On-Stream Thickness Monitoring Versus Conventional Inspection in Refineries

Introduction

Fixed equipment in a refinery can be susceptible to corrosion from the process side, necessitating an inspection strategy to understand the condition of that equipment. Historically, these programs have primarily relied on periodic manual inspections to provide that data. Often, manual inspection is sufficient to provide that information, but in many cases it may not be if process conditions change or are erratic. In recent years, new technology has become available to supplement manual inspection with a system that can provide data automatically and on demand. This data can then be combined with other relevant data to create detailed logs, which helps make the owner-user aware of the health of the plant and enables them make informed decisions as to what action, if any, is necessary to maintain the integrity of their assets. Up to date knowledge of the current state of equipment integrity allows operations to remain safe and reliable, even as process conditions and subsequent corrosion conditions change within the plant environment. Online measurement of pipe and pressure vessel wall thickness is made possible by installing wireless, semi-permanent thickness monitoring sensors.

In this article, the need and requirements for online corrosion monitoring and the methodology for choosing the monitoring locations are explained. The quality and frequency of the measurements delivered from the monitoring system is compared with manual ultrasonic thickness (UT) measurements. And finally, a short discussion of the value derived from the online wall thickness measurements at a US West Coast refinery is provided.

The Need for Online Corrosion Monitoring

In order to know whether refinery equipment has withstood the demands that have been placed upon it, and that it can withstand the demands being placed upon it now and into the future, a current picture of asset health is needed. Manual inspection can provide a snapshot of plant integrity, but is normally only feasible on a limited frequency due to the cost and practicality of accessing piping and pressure vessels. Much of a refinery’s process equipment, is simply too hot to obtain a reliable reading while the plant is operating. Additionally, access to certain locations can be expensive if staging is required, and can put personnel at risk. As a result, manual inspection is often carried out during a period with the plant is shut down. An automated corrosion monitoring tool can be used to supplement periodic manual inspection and bridge the information gap.

In addition, if online measurements are of sufficient quality and frequency, then small changes in the integrity of the plant can be identified in near real time. Knowledge of where, when, and how corrosion is developing, as well as an accurate measure of its severity, without the need for plant shutdown, better enables owner-users to make a multitude of proactive decisions, ranging from loss of containment avoidance to profit optimization.

Additional benefits can be realized using fixed, online sensors. For example, the same spot is measured every time, eliminating measurement error inherent to manual measurements. Other potential sources for error, such as the use of different technicians or alternative equipment, are also eliminated. The resulting data is more consistent and therefore more useful when comparing consecutive measurements for corrosion rate calculations. The wireless online UT system discussed here can provide direct wall thickness measurements to allow for real-time integrity assessment and the detection of small changes in corrosion activity.