Protecting Your Pipes – Using Pulsed Eddy Current as an Effective Inspection Tool
Figure 1: Pulsed Eddy Current Enables a Single Sensor to be Used to Monitor Many Different Location
5.3 5.35 5.4 5.45 5.5 5.55
0 Change in flow rate 0.1 mm 50 100 Time (days)
A unique feature of PEC is its portability. With PEC, a single sensor can be used to monitor many different locations. Positioning frames and centre pop marks are used to ensure that the PEC probe is accurately located in the same monitoring position for each measurement.
PEC = pulsed eddy current. Case Studies
Ammonium bisulphide (NH4HS) corrosion is a well-known corrosion problem for hydrocrackers at refineries. During regular standard inspections it was found that, due to fouling and more severe operation conditions than expected, the flow-assisted NH4HS corrosion appeared to have increased beyond the design corrosion rates.
Although UT wall thickness measurements on the air cooler outlet piping gave a reliable measure of the minimum wall thickness, the data could not be used to reliably estimate the corrosion rates over intervals of a few weeks. PEC was therefore chosen as a complementary NDT technique for estimating the corrosion rates and trends. PEC typically achieves an in-service measurement repeatability of 0.2 % (e.g. 0.02 mm on a 10 mm wall thickness). With such a high repeatability, PEC is able to establish corrosion rates in less than one-tenth of the time needed by UT.1
In May 2000, the supporting legs of a liquid petroleum gas (LPG) storage tank sphere failed during a hydrotest. The cause of the failure was identified as corrosion of the steel legs underneath the layer of concrete fireproofing. Water deflectors had not been fitted correctly and water was able to enter the interface between the concrete and steel, with disastrous consequences. Since this failure, PEC testing has been used to inspect the legs of many storage spheres worldwide. PEC is an attractive option as, unlike UT inspections, there is no requirement to remove fireproofing. Also, PEC is not restricted by the diameter of the legs, which are too large for radiography to be used. PEC can be used to inspect structural members through up to 250 mm of concrete fireproofing.
In 2006, an ageing Gulf Coast petrochemical facility experienced a leak from a four-inch hydrocarbon line. After several minutes, this leak found an ignition source, causing a massive fire which destroyed half the unit and cost the company US$50 million. Despite the advances in materials and inspection and maintenance practices, the insidious problem of CUI still costs our industry millions of US dollars a year. The company was concerned that there were other piping systems within the plant that also had the potential of causing such a fire. A systematic piping programme was initiated that included a risk assessment and visual inspections followed by PEC examination of susceptible areas. Where visual inspection identified
150 200
areas that were showing signs of CUI, further evaluations were conducted. These included the PEC technique to determine wall loss without removal of the insulation – this was used as a screening tool to identify corrosive areas – and when critical areas were identified (either visually or with PEC), stripping insulation and conducting further visual and UT inspections. Another project involved the examination of the utility piping (high-pressure steam lines) of a large Gulf Coast chemical plant. FAC is metal loss through dissolution of the protective oxide film in piping serving water or wet steam. Under certain water chemistry, fluid velocity and operating conditions, FAC can cause internal wall thinning of piping, such as condensate or feedwater piping.
In some cases, FAC thinning has led to catastrophic failures and fatalities. For this project we developed the following approach: •
identify and rank high-priority examination areas; • •
conduct PEC online or offline non-intrusive examination of high-priority areas;
UT examination of selected locations; and • engineering analysis.
A unique feature of our approach to FAC examination is our non-intrusive wall thickness measurement system using PEC technology (see Figure 1).
As pulsed eddy current is a non-contact non-destructive testing technique, it can also be applied for wall thickness monitoring at high temperatures, online and through insulation.
The PEC system is especially attractive to industrial and power plant clients because it can be used:
• • •
•
while the plant is online or offline – no turnarounds required; without removing insulation and lagging;
at a substantial cost saving, particularly when asbestos insulation is present;
wall thickness accuracy: better than ± 5 %;
• pipe temperature: -150–950 °F; • •
insulation thickness: 0–4 inches; and pipe wall thickness: 0.15–2.6 inches.
The components typically examined by PEC include condensate and feedwater piping, feedwater heater drain piping, boiler tubes (refractory-covered and finned), feedwater heater shells, de-aerators, tanks, vessels, coal silos and piping, and any other ferritic material. These examinations allow management and inspectors to identify problems before they create dangerous conditions or cause expensive shutdowns. n
1. Crouzen P, Munns I, Pulsed Eddy Current Corrosion Monitoring in Refineries and Oil Production Facilities – Experience at Shell, Amsterdam, The Netherlands: Shell Global Solutions International, 2006.
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HYDROCARBON WORLD – VOLUME 6 ISSUE 2
Wall thickness (mm)
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