Reciprocating Machinery

ØDS has 20 years of experience with noise and vibration control of process and drivetrain installations involving reciprocating machinery.

The ØDS Machinery Dynamics Group is specialized in the design and the dynamic properties of high-power machinery.

Because ØDS combines experience with design expertise of reciprocating machinery, we can provide high-level troubleshooting

• Natural Gas Compressors
• Industrial Refrigeration Compressors
• Propulsion Engines
• Generator Sets
• Automotive Engines
• Torsional Dynamics
• Bearing Reaction Forces
• Foundation Vibrations
• Piping Vibrations
• Condition Monitoring
• Troubleshooting

Reciprocating Machinery

General Aspects
Despite significant differences in design and operation, most reciprocating machinery has similar excitation mechanisms:

• inertial forces from rotating and oscilating masses
• process forces
• pulsating flow in the discharge piping

These types of excitation mechanisms have many harmonic frequency components (incl. half-orders for 4 stroke engines), and the risk of resonant conditions is therefore considerable. This implies that reciprocating machinery is prone to vibration problems.

Torsional Dynamics
The exciting torque from piston inertial forces and cylinder pressure reaction can have components up to the 15th harmonic of running speed.

ØDS uses the finite element (FE) method to determine train torsional natural frequencies.

In some cases, it is necessary to resort to tuned absorbers or dampers to handle resonant conditions. ØDS can assist in dimensioning and selecting such devices.

If necessary, forced torsional response can be calculated to assess operating stresses and fatigue safety margins. We have experience with estimating uncertainties in torsional stiffness and amplification factors for the different frequency ranges.

Should torsional vibrations be suspected in operating machinery (e.g., high axial vibrations at thrust bearing), ØDS can perform verification measurements by using:

• shaft encoder
• laser torsiometer
• strain gauges with telemetry

For further information, please refer to our brochure on torsional vibrations.

Foundations and Auxiliary Equipment
The crankshaft bearing reaction forces are not in phase. Neither are the guide force moments. This means that the foundation must sustain not only reaction forces but also substantial unbalance moments.

For concrete foundations, the most critical parameters are tie bolt dimensioning and pre-tension as well as concrete integrity.

In marine and offshore applications, the support flexibility is higher, and resonance can arise with structural natural frequencies in the foundation. Furthermore, the risk of transmitting harmful excitation forces to auxiliary equipment increases.

ØDS has extensive experience with FE modelling of foundations as well as with on site vibration measurements. Depending on the assignments, the solutions have been:

• realignment
• barred speed range
• structural modifications
• dynamic absorbers
• resilient mounting
• compensators

Pressure Pulsation and Piping Vibration
The many harmonics in the discharge pressure can cause damaging resonant vibrations in discharge piping, nozzles, valves and measurement equipment.

Resonant condition can occur both with regard to structural and acoustic piping natural frequencies. This must be considered at the design stage by performing a dynamic piping analysis. If piping resonance arises nonetheless, the solution may consist of:

• restricted operating windows
• piping modifications (quarter-wave stubs, acoustic filters, etc)
• additional supports
• Helmholtz resonators

However, the excitation forces are not always due to reciprocating machinery. Other phenomena, such as cavitation and vortex shedding in piping, should also be taken into consideration.

Condition Monitoring
Reciprocating machinery requires frequent maintenance. The potential savings by use of condition monitoring are therefore considerable.

ØDS can assist in selecting the monitoring strategy, the parameters to be monitored and the actual condition monitoring system. This selection should be based on an analysis of the likely wear and failure modes, including the time of development and the consequential damages.

The monitoring should be concentrated on the critical components such as valves, piston rings, piston rod rings and crankshaft. The monitoring could include:

• p-V diagram: efficiency, wear of valves and seals
• rod drop: wear of piston rod rings
• torsional natural frequencies: cracks in crankshaft
• oil condition: oil deterioration, wear
• casing vibrations: damaged springs and valves, unbalance
  

The evaluation should be based on monitoring of the relative changes of these parameters.