El modelo de la activación en cascada

lined pipe has been used in the United States for nearly 100 years. Cement-lined steel pipe combines the physical qualities of steel with the protective qualities of cement mortar. The lining creates a smooth, dense finish that protects the pipe from tuberculation (the formation of scale or other nodules on the inner surface of the pipe) and provides a relatively high flow coefficient. In addition to acting as a physical barrier between the steel pipe and any potentially corrosive fluid, the cement lining also creates an alkaline environment near the steel wall that helps inhibit corrosion [1].

Chevron has successfully used cement-lined pipe for many years in both producing and refining applications. Cement-lined pipe is most often used to protect carbon steel pipe from corrosion in water/brine environments. Typical applications include water injection systems in oil fields and fire water systems in large plants.

Applicable Specifications

General. There are several specifications you may use for cement lining steel pipe:

• API RP 10E, Recommended Practice for Application of Cement Lining to Tubular Goods, Handling, Installation and Joining.

• PPL-MS-1632, Materials and Fabrication of Cement-Lined Piping and Tubular Goods. PPL-MS-1632 modifies API RP 10E, which is intended for oil produc-tion related uses.

• AWWA STD C602-83, Cement-Mortar Lining of Water Pipelines in Place.

• AWWA STD C205-85, Cement-Mortar Protective Lining and Coating for Steel Water Pipe—4 In. and Larger Shop—Applied.

• ANSI Standard A21.4, Cement-Mortar Lining for Cast Iron and Ductile Iron Pipe and Fittings for Water.

API RP 10E and Company Specification PPL-MS-1632 (used in conjunction) are the recommended specifications for cement-lining of pipe for produced water, rein-jection water, brine, and salt water service in the oil field.

For some piping intended for fresh or brackish service the AWWA Standards are often good enough. Many small applicators use the AWWA standards and are not familiar with API RP 10E. Refineries and chemical plants have successfully lined pipe to the AWWA Standards for fresh and seawater service.

How to Specify. API RP 10E is the recommended specification for cement lining both new and used pipe. Specification PPL-MS-1632 should be used in conjunction with API RP 10E for cement-lining new pipe. In-place (in-situ) cement lining is sometimes done on existing pipe to extend the life of internally corroded pipe, as well as for lining new pipe on site. PPL-MS-1632 should still be used along with the API specification even though it does not directly address in-place lining. PPL-MS-1632 still gives guidance on cement selection, gasket selection, etc.

API RP 10E is preferred primarily because it covers sulfate- resistant cements with low tricalcium aluminate (C₃A) and is a more stringent specification and more appropriate for oil producing or refining services. Company specification PPL-MS-1632 is based directly on API RP 10E and modifies it by adding or deleting require-ments from several paragraphs.

Many cement lining vendors are not familiar with the API practice and commonly use the AWWA standards. In several instances the AWWA standards have been used in lieu of the API standard. The Richmond Deepwater Outfall Project is one example. Generally, if the water being piped is fresh or mildly brackish and the piping system is not deemed critical, specifications other than the API RP 10E are adequate. Examples of noncritical systems are potable water, domestic drainage, and sewage systems.

The AWWA Standards have been used in the past with requirements added for steel pipe, curing, joining, etc. Contact the CRTC Materials and Equipment Engineering Unit for assistance with the specific requirements for your project.

Steel Pipe Requirements

Company Specification PPL-MS-1050 supplements API SPEC 5L. See Section 310.

Thickness and straightness are two very important requirements for steel pipe to be cement-lined. Thickness is more important than the grade. If the pipe is thin-wall, it

is more apt to be dented, formed out-of-round, or to become bent. Section 2 in API RP 10E and PPL-MS-1632 cover pipe thickness requirements.

Pipe should be straight to within 1/8 inch per 10 feet of length. This is not covered in API RP 10E but is used by several cement lining applicators.

Spiral-welded pipe is not used for cement-lined pipe that conforms to API RP 10E because the cement cannot be applied by the rapid spin method. The raised weld profile causes the pipe to vibrate severely, and bounce or wobble during applica-tion, when spinning speeds can reach 700 rpm. This prevents the application of a good, dense lining and may damage equipment.

However, spiral welded pipe has been successfully lined using other lining

methods. These methods utilize a heavy slurry sand cement and involve slower spin-ning speeds and hand trowelling. This type of cement-lined pipe is covered in the AWWA specifications and is appropriate for low corrosive and noncritical systems, as mentioned earlier. Richmond Refinery made successful use of spiral-welded cement-lined pipe for the deep water effluent outfall line.

