2.18

DESIGN OF STEEL CONDUITS (Continued)

STIFFENER RINGS AND SUPPORTS (Continued)

Circumferential stresses in support rings for use with saddle-type supports
can be computed by use of the formula and graph shown in Figure 24. A value
for p
of 120 degrees is usually used.

.18 A typical rocker-type support assembly and details of the different parts are shown in Figures 25 and 26, respectively. Supports of this type offer little resistance to movements of the conduit resulting from temperature changes and are usually used for conduits over about 4 feet in diameter where conditions permit. An over-all friction coefficient of 0.15 may be assumed for design purposes. Vertical reactions, forces due to frictional resistance, wind loads, and concrete bearing values should be considered in the design.

.19 A typical sliding-type support is shown in Figure 20. Supports of this type are
used principally in open tunnels where clearances are restricted and where the
sliding plates are not subject to abrasion by blowing sand or other causes.
Friction coefficients of 0.15 may be assumed for design purposes where self-
lubricating bronze plates are used for one of the contact surfaces. Where two
nonlubricated steel plates are in contact, a friction coefficient of 0.50 may be
assumed. Vertical reactions, forces due to frictional resistance, wind loads
if the installation is outside of a tunnel, and concrete bearing values should be
considered in the design.

.20 A typical saddle-type support is shown in Figure 21. Supports of this type may be used where clearances are restricted. The frictional resistance may be reduced by using two layers of graphited asbestos sheet packing between the flange of the support ring and the bearing plate, with the graphited surfaces in contact. Although this type of support when new has a friction coefficient of about 0.25, a coefficient of 0.40 should be used for design purposes on the assumption that the sheet packing may deteriorate or become dislodged by temperature movements of the pipe.

.21 Transverse earthquake loads of from 0.10 to 0.20 of the gravity load, depending on local records, should be included for installations to be made in locations subject to seismic disturbances. When a pipe line is located close to an earthquake-producing fault zone the above seismic coefficients should be increased in accordance with local records.

BENDS, BRANCH OUTLETS, AND WYES

.22 Bends (or elbows) are required where changes in direction, either horizontally or vertically, occur in a conduit. Plate-steel bends are made up of short segments of pipe with mitered ends. Wherever possible, bends should be designed with deflection angles between segments of from 5 to 10 degrees and with radii of from 3 to 5 diameters. For penstocks where the conservation of head is very important, deflection angles of from 4 to 6 degrees may be used. Where the point of intersection of a horizontal angle coincides with that of a vertical angle, or where these points can be made to coincide, a single bend, called a combined or compound bend, designed to accommodate both angles should be used. The combined bend should have a pipe angle equal to the developed angle (X) obtained by the use of the appropriate formula shown in Figure 27. Reducing bends designed in accordance with the layout and formulas shown in Figure 28 combine the functions of a uniform-diameter bend and a reducer, and should be used where practicable. Typical uniform-diameter bends are shown in Figure 29.

.23 Branches and wyes are used where the flow in a conduit diverges into two or more streams or where the flows in two or more conduits converge into a single stream. The most important features to be considered in the design of fittings of this type are structural safety and hydraulic efficiency. For safety, reinforcement of some type must be provided to offset the effect of the

ROCKERTYPE SUPPORTS

SLIDINGTYPE SUPPORTS

SADDLETYPE SUPPORTS

EARTH-
QUAKE
LOADS

BENDS

BRANCHES

&

WYES

2.23A

Hydraulic
Efficiency

Reinforcement

Stress Analyses

DESIGN OF STEEL CONDUITS (Continued)

BENDS, BRANCH OUTLETS, AND WYES (Continued)

unsupported pressure areas resulting from the removal of part of the material
from the conduit walls. The locations of these areas, and the loading diagrams
assumed for design purposes are shown on Figures 30 and 31 which illustrate
a number of different types of branches and wyes. Several different methods
of reinforcement are also shown.

