Special Linings for unusual service conditions. Please contact ACIPCO.

Transcription

1 Linings

2 Linings Along with technical and metallurgical advancement in piping materials, research on lining requirements for pipe and fittings has resulted in the development of linings to meet many different service requirements. ACIPCO offers several types of linings, the most common being cement lining. Pipes and fittings furnished by ACIPCO are offered unlined or with linings as follows: Cement lined per ISO 4179 and AWWA C104 for water transmission lines and sewer force mains. Protecto 401 Lined Ceramic Epoxy Lined for septic gravity sewers and sewer force mains. Special Linings for unusual service conditions. Please contact ACIPCO. CEMENT LINING Cement-mortar lining for ductile iron pipe and fittings for water service is in accordance with ISO 4179 and AWWA C104. Cement-lined pipe is also furnished for sewage service and a number of other applications. ACIPCO applies a sulphate resisting cement-mortar lining to ductile iron pipe. The sulphate resisting cementmortar lining is a Portland type cement per ASTM C150 and meets the chemical requirements of BS The lining is applied by using a high-speed centrifugal process. By using this method, excellent quality control of the cement-mortar lining is maintained. The cement linings ACIPCO produces are dense, smooth, uniform, and well bonded to the pipe wall, and offer very little frictional resistance to the flow of water. The pipe is spun at a very high rate accompanied by vibration to produce a dense lining. This high-speed lining brings water and latence to the lining surface. The latence is immediately washed out of the pipe with water. By using this unique process developed by ACIPCO, there is no need to grind linings or use additives to the cement mortar. The immediate result is a smooth, dense, and well-compacted cement-mortar lining. After the application process, the linings are then cured in a controlled environment to prevent too rapid a loss of moisture from the mortar. Standard Cement Lining Thickness Pipe Size (mm) Nominal Thickness (mm) This table shows recommended thicknesses from ISO 4179 for cement-lined ductile iron pipe. For some service conditions, greater lining thickness may be preferred. Consult ACIPCO for specific details. RESISTANCE TO SOFT AND ACIDIC WATERS When cement-mortar linings are subjected to very soft water, calcium hydroxide, CA(OH) 2, is leached. The concentration of leachates increases with the aggressiveness of the water and its residual time in the pipe and is inversely proportional to the diameter of the pipe. These waters will also attack calcium silicate hydrates, which form the larger portion of cement hydrates. Although calcium silicate hydrates are almost insoluble, soft waters can progressively hydrolyze them into silica gels, resulting in a soft surface with reduced mechanical strength. ACIPCO 36 International Pipe Manual

3 Seal-coat will retard this leaching and attack to a great extent; however, as mentioned before, there are very few countries that have sufficiently aggressive waters to necessitate the use of a seal-coat. Also, such aggressive waters may cause toxic metals to leach from piping in customers homes, making it difficult to pass water quality standards requiring tests at first draw from customers taps. Therefore, water quality standards requiring better balanced water chemistry may cause these few communities to treat their water, and further diminish the need for seal-coat. Utilities or municipalities that are concerned that their water may be aggressive to cement-mortar linings without a seal-coat are encouraged to follow the procedure detailed in Section II.A., Use of Seal-Coat, in the Foreword to the ANSI/AWWA C104/A21.4 Standard to determine if a cement-mortar lining without seal-coat will impart objectionable hardness or alkalinity to the water. Also in instances where utilities or municipalities are concerned that water may be aggressive, they may want to consider specifying thicker cement linings. Please contact A C I P C O for details on cement linings thicker than AWWA C104 or ISO Standard non-seal-coated, cement-mortar-lined ductile iron pipe is generally considered to be suitable for continuous use at ph between 6 and 12. For service with ph outside this range, consult A C I P C O. Friction head loss or drop in pressure in a pipeline is an everyday concern for the waterworks engineer. Head-loss calculations are based on equations developed by hydraulic engineers after conducting numerous flow tests on actual working mains. Several formulas were developed by Darcy, Chezy, Cutter, Manning, Hazen-Williams, and others. Of these, the formula and tables prepared by Hazen-Williams have proved to be the most popular. F L OW T E S T R E S U LT S O N C E M E N T- M O R TA R - L I N E D D U C T I L E A N D G R AY I R O N P I P E A pipe lining, to be satisfactory, must provide a high Hazen-Williams flow coefficient C initially and must have sufficient durability to maintain a high flow coefficient over many years of service. Unless the lining meets the above requirement, its other properties, chemical or physical, are of little significance. Numerous flow tests have been made on operating lines throughout the United States to determine how well cement-mortar linings meet these basic requirements. Tests on both new and old water mains have established the average value of C that can be expected of new cement-lined iron pipe, and have also provided a measure of the continued effectiveness of such linings over extended periods of service. ACIPCO Cement-mortar-lined ductile iron pipe has a Hazen-Williams C value of 140, a realistic value that is maintained over time. 37 I nt e r n a t i o n al P i p e M an u a l

