There must be more to this question than what is currently stated, because the answer seems so obvious. You need to know how much gas was vented to the flare and the CH4 concentration in that gas. If you don't know both of those two things, then the only other option is to guess.

Dear don1980
I could not follow you.
I just want to know amount of flaring, don't need CH4 concentration.

The quesrion is: how could I predict or calculate it (knowing CH4 introduced in main tank)

As I understand it (correct me if I'm not understanding it right) the operator is manually opening a vent valve to get rid of the methane which accumulates in the condenser because it obviously won't condense. So, the amount of venting is directly a function of the amount of methane that enters the system- none of the methane condenses so it will all be vented to the flare. But, the vent stream will also have a high concentration of propane. So, the amount of gas that's vented to the flare is a function of the amount of methane entering the system and the concentration of the vented stream (the amount of propane that's swept out along with the methane).

Yes exactly, how can I cal. It?

You can get a rough estimate based on a known amount of methane in the condenser feed stream, a calculated propane concentration in the vent stream (based on known T&P in the condenser), and an assumption that the operator doesn't leave the vent valve open any longer than necessary.

Have the operators log:

1. pressure before flaring
2. pressure after flaring
3. time of flaring

Characterize the pipe and fittings from the chilling compressor discharge to the flare into a fluid flow problem, then solve for flow.

Good Luck,
Latexman

Dear Latexman .
Good idea. But the pressure control system is automatic. So the pressure of the vessel wouldn't change.
PV valve open until the vessel pressure does not increase.

Then it is easier, constant pressure, constant flow. My advice is still valid. You can calculate the flow rate x the time of flow = mass lost.

Log the data until you see a consistent frequency, then you can have a consistent loss.

This frequency will be consistent with how long it takes the incoming CH4 concentration to build up to the CH4 concentration in the condenser that causes high chilling compressor discharge pressure. To improve the consistency, specify to the operators the exact chilling compressor discharge pressure, not a range, you want them to start flaring. And, if it makes sense, it may not, specify to the operators the exact chilling compressor discharge pressure, not a range, you want them to stop flaring.

Good Luck,
Latexman