ACCEPTABLE LOADS
Centrifugal Pumps API 610 Nozzle size.
Centrifugal Compressors API 617, 1.85 times NEMA
SM-23 allowable Nozzle size, material.
WRC 107, WRC 297 Nozzle size, thickness, reinforcement details, vessel/exchanger diameter, and wall thickness.
Tank Nozzles API 650 Nozzle size, tank diameter,
height, shell thickness, nozzle elevation.
Steam Turbines NEMA SM-23 Nozzle size.
Analysis Considerations for Rotating Equipment
Rotating equipment piping represents one of the more difficult systems to design for thermal flexibility. The difficulty increases as the design temperature and pipe diameter increase. Use pump piping systems as an example. The loads on the pump nozzles must be within acceptable limits for all possible spare pump operating conditions. The overall design of piping systems connected to rotating equipment is generally governed by the load limitations on the equipment rather than the thermal expansion stresses in the pipe. If the equipment load criteria are met, the thermal expansion stresses will generally be well within their acceptance limits. The allowable loads for rotating equipment are based on nozzle diameter, and are either read from a table or calculated from simple equations that are contained in the appropriate industry standard.
The analysis should consider all pertinent branch runs that are connected to common manifolds. For a pump system, one set of operating conditions is all pumps operating. The effect of each pump being used as a spare or being blocked off for maintenance must also be checked. For pumps that are on standby, unless they have warmup lines, the entire dead leg from the manifold branch connection to the pump is assumed to be at ambient temperature.
Nozzle load limits must be satisfied for combined thermal, weight, and friction loads. Spring supports are often needed near the pump nozzles to effectively reduce the weight load on the pumps while free pipe thermal expansion is still allowed to take place.
Considerations for Stationary Equipment
Air-Cooled Heat Exchangers – The most common configuration for air-cooled heat exchanger piping uses short, stiff, straight sections of pipe to connect the pipe manifold to the exchanger nozzles. The manifold is located directly above or below the exchanger header box. The heat exchanger tube bundle is allowed to move laterally to accommodate the thermal expansion of the pipe manifold. The flexibility analysis should include the restraining effect of friction from movement of the exchanger bundle, which will resist lateral movement of the bundle.
The reaction loads at the exchanger nozzles that are obtained from the results of the flexibility analysis are compared to the allowable loads contained in a table in API 661 based on nozzle diameter.
Pressure Vessels and Shell-and-Tube Heat Exchangers – The need to evaluate the loads that are imposed on pressure vessel or shell-and-tube heat exchanger nozzles is not as obvious as for rotating equipment or air-cooled heat exchangers. The comparison basis to employ is not as simple either. These equipment items are not as "load sensitive" as the other two categories. There is also no readily available industry standard that simply relates nozzle size to allowable load.
Evaluation of nozzle loads that are imposed on these items requires calculating the local stresses in the nozzle and vessel or exchanger shell resulting from these loads, combining these with the stress due to design pressure, and limiting these combined stresses to allowable limits. Performing this analysis requires consideration of the nozzle design details at the vessel shell and the strength and stiffness of the vessel shell itself. Accepted procedures to perform this evaluation are contained in Welding Research Council Bulletins WRC-107 and WRC-297. Discussion of these procedures is beyond the scope of this course.
Fortunately, it is normally not necessary to perform such nozzle-load evaluations. In the majority of cases, as long as the vessel nozzle details have been adequately designed for pressure, and the piping system thermal expansion stresses are within allowable limits, the loads on the vessel nozzles will normally be acceptable and do not require separate verification. Unfortunately, there are no specific guidelines to indicate exactly when loads on these equipment items should be checked. Evaluating nozzle loads should be considered in situations where the pipe is of relatively large diameter [over about 600 mm (24 in.)], especially if it is connected to a large-diameter/relatively thin-walled vessel (such as an atmospheric or vacuum pipestill tower).
Tank Nozzles – The loads that are transmitted from piping to the shell nozzles of large-diameter storage tanks are a major concern of tank designers. The loads are a result of shell radial movement and nozzle rotation while filling and emptying a tank, thermal expansion of piping, differential settlement between the tank and the piping supports, and the weight of piping, valves, and other system components. As with pressure-vessel nozzles, determining the allowable loads on tank nozzles is not a simple "table lookup" procedure based only on nozzle diameter. It involves considering nozzle and tank geometry, nozzle location on the tank shell, multiple equations and graphs, and a graphical solution. Discussion of this procedure is beyond the scope of this course.
Extent of Analysis
The extent of a piping system flexibility and weight analysis depends on the situation.
Because the overall purpose of the analysis is to provide enough flexibility for the system, the engineer must analyze the right combination of operating conditions to determine where, and if, additional flexibility is needed to reduce pipe stresses or loads at end points. The engineer must also decide if it is desirable and acceptable to not include portions of a large, complex system in the analysis in order to simplify the modeling. For example, including a 100 mm (4 in.) diameter branch run in the model of an extensive, 600 mm (24 in.) diameter main system may not be necessary. Judicious installation of anchors or other restraints in a large system could also help simplify the modeling.
The following steps are involved to confirm the acceptability of the planned piping design:
• Define line size, wall thickness, material, number of temperature cycles, layout, maximum differential temperature, and any alternative operating scenarios.
• Determine conditions of end restraint and anchor movements, including deflections of vessel shells and equipment casings.
