Vertical Pump Resonance Problem

Vertical Pump Resonance Problem

Gus Altimany, Air Products and Chemicals

Patrick Smith, Air Products and Chemicals

T U R B O M A C H I N E R Y & P U M P S Y M P O S I A

Biographies

• Patrick J. Smith is a Principle Engineering Associate - Machinery in the Operational Excellence Technical Team at Air Products & Chemicals. He is also an AP Fellow. He is based in Allentown, PA and has over 35 years of rotating machinery experience.

• Gus Altimany is a Machinery Engineering Manager at Air Products & Chemicals. He is based in Allentown, PA and has 25 years of rotating machinery experience.


Pump Overview• 17- stage vertical cryogenic pump

• Driven by 800 HP variable speed motor

• Pump and motor hard coupled

• Pump operated at a preset speed between 3300 RPM (55 Hz) and 3600 RPM (60 Hz) depending on process duty requirements

• Motor fitted with DE casing vibration sensor

• Motor fitted with DE and NDE bearing temperature probes

• Pump mounted in a pit

Pump on test stand in shop


Problem Statement

• During commissioning pump experienced periods of high vibration

• Magnitude of vibration affected by speed, bearing temperature, ambient conditions, and ice build up

• See next slide for plots


Problem Statement


Initial Findings• Vibration all at 1X running speed

• Bump check showed a clear resonance at around 3540 RPM (59 Hz)

• Coast down vibration plot showed possible resonances at running speed and at around 3300 RPM (55 Hz)

• Five identical pumps at site showed the same behavior

• High vibration was not observed during shop testing

• Pumps were an old design with many references. However, the new application was a higher power and used larger, heavier motors.


Initial Analysis• Rotordynamics reviewed – concluded there were no problems

• Suspected issue with structural resonance. In a vertical pump, the motor is a large cantilevered mass. The natural frequency associated with a cantilever mode is sometimes referred to as a reed critical frequency due to the mode shape being similar to that of a musical reed. If the pump motor combination reed critical frequency occurs in the speed range of the pump, high vibrations can occur.

• Motor vibration provides the exciting force.

• Natural frequency is function of stiffness and mass. To change the natural frequency, the stiffness and/or the mass can be changed.

• Lowering the excitation force can lower the vibration response, but does not change the natural frequency.


Initial Field Fixes• Several attempts were made to stiffen the motor/pump

laterally by adding temporary supports. However, this resulted in no change in the vibration behavior.

• In parallel the motor was field balanced in an effort to reduce motor excitation force. This resulted in slightly lower vibrations, but still not acceptable.

• 896 kg of weight was hung from nozzle head, resulting in low, stable vibration. Note the following:

• Weight of nozzle head = 600 kg

• Weight of motor = 2405 kg

• Weight of smaller motors used in the past < 1500 kg


Path Forward• The hanging weights were considered a temporary solution.

• Similar vertical pumps were being installed at many other sites. These pumps used newer, high head/stage hydraulics. This resulted in significantly less stages, but this only impacted the cold end. The warm end and motor sizes were similar to the pumps with the vibration problem.

• An engineering solution was desired that could address the existing pumps and the pumps that were yet to be installed.


Finite Element Modal Analysis – Simulations

• Components model included nozzle head, barrel w/mounting flange, soleplate, bowls, suction bell and motor

• Further models included simulation of:

➢Nozzle head ice build-up

➢Hanging weights (steel curtains)

➢Horizontal & vertical supports

➢Nozzle head windows

➢Vertical rods support


Finite Element Modal Analysis - Results

• Identified various modes of system natural frequencies: Main focus were torsional & second bending

• Field System Natural Frequency (SNF) Tests Revealed Close Proximity to Design Speed of 59HZ → Excitation Leading to High Vibrations

• Industry Preferred Separation Margin away from SNF is ±20% (10% Acceptable)

Mode 10 –53.8613 Hz

Mode 11 –61.1212 Hz

Natural Frequencies


Attempted Modifications & EffectModification What was done Effect Result

Nozzle Head Windows

Removed/Cut material from nozzle head

Reduction of torsional mode

Bending mode up closer to running speed

Nozzle HeadTie Rods

Added angular rods in both tension and compression

No change No change

Support Rods Added 4-vertical rods under motor corners

Vibration amplitude reduction for short period but trend up w/minor excitation

Still within 5% margin to system natural frequency

Steel Curtains Added various hanging weights

Vibration amplitude reduction=F(mass)

Tried progressive weights


Curtains Rods Structure


Adopted Modifications

Modification What was done Effect Result

Engineered Steel Structure

Built structure around nozzle head w/adjustable braces(horizontal/vertical)

Radial (bending) reinforcement of motor DE/nozzle mounting flange

Torsional mode remained close to running speed

Steel Curtains Added hanging weights to nozzle head ribs

Vibration amplitude reduction below 10% margin

Optimized on 1000Kg after head stress analysis

Engineered Vibration Absorber

Hanged detuning panels (2x head @ 90°) from head

Vibration amplitude reduction @ specific frequency

Required frequent tuning / adjustment


Engineered Support Structure

Vertical Posts bolted to

plates welded to soleplate

and located above concrete

– transferring dynamic

loading to concrete

• Design & fabricate SS structure around nozzle head. Tried 18 different variations to find a structure that meets the design requirements, considered piping attachment stiffness, weights, and conducted sensitivity analysis/checks → complete integrated comprehensive analysis


Engineered CurtainsAttach detuning engineered vibration absorbers to nozzle head –these are not damping devices that change system natural frequency but detuning devices that absorb the specific HZ vibration amplitude

Vibration Absorber


Location of Field Data Vibration Measurements


Field Study – System Dynamic Analysis • Field data measurements and analysis

• Developed FEA models based on collected data and operating deflection shape (ODS) measurements

• Higher damping of the system natural frequency mode typically indicates that this mode will not lead to high vibrations

• Proposed plan was to evaluate simple modifications to the support structure and measure the effect on the various modes➢Support structure with horizontal and vertical braces around the nozzle head➢ Loosen bolts and remove braces → Measure impact progressively

• Alternative plan was to engineer vibration absorber – 2xhead for directional detuning


Final Solution

• The solution was to leave the hanging weights installed on the pumps. This was the simplest solution and provided the most consistent results of all the attempted modifications.

• There have been no structural resonance issues to date.


Conclusions• High vibration was due to a resonance problem, but the cause

was different depending on the mounting method of the pump

• If the pump was mounted on a steel structure, the structure behaved more like a simple mass and spring system. In this case the flexibility of the steel structure largely determined the structural natural frequency. This is why this wasn’t seen during shop testing.

• If the pump was mounted on concrete in a pit, the structure behaved more like a cantilever spring and the stiffness and mass of the nozzle head largely determined the structural natural frequency. This is why stiffening the mounting had little to no effect

m

k

Mass and Spring

m

L I

Cantilever Spring


Conclusions

• The hanging weights resolution could have been replaced with a heavier weight nozzle head assembly if the FEA were conducted accurately during early design phase


Lessons Learned

• When reviewing a pump for a new installation, it is important to understand there are risks when stepping outside of a suppliers experience. The pumps discussed in this case study were typically driven by much smaller (power and weight) motors. The size was a step out.

• When installing large motors on vertical pumps, consider performing a system finite-element analysis (FEA) during the design phase to ensure there are no structural resonances within the operating speed range. The system should include the pump assembly, supporting structure and piping connections to the nozzle head.

Vertical Pump Resonance Problem

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