ArticleVibration Reduction With A Dynamic Absorber
by Gary Einarson
This brief paper describes the application of a dynamic absorber to a pump with chronic vibration problems. Piping and base problems conspired to place this pump in a state of resonance at its operating speed. The resonant condition predictably resulted in mechanical seal failures, coupling failures and worn pump bearings. Installing the dynamic absorber reduced pump vibration by more than ten times!
“This problem has to do with two Worthington ERPN 80 centrifugal pumps driven by 125 HP motors at 3,562 RPM. The pumps are used to transfer liquid ammonia during production; at least one pump operates 24 hours a day. Each pump contains two mechanical seals that cost $3,200 per set. The pumps have common suction and discharge headers and are spaced about four feet apart. They are located outside and thus are subjected to ambient temperature variations from -40°C to +40°C.
The pumps had been modified in several ways. The suction piping supports were altered because shifting was causing misalignment so severe that the coupling shim packs broke apart. Mechanical seal failures had to be repaired. Bearings wore prematurely due to lubrication problems; the interior of the fabricated component of the pump base had to be grouted. Modifications have been made to the discharge piping and hanger supports on the minimum flow line.
Pump A always ran more smoothly than pump B; vibration amplitudes were close to 0.2 IPS on A and 0.3 IPS on B. The check valve and piping on the B pump were worked on during a major shutdown. Upon start-up the vibration level increased to almost 0.6 IPS, sufficiently high to cause early mechanical seal failure. The spectrum was dominated by vibration at operating speed.
One possible cause of the increase in vibration level was that something had fallen into the impeller while the discharge piping was open during the shutdown. However, no damage was found on the impeller or inducer when the pump was removed from service and inspected. Runout, impeller unbalance, and signs of rubs were checked. The pump was carefully re-installed with attention to coupling, alignment, and soft foot. Vibration levels remained high, sometimes to 0.8 IPS. The pump was reserved as a spare for short-term use whenever maintenance was to be performed on the A pump until a solution could be found.
When the pump was again operating, it was discovered that loosening poorly installed pipe hangers on the minimum flow line decreased the vibration level between 0.3 IPS and 0.4 IPS. The decrease was tolerable for short-term operation but was higher than it should have been.
The vibration levels along the piping were measured to determine the locations of nodes and antinodes. Rigid hangers were installed at appropriate locations to achieve an acceptable decrease in pump vibration. However, the vibration levels again increased to almost 1.0 IPS overall.”
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