• Locate intermediate points of restraint and define any limitations that they impose on piping movement. This includes spring hangers and counterweights that are installed for sustained weight loads.
• Select a suitable analysis method and calculate the loads and stresses.
• Compare the results with the allowable stress range for thermal expansion stresses, the allowable stress at design temperature for weight-plus-pressure stresses, and the applicable load criteria for connected equipment.
Providing Additional Thermal Flexibility
The initial piping system layout may not be satisfactory for thermal flexibility stresses or loads on connected equipment. The following guidelines may help the situation.
• Provide more offsets or bends, or use more expansion loops within the same space.
These make the system more flexible and reduce the thermal stresses.
• Install expansion joints. However, SAES-L-011, Paragraph 2.8, makes this the exception by imposing the following restrictions:
- The use of corrugated metal pipe sections or creased bends to reduce the stiffness is prohibited for all pressure services. Swivel joints, expansion joints, flexible pipe or hose, or similar devices shall be used only when approved by the Chief Engineer through the waiver process.
- Expansion joints represent a "weak link" in a piping system. They may affect the life of the system since they are more susceptible to damage, and can create maintenance and operational problems. Thus, the use of expansion joints should only be considered as a last resort, and only through the waiver process.
• Make use of cold spring to prestress a piping system so that loads and moments are minimized when the piping is hot. However, cold spring does not affect piping flexibility stresses because the stress range from cold to hot must be considered. Cold spring should be avoided for piping that is connected to rotating equipment since it is difficult to control accurately.
• Strategically locate restraints such as guides, directional anchors, and limit stops, to minimize thermal and friction loads at equipment. Restraints could also be used to direct pipe thermal expansion into a section of the system that has more inherent flexibility to absorb it.
• Use spring supports if large vertical thermal movements are expected, or if thermal expansion causes pipe to lift off fixed supports. Avoid fixed supports that result in large thermal stresses.
• Use Teflon bearing pads at supports for large-diameter pipe or other large weight loads if friction loads are excessive. Friction loads can accumulate along the line and create unacceptable loads at equipment connections, or create the need for stronger structural members.
Sample Problem 3
Refer again to Figure 6 and the information that was provided in Sample Problems 1 and 2.
For the system illustrated, either P-101A or P-101B is required to be in operation for the process application. Assume that all the supports and restraints discussed in Sample Problems 1 and 2 will be installed. There are no similar systems to this in operation. It is now necessary to answer the following questions:
a. Is a formal piping flexibility analysis required for this system?
b. What design conditions and operating variations should be considered?
c. Specify whether equipment nozzle loads must be considered and if so, the basis to be used for this evaluation and the loads that must be considered.
d. Assuming the thermal loads imposed on the pump nozzles are found to be too high, what design concepts might be worth considering to try to reduce them to acceptable limits?
Solution
a. The simple criteria from ASME/ANSI B31.3 to exempt systems from formal analysis cannot be used since this system has more than two fixed points and is connected to load-sensitive equipment. Referring to the table in Work Aid 1, this system far exceeds the diameter guideline for when a formal analysis should be done for pump piping.
Therefore, formal flexibility analysis is required.
b. Note that the following additional information must be obtained before a formal analysis can be done.
• Some layout dimensions are missing.
• The type of elbows must be determined.
• The type of branch connection South of Location 4 must be specified.
• The thermal movements at the pump nozzles must be determined. These will depend on the location of the nozzle with respect to the fixed point on the pump casing.
• The thermal movements at the T-101 nozzle must be determined. This will depend on the elevation of the nozzle with respect to the tower support and the radius of the tower.
A thermal analysis, a weight analysis, and a combined thermal-plus-weight analysis would definitely be required to design the system. A wind analysis could be considered for completeness. However, windloads on the pipe are not likely to govern the design, especially with the guiding assumed in the vertical run at Location 6.
Three different scenarios must be analyzed to account for the pump operating cases: P-101A operating with P-101B spared; the opposite case; and both pumps operating, corresponding to the period of time when the pumps are being switched. The portion of pipe between the branch connection and the nonoperating pump is considered to be at ambient temperature for the first two cases, while the rest of the system is at design temperature.
c. Nozzle loads at P-101 A/B must be evaluated using API 610 based on the 300 mm (12 in.) nozzle diameter. To be precise, the nozzle loads at T-101 could be evaluated as well, but in this situation should not be necessary. The pipe diameter is not that large, and the pump nozzles are much more sensitive than the vessel nozzle. Thus, any piping system design that achieves acceptable pump-nozzle loads will result in very low piping stresses, and it is unlikely that the nozzle loads will be high. Should a vessel-nozzle load evaluation be considered necessary, additional information regarding vessel and nozzle details would be required.
d. If thermal loads on the pump are still too high, the two most likely possible actions to consider are to add more restraint(s), or provide additional flexibility by adding more offsets or an expansion loop. The first approach is the more attractive if it works since it would be much less expensive and not require additional plot space.
In this situation, it is necessary to identify the direction in which the high force or bending moment is acting. This is easily found from the results of the flexibility analysis. Then try to determine what might be causing it, and then make a design change to counteract it. For example, if East/West deflections at Locations 1 and 2 cause high bending moments at the nozzles, adding East/West stops at these locations might solve the problem.
USE OF ANCHORS FOR BURIED PIPING SYSTEMS AND DESIGN TOOLS