Piping buried, insulated

In addition to line burial and the addition of heat at the wellhead, insulation of exposed areas near the wellhead maintained higher pipeline temperatures, thereby reducing the amount of methanol needed for hydrate inhibition. Figure 8.6 displays the temperature increase in the buried and heated pipeline when exposed pipes were insulated. A combination of the methods causes the pipeline fluid to be outside the hydrate formation region (to the right of the curve marked 0 wt% MeOH), and methanol addition is no longer needed. 

Natural convective flows in porous media occur in a number of important practical situations, e.g., in air-saturated fibrous insulation material surrounding a heated body and about pipes buried in water-saturated soils. To illustrate how such flows can be analyzed, e.g., see [20] to [22], attention will be given in this section to flow over the outer surface of a body in a porous medium, the flow being caused purely by the buoyancy forces resulting from the temperature differences in the flow. The simplest such situation is two-dimensional flow over an isothermal vertical flat surface imbedded in a porous medium, this situation being shown schematically in Fig. 10.25. 

In the early years of industrial development in the United States, many plants buried their outside pipelines. The initial cost for this type of installation is low because no supports are required and the earth provides insulation. However, location and repair of leaks are difficult, and other pipes buried in the same trench may make repairs impossible. Above-ground piping systems in industrial plants have proven to be more economical than buried systems, and, except for major water and gas lines, most in-plant piping systems in new plants are now located above ground or in crawl-space tunnels. 

The potential of a cathodically protected stmcture is determined ideally by placing the reference electrode as close as possible to the structure to avoid an error caused by IR drop through the electrolyte. Any IR drop through corrosion-product films or insulating coatings will persist, of course, even where adequate precautions are exercised otherwise, tending to make the measured potential more active than the actual potential at the metal surface. In practice, for buried pipelines, a compromise position is chosen at the soil surface located directly over the buried pipe. This position is chosen because cathodic protection currents flow mostly to the lower surface and are minimal to the upper surface of a pipe buried a few feet below the soil surface. 

Cathodic protection cannot work with prestressed concrete structures that have electrically insulated, coated pipes. There is positive experience in the case of a direct connection without coated pipes this is protection of buried prestressed concrete pipelines by zinc anodes [38], Stability against H-induced stress corrosion in high-strength steels with impressed current has to be tested (see Section 2.3.4). 

Underground transmission lines are preferred in places where rights-of-way are severely limited because they can be placed much closer together than overhead lines. They are also favored for aesthetic reasons. They may be directly buried in the soil, buried in protective steel or plastic pipes, or placed in subterranean tunnels. The conductors are usually contained within plastic insulation encased in a thin metallic sheath. The conductors enclosed in steel pipes may be immersed in oil, which may be circulated for cooling purposes. For all types of underground lines, the capacitance is higher than for overhead lines, and the power transfer capability is usually limited by the resistive losses instead of the inductance. Wliile not exposed to environmental... 

The amount of corrosion damage resulting from the operation of d.c. railways is generally less than the corrosion caused by street tramway systems since the railway track insulation is better, the frequency of service is less, and pipes and cables are usually buried further from the running rails. 

A substantial reduction in the amount of stray current picked up by nearby buried pipes or cables may be achieved by interrupting the longitudinal conductivity of the structure by means of insulating gaps or joints. Care must be taken in siting the gaps, and they should preferably be placed in localities where the current tends to enter the structure and at points on each side of the track where the pipe or cable crosses under the rails. 

The concepts are similar for both onshore and subsea pipelines. In the above conceptual picture, it is assumed that the pipeline wall temperature is constant at 39°F. If a line is insulated, hydrate dissociation becomes much more difficult because the insulation that prevented heat loss from the pipe in normal operation will prevent heat influx to the pipe for hydrate dissociation. Alternatively, if the pipe is buried, the pipe wall temperature will be greater than 39°F and the system may be insulated by the ground. 

