1.3 Condition Monitoring Locations (CMLs) & Coverage

1.3 Condition Monitoring Locations (CMLs) & Coverage

Condition Monitoring Locations (CMLs) are designated areas along a piping circuit where periodic thickness measurements and non-destructive examinations (NDE) are conducted. The strategic selection, density, and monitoring of CMLs are critical to detecting localized and uniform wall thinning before a failure occurs.

Selecting CML Locations

CMLs should be established in areas that are most susceptible to corrosion, erosion, and environmental cracking. The selection process, guided by API RP 574, focuses on the following high-risk zones:

1. Elbows, Tees, and Reducers

Fittings that change flow direction or piping cross-section are subject to turbulent flow, impingement, and erosion-corrosion.

• Elbows: The outer radius of elbows is directly in the path of the high-velocity fluid stream and particulates, making it highly susceptible to erosion. CMLs must be placed on the outer curve of the bend.
• Tees: The branch connections of tees experience severe turbulence and vortexing. Thickness readings should be taken around the branch intersection.
• Reducers: Changes in diameter accelerate or decelerate flow. CMLs are placed on both the sloped transition wall and immediately downstream of the reducer.

2. Downstream of Flow Disturbances

Severe turbulence occurs downstream of control valves, regulators, orifice plates, and flow meters. Cavitation and liquid droplet impingement in these regions can cause rapid wall thinning. CMLs must be placed immediately downstream of these components, typically extending 10 pipe diameters downstream.

3. Soil-to-Air Interfaces

For piping that is buried underground and rises to the surface, the soil-to-air interface is highly corrosive. The combination of soil moisture, oxygen, and atmospheric contaminants accelerates external corrosion. CMLs must be established at the interface, and thickness measurements must be taken both at the ground line and just below the surface (requiring excavation of 6 to 12 inches of soil).

4. Injection Points and Dead Legs

As detailed in Section 1.2, these locations are highly susceptible to localized damage. CMLs on injection points must monitor the injection nozzle weld, the pipe wall directly opposite the nozzle (to detect impingement), and the mixing zone downstream. CMLs on dead legs must be positioned at the low points where corrosive water or deposits settle.

CML Density and Spacing Guidelines

The number and spacing of CMLs within a circuit depend on the piping's classification, corrosion rate, and predictability of the damage. API 570 categorizes piping into four classes based on safety and environmental consequences:

• High-Corrosion or Localized Circuits: Require a high density of CMLs. If a circuit is prone to localized pitting or erosion, CMLs must be spaced closely, and thickness measurements must involve scanning rather than single-point readings.
• Low-Corrosion, Uniform Circuits: CML density can be lower. If historical data shows a highly predictable, low rate of uniform corrosion, a few representative CMLs per circuit are sufficient. CMLs can be reduced or eliminated only after a statistical analysis and approval by a piping engineer.

Thickness Measurement Techniques

Two primary NDE methods are used to measure wall thickness at CMLs:

Ultrasonic Thickness (UT) Testing

UT is the most common method for measuring wall thickness. It utilizes high-frequency sound waves that travel through the pipe wall and reflect off the back surface.

• Scanning vs. Point Measurements: For localized corrosion, a single-point UT reading can easily miss adjacent deep pitting. In localized circuits, the inspector should perform a grid scan (moving the UT probe continuously over a defined grid, such as 2-inch by 2-inch) to locate the absolute minimum thickness.
• Limitations: UT requires direct contact with the pipe metal, meaning insulation must be removed or plug holes cut at CML locations. It can also be difficult to obtain accurate readings on small-bore piping (NPS 2 and under) due to surface curvature.

Profile Radiographic Testing (RT)

Profile RT is the preferred method for monitoring small-bore piping (NPS 2 and under).

• Advantages: Profile RT provides a radiographic image of the pipe wall in cross-section. This allows the inspector to measure the wall thickness directly from the image without removing insulation. It also reveals internal deposits, plugging, and localized erosion that UT might miss.
• Limitations: Radiation safety zones must be established, which can disrupt nearby maintenance work.

CML Optimization Scenario

Consider an NPS 6 Class 1 hydrocarbon piping circuit carrying flammable gas. The line operates at 150°F (66°C) and is insulated.

• During a routine turnaround, profile RT on a Class 1 circuit elbow revealed severe localized erosion-corrosion, thinning the wall to 0.190 inches. The adjacent straight runs remained at 0.280 inches. The minimum design wall thickness (t_required) is 0.170 inches.
• In response, the inspector must optimize CML coverage:
  1. Add CMLs: Establish new CMLs on all elbows and tees within the circuit.
  2. Implement UT Scanning: Transition the CML on the thinned elbow from single-point UT to a grid scan to map the localized thinning.
  3. Corrosion Rate Adjustment: Recalculate the corrosion rate using the minimum reading (0.190 inches) to determine the new remaining life and next inspection date.

Worked Example: Determining CML Quantities and Optimizing Thickness Readings

An inspector is establishing the CML plan for a newly installed 400-foot NPS 10 Class 2 hydrocarbon line.

1. Determining Baseline CML Quantities (API RP 574 Guidelines): Under standard Class 2 piping circuit guidelines, the inspector identifies all fittings and high-turbulence points. The circuit contains:
   • 10 elbows (direction changes)
   • 2 tees (mixing points)
   • 1 control valve bypass assembly
   • 4 straight runs Initially, the inspector assigns CMLs to all 10 elbows (outer radius), both tees (run and branch), 1 CML downstream of the control valve, and 1 CML per straight run, resulting in 17 baseline CMLs.
2. Optimizing Thickness Readings: To balance cost and safety, the inspector specifies different NDE methods:
   • Grid-Scan UT: Selected for the 10 elbows and 2 tees (where localized thinning is likely) to scan a 2-inch by 2-inch grid.
   • Single-Point UT: Selected for the 4 straight runs (where uniform corrosion is expected).
3. Data-Driven CML Optimization: After the first inspection interval:
   • 8 of the elbows show zero wall loss (0 mpy corrosion rate).
   • 2 elbows downstream of the control valve show localized turbulence thinning at 6 mpy.
   • The straight runs show a predictable 0.5 mpy uniform rate. The inspector optimizes the circuit plan by reducing CMLs on the 8 non-corroding elbows to a single representative CML while keeping the 2 active elbows under tight grid scans and adding 1 CML downstream of the mixing tee. This reduces the total CML count from 17 to 11 optimized CMLs, concentrating inspection budget where active thinning is verified.