4 For laminar, fully developed flow in a pipe, friction depends only on the Reynolds number (a function of velocity, inside pipe diameter, and the kinematic-viscosity of the fluid being transported). It is interesting to note that the roughness of the pipe wall is not considered. The reason is that, for the parabolic laminar flow velocity profile, very little of the flow comes in contact with the roughness elements of the wall surface; the velocities in the vicinity of the wall surface are quite low. When laminar flow exists, the fluid seems to flow as several layers, one on another. Because of the viscosity of the fluid, a shear stress is created between the layers of the fluid. Energy is lost from the fluid by the action of overcoming the frictional force produced by the shear stress. FLOW COEFFICIENT OF CEMENT-MORTAR LINED DUCTILE IRON PIPE For turbulent flow of fluids in circular pipes, there is a layer of laminar flow adjacent to the pipe wall called the laminar sublayer. Even in turbulent boundary layers, this sublayer exists where laminar effects predominate. In the case of a pipe, the greater the Reynolds number, the thinner the laminar sublayer is. It has already been noted that the roughness has no effect on the head loss for laminar flow. If the laminar sublayer is thicker than the roughness of the pipe wall, then the flow is hydraulically smooth and the pipe has attained the ultimate in hydraulic efficiency. If this flow were plotted on the Moody diagram, it would coincide with the smooth pipe curve. DIPRA and its predecessor, CIPRA, have long advocated a Hazen-Williams C value of 140 for use with cementlined gray and ductile iron pipe. This recommendation of a C value of 140 for design purposes is sound. It recognizes that the real world of pipelines is a far cry from the gun-barrel geometry of the laboratory pipeline. Furthermore, DIPRA s continued field testing of operational pipelines has shown a C value of 140 to be realistic, and one that is maintained over time even when transporting highly aggressive waters. In all normally specified pipe sizes, cement-mortar-lined ductile iron pipe has an internal diameter that is larger than the nominal diameter, which is larger than the nominal pipe size. For most substitute pipe materials, the inside diameter is equal to or in some cases, even less than the nominal pipe size. The head loss encountered in a piping system is much more sensitive to available pipe inside diameters than normal flow coefficients. Cement lining will withstand normal handling; nevertheless, pipe or fittings may be found at times to have damaged linings which need to be repaired before placing in service. FIELD REPAIR OF DAMAGED CEMENT LININGS AWWA C104, EN545 and ISO 4179 provide that damaged linings may be repaired, and the following repair procedure is recommended by ACIPCO: 1. Cut out the damaged lining to the metal. Square the edges. 2. Thoroughly wet the cut-out area and adjoining lining. 3. With the damaged area cleaned and the adjoining lining wet, spread the mortar (see recommended on next page) evenly over the area to be patched. After the lining has become firm and adheres well to the surface, finish it with a wet paint brush or similar soft-bristle brush. 4. The repaired lining should be kept moist by placing a wet burlap over the required area of the pipe or fitting for at least 24 hours. ACIPCO 38 International Pipe Manual

5 R EC O M M E N D E D C E M E N T M I X Cement mix by volume: 3 Parts Portland Cement; 2 Parts Clean Sand; necessary water for slump of 125mm to 200mm. The sand should be free of clay and screened. P R EC AU T I O N S 1. Mortar for lining should not be used after it has been mixed for more than one hour. 2. Too rapid a loss of moisture from fresh linings due to hot weather or high wind will prevent proper cure, resulting in the lining being soft and powdery. To prevent this loss of moisture, (a) do not line hot castings and (b) close the ends of the castings with wet burlap. 3. Fresh linings which become frozen will not be serviceable. Avoid lining in freezing weather. Pipe and fittings lined with special coatings are available from A C I P C O on a special order basis. For more detailed information regarding lining selection, application parameters, and typical field topcoats, please contact A C I PC O. Ductile iron pipe is especially suited for pressure sewer applications because its standard wall thicknesses provide for high operating pressures with a minimum 20% surge allowance. Surges, or hydraulic transients, are a very serious problem for pressure sewers, as pump stations are vulnerable to power outages and surge control devices for raw wastewater are not always dependable. The surge forces, which can rupture some piping materials, are caused by momentum in the liquid due to change in velocity. Pressure sewers are designed much the same as water transmission lines. The Hazen-Williams formula is the most popular flow formula. The Hazen-Williams friction coefficient, C, is generally accepted as 140 for cementmortar linings and 150 for epoxy linings. Special linings are not usually specified if the pipe is always flowing full and the waste stream is domestic sewage. However, the designer has the option of special linings to resist H2S as discussed under Gravity Sewers. In either case, the hydraulic efficiency of ductile iron pipe, coupled with its largerthan-nominal inside diameter, results in reduced pumping costs over the life of the pipeline. Piping for pressure sewers can be provided not only with push-on joints, but also with restrained joints having deflection capabilities after installation. All ductile iron force main piping can be interfaced with pumps, lift stations, and plant piping with standard fittings and factory-supplied spools. Ductile iron pipe is often used in difficult installations because of its great strength, joint integrity, and its versatile and readily available joints and fittings. ACIPCO 39 I n t e r n at i o na l P i p e Ma n u a l