Right-angle branches and cylindrical outlets or inlets should be avoided
where hydraulic efficiency is important or where the question of cavita-
tion is involved. The use of conical connections with side-wall angles Ø
equal to from 6 degrees to 8 degrees reduces hydraulic losses to about
one-third of those resulting from the use of cylindrical connections.
Hydraulic losses may be further reduced by joining the branch pipe to the
main pipe at an angle e less than 90 degrees as shown in Figure 30(c).
This deflection angle varies in practice from 30 to 75 degrees. For branch
outlets (Figure 30) it should not be less than 45 degrees and the corres-
ponding angle for wyes (Figure 31) should not be less than 22-1/2 degrees
or 45 degrees between two branches, as smaller angles introduce fabrica-
tion difficulties. Branches and wyes are usually designed so that the
longitudinal axes meeting at a common point will lie in the same place.

B. Branches as shown in Figure 30 (a), (b), and (c) may be reinforced with a
simple curved plate designed to meet the requirements of the API-ASME
Code referred to in Paragraph 2.16. Branches and wyes of the types
shown on Figure 30 (d) to (g) inclusive, and on Figure 31 (b) to (f) inclusive,
may be reinforced by the use of one or more girders or by a combination
of girders and tie rods. The type and size of reinforcement to be used
depends upon the pressure in the vessel, the extent of the unsupported
pressure area, and clearance restrictions. When branch outlets intersect
in a manner as shown in Figure 30 (d) and (e) and wyes as shown in Fig-
ure 31(f), three or four exterior horseshoe girders may be used, the ends
of which are joined by welding. This welding will be facilitated by insert-
ing a round bar at the junction. For wyes used in pipes with low-velocity
flow an internal horseshoe girder, also called a splitter, as shown in Fig-
ure 31(a) may be used. The splitter is structurally effective, being sub-
ject to direct tension and bending loads, but will cause disturbance if the
flow is unequally divided. Branches of the type shown on Figure 30 (f) and
(g) are not desirable when D1 is over about three-fourths of D, as the
lateral curvature of the reinforcing girder becomes very sharp under this
condition. In these cases, the design indicated on Figure 30(e) is prefer-
able. Wyes reinforced as shown on Figure 31(a) are satisfactory for con-
verging flows, but should be used for diverging flows only in exceptional
cases where it is impracticable to place reinforcement on the outside of
the wye.

C. The method of stress analysis used for branches and wyes is approximate.
Simplifying assumptions are made in the analysis which yield results of
sufficient accuracy for practical design purposes. The reinforcement is
proportioned to carry the entire unbalanced load as indicated by the shaded
areas in Figures 30 and 31. A portion of the pipe shell is considered to be
acting monolithically with the girders as in the case of stiffener rings.
Excess material within the area of reinforcement may be considered as
load-carrying. In the analysis of the ring girder type of reinforcement
shown in Figures 30 (f) and (g), it is assumed that the curved girder is
acting similar to a plane ring, that the loads in both directions are uni-
formly distributed, and that the ring is circular. The first assumption is
believed to be reasonably accurate because the ring girder is supported
along its entire perimeter by the pipe shell and cannot be appreciably
twisted or deflected laterally. Assumption of a uniform load distribution
is on the side of safety. As to the assumption of circularity for the ring,
however, it should be noted that the ring girder becomes egg-shaped for
branch outlets with small deflection angles, but for larger deflection
angles the assumption of circularity is nearly correct.

2.23D

DESIGN OF STEEL CONDUITS (Continued)

BENDS, BRANCH OUTLETS, AND WYES (Continued)

The ring girder is statically indeterminate and its dimensions must be
assumed for the stress analysis. Using a uniform cross section will sim-
plify the work. Where tie rods are used the deflections of the girder at
the junction with the rod must be computed; these are then equated to the
elongations of the tie rods and the forces in the tie rods obtained by
solving the equations.

The reinforcement for wyes as shown in Figure 31 (a) and (b) are statically
determinate, and the bending and direct stresses can be computed without
difficulty for any section. Increases in the bending stress due to the small
radius of curvature at the throat of the girder may be evaluated by apply-
ing a correction factor to the bending formula for straight beams. In
some cases it is desirable to check the computations by a hydrostatic
pressure test at a pressure 50 percent in excess of the design value. Also,
branch outlets and wyes may be stress-relieved after completion to reduce
the residual stresses due to welding.

D. Figure 32 shows a 15- by 7-1/2-foot branch outlet for the penstocks at
Anderson Ranch Dam. The unbalanced load is carried by a curved T-
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 branches bring the branch lines
parallel to the main line.

EXPANSION JOINTS

.24 Expansion joints are installed in exposed conduits between fixed points or anchors to permit longitudinal expansion and contraction when changes in temperature 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.

