A Self Cleaning Wet-well - empoweringpumps.com
Transcript of A Self Cleaning Wet-well - empoweringpumps.com
A Self Cleaning Wet-well
The ‘Inside story’ on a radical design approach
Is the cleaning of a wet-well just a costly and troublesome fact of life? Or is there truly an effective, cost efficient alternative?
Hydro Innovations © 2009
Topics:
1. The Typical Wet-Well
2. Why does fat build-up happen
3. Some Cleaning methods
4. Some “automatic” alternatives
5. The solution
1. The Typical Wet-Well
The typical wet-well consists of a pair of submersible pumps sitting at the
bottom of a wet-well (or pit) that could be between 1.5m and 3.0m in
diameter. The pumps are fitted to guide rails, which guide the pumps onto
discharge elbows located at the base of the well.
The pumps cycle alternatively during inflow periods (generally heaviest around
meal times), and pump between the “on” and “off” levels in the well. These
levels are generally determined by the level of the gravity influent line (invert
level) and the level equivalent to the top of the pumps.
Preventative measures fail to capture these products
Grease, fats and oils enter the systems when grease traps or other
preventative measures fail to capture these products, or when operators of
food out-lets are not diligent, or have accidental spills. To a lesser extent, the
householder, not aware of the consequences of their actions, could be using
their sewerage system to dispose of waste oil products.
The pumping stations that will generally “see” the most inflow of fats and oils
will be those located near high concentrations of restaurants, food outlets and
shopping centres. It is at these locations that operators are generally most
concerned about the effects that a build-up of fat in their wet-wells will have
on odours produced and the consequent need to clean them.
2. Why does Fat Build-Up Happen?
Fat build-up happens in the wet well as a consequence of the specific gravity of
the fats and oils, and the design of the “typical” wet well and its “typical”
equipment.
The fats and oils, being lighter than water, will naturally float on the surface.
Now visualise the DESIGN of the typical wet well trying to cope with this. The
pumps start when the “on” level is reached in the well, and stop when they are
somewhere near the top of the submersible pump motors. At this level in the
pit, the well is still at its full diameter and the pump suction could be upwards
of 400-500mm below this level, giving the pump virtually no chance of
removing any of the oils or fats.
“Grease blanket”, then rides up and down
This “grease blanket”, then rides up and down with the water level, being built
upon with each and every inflow, with virtually no chance of any volume of it
being taken away by any of the standard pump cycles.
3. Some Cleaning Methods
Lowering the “off” level would seem to be a logical method of “getting” to the
fat with each pump cycle. This has 2 major drawbacks from being the perfect
solution.
Firstly, any lowering of the “off” level would expose the submersible
electric motor to possible cooling problems.
Secondly, and most importantly, any lowering of the level could cause
the submersible pumps to vortex and possibly become air-bound on
their next cycle. This naturally would mean a call-out for the operators
and a re-priming of the pumps. And in any case, lowering the off level
could still mean that the oils and fats still sit above the pump suction.
Problems require addressing odour issues and functionality
So, having written this approach off, all that is left for the “typical” system is to
clean the well after it has reached a point with odour or functionality issues
that require addressing of the problem.
The common approach here is to use a combination of chemicals to break
down the greases, high pressure cleaners and/or some sort of self priming
pump or “vac truck” to completely remove wet well contents.
Expensive, Labour Intensive and with O.H. & S. risks
Any of these things is not inexpensive. They also all require the removal of the
wet well “lids” and safety grills to allow access to the well. This is not only
another expense, but increases the O.H. & S. risks and brings into play the
confined spaces regulations.
It becomes a minimum of a 3 person operation. More likely 5 or 6 when you
consider the need to operate safety lifting equipment, operate the pressure
cleaner, and have the mandatory observer. It may also require an electrician to
disconnect pump cables to minimise the risk of cutting them with the high
pressure cleaning equipment.
Cleaning wet-wells this way, is an expensive exercise.
4. Some Automatic Alternatives - limitations
Automatic wet well cleaning systems
We have come across several “automatic wet well cleaning systems”, but they
are invariably complicated mechanically and/or electrically. They are generally
based on some kind of water jetting to wash down walls and/or churn up the
“fat blanket”, but then still rely on the station pumps or a surface mounted
pump to remove the fats.
Flushing valves
Another popular (but possibly not with operators) method is the use of various
flushing valves. These valves are supposed to stir up greases and fats so that
they mix with the water and can be pumped away. This sounds good in theory,
but oils and fats are still lighter than water and still need to have “off” levels
close enough to the pump suction to draw fat into the pump. This again runs
the risk of pumps vortexing and becoming air-bound on a subsequent pumping
cycle.
Reliance on equipment at the bottom of the wet-well
In addition to the limited success these types of systems have, they still rely on
a piece of mechanical equipment that is located at the bottom of the wet-well
to function properly. It is equipment that will require maintenance and repair
from time to time and just another thing in the pump station that can go
wrong.
5. The Gorman-Rupp Solution
The best solution is prevention (that works), rather than a cure (cleaning after
fat build up).
Gorman-Rupp self priming pumps, when used for the main pumps at the
station, can be “set up” to provide a “self-cleaning” function:
- without adding one extra piece of mechanical equipment to the
station
- without adding a single electrical component to the control
system
- and without having to enter the wet-well or even open the wet-
well lids.
The answer lies in taking advantage of the superior air handling capabilities of
the Gorman-Rupp self-priming centrifugal pump.
Provided suction conditions are right (and Gorman-Rupp Super-T series and
Ultra-V series pumps are designed to be able to prime to 7.5 metres), suction
lines can be set very close to the bottom of the well, and pumps run until they
lose prime (are basically ”sucking air”).
No chance for grease build-up
This means that virtually all of the contents of the wet well are removed
(including the oil and fat) with each and every pump cycle, not giving greases a
chance to build up at all.
To achieve this result, pumps can be retrofitted to existing wet wells with
conical shaped bases, or older style rectangular wet wells, as well as purpose-
built new ones.
In all these cases, because self-primers only need a small amount of space for
the suction lines to fit into, they can be positioned at the deepest and
narrowest part of the well to ensure that the surface area of un-pumped fluid
after the pumping cycle is much smaller than the maximum diameter of the
well. In some cases, the surface area remaining can be as little as one tenth the
surface area of the main chamber of the wet well, so at the very least, in this
instance, fats will take up to ten times as long to build up.
There are numerous other benefits to using self-priming pumps in wastewater
applications which include:
• Lower maintenance costs
• Longer service life
• Greatly improved O. H. & S. conditions
• Safer operation
• Virtual elimination of Confined Spaces issues
We trust this paper has been thought provoking and of value to you.
There are several other papers that Hydro Innovations can make available to
designers of pumping systems. We also have information discs and
presentations on these subjects.
Call us on 02 9647 2700 so you can have a real ‘person’ contact when you
have ANY questions on pumps and their applications. We’ll also arrange for
you to receive priority advice on any new technical developments, new
product releases, tips on cost savings and maintenance, etc. Our mission is to
be a valuable resource to help YOU do your job better.
Hydro Innovations General Manager & author of the white paper series, Garry Grant
Hydro Innovations © 2009