Long, Short, and Baseline Readings

The CML measurements to be used for the Corrosion Rates (CR) calculation are shown on the CML Measurement History tab of the Circuit. The readings are classified as long, short or baseline readings.

When calculating Corrosion Rates (CR) for each Condition Measurement Locations / Measurement Point (CML/MP), IMS uses three good thickness readings for the long- and short-term CR. Typically, these are: The Latest Reading, the Previous Reading and the First Reading (baseline or nominal thickness).

The table below summarizes how the thickness readings, used in the CR calculations, are indicated. The figure below shows the CML Measurement History tab on the Circuit (see PEI Circuits - Details Page). You can find these indicators back here.

Indicators for the readings used in the CR calculations:

A Circuit’s CML Measurement History. Notice the L, S and B indictors.

In the table below an example of thickness readings is given:

Short-term CR is calculated using the previous and last thickness reading. While the Long-term CR is calculated using the first and last thickness reading.

Nominal Thicknesses and Base Readings

Often the initial reading for a CML may have been inputted as a Nominal Thickness.  This means that the actual thickness was not an actual measurement.  For a plate, the nominal thickness is the specified plate thickness, i.e. 11.11mm (0.437”).  For a pipe, the nominal thickness is based on the pipe schedule, i.e. 6 NPS Schedule 40 pipe is 7.11mm (0.280”).

The problem with Nominal thicknesses is that they are not accurate, especially for seamless pipes! Most seamless pipes have a tolerance of +/–12.5%, so a new 6 NPS Schedule 40 pipe could have a thickness as low as 6.22mm (0.245”), or as high as 8.00mm (0.315”), and still be within manufacturing tolerance.

As an example, you may enter a Nominal thickness of 7.11mm (0.437”) and 2 years later take a UT reading and measure 6.35mm (0.250”).  The question then is: Has the pipe really lost 0.76mm (0.187”), or was it just thinner than nominal when it was installed?   You really do not know unless you take baseline readings. 

Example

This example will show how IMS identified, for a 6 NPS pipe, the thickness readings as “B”, “S” and “L”, when the initial thickness is a Nominal thickness.

On 1-June-1998: The Nominal thickness of 7.11mm (0.437”) was entered in IMS.

Thickness readings:

On 1-Jun-2003: The 2nd real measurement is entered - an RT reading – 6.20mm (0.244”).

As soon as 2 non-Nominal readings are entered, IMS no longer identifies the Nominal reading as the Base reading. The Base reading is now the first non-Nominal reading.

Thickness readings:

We now have much better CRs. But notice that the Long and the Short-term CR are still the same. When a third set of readings are added in June 2008, the CR change again.

Thickness readings:

The Long & Short Corrosion Rates are now different!