Pipe supports bear the dead loading, live loading, wind, snow, and seismic loadings, as well as the loads imposed or caused by variations in temperatures, both ambient and the contained fluid. Pipe supports must prevent exceeding the stress limit of the piping material, and prevent excessive forces and moments on the equipment to which the piping is attached. Also to be considered are the fluid dynamic forces, such as water hammer and steam hammer.
The American Society of Mechanical Engineers (ASME) publishes The Code for Pressure Piping B31. The Power Piping Code (B31.1) is one of several sections of this code that applies to “power and auxiliary service piping systems for electric generating stations; industrial and institutional plants; central and district heating plants; and district heating systems, both on the property of and within the buildings of the users”. This course will refer to B31.1 as the governing code for the subject covered herein. Pressure piping systems in other facilities, such as chemical plants and refineries, each have their own governing codes, and should be referred to, as appropriate. The requirements for pipe supports and hangers are, however, very similar to those of B31.1.
Nothing in this course is intended to modify or supplant anything in B31.1 or any other governing codes.
The term “pipe supports” is used in a generic sense that includes pipe hangers and restraints, and any other structure or device that bears the weight or restricts the movements of piping.
Pipe support illustrations are taken from ITT Grinnell catalog PH 81.
Steps in Designing Pipe Supports
There are three loosely defined steps in pipe support design:
1. Developing a preliminary piping layout 2. Performing a piping stress analysis 3. Selecting the individual pipe supports configurations and components
The preliminary piping layout is of necessity done by the piping designagency, and is described in the next section.
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The pipe stress calculations can be done by the design agency or by the pipe support contractor. It often is more convenient to assign both the stress analysis and the detailed pipe support configurations and supply to the general construction contractor, who will then subcontract it to a pipe support subcontractor. This is because the stress analysis and the pipe support configurations are closely related, and often reiterative processes, and therefore fit nicely into a single scope of responsibility package. The argument against this is that the pipe stress calculations and decisions should remain with the design engineer of record, and not delegated to the general contractor or to a pipe support supplier. The latter approach is strongly recommended. If, however, the pipe stress is delegated, the stress should still be performed under the supervision of a Professional Engineer, and the stress analysis so stamped.
The detailed configuration of the supports is usually left to the pipe support supplier or contractor because the details can vary with suppliers, and the suppliers can most efficiently and effectively perform this function. The final configuration drawings should then be reviewed an approved by the design engineer of record.
Preliminary Layout
The physical layout of piping systems must take into account how the systems are to be supported. Two primary considerations at this stage are inherent flexibility of the piping system, and structural attachments for the supports. Inherent flexibility is necessary to deal with the effects of thermal expansion and contraction. It is accomplished by including bends or loops in the piping, and is the work for an experienced piping designer. Also, of course, the piping must be routed where there is concrete or steel from which to support it.
In preliminary piping layouts, the experienced designer will take into consideration such things as the pipe size and wall thickness, and the maximum range of temperatures the piping will be subjected to. He or she will then, either by rule of thumb or guesswork, lay out the piping using bends, loops, and offsets. Expansion joints or ball joints may also be included for additional flexibility in tight areas. Anchor points and tentative pipe support locations are selected. Allowable forces and moments at equipment nozzles are assumed, pending input from the equipment vendors. At this point, the responsible engineer must decide the degree of pipe stress analysis that will be performed.
The Power Piping Code
The Power Piping code (ANSI / ASME B31.1) requires that “power piping systems subject to thermal expansion or contraction or to similar movements imposed by other sources shall be designed in accordance with the requirements for the evaluation or analysis of flexibility and stresses specified herein”, and for all piping to take thermal stress “into consideration”.
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The Power Piping Code states the degree of stress analysis that must be performed to conform with the code. I will quote:
“All piping shall meet the following requirements with respect to thermal expansion and flexibility:(A) It shall be the designer’s responsibility to perform an analysis unless the system meets one of the following criteria.(A.1) The piping system duplicates a successfully operating installation or replaces a system with a satisfactory service record.(A.2) The piping system can be adjudged adequate by comparison with previously analyzed systems.(A.3) The piping system is of uniform size, has not more than two anchors and no intermediate restraints, is designed for essentially non-cyclic service (less than 7000 total cycles), and satisfies the following approximate criterion:
(a) English units DY / (L-U)² is less than or equal to 0.03
(b) SI units (6944.44) x [DY / (L-U)²] is less than or equal to 208.3
Where:D = nominal pipe size in inches or mmY = resultant of movements to be absorbed by the pipelines in inches or mm.L = developed length of line axis in ft. or m.U = anchor distance (length of straight line joining anchors in ft. or m.)
