Medium Flow
Temperature changes due to a loss of cooling medium flow should be considered. This includes pipe that is normally at ambient temperature but can be blocked in, while subject to solar radiation.
Steamout for Air or Gas Freeing
Most on-site equipment and lines and many off-site lines are freed of gas or air by the use of steam. For 862 kPa (125 psi.) steam, 149°C (300°F) is used for the metal temperature. Piping connected to equipment which will be steamed out, especially piping connected to upper parts of towers, should be checked for the tower at 149°C (300°F) and the piping at ambient plus 28°C (50°F). This situation may govern the flexibility of lines connected to towers that operate at less than 149°C (300°F) or have a smaller temperature variation from top to bottom.
No Process Flow While
Heating Continues If process flow can be stopped while heat is still being applied, the piping flexibility should be checked for the maximum metal temperature. Such situations can occur with steam tracing and steam jacketing.
Metal temperatures that govern the flexibility design of a piping system are not necessarily the ones associated with the most severe coincident pressure and temperature which govern the wall thickness of the pipe. Piping flexibility depends only on the temperature. Therefore, a condition of high temperature and low pressure may govern the piping flexibility design while the wall thickness is based on a higher pressure but a lower temperature. However, note that the design pressure is considered with the pipe weight when calculating the total longitudinal stress in the pipe during a weight analysis.
For restrained pipelines, SAES-L-011, Paragraph 2.4, specifies the following:
• A tie-in temperature range shall be established for the design and construction of all buried and aboveground fully-restrained pipelines. The design shall be based on the expected temperature rise during operation as well as the maximum anticipated decrease in temperature after tie-in.
• This requirement results in the use of the maximum expected temperature range in the design of a fully-restrained pipeline. This then yields the maximum possible thermal
Number of Cycles to be Considered
The number of times that a line experiences the combination of temperature and end movement influences piping flexibility design because the flexibility stress basis is based on fatigue failure. ASME/ANSI B31.3 includes a factor "f" in the equation for the allowable stress range to account for the number of cycles as shown below. A plant life of 20 years should be used to estimate the number of cycles. One cycle a day for 20 years is about 7,000 cycles. If the number of cycles exceeds 7,000, the number of cycles should be indicated in the design specification for the affected lines.
SA = f (1.25 Sc + 0.25 Sh) where: SA = Allowable displacement stress range, psi.
Sc = Basic allowable stress at minimum metal temperature expected during the displacement cycle under analysis, psi.
Sh = Basic allowable stress at maximum metal temperature expected during the displacement cycle under analysis, psi.
NUMBER OF CYCLES f
7,000 or less 1.0
Over 7,000 to 14,000 0.9
Over 14,000 to 22,000 0.8
Over 22,000 to 45,000 0.7
Over 45,000 to 100,000 0.6
Over 100,000 to 200,000 0.5
Over 200,000 to 700,000 0.4
Over 700,000 to 2,000,000 0.3
Note that the allowable stress range for thermal flexibility stresses does not use the longitudinal weld-joint efficiency factor for any type of pipe. Therefore, the cold and hot stresses in the equation will be the same for seamless and welded pipes.
Load Limitations On Equipment
A poorly designed piping system that is connected to rotating equipment can cause damage to the equipment. For example, an excessive load on a pump can cause high vibration or bearing and seal wear problems that will lead to excessive maintenance requirements. In an extreme case, the single application of an excessive load can result in immediate damage and require a shutdown.
Rotating equipment, i.e., pumps, turbines, and compressors, are the most sensitive type with respect to imposed piping loads due to the moving parts and small clearances involved in their design. However, pipe loads that are imposed on stationary equipment items must not be allowed to become excessive either. This will be discussed further below.
Loads that are imposed by the piping system on connected equipment are determined from the results of the piping flexibility analysis. These loads are then compared to allowable values based on industry standards for particular types of equipment to determine if they are acceptable. For some equipment items, the allowable loads may just be read directly from tables that are contained in the applicable industry standard. In other cases, the allowable loads must be calculated based on criteria contained in an industry standard. In still other cases, the stresses that result from the imposed loads must be calculated, and the stresses then compared to allowable values. Equipment vendors will sometimes also have allowable load criteria that must be considered.
SAES-L-014, Design of Pump and Compressor Station Piping, also requires that the additional loads caused by slight misalignment between pipe and equipment flanges be considered. Flanges must be aligned to within relatively small tolerances to ensure that pipe installation and flange boltup do not impose excessive loads on equipment nozzles.
Discussion of the actual allowable equipment loads is beyond the scope of this course.
However, the following table summarizes the industry standards that apply to equipment nozzle load evaluations, and the parameters that are used to determine the allowable loads.
EQUIPMENT ITEM INDUSTRY CODE PARAMETERS USED