Case Study: Design and Performance Verification Testing of a Screw Compressor Discharge Silencer
The paper presents the design and performance verification testing of a discharge silencer for a screw compressor unit installed in a natural gas transmission station in Kuwait. The four-lobe oil free screw compressor, operating at 4,380 rpm (73 Hz) and with a design discharge pressure of 66.5 psig at a flow rate of 7.5 MMSCFD, initially had a horizontal discharge silencer arrangement. However, in spite of the existing silencer, the discharge flow had high amplitude pulsations resulting in high amplitude vibration in the compressor casing and downstream piping. A new Helmholtz resonator type acoustic silencer was designed to reduce the downstream pulsation amplitudes to ~5% (pk-pk) of the static discharge pressure and to minimize the overall piping response. The new silencer is designed such that its acoustic responses avoid the screw compressor lobe passing frequency (4X) and discharges flow with significantly reduced pulsation amplitudes. Following the manufacturing and installation of the silencer unit, field testing was conducted to confirm the agreement of the system performance to the design predictions. The silencer upstream and downstream dynamic pressures and discharge piping vibration amplitudes were recorded for various operating conditions and for two different silencer discharge orifice configurations. In addition, acoustic measurements were also performed around the compressor units, which revealed a noticeable reduction in sound levels compared to the operation with the previous silencer design. In summary, the new silencer design enables the operation of the screw compressor with significantly reduced discharge flow pulsation, and with piping vibrations that are within allowable limits.