Using EMAT technique with panametric probes, TCR Arabia can take the high temperature thickness measurement only upto a surface temperature of around 400 Degree C. Above this temperature the thickness readings are not stable, reliable and repeatable. Hence thickness measurement shall not be done for lines and equipment with temperature above 500 C.
The surface for thickness shall be clean from rust, scale or any other kind of deposits and shall be fairly smooth. A metallic file, wire brush, small chisel and emery paper can be used for cleaning and getting a clean surface for thickness survey. No hammering is permitted for removal of scale/deposits. In case the above method does not yield the desired cleaning, then mechanical cleaning by power brush should be used. Grinding shall be used as a method for cleaning only as an exceptional case with prior permission from inspection engineer.
Thickness can be taken on painted surface provided the paint is visually seen to be adhering to the surface without any blisters. For critical measurement where the corrosion rate calculations are important from remaining life point of view, paint removal should be done before doing thickness survey.
In case of piping, corrosion loops shall be the basis for carrying out thickness survey for all on-site piping. Offsite piping and tank farm piping may have special loops made for thickness monitoring. Each corrosion loop in case of on-site piping shall have a combined isometric shown. The TML’s are serially marked on the combined isometric. Base readings if any being taken (before commissioning) shall be random values measured on the components. Routine on stream or shutdown thickness measurement at these locations shall be done in the form of a scanning. The scanning format shall be in a grid of size 1.5” x 1.5”. The component on which the scanning is to be done shall have the grid marked with a chalk before thickness scanning. All the TML’s need not be scanned. Out of all the locations some TML’s shall be identified for regular scanning. The select TML’s shall be identified by inspection engineer based on probability of corrosion at these locations (as compared to other locations in the loop) and accessibility considerations. Access for thickness scanning by way of ladders, scaffolding or portable trolleys shall be provided by respective maintenance department. In case corrosion is observed in these TML’s then other TML’s in the loop shall also be included for thickness scanning.
Hot Tap Locations
In case of thickness survey of equipment piping for hot tap locations the following steps shall be follows.
Maintenance shall mark the location of the new nozzle. The locations shall be as per exact type and dimensions of the component to be welded on the parent pipe. Inspection engineer shall also verify the type of component to be welded viz, weldolet, pipe of pipe connection, nozzle with reinforcement pad, split sleeve nozzle etc.
Inspection engineer shall mark the center line of the proposed weld joint. A width of 1.5” to 2” shall be marked on either side of the proposed weld center line. A close thickness survey shall be taken along the center line and on the either sides. The minimum thickness measured shall be reported in the hot tap file.
If the thickness measured is comparable to nominal or the previous measured values (if available at the same locations or at different locations in the same pipe) then it could be assumed that there is no corrosion at the location.
If the thickness measurement indicates severe corrosion, and thickness measured is very close to the minimum allowable for hot tapping then hot tapping should be avoided at the location as it will be difficult to pick up a thickness point with minimum thickness.
Minimum thickness required for hot tapping is 4.8mm. If the pipe is corroded and actual thickness is in the range 6 – 8mm, then alternate methods should be used to check the pipe thickness and certifying the same fit for hot tap.
Thickness Locations In Tanks
In case of storage tanks, thickness shall be measured from outside as follows on the first and second shell course from bottom. In all other shell courses thickness shall be measured along the staircases. Few thickness points shall be taken near the weld and few at the center of the shell course plate. In case of roof plate, thickness shall be measured on each plate. Two thickness points at the center of each plate and one thickness point at the corner of each plate. In case of bottom plate, thickness measurement is possible only during an internal inspection. Thickness shall be measured on each plate. . Two thickness points at the center of each plate and one thickness point at the corner of each plate.
Recording Of Thickness Measured
We will measure the thickness and record it on a format that has the following details:
In case of piping, where spot readings have been measured at TML’s the TML number shall be written and the readings measured shall be entered against the TML number.
- Tag Number of equipment/pipeline or appropriate description in case tag number does not exist for the component/job.
- Date of measurement
- Sl. No Meter used for thickness measurement
- Details of the meter used for thickness measurement like frequency etc.
- Identification number of the standard block used for calibration of the meter before starting the job
- Nominal thickness of the component being checked for thickness
- Name of the technician measuring the thickness
In case of equipment a development drawing of the equipment shall be submitted approx. showing the location of thickness measurements. The thickness may be entered on the sketch itself. Alternately the TML’s can be marked on the sketch and corresponding thickness valves for each TML may be submitted separately.
In case of close scanning of a location in an equipment or a piping location, the readings shall be submitted as a grid. The grid will have the orientation of the grid (N/S/E/W) with possible reference from a nearby nozzle, weld etc. The grid identification at site is required so as to check the thickness at the same location and compare the same for corrosion, if any. The grid size shall be clearly mentioned on the sketch.
Measured thickness data shall be evaluated by the respective inspection engineer.
Random thickness measurement shall be compared with the nominal thickness of the vessel and also with the previous measured thickness. In case the thickness values (min/max) compares well with the previous measured thickness or nominal thickness, the values measured shall be considered to be satisfactory.
In case random thickness survey indicates a sharp difference from the previous readings (apparently indicating a corrosion rate of more than 0.5mm/year), then a detailed scan has to be done at that location (grid scanning). The inspection engineer will also find out whether such steep corrosion rates are possible by discussions with operations/CTS. This will help in identifying other corrosion prone areas in the equipment/piping. Based on the grid scanning results, if corrosion is confirmed then thickness measurement is repeated should be repeated after a short period (period to be decided based on the corrosion rate, remaining corrosion allowance and the ability to do repeated UT survey and the criticality.
During any scan, for each TML, the minimum thickness can be compared between successive thickness measurements surveys and corrosion rate can be calculated.