The outer walls of insulated pipes are susceptible to corrosion if the insulation stays wet for long periods of time. Corroded pipes have the potential for bursting without warning, releasing steam or combustibles and insulation debris which may contain asbestos, requiring significant cleanup and repairs. Managing the associated risks requires the consideration of a number of factors such as corrosion rates, examination methods, prioritization of inspection locations, and fitness for service evaluation and repair methods.
A number of factors affect the corrosion rate such as pipe material and geometry, proximity to saltwater environments, local weather, and prevailing wind directions, as well as insulation type and condition. Some locations on the piping system require additional scrutiny, such as at pipe supports where rain water can cascade down hanger rods and into openings in the insulation, and at the bottom of vertical pipe runs where moisture is prone to becoming trapped in the insulation cladding. To prioritize inspection locations, risk factors such as exposure of personnel, the potential amount of energy released, the postulated corrosion rate and failure potential based on pressure and temperature need to be taken into account. The most susceptible materials are carbon and low alloy steel. Pressurized dead leg small bore pipes are of concern due to the small wall thickness and low operating temperatures that do not provide an opportunity to dry out the insulation.
Digital Radiography Testing (DRT) through insulation provides excellent results for most 6” or less diameter piping systems. Highly sensitive digital exposure plates require less radiation compared to conventional Radiography, resulting in small radiation safety zones that allow work in surrounding areas to continue during inspection. The technique is effective for the evaluation of corroding fillet welds and threaded connections, and provides a digitized image that is database friendly. It provides information on pipe diameter and schedule, which aids in the preparation of repair sketches and bill of materials before an outage occurs.
Repair options include replacement and operational changes which allow abandonment as well as improvements or elimination of insulation.
Corrosion Under Insulation (CUI) damage is a significant issue at some plant sites where limited run time doesn’t allow the insulation to dry. In the near future, older, less economic base loaded plants that will run less frequently may fall into this category.
To evaluate risk, the probability of failure needs to be considered with its potential consequence. For a pipe with CUI damage, probability of failure is dependent on the corrosion rate, pressure, pipe diameter, and thickness. The consequences are a potential injury and lost production.