WHITE SMOKE FROM ENGINE ONLY ON PETROL

have the coolant system pressure checked for a head gasket leak.

gasket is fine i have done the tests and there is no leak any other idea

the only other thing that I cold think of is that the pcv system may not be working right and causing this.white smoke is usually an indication of water or coolant leakage.so check yoru pcv valve to see if it's working properly . Other than that I can't think of anything

could this be the piston ring problem ? they are damaged or weak ?

if it were a piston ring problem the it would do it all the time. YOu can test compression though and if there is a 20% variance between any cylinder than it may be of concern but I kind of doubt it. the thing is when you switch between petrol (gas) and CNG you are going from a liquid to a gas so it may be something in the fuel as well causing it.

exactly thats what i am thinking but i still want to confirm it from you as you are an expert if the piston rings were some how weak it should produce this white smoke on both cng and petrol right ?

yes and per your statements it's not. so check compression other than that without seeing this myself I have no further comments or suggestions.