(B) All systems not meeting the above criteria, or where reasonable doubt exists as to adequate flexibility of the system, shall be analyzed by simplified, approximate, or comprehensive methods of analysis that are appropriate for the specific case.(C) Approximate or simplified methods may be applied only if they are used for the range of configurations for which their adequate accuracy has been demonstrated.(D) Acceptable comprehensive methods of analysis include: analytical, model tests and chart methods which provide an evaluation of the forces, moments, and stresses caused by bending and torsion form the simultaneous consideration of intermediate restraints to thermal expansion of the entire piping system under consideration, and including all external movements transmitted to the piping by its terminal and intermediate attachments. Correction factors must be applied for the stress intensification of curved pipe and branch connections, as provided by the details of these rules, and may be applied for the increased flexibility of such component parts”.
A comprehensive pipe stress analysis of piping systems generally means applying a reiterative computer program and finding the optimum combination of stiffness, flexibility, and growth If the system cannot be made to “stress out”, a reconfiguration is sometimes necessary.
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Guidelines
That being said, there is still latitude for the responsible design engineer to exercise judgment as to the degree of stress analysis to be undertaken.Unfortunately, comprehensive stress analysis is complex, time-consuming, and expensive. For many projects, the following guidelines are followed:
1. Piping 2 ½ inches diameter and larger with temperature ranges of 250 deg F or greater is to be formally computer analyzed.
2. Piping 2 ½ inches in diameter and larger which experience temperature ranges between 100 and 250 deg F is to be evaluated in accordance with Code approved manual techniques or stress table techniques.
3. Piping less than 2 ½ inches is generally not analyzed because this piping is considered to be inherently flexible. Furthermore, this piping is generally shown diagrammatically in the piping drawings, is not fully dimensioned, and is field-run by the erector. There may be exceptions to this guideline, for instance in the case of boiler blowdown and drain piping. Such piping is high-pressure and high temperature, and can tend to have straight runs if not carefully analyzed.
4. Piping carrying hazardous fluids or having long straight runs should receive special consideration.
Again, the interpretation of the Code, including these exclusions from comprehensive computer analysis, is the responsibility of the responsible engineer, who is usually not the piping designer. Each flexibility problem should be analyzed by a method appropriate to the conditions. Regardless of the method used, the steps to determine the adequacy should be recorded in the design calculations.
The following example is taken from the Giffels and Rossetti Reports appearing in The Piping / Plumbing Engineer. It illustrates an application of the equations given in paragraph (A.3) above.
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Using Fig. 3, follow the six-inch pipe dia. line horizontally until it intersects the vertical line at (L-U) = 18 feet. The intersecting point indicates that the system can safely absorb expansion equivalent to a resultant movement Y = 1.5 inches. Since the actual resultant Y = 1.235 inches, is less, the system has adequate flexibility and no further calculation is necessary.
Cold Springing
Cold springing is a procedure sometimes used to reduce thermal stresses in
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piping. The pipe is installed in a pre-stressed condition when cold in such a way that the stress is relieved when the piping reaches its operating temperature.Larger stress values are allowable at ambient (cold) temperatures than at operating (hot) temperatures. This procedure must be carefully designed and executed, and caution taken if the piping is subsequently cut for future alterations.
Other Pipe Support Loads
Pipe support loads other than those caused by thermal effects must also be accounted for, such as:
Piping dead weightLive weight, which is the contained fluid. In steam and gas piping condensation and hydro test liquids must also be accounted for.Wind, snow, and any other temporary loads.Seismic loads.
Reactions at Piping Termination Points
After the resultant of all the piping loads and growth have been calculated or estimated, a second look should be given to the preliminary locations of anchors and supports previously chosen. A judicious placement of anchors and supports can help reduce the force and moment reactions at the piping termination points at the equipment nozzles. The allowable forces and moments on the equipment nozzles have to be agreed to with the equipment manufactures. Rotating equipment such as turbines and pumps are particularly sensitive to nozzle loadings. Tanks and heat exchanger nozzles often can be reinforced by the manufacturer to withstand heavier loads. The use of expansion joints and ball joints may also be considered to reduce piping reactions.
Transfer of Forces and Moments to Any point
Flexibility analyses yield forces and moments exerted by piping on its terminal ends in three coordinate planes. It is sometimes desired to determine the piping reactions at some other points remote from the piping connections. It is then easier to recognize the effects of these forces on other equipment and structure, such as the strength required of a hold-down bolt on a pump or tank.
The reactions at any remote point are easily determined in each plane. The force and the moment may be transferred anywhere in their plane, but the product of the relocated force and the distance the force is moved must be added algebraically to the moment.
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Moment at point P = + Mz - Fx a Force at point P = Fx
The signs given, + and - , show the directions of the forces and moments exerted on the pipe by the equipment and on the equipment by the support. Reversed signs show the directions of the forces and moments exerted on the equipment by the pipe and on the support by the equipment.
