Hai Lin Zhu Zou Min
Jiangsu Institute of Petrochemical Technology
Peoples Republic of China
Monitoring the condition of fluid-power systems and accurately diagnosing the sources of problems that arise can prevent unexpected failures and reduce unplanned maintenance and downtime. And with today’s heightened emphasis on making hydraulic equipment more productive, it is important to catch minor problems before they become major headaches.
Fortunately, troubled fluid-power systems usually exhibit one or more of the following symptoms:
• Insufficient pressure rise.
• Inadequate flow.
• Excessive temperature.
• Leaking oil.
• Vibration or noise.
Thus, tracking data on pressure, flow, temperature, vibration, and oil-sample debris provide useful information on system conditions.
For instance, dynamic pressure signals directly reflect the working condition of a fluid-power system. Furthermore, pressure is easy to monitor and has a high signal-to-noise ratio.
Analyzing oil samples helps monitor component wear, since contamination leads to approximately 80% of all fluid-power system failures. A widely used technique called Ferrum Spectrum pinpoints the degree, location, and causes of wear.
An unusual rise in fluid temperature often indicates a design problem or component malfunction. And transducers that convert vibrations into electrical signals make it simple to track performance changes in rotating equipment.
In contrast to these well-established techniques, flow signals have not been widely used for monitoring hydraulics. Nonetheless, they can provide useful information based on the relation between flow and system operation.
For instance, it goes without saying that it requires a predictable flow to maintain speed and precision in hydraulic cylinders and motors. When flow characteristics diverge from the norm, so does system performance. Here are some common faults that can be readily detected by monitoring flow.
Insufficient flow is a common problem that occurs when the outlet filter clogs and the hydraulic pump cannot supply enough fluid to the system. A lack of pressurized fluid means hydraulic actuators do not function as intended.
Leakage from hydraulic pumps, cylinders, valves, and pipes is another such case. This not only increases oil consumption, but also decreases cylinder force and motor torque because the system cannot generate maximum design pressure. A similar problem involves internal and external leakage from fittings or through excessive mating clearances. This will cause oil temperature to rise, which could be indirectly reflected through the flow signal.
Vibration and noise in hydraulic equipment causes frequent flow pulses that can greatly reduce filter performance. As a result, oil contamination cannot be controlled which endangers the system. Vibration and noise mainly stem from flow pulses and pressure shocks in components such as pumps and valves. Monitoring changes in flow and pressure helps identify sources of the problems.
Cavitation erosion mainly arises from flow and pressure fluctuations. For instance, if pump outflow decreases because it cannot draw enough oil, cylinder and motor speed will drop. One cause is air mixing with the oil. Flow fluctuations and low-frequency pressure pulses at the pump exit are also signs of cavitation.
Creeping is an abnormal cylinder motion caused mainly by air entering past damaged cylinder seals, wear of components, and unstable flow feeding. So creeping failure could be mirrored by the flow signal.
Locking stems in hydraulic valves or actuators often have excessively high resistance when shifting valves from a stationary state. This fault is also reflected in flow and pressure fluctuations. Thus, flow signals can be a good source of information for system monitoring and fault diagnosis.
Generally, the flowmeter value (average flow) is of interest because this gives an indication of leakage and efficiency in the fluidpower system. For example, comparing pump flow under no-load and full-load conditions can determine the degree of internal pump leakage as well as the volumetric efficiency of the pump and system. Equipment condition can be monitored through time domain and spectral analyses of flow pulses from the meter.
Two ways to monitor flow are with invasive or noninvasive instruments. Invasive devices are widely used, moderately priced, and place the flowmeter’s sensing element, such as the turbine in a turbine flowmeter, in the flow path.
However, invasive sensors require additional pipe connections, disturb the flow field, and reduce system pressure. Fluid flow wears and corrodes the sensing elements and, thus, shortens meter life. Also, physical constraints within the system could prevent flowmeters from being installed in certain locations.
Noninvasive measurement tracks flow with transducers mounted outside the pipe wall, and units such as ultrasonic transit-time and Doppler flowmeters overcome many of the shortcomings of invasive flowmeters. For instance, a Doppler flowmeter from Controlotron, Hauppauge, N.Y., distinguishes the true flow signal from noise with an advanced technique called isochronous modulation and displays tendency curves of flow changes with time. This makes it well suited for condition monitoring and fault diagnosis.
In general, clamp-on noninvasive meters measure flow in many pipe positions and locations, but they are still rather expensive compared with invasive meters.
Keep in mind that this type of condition monitoring is still evolving because flow is a highly complex, dynamic condition that is not fully understood. A wide range of factors can affect readings, such as viscous friction, unstable vortices or secondary flows, changes from laminar or turbulent flow, boundary conditions such as pipe roughness, and changing environments. Thus, further research and advances are required to perfect these monitoring techniques.