Rotating machinery acceptance
The vibration acceptance test serves to ensure the mechanical operation of a machine or to identify any possible defects. It concerns machines classified as criticality 1, which means vital or of strategic significance (unlined machine, for which there are no replacement parts because they are expensive or because they are no longer on the market, etc.).
To carry out vibration acceptance, it is necessary to identify the type of machine concerned, but also to follow a methodology according to the history of this machine and determine the best time to perform it.
Carrying out vibration acceptance:
As there is a considerable range of machinery, no two machines are the same. Indeed, made by different manufacturers but also under environmental (frame, pipes, insulation, support, temperature and air pollution, etc.) and operating conditions (speed, flow rate, pressure, temperature nature of the medium, etc.) that are rarely the same, there are various sources of malfunctions (faults) that generate characteristic vibrations.
We can classify them into eight families:
- Periodic phenomena directly related to the machine’s kinematics (imbalance, alignment, meshing, etc.)
- Periodic phenomena not related to the machine’s kinematics (bearing instability, acoustic coupling, ghost frequencies, shaft cracking, etc.) and phenomena related to thermal expansion
- Random phenomena (cavitation, pumping, rotation);
- Transient phenomena (shock);
- Impulsive periodic phenomena (periodic shock), such as a damaged tooth on a pinion, a broken fixing foot, etc.;
- Proximity of excitation frequencies close to resonance frequencies or shaft critical frequencies;
- Direct or indirect isolation phenomena;
- Phenomena relating to operating conditions (acoustic tuning, von Karman vortices, cavitation, etc.).
In order to carry out vibration acceptance, the machines need to be split into two categories according to the machine family:
- machines of variable or fixed speed;
- machines with slide or roller bearings
dB Vib Consulting basic methodological approach involves:
- Qualifying the dynamic behaviour of the machine’s bearings by measuring transfer functions when the machine is at standstill. This step helps identify structure resonance frequencies that could influence the vibratory response.
- Record vibratory signatures during stop and/or start-up phases in order to of the rotation frequencies. Comparing the vibration responses of the machine excited by imbalance and its harmonics with the results obtained at standstill enable us to separate the modes of the structure from those of the shaft.
- Record the vibration signals for the operating conditions (load, flow/pressure, rotation speeds, etc.) and during transitory phases. The correlation of the vibration levels with the operating parameters enables us to qualify the sources of excitation and perform our diagnosis.
According to the history of the machine, operating constraints and observations, this methodological approach can be adapted on a case-by-case basis.
As for the measuring instruments used, they differ according to each specific case. Where possible, our experts use their own sensors (accelerometers, pressure transducers, strain gauges, proximity probes) as well as those permanently installed on the machine (accelerometers, velocimeters, proximity probes, process data). They also use synchronous multi-channel analysers/recorders (4 to 40 channels per system) to collect, record and analyse vibration signals. The choice of this type of equipment is essential for analysing the transitory phases and variations in operating conditions.
When should it be carried out ?
Vibration acceptance can be carried out:
- During acceptance of a new installation at the plant or on-site,
- At the end of the warranty period to qualify the evolution of the machine’s vibration behaviour compared with its commissioning,
- To establish of a zero condition for a critical machine in order to conduct periodic monitoring,
- Before scheduled maintenance work in order to adapt the nature of the work to the requirements or to extend stoppage,
- At the end of maintenance work to validate the work carried out,
- To explain significant vibration levels or recurring failures.