Once the compressor manifold system natural frequencies are understood, practical means of controlling these modes must be applied. Keep in mind that the two main sources of excitation are mechanical shaking forces and pulsation induced shaking forces. The mechanical shaking forces are most significant at the lower harmonics of compressor speed. The pulsation induced shaking force frequencies are determined by the acoustic design. Ideally, the mechanical analysis of the manifold system is done early in the overall system design. By knowing the mechanical natural frequencies, decisions on bottle sizes and nozzle lengths and wall thickness' can be influenced. Suction nozzles must not be so long that a mechanical resonance occurs.
One of the most effective means of controlling system mechanical natural frequencies is the use of proper discharge bottle supports. A high percentage of all compressor installations designed according to API 618 specifications utilize some sort of discharge bottle support. A typical strap and wedge-type support can significantly affect the low mode, the discharge bottle cantilever mode, the rotary mode and the angular mode.
Dynamic cylinder supports can be utilized to raise the low mode above the range of expected excitation frequencies. A plate-type support attached to the cylinder head bolts is often used. Some compressor manufacturers provide attachment holes for this type of support so that the pressure containing bolted joint is not used.