Transient Analyses
Transient or external loading analysis (seismic, water hammer, etc.) is an iterative technique whereby the maximum stress levels, terminal reaction, and piping deflections are determined by computer analysis for each load case.
For seismic analysis, the piping is subjected to horizontal and vertical force fields which represent the maximum accelerations expected during a seismic event in the particular geological area. A typical loading would include three cases, one with a specified horizontal east-west field, one with a horizontal north-south field, and one with a vertical field. The resultant stress, terminal reaction, and deflections are the root mean squares of the individual case loads.
For steam or water hammer, an equivalent static loading is developed to envelope the accelerations expected of the dynamic reactions. For each length of pipe bounded by two elbows, two pieces of equipment, or one elbow and one piece of equipment, the maximum differential pressure that could exist in that section during the transient is determined and applied over the pipe cross-section to develop the maximum expected loading on this section. Each section is treated in the same manner. The piping system is then modeled and computer analyzed with all of these loadings applied to each section.
The results of such a transient analysis are investigated for acceptability of stress in accordance with the Power Piping Code, terminal reaction in accordance with the manufacturers’ requirements, and excessive deflections which might cause interference with other piping or structures.
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For systems whose results are unacceptable, there are two solutions. The preferred solution is to locate restraints so that the results are acceptable. This involves a dual iterative technique because not only will a restraint location have to satisfy the transient analysis, but it is also subjected to another thermal flexibility analysis to assure that the new restraint location does not cause the thermal stress limits to be exceeded.
The second solution, if the restraint does not work, is to install a hydraulic snubber or a travel stop. The hydraulic snubber is a device that allows thermal expansion, but resists the accelerations cause by transients.
Pipe Support Configurations
Pipe hangers and supports are devices which transfer the loads from the pipe or the structural attachment to the supporting structure or equipment. They include rod hangers, spring hangers, sway braces, turnbuckles, struts, anchors, saddles, rollers, brackets, and sliding supports. Structural attachments are elements that are welded, bolted, or clamped to the pipe, such as clips, lugs, clamps, clevises, and stops.
Pipe supports come in many configurations, and are designed to constrain pipe motion in one, two, or three space coordinates. Only the most common types will be generally described in this course. Manufactures have catalogs that thoroughly illustrate supports of all types.
In the following pipe support sketches, the dots (.) indicate suggested locations
for thermocouples.
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Rod Hangers
Probably the most common is the rod hanger that attaches to the pipe by a U bolt or clevis and clamps to structural steel above. This is illustrated below.
The rod hanger provides support in the vertical direction and allows limited motion in the horizontal direction. Adjustment in the vertical direction can be accomplished by threads or a turnbuckle.
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Riser Clamp
Another common support system is the riser clamp, illustrated below, which constrains vertical runs of pipe.
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Travel Stop
A positive vertical restraint is the travel stop, which fastens to lugs welded to the pipe.
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Trapeze Assembly
This support consists of a pipe covering protection saddle and two channels.
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Variable Spring Hanger
A variable spring hanger is shown next. These devices are installed in locations where stresses are not considered to be critical, and where movement permits their use. The supporting force varies with the spring deflection. Movement of the pipe causes the spring to extend or compress. Since the weight of the pipe is the same in either the hot or cold positions, the variation in the spring force results in pipe weight transfer to equipment and adjacent hangers, and consequently additional stresses on the piping system. Since it is desirable to support the actual weight of the pipe in the hot position, when the stresses become more critical, the hot load is the dead weight of the pipe. The cold load is actually under or over supporting the pipe, depending on the movements from hot to cold.
Constant Support Hangers
Constant support hangers provide a constant supporting force for the piping system throughout its full range of vertical pipe movement. This is accomplished through the use of a spring operating in conjunction with a lever, in such a way that the spring force times the distance to the lever pivot is always equal to the pipe load times its distance from the pivot point. This type of support is thermally invisible, as the supporting force equals the pipe weight throughout the entire thermal cycle. These hangers are used on systems and at locations where the stresses are considered critical.
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This is a small sample of the wide variety of hanger and support arrangements that can be provided by competent vendors to accommodate any number of situations that present themselves in power plants, process, or other facilities that utilize a variety of piping configurations.
Conclusion
The design of the system and devices that support piping systems in the manner prescribed by the Power Piping Code ANSI /ASME B31.1, or any other governing code, including the stress analysis and other analyses that the support design depends upon, is the responsibility of the Design Engineer of Record, that is, the Professional Engineer who stamps or seals the piping design drawings. This may not be the direct quotation of any code of state regulations, but it is a generally accepted principle in the trade. This course provides some of the generally accepted procedures in designing pipe support systems, but by no means is it comprehensive.
Pipe support design must be considered an integral part of the design process.
