The vibration analysis equipment enables telling when a normal vibration exceeds acceptable levels and can end up becoming a problem. It also enables determining the source and the cause of the vibration, which is why it is helpful in preventive maintenance and resolution of failures.
FACTORS THAT AFFECT THE VIBRATION
- Unbalancing of revolving parts and balancing of the rotor
Unbalancing of the pump rotor can generate large unbalancing forces, resulting in excessive loading of the bearing and shaft and induction of high levels of vibration.
The magnitude of the pump vibration caused by unbalancing of the rotor is also influenced by the mass of the pump housing: as the pump housing increases, the magnitude of the vibration measured at the bearing support diminishes.
- Natural frequency and resonance
Operation of a pump at a revolution close to the pump’s passage frequency, or to one of the lower natural frequencies of the structure, can result in a resonant condition.
- Hydraulic resonance in the piping
Vibration problems can be caused by hydraulic resonance of the liquid inside the pump/piping system. When pressure pulsations induced by the pump are reflected by the piping system and added in phase to the source pulse, the amplitude of the pulse is increased. High resulting pressures can cause mechanical failures both in the piping and in the pump’s components, may generate unacceptable levels of noise, and cause vibration of the system’s components.
Experience has shown that the following measures can prove effective in the correction of hydraulic resonance:
a) modify the resonant piping;
b) change the pump’s revolutions;
c) change the pump’s internal design characteristics;
d) insert a damper in the pump/piping system.
Changes to the pump, piping or support structures that do not change the pulsation response (natural hydraulic frequency) of the pump/piping system will not correct the resonant condition.
- Hydraulic disturbance
Below is a list of some hydraulic disturbances:
a) recirculation and radial forces at low flows;
b) separation of the fluid at high flows;
c) cavitation due to NPSH problems;
d) disturbances in the pump’s intake flow due to inappropriate design;
e) air intake or aeration of the liquid;
f) hydraulic resonance in the piping;
g) solids contained in the liquid, such as residual water, colliding with the pump and causing unbalancing.
- Other mechanical problems
Misalignment of the shaft, damaged bearings, curved shaft and inadequate piping supports are also frequent causes of vibration.
An example of a vibration survey datasheet is shown below.
|
