CompressorTechTwo - 2010 (Sample)

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Alternative Gas

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Harmonic Components(1st-20th)

Figure 3. Pressure pulsation values at the pulsation vessel flange vs.
harmonic component.

is achieved by unloading one side of the double-acting cylinder. For alternative gas operation (nitrogen in start-up), pulsation simulation shows relatively considerable pulsation in the 19th harmonic (103.55 Hz). This is because of resonance with the mechanical system. It requires a low-pressure orifice to mitigate risk. Based on simulation results, significant effects and problems are expected with higher molecular weight and pressures (in this case nitrogen).

Practical Recommendations

In the design stage for special-purpose machines, pulsation limits are recommended around 95% to 85% of API 618 (Approach 3) limits to have some margin (around 5 to 15%) to mitigate risk during construction and installation periods as well as unpredicted deviations and problems.

Overhead horizontal vessel design leads to shorter length of piping between vessel and cylinder, less risk for harmonic resonance with vessel-cylinder connecting pipe and less required footprint. However, this design requires much more support, may introduce more risk for vessel-support shaking forces and difficulties in maintenance. This design (overhead horizontal vessel design) is common and reflects traditional design techniques. However, it may seem difficult to implement for big packages because of difficulties in support and vibration of overhead extra large-sized horizontal pulsation vessels. Vertical vessel design has attracted considerable attention in recent years, especially for heavy-duty, big reciprocating compressors. However, optimum selection leans toward horizontal suction and discharge vessels for nearly all applications.

Arranging pulsation studies and pulsation vessel fabrication requires: 1. Final piping isometric drawings and facility data be available for the final pulsation study before start of the fabrication of pulsation vessels. 2. To minimize the risk of final data not being available, a proper “Preliminary Pulsation Study” should be performed immediately before the fabrication of vessels. The preliminary analysis is to be done considering a plant without reflective lines. The maximum allowable pressure pulsation levels must be 80% of the allowable values of defined pulsation limits (70% when two or more pulsation suppression devices are connected to a common piping system).

If the pulsation value margin to mitigate risks, however, is not enough or in case of resonance the only adjustment methods will be the installation of orifices and piping and support modifications, the preliminary analysis must consider

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