Short answer
A condition monitoring location (CML) is a fixed, marked point on an asset where an inspector measures the wall thickness again and again. The readings at a CML give the corrosion rate and the remaining life. In older documents, the name is thickness measurement location (TML).
Where to place CMLs
Place CMLs where the damage mechanism is most active, or where you expect the worst wall loss. Typical locations for piping and vessels are:
- Elbows and tees, where the flow changes direction
- Injection points and mix points
- Deadlegs and low points, where water can collect
- Areas downstream of control valves and orifices
- Inlet nozzles and impingement zones on vessels
- Heat-affected zones next to welds
API 570 and API 574 give guidance on CML selection for piping. API 510 covers CMLs on pressure vessels. The number of CMLs depends on the damage mechanism, the consequence of failure and the history of the circuit.
What data each CML needs
- A unique ID, and a location that a field inspector can find again
- The nominal thickness and the minimum required thickness (t-min)
- Each reading with its date, value, method and inspector
- The corrosion circuit or the component that the CML belongs to
Reliatic stores this data for each CML. It calculates the short-term and long-term corrosion rates, the remaining life and the next inspection date for each CML. The CML with the shortest remaining life sets the remaining life of the asset.
CMLs and corrosion circuits
A corrosion circuit is a group of piping or vessel parts with the same material and the same process conditions. You expect a similar corrosion rate in all of them. You place CMLs in each circuit, so that the readings represent the whole circuit.
Common problems
- A CML that the field team cannot find, or that moves between surveys.
- A reading that is thicker than the last one. Check the location and the gauge before you accept it.
- No t-min on record. Without t-min, the remaining life is not valid.
- Too few CMLs on a circuit with local corrosion. Spot readings can miss the local wall loss.
Questions and answers
What is the difference between a CML and a TML?
TML (thickness measurement location) is the older name. Current editions of API 570 and API 510 use CML, because a location can also record other types of condition data, not only thickness.
How many CMLs does a piping circuit need?
No single number applies. The number depends on the damage mechanism, the consequence of failure and the history of the circuit. API 570 and API 574 give the guidance.
How does Reliatic calculate remaining life for a CML?
It takes the higher of the short-term and long-term corrosion rates. It divides the thickness above t-min by that rate. The next inspection is the lesser of half the remaining life and the code maximum.