« PreviousContinue »
DESIGN OF STEEL CONDUITS (Continued)
The ring girder is statically indeterminate and its dimensions must be Stress
The reinforcement for wyes as shown in Figure 31 (a) and (b) are statically
D. Figure 32 shows a 15- by 7-1/2-foot branch outlet for the penstocks at Typical Anderson Ranch Dam. The unbalanced load is carried by a curved T- Installation shaped girder and by two 7-inch diameter tie rods, the ends of which are welded to the girder. In addition, the branch opening is reinforced with an extra heavy plate for one-third the circumference of the header pipe. A typical wye branch for a 39-inch outlet pipe is shown in Figure 33. This wye is reinforced with an exterior horseshoe girder of T-shape. Bends welded to the downstream ends of the brahches bring the branch lines parallel to the main line.
.24 Expansion joints are installed in exposed conduits between fixed points or EXPANSION anchors to permit longitudinal expansion and contraction when changes in tem- JOINTS perature occur, and to permit vertical movement when conduits pass through two structures where differential settlement or deflection is anticipated. A typical expansion joint intended to accommodate longitudinal movement primarily is shown in Figure 34, and a typical expansion and deflection joint designed to accommodate both longitudinal and vertical movements is shown in Figure 35. For large, fabricated steel pipe the sleeve-type expansion joint is generally used. This consists of an inner and an outer sleeve, a stuffing box with packing, held by a retainer ring and compressed with a packing gland. The inner ring is usually provided with nickel cladding on the outside to prevent corrosion and facilitate temperature movements in the joint.
The inner sleeves should be designed to withstand the external pressure exerted by the packing. The clearances at the ends of the sleeves and the distances from the ends of the sleeves to the packing-retainer rings should be ample to permit the maximum movement expected.
The glands should be designed in accordance with the formulas shown in Figure 36. The bolts or studs should be of sufficient size to exert the force required to develop a pressure between the packing and the inner sleeve of from 1.25 to 1.50 times the maximum normal internal operating pressure with a spacing of from 12 to 14 inches. The packing should consist of from four to eight rings of Square, lubricated, braided, long-fiber flax rings, the number depending upon the internal pressure. The size of the packing may vary from five-eighths inch to one and one-quarter inches depending upon the size of the expansion joint.
Frictional resistances in expansion joints may be assumed at 500 to 1,000 pounds per linear foot of circumference.
AIR INLETS .27 & OUTLETS
Manholes for inspection and maintenance purposes should be placed in all conduits large enough to permit entrance. Typical designs are shown in Figures 37 and 38. The manholes shown in Figure 37 are intended for high heads. As the limiting pressures shown in the tables of this figure are for steam, the corresponding cold-water pressures will determine the class of manhole to be used.
Where practicable, manholes should be placed from 400 to 500 feet apart. They
Nozzles for connecting drains and filling lines should be attached to conduits
Air inlets should be provided at the upstream ends of penstocks and outlet
7/ Durand, W. F., “Hydraulics of Pipe Lines,” published in 1921 by D. Van
Norstrand Co., New York, N. Y. (out of print).
8/ Enger and Seely, “Vents on Steel Pipes,” Engineering Record, Vol. 69, No.
21, May 23, 1914.
Ledoux, J. W., “Pipe Line Inlet Air Valves,” Water Works, July 1926.
2.29 ACCESSORIES (Continued)
Flanged joints are required where conduits join gates, valves, pumps, turbines, FLANGED or other facilities having flanged openings. Welding-neck-type flanges of JOINTS forged steel should be used for high heads and large conduits. Slip-on or plate flanges may be used for low heads and small conduits. The face of a flange should not be machined until after the flange has been welded to a section of pipe of sufficient length to avoid warpage of the flange. Flanges of standard design can be used in some cases, but flanges of special design are frequently required. Design procedures are covered in the API-ASME Code referred to in Paragraph 2.16. Bolt sizes and spacings and the types of gaskets used are determined by the construction of the flanges with which the conduit is to connect. Figure 39 shows typical flange connections using the welding-neck type, the slip-on type, and the plate type of flanges.
Closing sections (sections of pipe with lengths in excess of the theoretical CLOSING lengths required) may in some cases be furnished for installation at appropri- SECTIONS ate points in the line to permit field adjustments in conduit lengths to compensate for shrinkage in field-welded joints, differences between theoretical and actual laying lengths of conduit sections, and discrepancies between theoretical
and actual field measurements.
Test heads are used where field conditions permit, and the magnitude of the TEST HEADS installation Warrants a proof hydrostatic pressure test after installation. In anticipation of such tests, it will be necessary to provide test heads unless the conduit can be closed by gates or valves. The test heads may be ellipsoidal, standard dished, ASME Code or hemispherical heads, as shown in Figure 40. For small pipes, flat heads or blind flanges may be used. The pressure for formed heads should be applied on the concave side. These heads should be welded to the pipe in the shop and should be removed after the test. Allowance should be made in the length of the pipe section receiving the test head for the welding and removal of the head and the preparation of the plate edges for the final weld after testing. For computations of thicknesses see the API-ASME Code referred to in Paragraph 2.16.
Walkways, stairs, and ladders should be provided where required to furnish WALKWAYS, access to conduits placed in open tunnels or installed above the ground surface. STAIRS, Walkways should be placed on one or both sides of the conduit, and ladders & should be provided to reach manholes, expansion joints, drain valves, etc. LADDERS Supports for walkways should be designed so that any required attachments to the conduit are welded to the support rings. A typical walkway and stair installation is shown in Figure 41.
