Anyone who has attended one of my Process Safety training courses has heard me use the phrase “circuitizing your piping” when we discuss the mechanical integrity aspect for piping. API 570 is our baseline RAGAGEP for our Piping Inspection Program, but API 574 is how these inspections will actually be conducted and this standard is chock full of great information and is a MUST READ for anyone developing a Piping MI program. A small and simple process may be able to have just one (1) pipe circuit, but there are VERY FEW processes where this will work. Most facilities, especially those who have multiple PSM/RMP covered processes will for sure have multiple pipe circuits in their MI inspection program. In fact, most facilities will have hundreds of piping circuits and although this may seem unmanageable at first, it actually makes managing the program much easier in the long run and process safety is a marathon – NOT a sprint! Here are some of the highlights from API 574 that we should consider when setting up a piping inspection program.
The single most frequent reason for replacing piping is from thinning due to corrosion. For this reason, an effective process piping inspection program will include monitoring piping thickness from which…
- corrosion rates,
- next inspection dates, and
- projected piping retirement dates can be determined
A good MI inspection program includes prioritizing the piping systems by identifying the consequences and potentials of piping failures. API 570 provides a detailed guide for CLASSIFYING PIPING according to the consequences of failure. The key to the effective monitoring of piping thinning is identifying and establishing thickness-monitoring locations (TMLs). TMLs are designated areas in the piping system where thickness measurements are periodically taken. By taking repeated measurements and recording at the SAME POINTS over time, corrosion rates can more accurately be calculated.
Some of the factors to consider when establishing the corrosion-monitoring plan for process piping are:
- Classifying the piping in accordance with API 570, 6.3.4 Piping Service Classes.
- Categorizing the piping into CIRCUITS of similar corrosion behavior (e.g., localized, general, environmental cracking).
- Identifying susceptible locations where accelerated corrosion is expected.
- Accessibility of the TMLs for monitoring.
Piping Circuits
A number of factors may affect the rate and nature of pipe wall corrosion. They include, but are not limited to, the following items:
- Piping metallurgy.
- Piping contents.
- Flow velocity.
- Temperature.
- Pressure.
- Injection of water or chemicals.
- Mixing of two or more streams.
- Piping external conditions.
- Stagnant flow areas, such as dead legs.
Complex process units or piping systems are divided into PIPING CIRCUITS to manage the necessary inspections, calculations, and record keeping. A PIPING CIRCUIT is a section of piping of which all points are EXPOSED TO AN ENVIRONMENT OF SIMILAR CORROSIVITY and which is of SIMILAR DESIGN CONDITIONS AND CONSTRUCTION MATERIAL.
When establishing the boundary of a particular piping circuit, the inspector may also size it to provide a practical package for recordkeeping and performing weld inspection. By identifying like environments as circuits, the spread of calculated corrosion rates of the TMLs in each circuit is reduced, and the accuracy of the calculated corrosion rate is improved. Proper selection of components in the piping circuit and the number of TMLs are particularly important when using statistical methods to assess corrosion rates and remaining life.
According to API 570, Section 4.2, ALL PROCESS PIPING MUST BE GIVEN A CONSEQUENCE OF FAILURE CLASSIFICATION. The inspector then reduces the uncertainty of the data obtained by assigning more TMLs to the lower classified piping and monitoring more frequently. This improves the ability to predict reliable retirement dates but also focuses limited inspection resources to areas that pose the greatest hazard.