More process plants are turning to online corrosion monitors to help extend equipment life and enhance operations. For instance, engineers at the 10-million-ton/yr refinery of Dalian West Pacific Petrochemical Company (WEPEC), Dalian, China, opted for such devices when they wanted to improve their understanding of the effects of processes and operational conditions on plant metallurgy.
They selected SmartCET units (Figure 1) from Honeywell Corrosion Solutions, Houston. WEPEC says the decision was based on three main factors: 1) the devices, unlike older instruments that only indicate generalized corrosion rates, provide information on localized corrosion through the pitting factor; 2) they offer additional corrosion variables that vastly improve the accuracy of the published corrosion rate and give an indication of how the corrosion mechanism changes in the process environment; and 3) WEPEC's own engineers could verify corrosion data and trends using ultrasonic thickness gauges.
In late 2009, the plant installed 11 transmitters in equipment with the highest level of corrosion risk, including the atmospheric distillation column and the condensate surge drum outlet.
Corrosion monitoring data and trends are displayed on operator consoles together with process parameters (temperature, flow, velocity, pressure) of each monitoring point. The data are shown on the same time axis, enabling the operators to correlate changes in corrosion with process conditions.
WEPEC hails the project as a complete success because plant engineers have gained a greater understanding of how process upsets affect plant metallurgy — and as a result have been able to reduce failure rates.
According to Honeywell, this success, coupled with similar results from refineries in the U.S., is generating a lot of interest from the chemical industry.
"Traditionally in the chemical industry, the corrosion problem has been dealt with on an after-the-fact/event basis. The processes used are typically very complex, as are the process chemicals, so trying to look at corrosion with any accuracy close to real time was a problem," says Sridhar Srinivasan, global business leader.
The online SmartCET technology used by WEPEC was originally trialled in a year-long project at BASF, Freeport, Texas, where 12 pilot devices found a significant correlation between the feed rate of a specific inorganic chemical and the level of corrosion in pipework. Traditional weight-loss coupons indicated corrosion of 2.965 mils/yr while SmartCET measurements showed 2.90 mils/yr. The coupons effectively validated online monitoring — so much so that BASF decided to abandon coupon use altogether.
"So we began to look at corrosion as a process variable, with the information being sent from the probe to the company DCS [distributed control system] in real time so that corrosion could be seen alongside other process variables impacting/causing corrosion," he explains.