Form V372, Rev. 12
- 13 -
12/18, ECN4334
reweaving and the creation of large rags. Therefore, careful nozzle sizing is required for
fermenter tanks.
e. Tanks downstream of influent screens with insufficient capacity for all possible flows (for
example, a rain storm) may experience nozzle blockage problems if too many
unscreened rags overflow the screen(s) and get into the downstream tank.
f. Coarse influent screens can allow large amounts of fiber to enter a tank. This fiber can
then reweave to create large rags, which then may cause nozzle blockage problems
under some conditions. Fine influent screens (
1/4” (6mm) or so) are best.
g. Any tank where influent screenings are dumped are not suitable for nozzle mixing
systems like Rotamix.
h. Corrective action to resolve nozzle blockage issues is to first consult with Vaughan Co.
or Vaughan’s Authorized Representative for help. The use of larger nozzle openings
may help, or in some cases, the nozzle system may have to be revised to use fewer and
much larger nozzles. Sometimes modifying the mixing pump to add finer chopper
capability may also be an option.
B. Nozzles need to be placed correctly in the tank as described by Vaughan Co. Rotamix nozzle
plan drawings for the project. The nozzles also must be aimed correctly in accordance with
Vaughan drawings. A proper evaluation of the nozzle installation prior to closing the tank by
Vaughan personnel or by trained Vaughan sales representatives is required and can head off
problems with an improper nozzle installation or nozzle aiming.
C. In multiple tank Rotamix systems, sometimes nozzle size from one tank to another are
different sizes. If nozzles are mixed up so that incorrect (too large or too small) nozzles are
used, a particular tank mixing system may not mix as well as expected. Again, a proper
evaluation of the nozzle installation prior to closing the tank by Vaughan personnel or by
trained Vaughan sales representatives is required and can head off problems with installation
of incorrect nozzle size.
D. A worn nozzle may show up as increased mixing within your digester or storage tank. Other
symptoms of a worn nozzle could be higher than normal mixing pump electric motor power
requirements (higher amps), and lower pump discharge pressure compared to your recorded
readings at startup. You may also hear pump cavitation noise, a crackling sound, similar to the
noise of pumping gravel. Many people incorrectly interpret cavitation as a pump bearing
problem. Nozzles on Vaughan Rotamix™ systems are made of glass-lined ductile cast iron
(and sometimes made of abrasion-resistant polyurethane). So nozzle wear should take many
years to achieve.
E. Operating a digester mixing pump on a Variable Frequency Drive or VFD can be very helpful
to reduce digester reaction rates to help control foaming or Rapid Volume Expansion (RVE)
events. However, operating the mixing pump at speeds slower than about 75% of full speed
increases the risk of nozzle plugging since flow and pressure generated by the mixing pump is
also reduced by lower speed pump operation. Too little flow and pressure through a nozzle
may not keep the nozzle clear of debris. Vaughan recommends a minimum mixing pump of
75% of full speed.
F. Reduced speed mixing is not recommended for sludge blend tanks or sludge storage tanks.
3. Troubleshooting mixing:
A.
Poor mixing performance from the Rotamix™ system may be caused by air or gas binding in
the mixing pump(s). Proper venting and filling of the pump casing at each startup is required
for the pump to work to its normal capacity. If the pump is partially full of air, it will not
generate normal pressure or flow, and therefore the flow through the nozzles will be
inadequate. Mixing increases Volatile Solids Reduction or VSR and this means that more
gases of decomposition
– methane and carbon dioxide – are generated. So, more gas
generation in a digester can also increase the risk of gas binding. The plant operator may not
know that gas binding is occurring. This is a problem because while he is operating the mixing
