3500/46M Hydro Monitor
Smart monitoring forthe intelligent machine age
Mark Snyder
Bently Nevada Renewables Application Engineer
Mark.Snyder@bakerhughes.com

Older machinery protection systems, and eventransmitters (Reference 1) and simple vibration
switches marketed today, only produce a“Direct” (broadband) vibration measurement.
Since the Direct value contains contributions
from all sources of measured vibration, it is
the one measurement that can provide acomprehensive representation of the overall
health of the machine. However, relying on thismeasurement alone has drawbacks.
One problem is that because the Direct
measurement is typically used to alert operators
or trip the machine, Direct alarm thresholds
are set relatively high to suit all situations andprevent false alarms and trips. Consequently,
these alarms and trips ignore notable vibration
changes in all but the worst-case operating
condition.

Another limitation is that Direct vibration alone merely
provides an indication that there is (or has been) a problem,
but provides little or no useful information to help you answer
the next logical questions of what is causing the problem,
and what operational or maintenance actions should betaken to mitigate or correct it.
This article describes how an effective machinery protection/
condition monitoring system solves these problems, and how
the 3500/46M Hydro Monitor addresses the unique needs ofhydro turbine generating units.

Phase measurement unlocks predictive
maintenance value
Today’s microprocessor-based machinery monitoring
systems can perform signal processing functions that once
required bulky portable analyzers. This enables the monitor
to break out several components and characteristics of thebroadband vibration that can serve as useful diagnostic“clues.” A good analogy is a musical chord. An untrained ear
can only tell that the overall sound is dissonant, but cannot
tell which of the notes is out of tune. However, if each notethat makes up the chord is played individually, even anuntrained ear can usually pick out which note is causing thedissonance. Similarly, exposing individual values that makeup the broadband vibration can help classify the type ofmachine problem.

The key to unlocking these individual values is to incorporatea once-per-turn phase reference measurement made by aKeyphasor* monitor. Previous Orbit articles have defined anddescribed the concepts of phase angle and the Keyphasor
measurement (References 2 & 3), and the following is asummary of the functionality and benefits that a Keyphasor
measurement provides.
• A Keyphasor transducer is necessary for in-place machinebalancing; anyone doing balancing needs the once-perturn reference that the Keyphasor channel provides.
• A Keyphasor signal enables the calculation of“synchronous” data – data that is taken by synchronizing
the vibration waveform sampling rate with the runningspeed of the machine. This preserves the accuracy ofphase angle measurements and the amplitudes ofvibration components that track with machine speed.
• A Keyphasor signal allows the monitoring system to filter
around (isolate) only the vibrations that have the same
frequency as running speed or multiples of running speed(1X, 2X, etc.) - even during machine speed changes.
• A Keyphasor measurement provides a phase angle value
associated with the filtered amplitudes of each transducer
measurement. Phase can be just as useful as amplitude
for detecting changes in the machine behavior. In fact,
phase angle may change even when there is no increase(or even a decrease) in vibration amplitude.
• Portable analyzers and data acquisition equipment needa Keyphasor signal to generate the values and graphicalplots necessary to definitively identify a machinery
problem.

The value of filtered amplitude and phase angle values(e.g., vector values) for machinery monitoring andpredictive maintenance is widely recognized. This concept
is acknowledged in a well- known international standard onhydroturbine vibration monitoring (Reference 4). Main points
are summarized here:• Broadband vibration amplitude (without frequency orphase information) is usually adequate for operational
monitoring and acceptance testing.
• However, vibration vector information is especially usefulfor machinery diagnostics and long-term condition
monitoring.
• Some changes in the dynamic state of the machine
cannot be detected and identified by broad-bandmeasurements alone.

3500/46M helps identify the problem
With a Keyphasor signal available in the 3500 monitoring
system, the 3500/46M (and other 3500 vibration monitors)
provides a number of useful variables in addition to overall
vibration (Figure 1). Guide bearing (shaft relative) vibration,casing absolute (acceleration, velocity) vibration, andgenerator air gap channel types will have different variables
as appropriate to their intended applications. All of thevariables’ values can be viewed and compared in real time
on a local display, and sent via Modbus to a SCADA or control
system for trending.
Changes in these values can provide clues to the cause or
characteristics of a machine problem.

Figure 1: 3500 configuration software screen showing the variables (in red rectangle) available from the3500/46M Multimode Hydro Radial Vibration channel type, and Trip Multiply settings (in blue rectangles).

Generally, changes in 1X Amplitude and Phase are related
to imbalance and misalignment. Phase angle is useful for
detecting shifts in the inertial, hydraulic or magnetic forces
acting laterally on the shaft or runner, such as from loss ofmaterial from one side of a runner, blockage of one bucket ofa runner, a deviation from the normal flow through a needlevalve, and other problems.
Changes in Not 1X vibration are typically symptoms of rubs,
fluid-induced forces in the bearings or at the runner, or roughload zone and vortexing. The “N” value in NX Amplitude canbe set to “2” to indicate rotor cracks and other changes
in stiffness, or to the number of runner buckets to detect
hydraulic imbalances associated with wicket gates ornozzles.
The 3500/46M also provides an average proximity probe
gap measurement for radial vibration measurements. Thiscan be used to monitor shaft journal position within theguide bearing, which could change due to misalignment
and preloads. Gap alarms can be set to indicate when thenormal bearing clearances have been reached or exceeded(corresponding to potential or actual bearing wear). Also, thisgap measurement, in combination with the NX measurement
set to the number of runner buckets, is used in the 3500/46M
Hydro monitor to calculate a “Composite” value that can
indirectly detect a shear pin failure in the wicket gate
positioning linkage.

