Failure assessment of crude oil preheating tubes in mono ethylene glycol–water mixture solution

Aqueous solutions of glycol are commonly used in many applications as heat transfer media. In this study, failure analysis and the leakage of oil tubes in a crude oil pre-heater of a desalting plant were assessed. Crude oil pre-heater (furnace) is a type of heat exchanger in which heat is transferred to oil tubes via ethylene glycol solution in order to increase its temperature for further processing. With the increase of temperature, the viscosity of the oil plus water mixture inside the tubes is reduced and desalting is facilitated. Reports show that, these tubes facing external corrosion in the form of smooth cavities finally lead to leakage. These tubes are made from ASTM A106 Gr B steel. The failure assessment of tubes was carried out by performing various chemical and metallurgical tests. Corrosion coupons were also immersed in ethylene glycol fluid at different places inside the furnace. The investigation revealed that degradation of ethylene glycol forming organic acids (acetic acid, carbonic acid, and formic acid), as the consequent reduction of the pH, take place progressively, being responsible for carbon steel acidic corrosion and eventually to the failure of pre-heating tubes. An action was taken to examine different pH stabilizers for mono ethylene glycol solution. The results obtained by polarization curves showed that, by using stabilizer EN231 at pH = 10.5, corrosion rate reduces to a minimum.