Stress

Analyses (Cont.)

Typical Installation

EXPANSION
JOINTS

2.25

ACCESSORIES

MANHOLES .25 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

DRAIN

& FILLINGLINE CONNECTIONS

used.

Where practicable, manholes should be placed from 400 to 500 feet apart. They
may be located at the top, at the side, in the lower quadrant, or at the bottom of
the conduit. In large conduits, manholes in the lower quadrant are usually the
most convenient to enter. One or more manholes, depending upon the length
and profile of the conduit, should be placed at the top to provide ventilation. In
siphons having steep rising slopes, manholes in the lower quadrant just ahead
of such slopes will facilitate the removal of silt which usually accumulates at
these points.

.26 Nozzles for connecting drains and filling lines should be attached to conduits
where required. A drain nozzle should be placed in the bottom of each conduit
at the lowest point of the line. Suitable gratings or bars, flush with the inner
surface of the conduit, should be installed across the opening for safety rea-
sons. The size of the drain is dependent on the time allowed for draining.
Sweep-type nozzles are generally used. Filling lines are provided for pen-
stocks only. They are used to fill the penstock with water and place the control
gate under balanced pressure to facilitate its opening. The intake nozzle should
be connected at the horizontal center, near either end of the penstock, but pref-
erably near the downstream end to permit filling under submerged conditions.
The line should be connected to the reservoir and should be of sufficient size
to complete the filling in about 8 hours, assuming a leakage through the turbine
gates at 1 percent of the rated flow to the turbine. Individual lines are usually
provided for each penstock, the flow being controlled with suitable valves
located in tunnels or gate chambers in the dam.

AIR INLETS .27 Air inlets should be provided at the upstream ends of penstocks and outlet &

OUTLETS

pipes and at the downstream ends of pump-discharge lines to prevent negative pressures while the conduits are being drained and to release air while the conduits are being filled. Air vents or float-operated combination air and vacuum-relief valves should be installed in all conduits at summits to prevent reductions in flow caused by air accumulations and to prevent the formation of partial vacuums in the conduits. Under some conditions, air pipes or air valves should be installed at points where the grade of a conduit changes abruptly from a flat slope to a steep slope. These connections will admit air into the line when drained either intentionally, or accidentally due to a rupture of the pipe section on the steep slope. All air valve connections should be protected with shutoff valves to permit servicing of the air valve. Methods of estimating the required sizes of air valves have been formulated by Durand 7/ and others. 8/

PIEZOMETER .28 Piezometer connections are frequently provided in penstocks and pump-
CONNEC-

TIONS

discharge lines for use in connection with turbine and pump performance tests. They should be located in straight sections of the pipe line removed from bends and branch connections. The connections are used in groups of four equally spaced around the periphery of the pipe, each group forming a separate piezometer line leading to a meter box in the powerhouse or pump house. Details of a typical piezometer connection are shown in Figure 2.

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)

.29 Flanged joints are required where conduits join gates, valves, pumps, turbines, or other facilities having flanged openings. Welding-neck-type flanges of 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 Čode 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.

.30 Closing sections (sections of pipe with lengths in excess of the theoretical lengths required) may in some cases be furnished for installation at appropriate 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.

.31 Test heads are used where field conditions permit, and the magnitude of the 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.

.32 Walkways, stairs, and ladders should be provided where required to furnish access to conduits placed in open tunnels or installed above the ground surface. 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. 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

.33 Piers are required for all rocker-type, sliding-type, and saddle-type supports.
A typical rocker-type installation is shown in Figure 42. Piers should be
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
the horizontal component of all forces divided by the coefficient of sliding fric-
tion at the base of the pier. The friction coefficient may vary from 0.35 to 0.65
depending on the underlying soil. Earthquake loads should be added where
appropriate. Recesses for base plates or cradles are usually left in the tops
of the piers, and anchor bolts are placed where required. The piers should be

9/ Merriman, Mansfield, et al, "American Civil Engineers' Pocket Book," published in 1916 by John Wiley & Sons, Inc., New York, N. Y., p. 590.

FLANGED

JOINTS

CLOSING
SECTIONS

TEST HEADS

WALKWAYS,
STAIRS,

&

LADDERS

CONCRETE
PIERS