Overflowing Fuel Oil In Transmitter

The causes could be many, and you need to work through the problem in stages. The tank sensor could be faulty, sending the wrong signal to the transmitter. If the tank sensor is flange mounted at the bottom of the tank, then there could be an orifice plate between the sensor and the fuel, and this could be choked. You need to disconect the sensor at the transmitter, and simulate the full input range, while checking for the correct transmitter output. The transmitter could be faulty. You need to check that the power supply is OK, and whether for full sensor input range you are seeing the correct transmitter output range. The controlling valve could be faulty. You need to simulate the full range input signal, and check that you get the full travel of the valve stem. If all these are Ok, then look at the valve seat? You say that the "bypass line was choked?" This would indicate that the fuel is heavy oil, in which case I would look at a blocked orifice plate at the sensor, or sticking control valve.

What is the measuring technique? If it is capacitance, for example, then a slight change in the dielectric constant of the material will cause false level signals to be presented; a similar problem occurred in a pharmaceutical plant many years ago because a change in a liquid was invoked without considering the consequences. This would not have happened if the hange had been subjected to a formal HazOp study.

Check to see if anything has changed...Assuming all instruments and valve are calibrated and sensor is OK:

1.Is the valve operating correctly?

2.Any foreign substance in the vessel? Water?

3.Mains Power/Local Air Supply adequate?

4. other, etc.

The tank overflowed because the control system design is inadequate for the application. You are relying on a single transmitter to control the level of an explosive liquid. In most places this is a violation of safety codes.

Over time, all sensors will fail. All an engineer can do is to reduce the chance of failure and to improve the chances of a safe failure. Explosive application should have at least a redundant level sensor (or two) to force the system to a safe failure mode if something failed. The simplest solution is to add a float switch as a High Level Alarm that would have controlled a secondary fill cut off valve.

suddenly the tank is over flowed ( due to bypass connection pipe also chocked )...why it is over flowed

You seem to have answered your own question! In your statement, you gave the reason before asking the question.

I'm assuming that you have a bypass valve on your control valve and the bypass has either stuck in the open position or it's leaking internally. When that happens, your control valve may be closed but the leaking bypass valve will allow fuel to continue flowing.

Another possibility is that your control valve itself has an internal leak. Same symptom, different cause.

You didn't mention if your level transmitter was indicating normal when the tank was actually overflowing already. It that is what happened then your transmitter may be defective.

If you're using a wet leg system, fuel oil may have solidified in your impulse piping rendering it useless.

These are just a few of several assumptions since your post doesn't have enough information for me to provide a definite cause.