Setting Effective Alarm Levels
Now that we have several values to give a more complete
picture of machine condition, the other challenge is to set
alarm levels that are appropriate for different machine
operating conditions. Using another musical analogy, mostcompositions contain both loud and soft segments, andthe softer passages usually cause the audience to quiet
down and listen more carefully to hear the subtle melodies.
Similarly, a good monitoring system should adjust its alarms
to “listen” more or less closely when varying conditions
change the “acceptable” vibration levels for long periodsof normal operation or for short periods during a start-up
sequence and other operational transitions.
One relatively simple way of dealing with the transientconditions is to use the Trip Multiply feature found in the3500/46M and other 3500 vibration monitors. Trip Multiply
temporarily elevates alarm levels by a factor set for eachchannel in the configuration software (see Figure 1). Notethat the Not 1X alarms can be excluded from the Trip
Multiply activity for that channel, which might be desirable if
vibration excursions other than 1X are of concern during thetransitory conditions. The Trip Multiply condition is actuatedfor all monitors in a 3500 rack using a pair of contacts on
the 3500/22 TDI Rack Interface I/O module, or for groups
of monitors using a Modbus digital input via the 3500/92
Communications Gateway.

Multimode Monitoring
Pumped-storage units have two directions of rotation
with vastly different hydraulic conditions. Kaplan units withadjustable blade angles can have significantly different
levels of “normal” vibration, and any hydro turbine usually
has at least one zone of high vibration associated withcertain unavoidable head, speed, and load conditions.
To accommodate these situations, the 3500/46M has a“Multimode” feature that allows you to establish up to eight(8) distinct machine modes, each with its own unique set of

Figure 2: Mode Setup dialog in 3500 Configuration Software. Mode drop-down list shows that four modes
have been defined in this example.

alarm parameters. The user-defined modes are enabled andnamed in the 3500 configuration software (Figure 2). Typicalmodes would accommodate operating conditions such as
forward rotation, reverse rotation, loaded, unloaded, highhead, low head, etc.
Once the modes have been defined, you can select eachmode and set the alarm parameters for the variables ofeach channel as appropriate for that mode. You can enable
or disable the setpoints, define the setpoint levels, and adjust
the alarm time delays as needed (Figure 3).
The multimode channels of a monitor must be “told” what
operating mode the machine is in, so that they can use
the correct alarm settings for each mode. Mode changecommands from the control system or other source can be
input via wiring terminals on the 3500/46M Multimode I/O
modules, or digitally via a Modbus write command feature of
the 3500/92 Communications Gateway Module.

Toward More “Intelligent” Machines
In the age of smartphones that incorporate music players,
cameras, portable digital assistants and mobile GPS devices,
it no longer makes sense to use a “dumb” monitoring system
to make intelligent machinery protection and condition
monitoring decisions. Microprocessor technology has madeit possible to embed powerful signal processing capabilities- once available only in bulky and cumbersome portable
analyzers - into every channel of an online machinery
monitoring system, while enhancing the effectiveness ofthese systems for reliable machinery protection.

real time and for trending in plant historians as part ofa predictive maintenance program. The features of the3500/46M Hydro Monitor can extend your monitoring systemfrom a device that simply tells you when to shut a machine
down, to a powerful predictive maintenance tool that helps
you keep your hydro turbine generating units hummingsmoothly.
In a future Hydro Corner article we will describe how theMultimode feature of the 3500/46M monitor can work
hand-in-hand with the State-Based Analysis features ofSystem 1* analytic software to enhance and simplify trending
and diagnostics across the varying operating states of amachine.

References
1. O rbit Vol.18 No.1, 1997. Sabin, Steve, “The limitations ofprotecting and managing machinery using vibration‘transmitters.’”
2. O rbit Vol.20 No.2, 1999. Forland, Clair, “Why phase
information is important for diagnosing machineryproblems.”
3. O rbit Vol.32 No.4, 2012. “Designing and Machining aKeyphasor Trigger”
4. ISO 10816-5, 2000. International Organization forStandardization, “Mechanical vibration — Evaluation ofmachine vibration by measurements on non-rotatingparts - Part 5: Machine sets in hydraulic power generating
and pumping plants”, Paragraph 5.7.

This enables the monitor to produce a steady streamof diagnostic-quality data both locally and remotely, inThe features of the3500/46M Hydro
Monitor can extend yourmonitoring system froma device that simply
tells you when to shuta machine down, toa powerful predictive
maintenance tool thathelps you keep your hydroturbine generating units
humming smoothly.

Figure 3: Setpoint configuration screen in the 3500 configuration software for one of the modes for a
Multimode Hydro Radial Vibration channel type. In this example, alarm settings are being established for the“Normal MW Generating” mode.

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