Types of Cement Linings

Cements. The Company specifies Portland cement conforming to ASTM C-150 Type I, Type II, Type III, or Type V depending on the sulfate levels of the water that the pipe will transport. Sulfate ions attack cement linings by reacting with the cement and forming gypsum, which occupies about 18% greater volume than the original cement. The gypsum in turn reacts with C₃A to form a complex hydrate crystal which expands to over 200% of the volume of the original constituents[2].

This causes the lining to spall and crack, and eventually to fail.

Types III and V cement are specified for high concentrations of sulfate (above 5000 ppm and 1500 ppm, respectively). Limits are placed on the content of C₃A in the cement. Cements with low amounts of C₃A are resistant to sulfate attack. Type II cement may be used for moderately sour water with sulfate levels below 1500 ppm.

Type I cement may be used for fresh water with sulfate levels below 200 ppm.

Fresh and potable water generally have less than 40 ppm of sulfates and seawater has around 2650 ppm. These levels vary; the engineer should obtain an analysis of the water the pipe will transport.

Experience. Two types of lining mixtures have dominated cement-lined pipe tech-nology over the last 25 years: pozzolanic cements, containing 60% cement and 40% pozzolans; and sand cements containing 60% sand, 35% cement, and 5%

pozzolans[2]. Pozzolans are fine particles of silica and alumina that react with lime to form calcium silicate and aluminates.

Experience has shown that pozzolan cements are more sulfate resistant, and sand cements are more acid-resistant [2].

CUSA Producing, Northern Region has used 60% cement/40% pozzolan linings successfully for many years in injection systems high in sulfates conditions. This type of cement has a lower permeability than sand cements and therefore provides more resistance to water diffusing through the pipe and corroding the steel [3].

Cement Lining Application

Chemical Attack. As with almost any type of coating or lining, proper application is one of the most important variables in the overall success and longevity of the coating or lining. For cement linings, proper mix proportion is equally important.

Shop-Applied. Straight sections of pipe are lined with a machine that spins the pipe joint and centrifugally applies cement linings to the interior of steel pipe. The entire pipe section is lined to a uniform thickness without interruption. Once the desired thickness is obtained, the rotation speed is increased to produce a dense cement with a smooth surface and a minimum of shrinkage.

Elbows, bends, and other shapes must be lined using mechanical placement, pneu-matic placement, or hand application techniques. The cement is often reinforced in these cases with a wire fabric reinforcement. The thickness may be varied to make a smooth transition with adjoining sections of pipe but is otherwise the same as centrifugally spun straight sections [4].

Several things can go wrong during the lining process and must be watched for:

• Excessive acceleration up to spinning speed leads to poor spreading of cement and results in lining eccentricity.

• Too high a spinning speed and too long a spin duration result in particle size segregation in the lining.

• Applicators must vary rotating speed for different pipe diameters to ensure proper centrifugal forces, which determine liner density.

• Holddowns, or rollers, should be spaced about one per every 7 feet of pipe.

This helps reduce vibration and eccentricity.

Field-Applied or In-Place. Field application is done in three basic steps. First, a mechanical scraper with wire brushes is run through the line enough times to remove heavy scale and deposits. Then, a rubber pig is run through the line to remove sand, debris and water. Finally, the cement coating is applied. Application is by a moving head that centrifugally shoots the mortar onto the steel pipe. A conical trowel almost immediately smoothes out the cement to a uniform thickness.

The pipe ends are then sealed to prevent moisture loss [5].

Curing. Specification PPL-MS-1632 requires all shop-applied cement linings be steam-cured. Steam curing accelerates the chemical (cement hydrolysis) curing process and brings the cement to full strength much quicker than an atmospheric cure will. Steam curing does not alter the lining’s chemical resistance properties.

Most, if not all, field applicators of cement linings are unable to steam cure. In these cases, the atmospheric curing requirements in API RP 10E, Section 3.4b, should be strictly enforced.

Quality Control Procedures. The Company should inspect the contractor’s plant during application to ensure proper lining procedures are being used. The Company should also inspect the finished product and review the applicator’s certification documents to ensure that the cement used for lining meets the required

specifica-tions. Certification and testing are covered in Sections 4 and 6, respectively, in API RP 10E.

Cement Lining Applicators. Listed below are several cement lining applicators.

The list is not complete, nor is it an “approved bidders” list.

