Guide to efficient tank cleaning

Previous: Part 3 - The quick win of improved mechanical action

Part 4 : Different tank cleaning nozzle designs

Spray Balls

These nozzles are spheres with multiple holes producing an omnidirectional spray. The jets from each individual hole do approximate to a solid stream but the spray pattern is best approximated to a full cone spray rather than solid stream. This is for the following reasons:

1- The holes in the spray balls are only very crude nozzles meaning that laminar flow in the resulting jets is limited. This is because only the thickness of the spray ball wall is present to form a coherent water jet. The streams that result break up easily and, generally, do not travel very far before breaking up.

2- The energy and water is divided. The water is split into scores of individual jets and so the energy of each is reduced. This means that each jet lacks momentum and reach.

What this means is that spray balls result in almost no meaningful impact on the walls of the tank. The vast majority of their cleaning power comes from the solvent properties of water cascading down the walls the tanks. This often needs to be boosted by the addition of cleaning chemicals or needs to be enhanced by long cleaning cycle times. In terms of water efficiency this type of cleaning is normally highly inefficient unless only very light and water soluble residues are being cleaned.

Spiral Wide Angle Nozzles

These nozzles produce a “cone” of spray up to 270° wide. This makes them suitable for cleaning tanks by inserting them towards the top of the vessel. The spray cone pattern is inefficient at energy transfer.

Multi-headed nozzles

Various nozzle manifold tank cleaners exist on the market. Several full cone nozzles will be positioned on a single head giving an omni-directional spray. Because multiple nozzles are being used to produce more direct sprays the impact per volume of water is increased. However the individual sprays are still full cones consisting of droplets and thus inherently inefficient. Such systems are suitable for small-medium sized tanks that require light to moderate cleaning.

Rotary Nozzles (Spinners)

These nozzles will have several flat fan sprays that rotate under the pressure of the fluid. The rotating fans will sweep the whole of the tank. As with static nozzles complete coverage happens only after a few moments but the nozzle spray will need to be working for while before any significant residue is removed. As a flat fan pattern is being used, there is a moderately efficient energy transfer resulting in a medium impact cleaning spray. These nozzles are often the most efficient choice for small to medium sized tanks that have moderately stubborn residues to remove.

Rotary Jet Cleaners

These machines have 2,4 or 8 solid stream jets which rotate sweeping the inside of the tank. A gearing mechanism changes the angle of the rotation so over time complete cleaning of the tank is achieved. These nozzles are by far the most energy efficient tank washers due to the solid stream jets being deployed. They are generally more expensive than other types of tank washer but for medium to large tanks or tanks with stubborn residues the additional capital expenditure is often paid for rapidly through efficiency gains.

Cycle times
Unlike the other types of tank cleaner mentioned above rotary jet cleaners need to run through a minimum set cycle time. This is because the jets need to be brought to bear on each part of the tank. Failure to run a complete cycle will mean that parts of the tank are simply not cleaned. Most rotary jet cleaners are geared to run fairly slowly. This is so that the jets have sufficient dwell time on each part of the tank to ensure maximum cleaning power. Also if the jets rotate too rapidly they tend to break up causing a loss in mechanical action. What this means is that for many tank cleaning applications rotary jet cleaners rotate too slowly. The mechanical action component is too high, each part is over cleaned but because the set cycle must be completed the overall water usage is higher than necessary. In short the tank is over cleaned, which is wasteful.

Fast cycle jet cleaners

These machines are smaller and lighter rotary jet cleaners that have been geared to run through their cycles much faster than normal. The resultant jets do not have as much dwell time on the surface of the tank and so do lose some of their cleaning power. Also, because they are spinning fast, the jets will tend to break up more rapidly. Nevertheless these cleaners will still give good mechanical action at distances below 2-3 meters from the nozzle.

The fast cycles means the jet cleaner can deliver a complete tank cleaning in a few minutes. The practical upshot is that for small to mid-sized tanks that have residues that do not require long jet dwell times to dislodge, this class of tank cleaner can offer some water efficiency gains. In effect this class of tank cleaner is designed to mitigate the limitations imposed by the long cycle times of standard rotary jet cleaners.

Impediments to changing cleaning heads

Different flows and pressures
Most spray balls, spinning spray balls and static nozzles will operate at low pressures between 1 and 3 bar. Flow rates can vary considerably with big spray balls using several hundred litres per minute and small spinner using as low as 20 litres per minute.

Rotary jet cleaners (both standard and fast cycle) tend to operate best between 5 and 10 bar pressure. Flow rates, again, vary considerably and depend greatly on the number of nozzles and pressure. Typically they will operate most efficiently in terms of water used per cleaning cycle at about 8 bar pressure.

Moving from spray balls to rotary jet cleaners will almost certainly improve water efficiency but it may necessitate a change of pump and / or pipe work. The good news is that the new fast cycle jet cleaners will normally be able to run at low pressures (1-2 bar). Their flow rates are often similar to that of existing spray balls so often can just be swapped in without having to change pumps. If this is the case it is still worth considering changing to a higher pressure pump because rotary jet cleaners will do a better job for less water per cycle at higher pressures.

Opening sizes
Spray balls will typically fit through openings smaller than 75mm. Some larger balls may require 100mm openings to fit through. Spinners are typically even smaller in profile with many models fitting through 50mm openings. Rotary jet cleaners will typically need 125mm plus openings to fit through but the new, smaller fast cycle models can fit through 75mm openings at a squeeze.

If you have openings above 75mm there may be no need to cut the tank to install rotary jet technology. If, however, you have under 75mm clearance then modifications to the tank will need to be made to install the more efficient jet cleaning heads. These might prove costly and so this needs to be taken into account.
 
ATEX
Spray balls and other static nozzle systems are effectively ATEX exempt. With no moving parts there is no chance of any sparking risk from such tank cleaners. As long as they are conductive and earthed then no charge can build as they are typically stainless steel attached to stainless steel pipes ATEX considerations are minimal.

Rotary jet cleaners and spinning spray balls do need ATEX certification if they are to be deployed in tanks that contain explosive environments. Thankfully there are many ATEX certified models on the market. It is, however, wise to look carefully at the ATEX claims of tank cleaner manufacturers. Getting an ATEX cert is an exhaustive process that covers not just the design of the device but also checks quality control and sourcing procedures to ensure compliance.

Some manufacturers have been known to use ambiguous phrases like “complies with ATEX design” or “ATEX exempt design” rather than simply stating the product is ATEX certified for X zone and Y temperature. If there is any doubt ask for the ATEX certification for the product. Compliance with ATEX is not something to take lightly or to have any ambiguity around. If the ATEX cert has been awarded then the manufacturer should be touting this fact in no uncertain terms because they will have had to go through a rigorous and costly process to get it!

Next: Part 5 - Conclusions and recommendations 

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