CONCRETE PIERS AND ANCHORS
Piers are required for all rocker-type, sliding-type, and saddle-type supports. CONCRETE A typical rocker-type installation is shown in Figure 42. Piers should be PIERS designed for the vertical reactions at the support, longitudinal forces resulting from frictional resistance due to temperature movement, and lateral forces caused by wind loads. The resultant of all forces under the most unfavorable conditions should intersect the base within the kern to prevent tension in any part of the footing. 9/
The vertical component of the resultant of all forces should not be less than
9/ Merriman, Mansfield, et al., “American Civil Engineers' Pocket Book,”
published in 1916 by John Wiley & Sons, Inc., New York, N. Y., p. 590.
CONCRETE PIERS AND ANCHORS (Continued)
constructed in advance of the time of conduit installation, when the support fixtures are grouted in place. The base of the pier should be placed below the frost line.
.34 For conduits supported above ground or in open tunnels, anchors are required at all bends subjected to forces exceeding the resistance of the weight of the section of conduit and water supported at the bend. Anchors are also required about midway between expansion joints where there are two or more expansion joints between two bends requiring anchorage. This condition occurs when the distance between any two bends requiring anchors is in excess of about 500 feet, as it is desirable to limit the distance between expansion joints to a value that will keep the longitudinal forces at the anchors within practicable limits. They may be of the type shown in Figure 43 or they may completely encase the conduit as shown in Figure 44.
Conduits placed underground usually require anchors at short-radius horizontal
The forces at an anchor may be estimated by use of the formulas in Figure 45. These forces should be calculated for both the expanding and contracting condition with the pipe both full and empty. Wind and earthquake loads should also be considered where conduits are above ground. The resultant of all forces, including the reaction at the anchor and the weight of the anchor, under the most unfavorable conditions should intersect the base within the kern to avoid tension in the concrete (see Paragraph 2.33). The vertical component of the resultant of all forces should not be less than the horizontal component of the resultant of all forces divided by the coefficient of sliding friction at the base of the anchor. The friction coefficient may vary from 0.35 to 0.65 under different soil conditions. Where conduits are buried the position of the resultant of all forces is not important, and a friction coefficient of 1.00 may be used.
Anchors should preferably be placed on a rock base if possible. The bottom of the anchor should be extended below the lowest frost line in areas subject to freezing. As actual forces may vary on both sides of an anchor from the computed forces, such possibilities should be considered in proportioning the anchor. At points where computations indicate a balance of forces and where theoretically no anchors are required, it is desirable to provide at least a nominal size anchor to prevent displacement due to reasons stated above.
FABRICATION AND INSTALLATION
.35 Steel conduits are usually fabricated in courses, each course having a length
equal to the net plate width after trimming. Figure 46 shows courses for a 15-
2.36 EAERICATION AND INSTALLATICIS (Continued)
Transportation problems usually determine whether conduits are fabricated in FABRICAthe contractor’s shop or at the site of installation. As transport by rail of con- TION duits over 12 feet in diameter is usually not feasible, conduits of such size (Cont.) should be fabricated in a field fabricating plant near the site of installation, either from plates shipped directly from the mill or from plates prefabricated at the contractor’s home plant. Figure 49 shows a portion of the field fabricating plant used for the 22-foot penstocks at Davis Dam. The plates were prefabricated, cut to size, edged and rolled, then transported to the field fabricating plant for completion. Internal spiders were used during fabrication to keep the pipe courses circular until the stiffeners were welded on.
The installation of steel conduits is frequently performed under a separate INSTALLAcontract. Where this is the case it is necessary to define the length of the pipe TION sections and the edge preparation for the field joints so that the amount of work to be done under each contract can be determined. It is also essential in such cases to specify the permissible tolerances in pipe lengths, trueness of ends, out-of-roundness, clearances in bell-and-spigot joints and in expansion joints, Spacing of Supports, etc.
Conduit Sections are usually transported from the nearest railroad station or from the field labricating plant to the site of installation by truck, trailer, or barge. Figures 50 and 51 show the transport by truck and trailer of 18- and 22-foot penstock Sections at Grand Coulee and Davis Dams, respectively, and Figure 52 shows the transport by barge of an 18-foot penstock section at Grand Coulee Dam from field fabricating plant to dam. Upon arrival at the site of installation the conduit sections are lifted in place by cableway, derrick, or other means suited to the site. Figure 53 shows handling by cableway of a 15foot penstock Section at Shasta Dam, and Figure 54 shows handling by derrick barge of an 18-foot penstock section at Grand Coulee Dam. Figure 55 shows handling by whirley of a 22-foot penstock section at Davis Dam, and Figure 56 shows its placement in the 27-foot octagon blockout in the intake structure.
After being set to line and grade on temporary supports, the conduit sections
Installations in tunnels or concrete blockouts are more complicated, due to limited clearances around the conduit which may require that all welding be performed from the inside of the conduit against a backing strip. If such conduits are to be backfilled with concrete, all welding, weld tests, and pressure tests should be completed and the exterior surfaces of the conduit cleaned before backfilling.
Among the possible nondestructive tests and inspections some of the following INSPECTION may be required by the Bureau of Reclamation to insure a safe welded job: & TESTS
A. Physical and chemical tests of plates, at mills.