TECHNICAL ARTICLE:
| Advances in Pipe Line Coating Using Induction Heating | ||||||||||
| By: John R Powell, Powell Engineering Services, Australia | ||||||||||
Introduction This article deals with the value adding to pre-manufactured pipe for use in the oil and gas industry achieved by adding a protective three layer epoxy polyethylene coating to provide a single service point of supply to the end user. The type of pipe used for oil and gas lines, where high pressure is required today, is to API standard and pipe may vary in diameters typically from 160mm to in excess of 1,000mm with varying wall thicknesses depending on end application and other factors. In a rapidly developing country like China, the requirement for oil and gas pipelines has been most positive with regular announcements of new lines running from just a short distance of <100km to >1,000km. Pipeline industries have developed in many countries what in some cases have been a secondary downstreamer opportunity for the tube and pipe manufacturer. This article will explore issues relating to the application of induction heating for pipe coating. The first such system utilising this technology was installed in Central China in 2003 and is meeting all expectations.
Essentially this is because of the high cost of pipeline repair in the field due to corrosion if adequate protection is not initially taken prior to the construction of the pipeline. Even more sensitive these days is the environmental impact of having leaking pipes and the safety regulations demand sound coating process to ensure long life of the pipeline. Field repairs are undesirable and costly and can be eliminated with a high quality coating practice after pipe manufacture.
Three-layer-coating systems provide specified performance properties for the external protection of steel pipes designed for safety and life long transmission service. The multi-layer pipe coating combines the benefits of two major advantages:
The most important requirements in the coating system for steel pipes are the following:
The pipe coating system is equipped with a wide range of performance properties:
The three-layer-coating meets many girth weld methods and offers no interference with the material properties of the basic pipe coating system. There are no limitations in the pipe field bendability using the conventional cold bending methods up to 2 per cent of OD. This pipe coating method reflects minimal pollution problems during the application and has no material environment limitations. The OD pipe coating process is performed as a continuous operation with applied process controls to secure consistent quality assurance information feedback.
Following is a description of the sequence of the operation and quality control methods. Figure 1 shows the pie chart of layering.
Pipe Surface Descaling Figure No.2 illustrates the basic coating system. Two shot blast machines are used to mechanically descale the dry and oil-free pipes. Roto-jet double disc turbines centrifugally apply the abrasive steel grit at a rate 500kg/min at a velocity of 80m a second toward the pipe surface. The pipes move helically through the shot blaster chamber that is protected with a coating of high wear-resistant material. Air shock cartridge filters, a separator unit and a dust collector work with a recovery system that establishes constant operation condition.
The pipes proceed continuously over the conveyor, with each roll driven by a thyristorised gear motor. Each conveyor roll meets the helical pipe angle securing pipe speed parameters in the circumference and the axial speed vector. The drive system within the process conveyor is operated in several independent sections. The new incoming pipe goes through a catch up cycle in order to synchronise closely with the preceding pipe. The cut-off pipe receives a speed-up signal when inside the water-quench section and stops at the conveyors end in order to be kicked-off toward the pipe storage area at the exit of the line. The operational sequences are programmed and controlled by a PLC controller, in which the software contains all diameter-related parameters. The main operating panel controls and monitors all production facilities over the whole process line.
The pipes proceed at a scheduled speed. Uniform heating is of paramount importance for good coating properties. Induction heating is the only method for this coating process and it is essential to serve a clean oxide free surface to the heater in order to obtain a constant application temperature.
Let us first review the basics of induction heating. Induction heating is the process where an electrically conducting workpiece is placed in a copper coil through which an alternating current flows. The magnetic field created by this alternating current causes current to flow in the workpiece. Figure 3 illustrates this principle.
It can be said then, that induction heating follows the well understood transformer effect where an AC voltage impressed on one winding will produce an AC voltage on the other winding. The work coil in an induction heater is the primary-winding of the transformer while the pipe acts as a short-circuited secondary winding. Since the pipe has an inherent electrical resistance to the current flow, heat is generated. Induction heating is unique in that the heat is developed in the pipe without any physical contact, without a radiated heat source and without producing any products of combustion. Since induction heating has no source related temperature limitations, its usage is commonly at energy levels or power densities much higher than those associated with indirect electric or fuel-fired heating methods and it is accurately controllable. The transmission of heat energy throughout a body is due to temperature differential and to the second law of thermodynamics that states that heat energy migrates in a direction from a body at a higher temperature to a body at a lower temperature. During the induction heating process, the majority of heat energy is generated within the pipe at a specific depth. Heat flows from this band in 2 directions, inward toward the centre of the pipe wall and outward from the pipe surface by radiation. The speed of this heat transfer is dependent upon its thermal conductivity, specific heat and density.
