To protect overspill of tank lorry during loading operation generally the automated gantry are provided with over spill sensor, which stop the flow of product immediately once the product touches to over spill sensor.

This is a only single measure available to control the over spill. But some time this sensor does not work due to issue in automation system, faulty over spill sensor etc etc.

To my mind the over spill at tank lorry gantry is a really a great risk specially if the product is motor spirit. I am looking the suggestion for secondary active control to avoid the over spill at Tank Lorry gantry.

Please suggest

Usually a metered delivery system is the main control - it will only allow a specified quantity to be delivered to the tanker. Each tanker is set up on the system so its capacity is known.

If a vapour recovery system any overfill will result in liquid to this system. It will be contained and can be detected by level detection in a knock out pot or similar.

Often a human is more reliable than an automated protective device. A dead-mans handle can be used to ensure the human is present to monitor the level.

Unless the overspill detection device is an integral part of the tanker, reliability is difficult to ensure. The ones I have seen are part of the gantry and have to be inserted to the top of the tanker for every fill. It is difficult to guarantee that it has been located correctly and they are vulnerable to damage.

Diasagree with #3, a human is not likely to be more reliable than a correctly assessed/designed Safety Instrumented System especially if a SIL2 is specified/used.

Section 5.1 items 1 & 2 covers the implied SIL rating of the control system.

Suggest you need to complete a SIL/LOPA risk assessment to determine what SIL rating you need. Probably SIL2

EN61508 and/or EN61511 cover functional safety determination.

After that, speak to a suitable C&I design company with knowledge of such work. They will be able to design and specify a SIS.