This article was originally published in the Aug./Sept. issue of COMPRESSORtech2. We only publish a fraction of our magazine content online, so for more great content, get every issue in your inbox/mailbox and access to our digital archives with a free subscription.
By Francisco Fierro, Angel Rivera and Benjamin White
Centrifugal compressor packages have become more popular for natural gas transmission applications in recent years as they are well suited to the necessary flow rate and pressure ratio requirements. Additionally, unlike reciprocating compressors, centrifugal compressors do not have the need for expensive pulsation bottles.
However, because the piping is connected directly to the compressor, a thermal analysis is vital to ensure compressor nozzle loads remain below the manufacturer’s specified limits. Exceeding these limits without approval from the manufacturer can lead to misalignment between the driver and compressor and can lead to coupling or bearing failures.
A proper thermal analysis should review and ensure a number of parameters are acceptable for operation. These include stress in the piping, compressor nozzle loads, cooler nozzle loads and pipe support reaction loads. Additionally, all mechanical natural frequencies should be identified, and design should reduce the likelihood of vibration problems.
Southwest Research Institute (SwRI) has performed many of these analyses and has identified common mistakes in centrifugal compressor piping layouts. These mistakes and resolutions will be presented along with general recommendations and good design practices. Following the provided information should result in a more robust design and will allow the piping designer to begin with a better initial design and account for possible problems in the design stage where changes and modifications are easily implemented.
Centrifugal compressors have gained more popularity for natural gas transmission applications. This can be attributed to a variety of factors, such as lower emissions, cost considerations, no significant compressor-generated pulsation and a flexible range of operating conditions.
However, centrifugal compressors are much more sensitive to the thermal growth of the attached piping. The added loads on the compressor nozzles may affect the alignment of the compressor enough that API 617 has an allowable load limit. Along with nozzle loads, equipment loads are also affected by thermal growth, which is the case with cooler nozzles, for example. Additionally, if not properly designed, the piping layout of the station may have high-stress sections due to thermal growth.
Thermal analysis/design considerations
Larger diameter piping stiffness
Recent industry trends have led to the use of larger compressors and larger-diameter piping. This allows for higher flow rates and a lower number of units. Larger-diameter piping generally provides more stiffness than smaller piping. This is also apparent in elbows, which are often used to provide a flexible point and allow the pipe to grow thermally.
A model was developed which was composed of a 6 ft. (1.8 m) vertical run followed by a 6 ft. (1.8 m) horizontal run. The start point and end points remained the same, but the elbow and line size between the two end points were varied between 4 and 48 in. (100 and 1220 mm). This model would be representative of taking a cooler riser design used on a smaller line and using the same design on a much larger line size.
Figure 1 presents the stiffness of a typical piping run for different pipe size. The stiffness of the curve reveals that the piping stiffness increases exponentially. Because of this trend, a similar piping layout can result in much higher reaction loads and thermal stress. Elbows are often used to relieve thermal stress, and it is critical to review the change in thermal stress when increasing the pipe diameter.
Oftentimes, the type of elbow used is determined by the station layout and the desired accessibility of certain areas to personnel. While 90° elbows are far more common than 45° bends, changing from one type of elbow to another can have significant thermal effects. Along with providing a lower pressure loss, 45° bends tend to provide a higher resistance to bending. Thermally speaking, additional stiffness in the piping run will usually lead to higher reaction loads at clamps and nozzles, as well as higher stress levels in the pipe. Figure 2 shows a comparison of a piping run with a 45° bend and a 90° elbow.
It is clear the 45° elbow nearly doubles the axial stiffness, which would lead to nearly doubling the loads and stresses in the area. Inversely, if a 45° elbow is installed and nozzle loads are excessive, replacing the elbow with a 90° bend will reduce the loads by the same stiffness ratio.
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