Rotary jets target tanks
July 1st 2008

Carefully planned and executed, tank cleaning does more than simply bring tanks back to mandated hygiene levels. It can improve process performance, slash water consumption and, as a consequence, reduce water, energy and disposal costs. Peter Rose of Alfa Laval explains

Some companies still regard tank cleaning almost as a necessary evil.

Legislative targets and mandated hygiene standards tend to dictate the thought and effort they put into the subject, since they really don't see that it can have a direct impact on their bottom line. With continually depleted resources and increased demand forcing up the price of both water and energy, it is an argument that doesn't really bear close inspection.

The key, of course, is selecting the correct methodology. For instance, there are several manual options available, ranging from the brush and bucket approach through the use of high pressure lances to a technique known as fill-boildump; a fairly precise description of what can be an imprecise cleaning method.

Basically, it involves filling a vessel with water and or a water/chemical combination, boiling it and letting it stew before dumping the contents. The process may have to be repeated several times to achieve acceptable levels of cleanliness.

Reliance on such techniques could be expensive not just in terms of time and labour but energy, water, chemicals and environmental impact.

Recognising this, many plant managers are turning to more automated tank cleaning methods which provide acceptable levels of hygiene, are less wasteful of materials and help to reduce time and labour. Of these systems, the most commonly used are static spray balls, rotary spray heads and rotary jet heads. Ostensibly, they work in a similar fashion, attacking dirt, grime and biofilm clinging to the interior of the tank with water and chemicals; however, the systems are different in terms of their approach, efficiency, cost and environmental impact.

The oldest is the Static Spray Ball (SSB), a ball-shaped, perforated machine which is fixed in place, individually or in clusters, inside a tank and attacks dirt and other deposits with high pressure jets.

Since the unit is static, the fluid jets always hit more or less the same spot on the vessel walls so, in those areas not directly impacted, the actual cleaning effect is zero. What the SSB technique depends on for cleaning is the flood of water cascading down the tank walls.

Consequently, SSBs require large volumes of water, high chemical concentrations, elevated temperatures and extended running times to achieve acceptable standards of hygiene. Powerful pumps consuming lots of energy are needed to drive the machines and heat the fluids. SSBs can become clogged with product particles carried by the cleaning fluid, leading to a reduction in efficiency and extended cleaning times.

Both the Rotary Spray Head (RSH) and the Rotary Jet Head (RJH) are more targeted in their approach, so energy costs, water and chemical use and environmental impact, are lower. They are also designed to be self-cleaning.

Rotary Jet Heads achieve high levels of cleaning efficiency and hygiene and reduce key operational parameters such as water and detergent consumption, waste and cleaning time by as much as 75%. As a consequence, energy consumption is also reduced. Unlike SSBs, rotary jet heads provide 360° coverage of all of the surfaces to be cleaned using programmed intensity and coverage rates to achieve maximum cleanliness. The motive power is provided by the cleaning fluid itself. Using relatively low pressures, it drives a turbine which rotates the nozzles around both vertical and horizontal axes. Nozzles are between 2.5 and 5.5mm in diameter, ensuring a long throw which brings even the largest vessels within the scope of the machines.

Cleaning is accomplished in a number of steps predetermined by a special computer program. An initial cycle is used to lay down a coarse pattern onto the tank walls after which subsequent cycles fill in the pattern until, by the eighth cycle, every square millimetre of the tank interior has been jet cleaned.

The fluid jets are also reflected back from the walls in the form of a spray which combines with the horizontal and vertical rotation to provide a mixture of deluging and turbulence that ensures even difficult to reach spots do not escape attention.

This is particularly important in more complex vessels and those containing agitators and baffles. In applications involving very sticky or stubborn products, the RJH can be fitted with an external air or electrically-powered motor to make jets even more powerful.

It is easy to test Rotary Jet Heads before they are installed. Software is available that simulates actual tank conditions so that a user can evaluate cleaning performance in terms of detergent distribution and wetting intensity. The optimum configuration can be determined before any equipment is installed, saving time and money.

Rotary Jet Heads, permanently installed inside a tank, will carry out a thorough clean, to high hygienic standards – in just a matter of minutes.

More articles from Alfa Laval Limited: