How to efficiently clean Tanks

Tank cleaning as a virtue
Peter Rose, Marketing & Communications Manager, Alfa Laval Limited

According to Sinner’s Circle ( Fig. 1) there are four parameters in any cleaning process that define the protocol and determine its efficiency:  Temperature, Chemistry, Mechanical action and Time. For any cleaning process to be completely successful, these four factors should be in balance.
If, for instance, the cleaning fluid’s temperature or volume is reduced, one or more of the other parameters has to be altered to restore the balance, otherwise the cleaning process will become less efficient, either in effect or cost; or both. This applies to the internal cleaning of tanks and other vessels as much as to any other cleaning operation and can be the key to achieving maximum storage or mixing efficiency with the least risk of contamination. So how is this balance best achieved? What are the options available for cleaning tanks and vessels internally?

Rub a dub dub

Despite modern technology, tanks can be – and frequently still are - cleaned manually using such basic equipment or techniques as:

• hand, bucket and brush
• high-pressure spray lance
• Fill, boil, dump (where the vessel is filled with boiling water which is agitated and then dumped.)

Results can be variable because of the human element involved and such procedures are generally regarded as inadequate to meet the stringent hygiene demands of the modern process industries. In addition, the bucket and brush approach would, nowadays, probably make a diligent Health & Safety officer’s hair stand on end. Nonetheless, these systems are still in use, both as legacy installations and in plants where cleaning and hygiene issues do not enjoy the highest priority.

Cleaning-in-place (CIP)
CIP is a method of cleaning whole plants or individual items of equipment without the need to open or dismantle them.  Generally speaking, the process is highly automated and involves little or no manual input, thus removing the opportunity for human error or omission.
There are three different types of automated CIP spray equipment normally used for internal tank cleaning. The different solutions depend on almost opposite approaches to achieve a clean tank: Either a combination of high volume and low pressure or the opposite; a mix of low volume and high pressure

Static spray balls (SSB) are normally used where the initial purchase cost is a prime factor in decision making, where hygiene requirements are limited, and where, generally, there is a lack of appreciation of the technology’s inherent limitations. Regarded as the traditional CIP solution, static spray balls are increasingly being superseded by more economical and more effective solutions such as rotary spray and rotary jet heads.

High volume/low pressure
The reason is simple. SSBs spray large volumes of low-pressure liquid onto the inner walls of the tank. However, because the jets from the static spray ball repeatedly hit the same, limited number of points on the tank wall, the cleaning effect depends almost totally on a flushing and rinsing effect caused by liquid cascading down the tank wall and over any built-in components.

The limitation of this approach is fairly obvious – the lack of control. Frequently, the only way to ensure that the specified hygiene level is achieved is to increase the volume or temperature of the cleaning liquid, the concentration of chemicals, the time spent cleaning or all of these parameters together.

Another consequence of limited controllability is that results cannot be replicated accurately and consistently to provide the kind of predictable cleaning results that are central to any meaningful quality assurance programme. This high-volume/low-pressure process struggles to remove product build-up, bio-films and bacteria cultures, which tend to cling tenaciously even to extremely smooth surfaces. SSBs, generally speaking, are not suitable for cleaning tanks used for products that foam, are highly viscous, thixotropic, or sticky.

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 cleaning efficiency and extended cleaning times. Since the machines are static, there is also an ever-present risk of product build up on the surfaces of the SSB; eventually making it part of the cleaning problem rather than the solution.

Low volume & high pressure
The alternative and more effective approach is to spray or jet a significantly lower volume of cleaning liquid at significantly higher pressure onto the surfaces to be cleaned. The physical impact of the spray or jet produces a scrubbing action which is far more effective than the cleaning achieved by merely rinsing with large volumes of liquid. Directing the jets or spray in pre-determined patterns to ensure maximum impact and coverage enhances the overall efficiency of the cleaning operation. Consequently, cleaning in this way requires less cleaning liquid and fewer chemicals. Since it is also much faster, it also increases productivity and reduces Downtime.


Rotary spray heads (RSH) are normally used for tanks that are relatively easy to clean and where a combination of good hygiene and low overall operating costs are important. The flow of the cleaning media causes the head of the spray head to rotate. The fan-shaped jets lay out a swirling pattern throughout the tank which generates the impact needed for the efficient removal of residual product; the cascading flow covers all internal surfaces of the vessel. A rotary spray device is a much more effective alternative to traditional static spray balls because of its low cleaning fluid usage and the low pressures needed for effective cleaning.

Rotary jet heads (RJH) are the equipment of choice for large tanks that are relatively difficult to clean. Rotary Jet Heads provide 3600 indexed cleaning that is computer designed to impact on every surface for a pre-determined period. Nothing gets missed and water usage is kept to a minimum. More importantly, the time required is so short that cleaning can often be carried out prior to start-up or after shut-down, with no consequent loss of production. Since the process is fully automated, labour can also be diverted to more productive tasks.

In action a RJH scrubs the tank’s internal walls  with fan shaped jets that cascade down to produce a vortex as they reach the bottom. An internal turbine and gear system uses the energy of the cleaning fluid itself to provide the motive force.  The nozzles are between 2.5 to 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 jets also rebound from the walls  to form a spray. This combines with the horizontal and vertical rotation to provide a mixture of deluging and turbulence that reaches every surface, including agitators and baffles. Tenacious residual product, bio-films and fouling are readily disposed of given the correct level of force and coverage. Thanks to the enhanced mechanical efficiency provided, other parameters from Sinner’s Circle – temperature, chemicals and time - can be reduced.


Alfa Laval Ltd
Camberley
Surrey

Can be contacted on

Tel: 01276 63383
Fax: 01276 685035