It happens time and time again, avoidable vibration problems torpedoing newly constructed or modified pump systems. It’s much more expensive to address them after construction then during the design stage. Often a dynamics design assessment is performed at the 60% plant design stage even before the pumps are fully specified or purchased. However, the scope of such a review may vary depending on the complexity of the system and the experience and objectives of the players involved. Owners, AE firms, OEMs, contractors, and subcontractors need to cooperate with each other from the start to ensure that the key upfront analyses are performed to avoid installation problems, cost overruns, and finger pointing after the fact.
It's a Pump System, Not Just a Pump
Actual pump structural vibrational behavior depends on the entire as-assembled system. The pump will vibrate differently depending on the particular motor or gearbox it’s attached to (especially for vertical pumps), the piping configuration, the baseplate/pedestal, and support structure. All these items must be properly accounted for to ensure that the pump, driver, and/or connected piping won’t go into a damaging resonant condition within its operating speed range. ANSI/Hydraulic Institute Guideline 9.6.8, Rotodynamic Pumps - Guideline for Dynamics of Pumping Machinery, is applicable after the pumps are purchased (but well before installation) and recommends that the system be considered. It also provides a risk-based process to help determine if a pump system analysis is needed. Making invalid assumptions, or not even considering the nearby floor or piping, can make the structural analysis less than worthless, as it provides those involved with a false sense of risk reduction, and in many cases, MSI troubleshooting revenue.
As many know, performance testing at the pump OEM shop will not properly characterize the as-installed vibration, since so many required variables are not the same. To reduce the risk of installed pump vibration, someone needs to model the entire system with all these components accurately accounted for in the analysis. MSI, acting as a subcontractor to the Owner, AE firm, or OEM, has performed many of these full system analyses, and we have often recommended specific design changes to successfully avoid predictable vibration problems at commissioning or over the operating life of the pump.
Some OEMs that MSI have worked with fully support a pump system analysis, as it has demonstrated a reduction in their warranty costs and protected their reputation, since the OEM will initially be blamed for just about any vibration issue, even if the pump is not the root cause. Other OEMs incorrectly consider the requirement some sort of critique of their pump and resist the effort. Some OEMs don't want to consider system components that are outside their scope of supply in an analysis. In the end, if the analysis is done effectively, the contractor avoids schedule and budget killing commissioning issues and the owner has lower maintenance costs over the life of the pump system. Everybody wins!
How to Make Sure It's Done Right
In many cases, although never identified or talked about, the real vibration problem root cause is an incorrectly worded specification for a high-value pump system. However, even AE firms with proper specifications can get burned because a local AE engineer or somebody else allows an analysis to simply “check the box”.
Finite element modeling techniques are employed for the vibration analysis. For the predictions to be accurate, the models have to properly represent the pump, motor, gearbox, piping, and foundation. This information comes from various sources, and sometimes collection of such data is the hardest part of the effort. Typically, the motor can be modeled in an idealized manner, such as a mass spring system that matches the mass, CG, and reed frequency, as long as the interface dimensions with the discharge head are spot on. The nearby foundation, baseplate or sole plate, and pump, on the other hand, need to be modeled in detail from manufacturing drawings or CAD solid models.
Therefore, the specification must make it clear that the OEM providing the technical data to the independent pump system analysis company is a condition of sale. MSI, and certainly many other consultants in the industry, have secured such information from many pump companies after NDAs are put in place. Usually it is not a problem, but every once in a while there can be issues with a pump OEM not understanding the agreed-upon pump system analysis requirements.
Owners and AE firms need to make sure that this important assessment work is carried out (and implemented!), and not short-circuited due to any issues with sharing of technical data. It could very well turn up deficiencies in the structural design of the pumping station floor or pedestal support, pipe support modifications, or pump discharge head stiffness, for example, with recommendations for correction. The animations below show discharge heads that were ribbed for stiffening to avoid structural resonance, and an example of how the floor cannot always be considered rigid.
All the players have to act as a team and share the necessary data to greatly reduce the risk of costly pump system problems. If you'd like to learn more about our Design Assessment capabilities, click the link below.
Pump system design assessment should not include an infinitely stiff floor
Stiffening the pump discharge head can shift a potential natural frequency resonance out of harm's way before it happens
