You Know What They Say About "Assume"...
Pump station retrofits are done to upgrade existing facilities either with new pumps for more appropriate flow/head for the station, or for better control/efficiency with Variable Frequency Drives. Many have seen these projects come in way over-budget or go far past the project schedule due to excessive vibration problems associated with resonance. In order to avoid problems, the focus for owners and engineering firms is typically on the new equipment requirements so that the project goals can be met and there are no problems in commissioning. However, time and again MSI gets called to troubleshoot vibration problems with brand new equipment even after it was specified, executed, factory tested, and installed perfectly per current industry standards. So what's going on?
You can find any number of case studies on MSI's website showing how important it is to think about more than just the pump in any installation. All of the pump system components, including the foundation and piping, are integral to the success of the operation and must be considered for realistic and accurate modeling so that the equipment isn't installed with preventable vibration problems. What we have seen is that Owners and Engineering Firms understand this and are taking great care to do the proper Pre-installation Pump System Dynamics Analyses for the new equipment before manufacturing even starts; however, we have seen surprises coming from how much the aged flooring, foundation, and piping can affect the new equipment.
When starting fresh with a new pump station, it’s quite easy to model the foundation accurately with some small assumptions about the quality of workmanship. In a retrofit project, it’s more like remodeling an old home - the project can quickly get in trouble if too many assumptions are made about the conditions of the existing structure. For example, a new bathroom on a rotting subfloor is a recipe for disaster.
In a retrofit Pump Station, assumptions may quickly pile up that leave a lot of potential for computer modeling to be inaccurate. In many cases, an Owner or Engineer may be confident that their Pump Station flooring is robust enough and in good enough condition that its effect on the new equipment can be confidently assumed and modeled accurately to real world conditions. However, many of these stations are well worn and not built to today's standards, which may cause the new equipment to see different, unexpected conditions (i.e. more flexible) than those modeled in the design assessment's structural analysis.
How Can MSI Help?
MSI's unique combination of field testing and analytical capabilities is particularly helpful in the following situations before you commit to your new pump system design:
- The Pump Station floor plans show flexible foundations, especially in multi-level buildings
- There is equipment on the same floor as the new equipment that has long standing, unresolved reliability issues
- Among the original pumps being replaced, there is a large difference in vibration characteristics between identical pumps
- All of the original pumps experience excessive vibration
Before the proper vibration dynamic analyses are performed, MSI completes thorough field vibration testing on the original equipment as well as the surrounding foundation and piping. Then the data from these tests is integrated into a structural analysis which focuses on evaluating the effect that the foundation and piping have on the existing equipment.
Representative models of the new equipment can then be incorporated into the structural analysis, and the Owner or Engineering Firm can confirm that the existing foundation, piping, etc. will not contribute to any pump system resonance issues. If the foundation and piping are expected to contribute significantly to the pump system natural frequencies, their effects can be precisely defined so that the manufacturer may take them into account in their analyses. The process is simply writing the proper language into the bid specification so that the pump manufacturers are responsible for properly taking the pump system, including the foundation and piping, into account.
This method takes a little more work up front, but makes for a much easier project flow that avoids adversarial back and forth during critical points in the project by allowing the Owner/Engineering firm to "hand-off" responsibility to the pump manufacturer at an appropriate time. At the end of the project, everyone involved can be happy that the new equipment is installed without busting the budget or the schedule due to preventable problems.
If you would like to learn more about when and how to perform a design assessment to anticipate and prevent vibration problems on your retrofit pump design, we recommend our whitepaper on Design Assessment to Prevent Pump Resonance, including details on the Hydraulic Institute's "Rotordynamic Pumps - Guideline for Dynamics of Pumping Machinery, Standard 9.6.8".
