In this issue:
New Monitoring System for Hydro Generating Units
GE introduces the Bently Nevada* ADAPT 3701/46 Hydro Monitor, the latest in a family of compact, high performance safety and machinery protection and condition monitoring solutions. ADAPT (Advanced Distributed Architecture Platform Technology) products excel at the intensive signal processing needed to identify machine-specific malfunctions and precursors to failure. The 3701/46 Hydro Monitor is ideal for small and medium-sized hydro turbines where a compact and powerful distributed monitoring or protection system is appropriate.
What it is
ADAPT Hydro consists of a 3701/46 Hydro Monitor module, one or two 6-channel input modules, and a relay output module. The six channels of each input module can be independently configured for Acceleration, Velocity, Radial Vibration, Thrust Position, Dynamic Pressure, Speed, and Keyphasor*, and there is also a seventh dedicated Keyphasor channel. Each measurement type has a default configuration, including a Rough Load Zone measurement for guide bearing vibration, and a Cavitation measurement for draft tube accelerometers. Vector (1X, nX, etc.), band-pass filtered, and peak detection measurements can be added to a channel to detect vibration characteristics associated with specific malfunctions tuned to your specific machine.
What it does
- Compact form factor for installation at the machine.
- Accepts a wide array of vibration, speed, and pressure sensors.
- Eight (8) configurable SPDT alarm and protection relays.
- Raw transducer signal buffered output connectors.
- Two independently addressable Ethernet RJ45 connections.
- Modbus TCP protocol allows levels and alarms to be viewed via industry-standard displays.
- Synchronous and asynchronous waveforms can be viewed real-time using the optional System 1* Basic display software.
- Configured and validated with Bently Nevada Monitor Configuration (BNMC) software.
- State of the art electronics and signal processing helps you focus on key machinery health indicators unique to your particular machine.
- Cost-effective solution enables you to expand reliable protection and remote monitoring to more of your hydroturbine generating units.
- New generation platform ensures robust, long term lifecycle support.
Where it’s used
The 3701/46 ADAPT Hydro Monitor is ideally suited for small and medium-sized Francis, Pelton, and Kaplan turbines where high channel count and generator air gap are not required, and where bearing and other temperatures can be taken directly into the control system. ADAPT Hydro easily connects to SCADA and unit controls so that specific machine conditions and problems can be remotely detected and managed. The combination of ADAPT Hydro and 3701/55 ADAPT ESD can serve as a complete small unit protection system.
Ditch the vibration switch, and let ADAPT Hydro help you make intelligent machine decisions!
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In each issue, we cover a machine malfunction or other technical topic relevant to hydro turbine generator units. In this issue we’ll cover unbalance and shaft bow. As we’ll see, although the root causes are different, the forces and resulting vibration characteristics are very similar.
Mechanical unbalance is a non-uniform mass distribution about the shaft geometric axis and results in a periodic excitation force at rotational speed. This force produces a synchronous (one times running speed, or 1X) vibration. For machines that rotate at low speeds well below their first balance resonance (critical speed) the vibration amplitude increases approximately as the square of the machine speed. The vibration phase angle may remain fairly constant, or it may increase somewhat (in the lagging direction), depending on the specific characteristics of the machine.
Some possible sources of unbalance are mechanical damage to the turbine runner, a bowed shaft, or distortion of the generator rotor due to loose banding or thermal effects. In the case of thermal effects, the vibration amplitude may not increase as the square of the speed initially, but may be dependent on generator loading which causes thermal distortion of the rotor.
If the mechanical unbalance is due to a combination of causes, the effects will depend on which effect is dominant. Any changes to the synchronous vibration amplitude or phase, whether increasing or decreasing, may indicate a mechanical balance problem.
Shaft bows in hydroelectric machines can be caused by the thermal effects of mechanical friction between rotating and stationary parts (i.e., bearings), or asymmetric heating in the generator rotor. Any bow in the shaft system will produce synchronous shaft motion. Shaft bow will introduce a 1X vibration or run out in exactly the same way as a mechanical unbalance.
If the bow is present at the start of the run, the machine will behave exactly as if an unbalance were present. The vibration amplitude will increase as the square of the machine speed at a phase lag angle that is constant, or that is increasing slightly. If the bow is due to a shaft rub or other non-uniform heating effect, the vibration amplitude and phase will quite likely change over time.
If the machine is tripped with a thermally-distorted rotor, the vibration amplitude will reduce as the square of the speed at a constant phase angle during the coast down. This is because the rotor will not have time to cool significantly during the relatively short time that it takes for the machine to slow down and stop turning. When the machine is allowed to cool before restarting, it will behave in a normal way with the vibration increasing again with time as the thermal effects become dominant.
Magnetic unbalance occurs when the magnetic field forces between the rotor and the stator are not uniformly distributed around the entire rotor. This will occur if an individual pole has a larger or smaller air gap than its neighbors, or if it has a smaller than normal magnetic field, due to shorted windings.
When an individual pole has a weaker than normal magnetic field, a unidirectional resultant force will be exerted on the rotor, in a direction that is 180 degrees away from the affected pole. If you can determine the direction of the magnetic unbalance vector (using the phase lag measurement), it may be possible to narrow down your diagnosis to the approximate area on the rotor where the shorted windings are located.
In order to observe purely mechanical unbalance separately from the combined mechanical and electrical unbalance, you will need to take measurements with the rotor at speed, but with the field de-energized. Depending on the size and orientation of the magnetic unbalance force, vibration amplitude and phase could either increase or decrease when the excitation breaker is closed. The important point is that a change occurs at the exact time when excitation is applied to the field. If magnetic unbalance exists on a running generator, a corresponding vibration change may be observed at the moment that the excitation breaker is opened during unit shutdown.
As we’ve learned, unbalance and rotor bow show up as high and/or changing 1X vibration, with or without changes in phase angle. To detect these malfunctions, a proper monitoring system should have guide bearing proximity probes and a Keyphasor* transducer. The monitoring system should have the ability to calculate and display the 1X vibration vector (amplitude and phase angle), and output these values to a SCADA system for trending. An additional tool that can help you distinguish between mechanical and electrical unbalance and rotor bow is diagnostic software. Systems like Bently Nevada System 1* or ADRE can display the 1X amplitude and phase data in useful formats such as bode, polar, and waterfall graphical plots.
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Links to More Information
ADAPT Product Family Webpage
Interactive Web Demo (click on “Product Animations” in the upper-left, and select the 3701/46M from the drop-down menu)
Download 3701/46 Data Sheet
Download 3701/46 Fact Sheet
System 1 Basic Product Webpage
System 1* Product Webpage
ADRE Product Webpage
To subscribe, or for more information on the contents, please contact your GE Bently NevadaTM product line representative or the author:
Mark Snyder, P.E.
Sr. Field Application Engineer
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