coupon code SFHZ35DD to access any of them. If you have any question or issue, feel free to contact me.
Have a nice day!
Mohamad
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