Avoid Low Flow of Process Fluid to a Fired Heater!

Hi

When a process engineer carries out the design of a system, he should be aware of the upset conditions that may lead to catastrophic scenarios. In this email, we shall talk about one of these scenarios.

A low flow of inlet fluid to a furnace is is one of the most critical upset conditions a fired heater may face. This is because the process fluid to be heated acts as a coolant to heater tubes. When this process flow is cut or highly reduced while the burner is still running, this can cause severe issues such as:

1. Hot spots in the tube leading to tube rupture
2. Overheating of process fluid inside the tubes which can cause issues such as coking for example which can lead to tube plugging

These issues are very critical and can lead to catastrophic consequences being a source of explosions and uncontrolled fire. That’s why there are common practices to avoid such a condition and ensure there is always enough flow rate of the process fluid:

1. It’s highly recommended to avoid adding isolation valves on heater inlet and outlet, This is to ensure an open flow path to the heater and avoid closing these valves by mistake. If isolation valves are used, a pressure safety valve would need to be installed with an orifice area that guarantees enough flow rate through the heater.
2. If the inlet flow rate to the heater is controlled, the control valve should consider failing in the open position. This is its fail safe position in this case.
3. In case of any other upset that leads to a low flow to the heater, a safety interlock should stop the heater burner or at least reduces its output to the minimum. If there is a total blockage that leads to no flow in the system, then the burners should be totally stopped. As we are dealing with a very critical condition that can lead to catastrophic consequences, we shall need to ensure that this shutdown function is highly reliable. That’s why its Safety Integrity Level (SIL) rating in most cases would be SIL 3.

So here, a process engineer should be highly cautious when specifying the required protection of a heater system in order to deliver a system which is safe to operate.

In our article Critical Role of Safety Shutdown Interlocks in Equipment Protection, we have talked about assessing the layers of protection in a system and their assessment. You can check it out if you like to understand the basic plant protection principles:

Check Article!

I hope you shall enjoy the article, and if you have any question or comment or suggestion, feel free to reply to this email.

Have a nice weekend!

Mohamad