Joining Cement-Lined Pipe

Gaskets. Gaskets are needed to protect the inner surface of steel pipe joints once the pipe is put into service. Gaskets for butt-welded joints must be heat-resistant to withstand the heat of welding. Asbestos has been used for many years and has been largely successful [3]. We believe that asbestos gaskets may still be used in compli-ance with environmental and health regulations because the gaskets are installed in the outdoors and never exposed once the joint is welded. However, the engineer must check the current regulations concerning the use of asbestos gaskets. The governing regulations are listed on page three of API RP 10E.

If the operating company or the governing regulations prohibit the use of asbestos gaskets, API RP 10E lists an alternative to asbestos. This is a new product with limited field experience. We believe that the flexible graphite sheet will work well, but it costs considerably more than asbestos.

Chevron Canada Resources has successfully used an Inconel wire-mesh-impreg-nated gasket for welded joints. These gaskets are available through Alberta Gaskets in Alberta. The CRTC Materials and Equipment Engineering Unit can provide assis-tance with selecting nonasbestos gasket materials.

Joints. The Company has used butt-weld, sleeve, and slip-on flange joints. Butt-weld joints are preferred because they are stronger and stiffer than sleeve joints.

Slip-on flange joints are hardly ever used because welding heat damages the cement lining. Screwed-on flanges are possible but not recommended. See Figure 300-12 for illustration of a butt-weld cement-lined pipe joint.

Armor Cote, TX 915/332-0558

Permian Enterprises, TX 915/683-1084

Ameron, CA 213/268-4111

Spiniello Construction Co., CA 213/835-2111

Heitkamp, CT 203/274-5468

Burke Industries, CA 408/297-3500

Progressive Fabricators, MO 314/385-5477 Thompson Pipe and Steel, CO 303/289-4080 U.S. Pipe and Foundry, AL 205/254-7000 American Cast Iron Pipe Co., AL 205/325-7701

Bitco, CA 415/233-7373

Shaw Pipe Protection, Alberta

Welding procedures for cement-lined steel pipe have traditionally been standard pipeline procedures, with incomplete penetration on the root pass to avoid damage to the cement and gasket. Electrodes have generally been of the EXX10 type. Some recent experience in Canada has suggested that these electrodes, which leave more hydrogen in the weld, may combine with the stress riser of the incomplete penetra-tion weld to produce root cracks.

A welding procedure using EXX18 low hydrogen electrodes and an inconel wire-reinforced composition gasket has been developed by Chevron Canada Resources.

The procedure allows more weld penetration (90+%) and thus a stronger weld. The procedure is downhill for NPS 2 pipe and uphill for NPS 3 and larger. The uphill procedure appears slower but will save on repair time. Contact the Design and Construction Group in Calgary for further information.

Branch Connections. Branch connections are preferably made with cement-lined tees. Branches may be made with bosses or weld-o-lets that have been fabricated into a pipe spool and cement-lined in the shop. Good advance planning and design will allow ordering shop lined branch components with connections and fittings attached. If field cutting must be done use a hole saw. A hole saw is a cylindrical saw attached to a drill. A cement-lined weld-o-let should be welded on and the internal lining repaired with a repair compound such as X-Pando.

Field torch-cutting for branches should be avoided as this damages the cement lining. Repatching these damaged areas is difficult, especially for small connections and fittings. Ordering extra tees, fittings, and flanges will prevent delays in field work and result in better lining integrity.

Typical Problems with Cement Linings

Chemical Attack. Cement linings can be corroded by many different chemicals [2]. Examples are:

• Strong acids with pH below 5.0

• Carbonic acids

Fig. 300-12 Butt-weld Cement-Lined Pipe Joint

• Sulfates (as described earlier)

• Magnesium chlorides

The pipe’s spinning during cement application and subsequent hardfinishing results in the lining being slightly thicker at the ends. If the gasket seal fails corrosion may start at the joint. However, the lining may not begin to spall or break until steel corrosion has progressed several inches away from the joint because of the slightly thicker lining near the joint.

Erosion. High fluid velocities, especially at tees and elbows, can also cause cement lining deterioration. Water and solids impingement can cause erosion and also continually replenishes corrosives to the lining. Unfortunately, the linings at elbows and tees are not centrifugally spun and therefore are not as dense and strong as linings in straight sections. Fluid velocities should be limited to roughly 10 fps (3 m/s) in order to avoid erosion damage (from the equation V = 100/ρ, where ρ = density of the fluid in lb/ft³).

Transportation and Handling of Cement-lined Pipe

Cement-lined pipe should be transported and handled with care so as not to crack or damage the lining. API RP 10E covers the proper procedures for loading/unloading, transportation, and installation handling for cement-lined pipe. Refer to API Recom-mended Practices RP 5L1, 5L5 and 5L6 for information on general handling of pipe.