Understand Design-Induced Pump Vibration

The system design is one cause of pump-vibration problems.

The system design is one cause of pump-vibration problems.

By Ron Eshleman, Vibration Institute

It’s no secret that many process pumps absorb massive amounts of vibration. Sooner or later, bearings, packing, or adjacent piping will fail and/or impellers will be damaged.

The five main causes of severe pump vibration include:

• mass imbalance
• resonance
• piping design and installation
• system design
• pump design.

It should be noted that all but mass imbalance involve either facility or pump design issues. Typically, either the external-piping arrangement is inappropriate or the size of the pump doesn’t reflect the needs of the application. The following tips focus on these two issues with regard to centrifugal pumps.

Piping design

There are situations where a pump must fit into an awkward space, leading to an inlet condition that creates chaotic flow at the suction. Disturbances occurring at the inlet can change the direction and velocity of the flow due to piping enlargements, bends, branches, and tees. This type of situation gets the flow started through the pump at an angle that doesn’t interface well with the impeller. Such conditions produce separation-trailing vortices and turbulence, which extract energy from the flow that manifests as noise and vibration. This condition requires re-designing the inlet piping or implementing some form of flow straightening. To ensure laminar inlet flow, it’s recommended that 10 pipe diameters of straight pipe precede the pump inlet.

Pump-system design

Centrifugal pumps are designed to operate efficiently in a rather narrow flow range—recommended by the Hydraulic Institute (HI), Parsippany, NJ, as 70% to 120% of the equipment’s best efficiency point (BEP), as defined by its pump curve. Operating outside these parameters, a pump will not only be inefficient, it will excite damaging vibration. Thus, the system designer must be cognizant of the pumping requirements and make provisions if variable-flow conditions are expected. (Note: Designers must also remember that if multiple pumps will be operating in parallel, the curve and BEP will change.)

If low flow occurs due to high discharge pressure, the pumped fluid will recirculate at the suction and cause excessive vibrations. It’s not uncommon for plants to have different flow requirements during different times of the day, or to purchase oversized pumps in anticipation of future needs. Both situations fuel vibration problems.

Recirculation causes a large amount of random vibration that, in turn, may cause failures in the bearings and packing, even the impeller. Similarly, if a pump operates in the high-flow area of its curve due to low suction pressure, cavitation will occur. As cavitation bubbles pass through the pump, they explode, resulting in noise (sometimes above hearing pitch) and vibration that lead to inefficient pumping and internal pump damage.

The most practical solution for suction recirculation is to relieve the discharge pressure by rerouting some of the flow through a controlled recirculation line to the inlet. Although the pump will operate quietly, it will be doing more work than necessary—lowering its efficiency. This, however, is a reasonable approach in variable-flow applications.

An alternative, given the fact that cavitation indicates excessive flow in a pump, involves lowering the flow rate. A good way to do this is to increase the suction pressure well above the vapor pressure so that the net positive suction head (NPSHA) has a margin of 30% above the suction pressure required to avoid cavitation.

Keep in mind

Dealing with these issues may require serious redesign of systems or modification of operational modes. Remember, though, that such actions can help prevent serious damage to equipment and processes caused by vibration. MT

Ron Eshleman is technical director of the Vibration Institute, Oak Brook, IL. For more information, contact Dr. Eshleman at reshleman@vi-institute.org, or visit vi-institute.org.