Flow induced vibration in a boiler occurs when the vortex shed by the passing flow coincides with the natural frequency of the component or the gas column. Flow induced vibration can be experienced in the tube bundles of heat exchangers, in the heat transfer surfaces in boiler, in piping conveying fluids, in rotating machines like fans, etc. However, this type of vibration is seen more commonly in the second pass of the boiler and also in the economizer and tubular air heater heat transfer surface. There are many ways to check this during the design stage, but all methods converge on checking the frequency of the vortex being shed with the gas column frequency. If these two fall in the range of 80 to 120%, then vibration and resonance can start. As it approaches resonance, this type of vibration has a loud sound resembling a lion’s roar, which is both very typical and unique for this.
The vortex shedding frequency mainly depends on the tube bundle arrangement adopted. Both the transverse and longitudinal pitching play an important role in deciding the vortex shedding frequency. The arrangement of the heat transfer bundle is correlated to a number called the Strouhal Number. This number can be obtained by knowing the transverse and longitudinal pitching of the heat transfer bundle and the tube diameter. The figure below gives an idea about this number, and many text books and boiler manufactures standards also provide this chart.
After selecting the Strouhal number to calculate the vortex shedding, it is necessary to know the gas velocity within the bank (not in the voids between banks) in inches per second. It is always required to use the maximum velocity within the bank. Knowing the outer diameter of the tube in inches, the vortex shedding frequency can be calculated by using the relationship: the vortex shedding frequency is equal to the product of the Strouhal number and gas velocity divided by the tube outer diameter.
There are many standard graphs available to calculate the velocity of sound in air at the temp at which the medium is flowing over the tube bundle. The maximum temperature of the medium is used to calculate this velocity. The sonic velocity can also be approximately computed from where T is the maximum temperature of the medium in degree R.
Knowing the sonic velocity and the vortex shedding frequency will enable the calculation of the standing wave length by using the formula: wave length in ft is equal to sonic velocity in feet per second divided by the Strouhal frequency in Hz.
Knowing the wave length, it is the practice to see that no cavity width is available 22 feet for λ/2 greater or equal to 22 feet and 11 feet for λ/2 less than 22 feet. It is to be ensured that no adjacent cavity is of the same width and will not be less than 2 feet.
The anti-vibration baffles are fixed based on these computations to avoid flow induced vibration. The mechanical fixing of these baffles can be done in many ways depending upon the designer’s requirement. In coal fired boilers with higher ash content, it may be even worthwhile to wait for some time to see if these vibrations die down after a few months of operation and starts and stops.