Inspectioneering
Inspectioneering Journal

TML Placement for Piping

By Jay N. Rothbart at Conam Inspection Inc. , and Michael Twomey at CONAM Inspection Inc.. This article appears in the November/December 1998 issue of Inspectioneering Journal

We have been asked many times by existing and prospective users of PCMS (a computerized, inspection database management system), how many TMLs (Thickness Management Locations) should be installed per piping circuit. These inquiries must be addressed indirectly, because each specific site differs in its operation, mission and objectives. We firmly believe it is up to each site to make choices based upon their own circumstances. When pressed on the issue we have pointed toward a couple of papers published or generated within our User Community. While these provide cookbook methods, they do not encompass all the issues facing the User Group. Based upon the continuing questions in this area we decided to provide a review of the issues.

Looking at data from the User Group does not provide a clear basis for setting numbers of TMLs. We obtained information from about 20% of the Group. For pipe and all other equipment, the number of TMLs in each User Site ranges from about 20,000 to over 175,000.

Taking into account plant size and the number of process units does not help. The average number of TMLs per Process Unit varies from about 1,000 to almost 6,000. The average number of TMLs per circuit varies from 5 to 50. The only conclusion we can reach is that TML placement has been very much site and process specific.

Given this lack of clear rules, we offer some broad parameters. The objective is to put some broad ranges around number and placement of TMLs as well the breadth of inspection while attempting to optimize the cost/benefit.

Section 1 - Important Factors

The number and placement of TMLs are dependent upon various factors. These include:

  • Defining the Circuit
  • Method of taking readings
  • The expectation of General or Local Corrosion
  • The expected Damage Mechanism(s)
  • The Criticality of the Piping

Circuitization

Thoughtful circuitization is the first step in the process of providing cost effective thickness data analysis. Perhaps a more accurate term for circuit is common corrosion environment. The environment includes the process fluid itself, plus the operating conditions of temperature, pressure and flow rate, and the metallurgy of the containing pipe. The objective is to compare like with like. For example, in a normal process system like a top pump around on a fractionating column, the piping downstream of the heat exchanger may be assigned a different circuit than upstream equipment if it is in a more severe corrosion environment. This may be due to increased temperature, even though the process material and the piping metallurgy are the same.

The basic approach to circuitization utilizes P&IDs (Process and Instrumentation Drawings) and Isometric Drawings as references. Based upon these the following are checked:

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