In order to specify an induction heating system for a hot working application, we need to define Depth of current Penetration. This factor determines the efficiency of the induction-heating coil. The Depth of Penetration is determined by the frequency of the current, the relative magnetic permeability of the pipe and the resistivity of the pipe. The formula for the Depth of Penetration or, as it is sometimes called, reference depth, is as follows: Depth of Penetration:
Figure 4 gives a table of current penetration depths for both above and below the Curie or magnetic transformation point.
From this formula we know that the current penetration increases as the resistivity increases and decreases as either the permeability or frequency increases. We see then that the higher the frequency, the shallower the heating effect (see Figure 5).
For non-magnetic materials and carbon steel above the Curie temperature (which occurs at a temperature of 760°C) permeability is equal to 1. For magnetic materials this relative permeability varies with power density and thus complicates the determination of reference depth (although this article will not delve into the calculation of magnetic state current penetration). Steel is shown for various commercially available induction heating frequencies. Figure 6 shows the electrical resistivity of various materials versus temperature. With this information one can calculate the depth of penetration using the standard formula given above.
Most coating operations are carried out below the Curie point or magnetic transformation point for steel. This has a direct bearing on frequency selection for the induction heating system to operate efficiently and in respect of tube and pipe, a family of curves are essential for correct sizing of the best operating frequency for a given application. It will be noted that the depth of current penetration has a direct relationship to the frequency as previously advised and this in turn has a bearing on frequency selection for varying wall thicknesses and diameters.
For the application of induction heating for pipe coating the line diagram in Figure 7 shows the induction system portion of the coating plant. This comprises of a solid state medium frequency induction power supply that ensures the correct voltage is applied to the coil which in turn heats the steel pipe to the pre-set coating temperature. Interconnection is made to the heating coil by flexible water-cooled leads.
The induction power supply is provided electrically by a mains frequency input power normally achieved through a step down transformer. This is due to the fact that most coating plants are of 1000kW or above and power is most economically supplied through a high voltage step down transformer. This power is connected to the medium frequency solid state power supplies incoming circuit breaker. The other component of service required is cooling water, which is required not only for the induction heating coil but also for the cooling of electronic componentry in the solid state power unit. This water is normally taken from a cooling tower or pond and recirculated since the plant for coating will run often 24 hours a day and it would be uneconomical not to circulate the water. Since the large proportion of losses occur in the induction heating coil as opposed to the induction power supply, the cooling water from the external source is diverted directly to the coil through water cool leads direct from the cooling tower. The branch of water to the induction power supply is generally connected to an external or internal distilled water circulation system since electronic componentry is a lot more sensitive to ionised water, which could result in electronic component failure prematurely, if raw tower water is used.
At a temperature of 220°C the pipes move helically through the spray booth. Some 3-6 electrostatic powder spray guns blow fluidised epoxy on the pipe surface. A small amount of excess powder is recovered and conditioned before being returned. The powder film melts within 60 seconds and starts the curing procedure. Still in the semi-liquid condition, the thin epoxy film is prepared quickly to receive the adhesive folio wrapping.
Two single screw extruders produce thin film folios through die-slots with constant thickness. A system of finely-tuned pressure rolls are set up geometrically in accordance with the helical pipe movement to guide the folios toward the pipe surface. The system builds up multiple folio layers and prevents air pockets or wrinkles occurring and assures the formation of the final homogeneous layer.
All pipe ends are prepared for field welding. This consists of: a) Removing the coating by rotary brushes to a specified length of, for example 150mm.
Pipe Coating Plant Layout Coated pipes may be manufactured with varying lengths for the pipeline installation, but for long distance runs they are typically 12m in length. One can imagine with a large pipeline how many pipe lengths need to be manufactured, stored, coated and shipped out to the installation site. A 500km pipeline would require 41,666 pipes of 12m in length. It is for this reason pipeline coating plants normally run 24 hours a day, often 7 days a week and must be reliable. Depending on the end application, pipe may be coated internally for gas application or externally or both. The majority of installations however, are only externally coated.
