In today's market of rising costs, many industries are finding ways to reduce downtime and increase production.  Mechanical Solutions, Inc. has developed an experimental modal analysis technique called Time-Averaged Pulse (TAP™) which effectively determines critical speeds of all rotating machinery under operating conditions. Since TAP™ is performed while the equipment is operating, downtime of the pump and nearby equipment is virtually eliminated.  This technique is able to identify any problem or potential problem and is especially useful in multistage high-energy variable speed pumps whose rotordynamic characteristics are speed and load dependent.

The most common method of determining a critical speed is "cascade" plotting, which consists of a succession of spectra as the machine runs up to speed or coasts down between the operating and static conditions.  However, incorrect conclusions can be drawn from this method.   TAP™ as designed knowing the dynamic coefficients of bearings and annular seals (Lomakin effect) can be substantially different between the transients of start-up and shut-down and the steady state operating condition.  Additionally, given the excitation source and level is unknown, the frequency response cannot be quantified to determine whether a vibration increase is critical or not, thus making TAP™ superior.

TAP™ incorporates the time averaging technique to rapidly improve the signal to noise ratio of the "bump" test response obtained under operating conditions. The purpose of time averaging lies in the reduction of the amplitude ratio of the random naturally excited component versus the impact-synchronous excited component in the signal processing.

A test on a fourteen stage horizontal centrifugal pump was conducted. The pump was rated at 970hp driven by an induction motor at 3600RPM.  Impulse input was made at the shaft coupling area to excite the shaft natural frequencies. At the beginning of the TAP™ test, the residual excitation showing up as multiples of running speed is very distinct in the spectrum. The shaft natural frequencies, still clouded by the response of the residuals, are emerging. At the end of the TAP™ test, the signal to noise ratio is greatly improved and the shaft natural frequencies can clearly be identified.

Numerous successful field troubleshooting projects have been performed by MSI using the TAP™ technique.