Form V372, Rev. 12
- 12 -
12/18, ECN4334
SECTION 8: TROUBLESHOOTING
Most Rotamix systems work extremely well, but once in a while we run into trouble. Historically the
biggest troubleshooting problems we have encountered with Rotamix installations have been: (a)
foaming and rapid volume expansion (RVE) events, (b) problems relating to gas binding in the mixing
pump that can negatively affect mixing, and finally, (c) a few problems with nozzle plugging. Trouble
can show up: (1) in the pump, (2) in one or more nozzles, (3) in the mixing.
1. Troubleshooting the pump:
A. See the associated Vaughan pump IO&M manual for the applicable troubleshooting chart.
Anything that could cause trouble for a pump in any other system could cause similar
problems for a pump in a Rotamix system. To head off future problems, a careful pump
startup is important. Verify correct pump rotation and take and record suction and discharge
pressure readings and motor amperage readings to compare against future readings.
B. Pump cavitation may be evident from the Rota
mix™ system mixing pump when mixing your
tank at abnormally low levels. Mixing at very low levels can cause vortexing in your tank,
which results in drawing large amounts of air into the mixing pump. Low-liquid-level mixing
can also cause the nozzle to discharge above liquid level, a situation that can cause aeration
of the fluid in the tank. Either of these situations can cause pump cavitation and vibration,
which if allowed to continue will shorten the life of your pump(s). To stop vortexing, Variable
Frequency Drive (VFD) motor control systems can be used. By using a VFD, you can slow the
pump (and pump motor) down as tank level decreases and thus reduce pumping power going
into the system, thereby avoiding some types of vortexing and cavitation problems. However,
Vaughan Co. recommends that the slowest VFD speed used be no slower than 75% of
normal pump speed. For example, if 100% speed is at 60 Hz. on your VFD, do not run the
VFD at less than 45 Hz. Too slow of a speed reduces pump and system pressure and can
result in nozzle plugging.
2. Troubleshooting the nozzles:
A. Historically about ½ of 1% of Rotamix systems experience a plugged nozzle, so while
blockage can occur, it’s not a common problem. Nozzle blockage may show up as reduced
mixing within your digester or storage tank. Other symptoms of nozzle plugging would be
reduced pump electric motor power requirements (lower mixing pump motor amps), and
higher pump discharge pressure than normal (which is typically about 15-17 psi differential
pressure across the pump).
a. Nozzle blockage can happen at startup because of debris left in the tank. Careful tank
inspection
by Vaughan Co. or Vaughan’s authorized representative before closing the
tank for filling and startup is required and can help avoid this problem.
b. Older systems can eventually experience nozzle plugging problems if the mixing pump is
not properly maintained to keep pump parts sharp and cutting clearances close. Routine
pump maintenance per the pump and Rotamix I,O&M manuals can help avoid this
problem. Pump cutting parts are designed for long life (3-5 years of continuous use), so
that such an event should not occur on a new system unless construction debris has
gotten into the system piping. Annual inspection of the pump as required in our pump
IO&M manuals should help prevent nozzle plugging because of loss of chopping ability.
c. Some systems, which allow large amounts of rags into the tank to be mixed may only be
able to use Rotamix with only a few nozzles and with very large nozzle exit diameters.
Even with effective chopping and reduced particle size, very high rag and fiber content in
the slurry may create a slurry that is so hard to shear that the slurry shape will not
deform to fit through any nozzle size.
d. Fermenter tanks in some treatment plants have the potential to cause nozzle blockage
because fermenter tanks tend to have high fiber content in the sludge, which can lead to
