Asset management | Pipe cleaning
Pipe crawler applies anticorrosive coating8 December 2011
A robotic pipe crawler has been used to spray molten metal on pipe interiors at Mexico’s Laguna Verde nuclear power plant. The robot improves uniformity of application and protects workers from radiation.
Metal spraying protects and greatly extends the life of a wide variety of products in the most hostile environments and in situations where coatings are vital for longevity. Metal spraying is carried out in a wide range of markets where corrosion is a problem, including oil and gas, construction, petrochemical and marine.
All methods of thermal spraying involve the projection of small molten particles onto a prepared surface where they adhere and form a continuous coating. To create the molten particles, a heat source, a spray material and an atomisation/projection method are required. Upon contact, the particles flatten onto the surface, freeze and mechanically bond, firstly onto the roughened substrate and then onto each other as the coating thickness is increased.
As the heat energy in the molten particles is small relative to the size of the sprayed component, the process imparts very little heat to the substrate. As the temperature increase of the coated parts is minimal, heat distortion is not normally experienced. This is a major advantage over hot-dip galvanising.
In the wire spray process, used for most anti-corrosion coatings, a wire is fed by a driven roller system through the centre of an oxygen-propane flame where it is melted. An annular air nozzle then applies a jet of high-pressure air, which atomises and projects the molten material onto the work piece. The driving of the wire is typically via an air motor and gearbox that forms part of the pistol. Wire diameters that can be flame sprayed as standard range from 1.6 mm to 4.76 mm (1/16- 3/16 inches). Wire is typically dispensed from coils or production packs (drums).
In the arc spray process, two electrically charged wires are driven and guided so that they converge at a point and form an arc. An air nozzle atomises the molten metal produced and projects it towards the work piece. The driving of the wires is typically either by air motor or electric motor and gearbox arrangement. The wires can be driven in three different ways, all which offer individual benefits: push only, pull only and push/pull.
Thermal spraying is not a new process. It has proved itself to be effective over its 90 years of existence in applications ranging from coatings in gas turbines to corrosion protection on park benches, according to UK thermal spray supplier Metallisation.
The general standard governing preventative measures for corrosion and oxidation at elevated temperatures is BS EN ISO 17834. This is the basis on which the coatings for the nuclear industry have been based, but there has also been a lot of development by companies in Japan and Spain for the specific applications in the nuclear industry (see also below).
Generally speaking, for corrosion and oxidation environments at elevated temperatures, coatings of aluminium- or nickel-based alloys are commonly used. With increasing temperatures, and increasing concentrations of sulphur and chlorine, nickel-based alloys with increasing chromium content would usually be selected. The coating thickness would typically be minimum 200 microns (0.2 mm) for aluminium coatings and a minimum of 400 microns for nickel-based alloys.
In 2009, Lainsa, a subsidiary of Spanish power plant maintenance company Grupo Dominguis, were asked to perform a metallisation coating in the straight stretches of the cross-under piping during the refuelling outages at Mexico’s Laguna Verde. These outages took place in February-April 2010 at unit 1 and during September-November 2010 at unit 2.
In Spain, Lainsa has provided metallisation mainly in the turbine building, on moisture separator reheaters (MSRs), on other heat exchangers and especially on vapour transportation (cross under) piping between the MSR and HP turbine.
Since Laguna Verde is a BWR plant, the interior of the piping system in the cross under can be potentially contaminated; the system is also located in a radiologically-controlled area. Therefore, plant owner the Federal Electricity Commission of Mexico (CFE) requested the use of a robotic system to prevent, as much as possible, people working inside the vapour pipes.
In the April-November 2009 period, Lainsa designed and built a prototype robot and a 1:1 scale model of the Laguna Verde cross under piping at its facilities in Spain. There it trained and qualified all the workers that were to be involved in the project, both Spanish and Mexican (through Lainsa’s partly-owned subsidiary company in Mexico, Hervi). After some final modifications to the robot and the software that controls it, Lainsa built the first Tirant 3 and used it for testing and qualification of the metallisation system. It designed the robot to standard SSPC CS-23/AWS C2.23M/NACE No.12, Specification for Application of Thermal Spray Coatings (Metallizing) of Aluminum, Zinc and their Alloys and Composites for the Corrosion Protection of Steel.
Once the system had received the necessary qualifications from Laguna Verde NPP, upgrade contractor Iberdrola Engineering & Construction and independent test laboratory AIMME (Metal Processing Technology Institute of Valencia), four final units of the Tirant 3 with a Metallisation 140 head were built and sent to Mexico for the execution of the project.
The remote-controlled robot, 732 mm long by 460 mm wide (without wheels) can operate in pipes with diameters 30-34 inches, travel up to 4 m/min with a range up to 18 m (limited by cabling). It consumes 600 W of power and operates with a compressed air supply of Q=1.5L/min at 6 bar.
A fundamental task in the metallisation process is the preparation of the surface. Such preparation is carried out in four phases: blasting of the surface to cleanliness SA 3 (the abrasives used for which must be of the correct material and must have the correct granulometry); extraction of dust and abrasive material used; visual inspection; final surface corrections.
The project team was made up of 19 people, seven from Spain and the rest Mexican personnel from Hervi. Hervi was responsible for services such as surface coating, decontamination, and mechanical maintenance at Laguna Verde NPP.
The system demonstrated its reliability through a study of test probes that were placed inside the pipe during the application. They were analyzed by Mexico’s National Institute for Nuclear Research (ININ) after the project ended. The 16 test probes showed that the first layer of the thermal-sprayed coating, an Ni-Al anchorage layer, was approximately 200 µm in thickness, and that an external layer of Ni-Cr-Mo-Nb was about 400 - 600 µm thickness. The total thickness of the test probe coating ranged from 514.0 µm to 832.4 µm, exceeding the minimum thickness of 500 µm recommended for thermal spray. The average test probe porosity ranged from 2% to 4%. Their superficial hardness varied from 79 HRB to 91 HRB. A probe adherence test gave a minimum resistance to tension of 9.11 MPa and a maximum of 29.05 MPa. In all cases this exceeded the requirement of 6.89 MPa.
On completion of the project, the four Tirant 3 units and related auxiliary equipment underwent the necessary radiological controls by the radiation protection department of CFE in Laguna Verde NPP. The four Tirant 3 units and all auxiliary systems (with the exception of some minor components) were returned to Spain in April 2011.
In a project like this, it was learned that planning of the different tasks, personnel involved, timing, materials used, humidity levels, temperature, adherence and hardness of the material applied, quality of the air, ventilation of the piping system should all be carefully coordinated.
At present, metallisation in elbows and is carried out manually. Modification of the Tirant 3 system to be able to perform automated application in elbows is under development, although modification will have to be tested and qualified before use.
This article was originally published in the November 2011 issue of Nuclear Engineering International magazine