What is an acceptable vibration level for the mixer used as an example in this blog?
An overall value as properly measured that is:
- Lower than 0.17 inches per second (ips) RMS per ANSI/Hydraulic Institute Standard 9.6.4 this is the value for a similar vertically suspended pump) or
- Lower than 0.13 ips RMS for machines (above 15kW) ISO 10816 – 3 or
- Lower than a value set by the specifying engineer. For example, 0.12 ips RMS
Figure 1. Answers the question: “At what point in a project timeline should vibration expertise be used to help reduce risk for a successful new or modified plant design?” This three-part blog discusses the Machinery System Dynamics Analysis and post-installation vibration testing (shown on the right). The case history at this link covers the earlier Plant Design Dynamic Assessment phase (shown on the left).
Three Parts to Vibration Risk Reduction
Reducing vibration risk in rotating machinery systems—such as mixers, pumps, centrifuges, blowers, compressors, and turbines—requires a proactive approach integrated into the plant specification, analysis, and testing phases. By addressing vibration risk early, owners and operators can avoid costly resonance-related failures, reduce warranty claims, and enhance system reliability.
Separation margin is the percentage difference between an excitation source (e.g., running speed) and the system’s natural frequencies. Key terms—such as natural frequency, resonance, amplification factor, and separation margin—are explained in this one-page document.
- The Specification Wording
The first step is to ensure that the specification sets clear requirements. The ANSI/Hydraulic Institute Guideline 9.6.8, Rotodynamic Pumps Guideline for Dynamics of Pumping Machinery, provides an excellent framework for pump systems, and its principles can be adapted for other rotating equipment. Effective specifications should:
- Define acceptable overall vibration levels.
- Require a separation margin to prevent resonance excitation.
AE firms are now including clauses that require an Experimental Modal Analysis (EMA) or impact test to verify separation margins. This ensures that vibration risks are addressed at the system level, not just the component level. MSI is available to assist with specification wording.
- The Dynamics Analysis
The second step is to conduct a dynamics analysis during the design stage. While no tool can perfectly predict future vibration issues, analysis provides the next-best assurance by estimating separation margins. Confirming that a sufficient margins exists helps ensure that a system’s natural frequencies will not coincide with running speed over the plant’s lifetime. Later, this analysis can be validated during the Vibration Acceptance Test (VAT). Part 2 of this blog will explore the dynamics analysis process in detail.
- VAT, including an EMA (Impact) Test
The third and final step is performing a Vibration Acceptance Test (VAT) after installation. Modern mixer specifications often require VATs that include an Experimental Modal Analysis (Impact) test to confirm that separation margins near critical natural frequencies are adequate.
Relevant standards include:
- ANSI/HI Standard 9.6.4: Rotodynamic Pumps for Vibration Measurement and Allowable Values
- ISO 10816: Mechanical vibration standards
- API Standards: For rotating equipment in the oil & gas and petrochemical industries
The specification for the mixer that is the subject of this blog required an allowable overall vibration of 0.12 in/s RMS or less at each bearing housing based on post-installation field testing. Factory tests may also be performed, but they cannot fully account for site-specific factors such as foundations, piping, and soil conditions, or other factors related to the actual installation.
Why Separation Margin Matters
Although ANSI/HI 9.6.4 does not explicitly require separation margin testing, leading AE firms now include it in purchase specifications for pumps, mixers, centrifuges, and blowers. Understanding the natural frequency separation margin and amplification factor provides valuable insight into how the system will behave over its lifetime, helping to prevent resonance-related vibration problems before they occur.
Looking Ahead
Part 2of this blog will discuss the analysis phase used to identify and correct potential pump system vibration issues. Part 3 will outline best engineering practices for performing a Vibration Acceptance Test, including an Experimental Modal Analysis (Impact) test, to confirm separation margins near critical natural frequencies.
Please contact MSI if you would like a vibration risk reduction tutorial or support in drafting specifications, performing dynamics analysis, or conducting field testing to minimize post-installation vibration risks in rotating machinery systems.
