Thermal Design Considerations For Centrifugal Compressor Piping Systems

Cooler riser design considerations

A key design factor when developing a piping layout is maintaining acceptable cooler nozzle loads. The cooler manufacturer will, in most cases, provide maximum allowable loads for the attached piping. If overly restrained, the thermal growth of the pipe will impart large loads on the cooler nozzles. This section of piping usually requires much higher flexibility than other sections of piping. Excessive flexibility can also increase the nozzle loads, as the weight of the pipe would then be supported by the cooler. As discussed earlier, larger-diameter piping provides significantly higher stiffness and a similar layout, which is acceptable for smaller piping and may result in excessive loads with a larger diameter line.

Figure 6 presents a common cooler riser design on a natural gas compressor station in the design phase. A 20 in. (508 mm) line splits at a tee, turns vertical and reduces down to 14 in. (356 mm) at the nozzle connection. This configuration resulted in a maximum load in the vertical direction of 102% of the allowable and 127% in the cooler lengthwise direction.

The vertical load is due to the thermal growth of the vertical run into the cooler. A pipe support near the tee forces the piping to grow upward towards the cooler nozzles. Additionally, the horizontal run between the buried header and tee cannot grow away from the cooler as the piping is buried just upstream of the elbow, and therefore grows towards the cooler.

Figure 5. Recently installed centrifugal compressor package.

The thermal growth is dependent on the operating conditions and the length of each run. While the operating conditions cannot typically be changed, the length of the piping runs can be modified by rerouting the piping. This would reduce the amount of thermal growth, but is not always preferred. The final option is to adjust the stiffness in the area by adjusting pipe clamp locations, line size, elbow types, etc.

Figure 6. Originally proposed cooler riser design and nozzle loads.

 

The final recommended system in this case involved shifting the pipe support away from the cooler to allow the vertical pipe run to grow downward and lowering the reducer from the top of the vertical run to the bottom. Shifting the reducer increases the bending flexibility of the vertical run and lowers the horizontal load on the nozzle. As shown in Figure 7, the vertical and horizontal loads were reduced to 80 and 73% of the manufacturer allowable loads.

Figure 7. Modified cooler riser design and nozzle loads.
Modeling techniques

Clamp modeling

After all the piping design factors have been considered and all criteria have been met, there is still the possibility the final design will fail and the thermal analysis results are invalid. This can be due to the modeling techniques used in the analysis. For mechanical piping reviews and general piping design, a pipe clamp is generally considered a fixed point.

While a full strap-type clamp as shown in Figure 8 does provide a stiff support, it cannot be considered rigid. Every clamp and clamp type have a translational and rotational stiffness associated with the clamp. U-bolt type pipe supports are much more flexible than a strap type clamp and provide low rotation stiffness. If the appropriate clamp stiffness is not considered in the model, the thermal growth, pipe stress and resultant nozzle loads are not accurate.

Additionally, a reaction load is calculated at each clamp location. These loads are a result of the thermal growth and the clamp stiffness. High clamp stiffness will result in unrealistic high reaction loads. High stiffness will also act a fixed point and reduce the amount of thermal growth at the compressor or cooler nozzle. For accurate modeling predictions, it is essential to accurately estimate stiffness in all directions.

Figure 8. Recommended strap-type clamp design.

Nozzle modeling

Similarly, the stiffness used in the connection between the piping and the compressor or cooler nozzle will have a significant impact on the resultant nozzle loads. A fraction of an inch in deflection of the piping can produce thousands of pounds of force on the nozzle if the connection stiffness is unreasonably high. The appropriate stiffness, which accounts for the equipment geometry, wall thickness and support, must be used. If the stiffness is not accurate, the resultant loads and the equipment nozzle checks are not valid. The equipment stiffness values can be determined through experimental testing or through finite element modeling of the equipment.

Temperature cases

When performing a thermal analysis, it is important to analyze all the expected operating conditions. According to 832.3f of ASME B31.8 Gas Transmission and Piping Systems [1], “The total range in temperature shall be considered in all expansion calculations.” This includes the hottest design case as well as the lowest cold case. Oftentimes, this can be during a shutdown of the compressor in the middle of winter. The attached piping and equipment will contract, imparting loads on all the clamps and equipment nozzles. It is important to ensure this contraction will not result in excessive loads or stress in the pipe.

Conclusions

Thermal analyses are critical to ensure the proposed piping layout is acceptable. A previous acceptable station design may yield different results when applied to a different location with new operating conditions, line sizes, elbows, etc. These changes can have significant effects on the predicted resultant stress and loads. Applying poor modeling techniques or improperly defining the temperature ranges of the thermal analysis model may lead to erroneously approving an inadequate design if maintenance is not taken to model the configuration accurately and/or conservatively. This can result in the costly redesigns, having to construct new piping or re-route existing piping, or failing piping/equipment, which can be very expensive.

Recommendations

Thermal stress analyses should be performed by experienced, competent personnel. Personnel should know the current piping codes as well as the modeling techniques detailed in this paper. Soil conditions should be clearly understood and applied to the models.

Thermal stress analyses should be performed as soon as the piping design/routing is complete. The analysis should evaluate the piping configuration as presented and clearly identify required changes.

A review meeting (to discuss the results of the analysis as well as the required piping design changes) is recommended. This meeting is important to allow the operating company to comment on the feasibility of implementing certain designs. Alternative solutions may be identified in this meeting that would require additional analyses to be conducted.

The piping system should be carefully monitored during commissioning for any high vibration levels. Particular attention should be paid to small-bore piping or any relief valve piping sections. These locations tend to be more flexible and thus more susceptible to vibration.

References

  1. Gas Transmission and Distribution Piping Systems, American Society of Mechanical Engineers, February 2004.

About the authors: 

Francisco Fierro is a research engineer at Southwest Research Institute. Contact him at: francisco.fierro@swri.org. Angel Rivera is an engineering design manager for natural gas pipelines at Kinder Morgan. Contact him at: angel_rivera@kindermorgan.com. Benjamin White, PE, is a research and development manager at Southwest Research Institute. Contact him at: benjamin.white@swri.org.

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