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Piston Compressors
MEASUREMENT OF VIBRATION AND ASSESSMENT OF TECHNICAL CONDITION OF PISTON COMPRESSORS
The measurement is limited to that of physical quantity sets which can be used to define and evaluate unambiguously vibration condition of the whole piston compressor with its drive, clutch and, possibly, with the gear.
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| View of a large piston compressor |
Such measurement is conducted in order to:
- Locate the sources of vibrations and determine their causes,
- Assess the vibration condition: current condition compared to the history of the machine,
- Collect data to make a complete analysis of the vibration state: current condition compared to the history of the machine,
- Interpret the vibration state observed: current condition compared to the history of the machine,
- Make a diagnosis regarding the technical state of the compressing unit: current condition compared to the history of the machine,
- Make a forecast abort future technical state of the compressor: current condition compared to the history of the machine,
- Determine the scope and the time of repair on the basis of the measurement of vibration.
The repair procedure can be planned according to the actual technical condition of the unit. The additional advantage of this approach is that it is possible to obtain proper spare parts beforehand. As a result, the time and the scope of repair can be reduced to a minimum.
Type and Scope of Measurement
We conduct measurements on piston compressor sets which are operating or disabled.
Measurement of an operating turbine set’s vibrations is conducted with:
- a monitoring unit mounted permanently: the so-called compressor set monitoring system,
- sensors of a stationary monitoring system mounted on the piston compressor set,
- a measuring system installed by us for a longer period of time,
- a measuring system installed by us for a period during which the measurement is carried out,
- a portable measuring apparatus.
In each of the above cases, if necessary, the measuring system can be supplemented with sensors essential for obtaining the best description of the piston compressor set’s technical condition.
The scope of measurement we conduct depends on requirements, objectives and a technical problems occurring while the piston compressor set is operating.
Practically, all piston machines are reciprocating machines. Each full working cycle of such a machine consists of a number of phases (intervals) during which largely diversified physical processes take place. For example, symptoms of the suction of a working medium to the cylinder differ from those of a compressed gas release. Symptoms of the elimination of looseness of the piston pin, or of the moment of the hydrodynamic stroke, or of the moment of the elimination and liberation of looseness of a slider differ from one another. Moreover, other symptoms occurring in the case of a broken spring or a broken valve and many physical phenomena characteristic for a single, full cycle of the machine’s work can be observed. The vibrations recorded during the operation of the piston compressor concur with specific, impulsive effects stemming from, for example, the work of a piston system or the dynamic unbalance of the mechanical system causing the displacement of weights (connecting-rod and piston, working medium).
The conclusion which can be drawn from these examples is that most measuring methods and diagnostic algorithms used for continuous assessment of rotor machines such as pumps or fans are of little use in the case of piston machines.
Therefore, a different approach allowing for the characteristics of vibration processes occurring in reciprocating machines is required.
The means of vibration inspection and diagnosis must be combined with special measuring methods and proper diagnostic algorithms in order to make an analysis of the state of reciprocating piston machines effective.
A mandatory requirement for inspecting piston machines is the use of a multi-channel, accurate gating before analysing proper lengths of time constituting a sequence of events during one cycle of work. The number of necessary vibration measuring channels is determined by the number of problems inspected. To make a correct diagnosis of piston machines by means of vibration signals, piaget replica watches it is necessary to know the actual phase diagram of the working machine. It is required for the selection of individual lengths of time from the whole signal. These lengths of time correspond to given phases of the machine’s work. For example, if an outlet valve of one cylinder is inspected, then only the length of time during which the valve is open should be picked out and examined. Failure to determine this length accurately may lead to unreliable interpretation of vibrations.
As long as a phase diagram of the machine’s work (with accuracy to a few degrees) is not known, no diagnosis should be made for the piston compressor, since information obtained in this way would be absurd, unreliable and unrelated to its actual technical state.
While, for example, an impeller pump, fan, water or steam turbine can be reliably examined without comprehensive knowledge of their construction, such a simplification is unacceptable in the case of piston compressors.
It should be underlined that all methods of piston machine diagnostics based on vibration signals are used only within the lengths of time which have been precisely picked out and limited. If, for example, a spectrum of the whole vibration signal was obtained during one whole work cycle, then the local diagnostic indices (each index is assigned to its own time length which constitutes a part of the whole vibration signal) would be blurred and decreased by tens of times, and diagnostic information would be irretrievably lost.
According as measurement conducted we are able to indicate and inspect the following basic faults of the piston compressing unit:
- Fault of the electric engine’s bearings,
- Misalignment of the engine’s shaft in relation to the compressor’s connecting rod,
- Seizures in seals,
- Looseness in bearing supports,
- Wrong arrangement of bearing supports,
- Misalignment of the rotor in the electric engine’s stator,
- Axial displacement of the rotor in relation to the engine’s stator,
- Faults of the main bearings of the compressor’s connecting rod,
- Fault or wear of the bearing liners,
- Fault and looseness in the connecting-rod joints,
- Fault of the shaft-bossing system,
- Faulty work of valves,
- Looseness in piston pins,
- Hydrodynamic strokes in the pumping chamber,
- Fracture of springs and valves’ plates.
We also carry out measurements geared to wards:
- Preparing the scope of repair work: selection of elements which must be inspected in the course of a repair,
- Assessing the vibration condition after the completion of the repair,
- Determining the frequency and the form of free vibration of piston compressors’s elements (eg. blade, shaft bossing, compressor’s cover, elements of bearings and their supports etc.) and machines’ units (e.g. pipelines, gates, support system, etc.)
- Measuring and assessing the influence of the flowing water on the vibration condition of piston compressor (influence of the hydraulic input),
- Assessing the influence of electromagnetic inputs of the engin on the dynamic state of the piston compressor,
- Searching for causes of instability of the rotating system’s motion,
- Eliminating all unfavourable and untypical phenomena occurring during the operation of piston compressors.
We indicate loosenings or any possibility of looseness in the clutch plate mounting on the shaft of the rotor. Measurement is carried out when the rotor is disabled, for example, during a repair of the unit. For this purpose, we use a method developed by the VIBROPOMIAR Company on the basis of modal analysis.
The VIBROPOMIAR Company also maintains a measuring team which responds to emergencies. It conducts limited measurement consisting of the evaluation of the rotating system’s vibration condition. This measurement serves the purpose of assessing fitness of a rotating system for further operation, making it possible to evaluate its technical condition .
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