Typically pipe is heated to a temperature of 220°C on the outside surface prior to entering the coating system. As previously mentioned heating frequency selection is determined considering diameter and wall thickness for different end applications, but in the case of pipe coating other factors must also be considered. There has been much discussion over many years on whether the pipe surface only needs to be heated since this is the coating area. Without much thought, one might concur with this supposition. However, the effect of surface radiation – and to a much greater extent, the cold well effect – of a heavy wall pipe are paramount considerations in determining the total mass to be heated and obtaining a strong bond adherence. As the pipe emerges from the induction heater, temperature radiation will start to occur over a large surface area. This loss is however small considering the loss to the inside wall of the tube if it has not been through heated. Figure 9 illustrates the effect of partial wall heating with different percentages of wall thickness of a pipe. It is for this reason – minimising rejects due to possible reduced coating temperatures – that the total wall thickness should be heated. This is unless the wall thickness becomes extremely heavy and it is possible to locate the coating plant very close to the heating outlet point of the induction coil.
This cold well effect is the robbing of temperature from the pipe surface by thermal conduction, to the inside diameter of the pipe. If the system is designed to heat the total wall thickness this conduction loss is eliminated and provides sound engineering design and minimal possibility of high reject after coating due to a stable coating temperature. The downside is that the Kg/hr of pipe mass to be heated is increased which increases the energy consumption, thus to total operating cost. Frequency choice related to audible sound is another consideration as certain pipes exposed to certain frequencies at high power density emit resonant sound waves and this aspect is also to be considered. The next aspect to determine, is the pipe coating speed achievable with different wall thicknesses and different diameters. Figure 10 illustrates the results of tests carried out to indicate the possible coating speeds per 1,000kW of power, operating at a frequency of 1kHz.
Normally a computer modelling of the various pipe diameters and wall thicknesses for accurate kW sizing – related to desired coating speed – is carried out for accurate power rating in kW. Another factor of kW rating is the effect of coil coupling. Since this is a below Curie temperature application, the de-coupling effects are not as pronounced as the above Curie applications but never the less must be considered. Coupling factor refers to the outside diameter of the pipe related to the induction-heating coil inside diameter. Naturally it is not feasible to have an individually designed coil for every pipe size to be heated. Modern induction power supplies of medium frequency are designed for accommodation of varying coupling factors in terms of power draw through variation of frequency and voltage, to keep the kW output constant and in turn, the temperature for coating constant. Naturally the window of kW adjustment will close after a certain range of power unit parameters have been met, depending on the inverter design. At that time power will start to fall away if an increasingly smaller diameter pipe is put inside a mismatched coil. It is normally desirable to change the induction coil so that the window is wide open again for the next range of tube or pipe sizes to be coated, thus reducing operating costs. Typical induction coils used for existing installations are shown on Figure 11. These coils are very large and are normally manoeuvred into final position with the aid of casters or wheels and jacked for correct pass line height. They are housed in a steel frame to ensure robust presentation and long life.
Some systems are built with a coil mounted on a frame, permanently located in the line and are moved in and out of position by a crane. As these coils are an expensive item, pipe diameter, wall thickness and coating speeds need to be accurately pre-determined to minimise the number of heating coils required.
As mentioned at the beginning of this article, the growth in the pipe industry for pipe coating have risen in recent years and the rebuilding of the Siberian pipeline is one installation that clearly shows the necessity for sound coating practice. Inductoheat supplied an induction heating system for the rebuild of this famous pipeline to a French company. The system had to operate in sub-zero temperatures for which Siberia is well renowned. The engineering group had to take careful consideration of equipment, design and parameters to allow satisfactory operation in a newly set up plant, built specifically for the pipeline rebuild in such climatic conditions. The areas of research and development consideration at present are: a) Under current consideration is a mobile coating plant which may be used for a big contract and upon completion of the contract, the plant can be dismantled and removed through shipping containers to a new location. The design of equipment for the induction-heating portion of plant has been made with this long-term plan in view by the pipe coating end-user. b) Another area of interest is to accommodate off-line coil changes using multiple cell coils. This is primarily to reduce turn around time with varying pipe diameters so that that production loss is minimised. The system envisaged would have the coils moving vertically or horizontally with 2 or 3 coils in a combined cell. This would only necessitate the electrical and water connections being changed from one to the next coil with movement of the coil being smooth and centre line adjustment eliminated. Pipe size change over time would then be very short. c) With the induction solid state power supply achieving efficiencies of some 95 per cent and the below Curie application showing coil efficiencies on large diameter pipe approaching 90 per cent, there is little room left for the induction manufacturer to make overall improvements in the hardware system. Interfacing electronic controls have been advancing rapidly in recent years. The total coating installation – with coating machine and induction heating system plus pipe movement into and out of the system – has been finely tuned to reflect an efficient high production cell, often seen today in modem coating plants. References The author acknowledges the following references:
The author John Powell retired from Inductoheat Pty Ltd at the end of 2003, but still remains active in the industry with consultancy work. For more information on the technology, either contact John Powell or Inductoheat directly. Inductoheat Pty Ltd
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