Two pipes are buried in an insulating material having k = 0.8 W/m - °C. One pipe is 10 cm in diameter and carries a hot fluid at 300°C while the other pipe is 2.8 cm in diameter and carries a cool fluid at 15°C. The pipes are parallel and separated by a distance of 12 cm on centers. Calculate the heat-transfer rate between the pipes per meter of length. 

People are sometimes careless at universities and bury steam pipes in the earth without insulation. Consider a 4-in pipe carrying steam at 300°F buried at a depth of 9 in to centerline. The buried length is 100 yards. Assuming that the earth thermal conductivity is l. 2 W/m2 °C and the surface temperature is 60°F, estimate the heat lost from the pipe. 

For the case of a buried pipeline, there are four resistances that contribute to the overall heat transfer coefficient (1) the convective heat transfer from the fluid to the pipe wall, (2) the conduction through the pipe wall, (3) the resistance due to the insulation, and (4) the conduction from the pipe to the soil. The overall heat transfer coefficient, U, is obtained from the following equation ... 

Outdoor steam pipes may be designed for low pressure drop and low heat loss. They should never be buried in the ground. For large installations, tunnels give good access and minimum loss, but are very expensive ( 20 upward per running foot). Overhead hues carried on poles or towers should have waterproofing outside the insulation. 

Paint/coating requirements for insulated piping and equipment or buried piping. 

Steel reinforcements in concrete, being passive, are noble in potential with respect to steel outside the concrete that is galvanically coupled to the reinforcements. The measured potential difference is in the order of 0.5 V [62]. The effect of large cathode area and small anode area has caused premature failures of buried steel pipe entering a concrete building [63]. In this situation, use of epoxy-coated reinforcements (to coat the cathode) or insulated couplings should prove beneficial. 

If insulating joints are installed in the above-mentioned pipe in order to reduce stray-current pickup, corrosion is now focused on the water side of the joint where any current that persists leaves the pipe to enter the water. Or, if a high-resistance joint exists between two sections of a buried pipe, corrosion may be more pronounced on the side where current enters the soil (Fig. 12.3). 

Older installations usually have an armoured paper insulated and lead sheathed service cable providing an SNE supply. The consumer s earthing terminal should be connected to the lead sheath of this service cable. Look out for earthing to the incoming water service pipe, which is no longer permitted. Check that the size of the earthing conductor complies with BS 7671, section 543, or, if the consumer has his or her own buried earth electrode, is in accordance with both sections 542 and 543. 

The resistance between the protected structure and the environment includes the resistance of any electrically insulating paint or coating on the structure. This is illustrated in Fig. 13.31. detailing the various resistive components when a buried pipe is polarized by an ICCP system. In this example, the desirable place to measure potential would be across the interface between the pipe and the environment, as is represented by the terminals marked "polarization potential" on the equivalent circuit of Fig. 13.31. 

Pipes of different materials, such as copper, steel mild steel and galvanized iron are often buried very close to each other in the same trench without any concern for galvanic corrosion. Figure 8.28 shows a method to prevent galvanic corrosion of mild steel pipe which is put in the same trench as close to the copper pipe. The copper pipe is coated and insulated to minimize galvanic corrosion. The mild steel pipe may be protected by a galvanic anode but this is not cost effective. 

In the discussion of cathodic protection monitoring, two important distinct areas can be identified. The first domain lies in monitoring the condition and performance of the CP system hardware. Monitoring of rectifier output, pipe-to-soil potential and current measurements at buried sacrificial anodes, inspection of bonds, fuses, insulators, test posts, and permanent reference electrodes are relevant to this area. The second domain concerns the condition of the pipeline (or buried structure) itself and largely deals with surveys along the length of the pipeline to assess its condition and to identify high corrosion-risk areas. 

Avoid, where possible, burying steel pipes in strongly acidic soils (lack of polarization) lead or aluminum should not be used for buried structures, equipment, and pipes in highly alkaline soils. Provide, if necessary, for a change of surrounding media (backfill, sand pads) use insulating coatings, cathodic protection and either separately or in combination. 

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