Case Study - Excessive Noise and Vibration on a Liquid Pumping Skid
The client had operated a unique design of liquid pumping skids featuring the same hardware (pump/motor/filters) for up to 20 years. During a routine ‘works tests’ of one of these skids, a complaint was received relating to excessive noise and vibration. Detailed inspection revealed significant damage to the pump/motor flexible coupling.
• A program of work was implemented to,
o Make a detailed review of the original design
o Investigate the source of the excessive noise emanating from the equipment
o Propose modifications to upgrade the existing design to permit the units to be released
o Procure hardware packages to allow design modifications to be implemented in the field
o Design an alternative skid featuring fundamentally NEW hardware to avoid the issues.
• A sequence of tests was carried out to determine the nature of the problem. These included,
o Pressure Measurements with traces showing very high amplitude oscillations on the suction and delivery sides
o Linear Vibration/Structure Borne Noise showing excessive pipe and skid base vibration
o All measurements featured a Fundamental Frequency at approximately 200 Hz, a standard pump tooth passing frequency with a 50 Hz motor
o Acoustic Measurements showed airborne noise at 1 m was in excess of 102 dB(a).
• A sequence of skid revisions were prepared to,
o Flexibly mount the pump/motor/bell housing from the support structure
o Introduce pulsation dampers at the fluid inlet and outlet positions
o Flexibly connect the inlet and outlet filters using stainless steel hose materials.
• Effects of initial modifications to the skid and extensive tests
o Measurements of pressures, vibration and noise viewed in the spectral domain still showed the same traits as those made before the modifications - although the vibration amplitudes were substantially lower. Airborne noise at 1 m was down to 90 dB(a).
• Root Cause
o A series of tests carried out with the pump manufacturer determined that the pump itself was creating pressure pulsations due to leakage at the “port opening” condition both into the inlet and outlet pipes. The pump was of an old “gear type pump” design with a small number of teeth and large pulsation amplitude superimposed over the delivery pressure.
• Final Design
o An alternative pump was selected (crescent internal gear) that featured a larger number of teeth, hunting tooth design, and carefully designed porting to minimise pressure pulsations both on the inlet and outlet sides
o The skid design was also modified to capture "lessons learned" from field experience to,
? Feature as few spools as is necessary
? Achieve a smaller skid footprint
? Make all service connections from one end of skid
? Provide Instruments mounted on a terminal box
? Provide a stiffer structure under pumps and filters
? Provide greater dirt holding capacity in the filters (same elements in all positions).