When size really doesn’t matter

Automated cleaning solutions for very large tanks

Tank cleaning systems are commonplace in many industrial and food processing systems. The size of the tanks being cleaned, along with the residue type, will dictate the most appropriate tank cleaning head to use. Most process tanks will vary between 1 metre and 10 metres in diameter. This size of tank falls well within the range of most tank cleaning heads. But what about very large tanks? Are there differences or additional points to consider when cleaning very large tanks? This article explores some of what we have learned on this topic over the years.

The systems in question

This article focuses on rotary jet cleaning systems. These are fluid powered tank cleaning heads that form powerful cleaning jets which rotate in a methodical way to provide complete, automated cleaning of the tank below. These types of tank cleaning head are commonly found in the food processing and chemical industry but are sometimes overlooked when it comes to very large tanks. The good news is that these systems scale up and can deliver cleaning jets more than 25 metres in length meaning a potential cleaning diameter of 50 metres from a single machine. Of course, for very large tanks multiple cleaning heads can be used.

Experience

A lot of our experience in very large tank cleaning has been gleaned from our work with the water industry in cleaning storm water attenuation tanks. These tanks can be 45 metres or so in length and 15 metres wide with volumes of 2700m3. The other type of very large vesselsthat are cleaned are, of course, the large oil tankers which have similarly size tanks. Our partner company, Dasic Marine, is a world leading expert in marine tank cleaning so,between us, we have decades of experience in cleaning these very large tanks. Below is some of what we have learned.

Reach / jet length

The first and most obvious point is that large tanks will need a tank cleaning head that can reach each part of the tank. Rotary jet cleaners will each be rated to have a certain effective cleaning jet length at a given pressure. The higher the pressure the longer this jet length will be. However, above certain pressures the velocity of the jets becomes too great and turbulent rather than laminar flow occurs. This means the jets break apart.

The optimal pressure for most jet cleaners is between 8 and 10 bar pressure. This gives a good balance between flow and power and yet keeps the velocities to a level where a coherent and laminar jet can be maintained over a long distance. Within this sweet spot of pressure, jet lengths of 25 metres or so can be achieved without the jet breaking up too much. Whilst it is tempting to up the pressure and so up the jet length, we have found that jets produced much above 10 bar tend to be unstable over the distances required. High pressure jets are great for cleaning when they can be within a few metres of the target but for the long distance required in large tank cleaning they are simply not viable. As such, 8-10 bar pressure is, in our opinion, ideal.

Time and flow

Large tanks will take longer to clean. The reason for this is that, to keep overall flow rates sensible, only two jet tank cleaning heads can be deployed. The large 4 or 8 jet tank cleaners do complete their cleaning cycles faster but at the higher pressures and larger nozzles sizes required to produce 15 or 20-metre jets their flow rates become very large. As such the cost of pumps and pipework become larger still. So, it becomes more sensible to use two nozzle systems which can still get the same jet length but at half the flow rate.

The good news is that with most very large tanks the time of the cleaning cycle is not usually a pressing concern. Unlike smaller process vessels which may need to be cleaned quickly tonot delay production, large tanks tend to be cleaned less frequently and with less urgency. This means the luxury of time is our friend when it comes to tank cleaning.

Surface area to volume ratio

Large tanks tend to be shaped somewhat differently to standard cylindrical process vessels. As tanks get larger their surface area to volume ratio gets smaller and smaller. If, however, tanks are shaped “less efficiently” then this ratio may increase. The most efficient shape in terms of surface area to volume ratio is a sphere so the less spherical a tank gets the more its ratio of surface area to volume will increase. Cylindrical tanks are closer to the spherical ideal than rectangular tanks. Tanks with more similar length, depth and width dimensions also have lower surface area to volume ratios.

What this means is that tank cleaners sized to clean a certain volume of tank cleaning vessel will not necessarily scale to clean a larger one. It’s not good enough to simply take a tank cleaner that will clean a 100m3 tank and then use five of them to clean a 500m3 tank. This is especially true if the tank is shaped less efficiently i.e.  it is relatively long and shallow. The less efficient designs of large tanks along with the general trend towards lower surface area to volume ratios for larger tanks may sometimes cancel each other out and result in some of the old rules of thumb working by chance.

For these reasons it is often a safer bet to use overall tank surface area as the critical variable rather than volume. When it comes to larger tanks we tend to recommend a minimum overall water usage per m2 being cleaned rather than per volume of tank being cleaned. This allows us to make some judgement on the total volume of water necessary to provide rinsing and carry off the dislodged residue.

Floor space

Another aspect of large tanks is that, in many cases, they tend to be much shallower than their smaller counterparts. A typical process tank will be twice as tall or more as it is wide. Very large tanks, like storm tanks or oil tankers, tend to be the opposite proportions or more. So, we have tanks with much longer lengths than their depths. This being the case the proportion of surface area taken up by the floor is typically much higher in larger tanks.

The greater proportion of floor space means we need to consider more carefully whether there is enough flow to allow residue to exit the tank and not simply be moved around. In large height to width ratio vessel this is not normally a consideration. Most vessels of this configuration will drain rapidly, and any dislodged residue removed quickly. In tanks with proportionally large floors, however, there is a danger that flow will not be sufficient to carry the residue away and hence it will simply be moved around the floor.

To combat this, we will calculate an overall volume of cleaning fluid per m2 of floor space. There are no hard and fast rules as to what the volume of cleaning fluid to floor space should be, as this would vary greatly depending on the residue. From experience we can generally advise on what will and won’t work. Our experience in Storm Tank cleaning has be crucial in developing this tacit knowledge. Storm Tanks can often have lengths 10 times their depth meaning a very great proportion of the tank surface area is floor. As such we know that we want to keep the total volume of fluid used in the clean above 60 litres per m2 of floor. This will mean there will be adequate flow across the floor to carry away a storm water residue. Other residues and types of tank will have different ratios of course.

The chart below shows tanks of the same overall volume and how the floor proportion changes. Larger tanks will tend to be shaped differently and so more commonly have higher proportions of their overall surface area made up by the floor and so tank cleaning systems need to be adapted accordingly.

Open tanks

Further problems can occur when trying to clean large open-topped tanks. Firstly, we obviously need to limit cleaning to a 180o downward pattern, this limits the choice of tank cleaner. Secondly, the positioning of the tank cleaning head can be difficult as it will need to be attached to a wall rather than hung from the non-existent ceiling.

In such situations we normally will mount the tank cleaning head on a positioning arm that is attached to tank wall. The arm will need to swing back to the side of the tank to allow it to be removed for maintenance. These swing arms normally have a practical limit of being about 2 metres in length. What this means is that for tanks above 4 metres in width the tank cleaner will not be on the centre line of the tank. This will result in a differential cleaning of the tank. The near side will be getting more impact and a greater proportion of fluid per m2 of area being cleaned than the far side.  

For very wide, open tanks we have started to experiment with angling the tank cleaners, so they get a greater proportion of the cleaning fluid towards the centre of the tank. The downside of this is that the top of the near side wall will be missed and one needs to be careful of over spraying the far wall. When the width distance is long there will be some natural arching of even powerful jets so with careful angling of tank cleaners we can avoid over spray problems on the far side (see diagrams)

The general objective of angling is to get as much of the flow concentred on the floor of the tank as possible. This is because the proportion of the area being cleaned that is made up by the floor is, as discussed above, much higher than in a standard “tall” tank configuration. Most tank cleaning heads are designed to concentrate a high proportion of the cleaning fluid on the tank floor as this is where residue concentrates. So, if we imagine a 90-degree cone below the tank cleaner, this is where the bulk of the cleaning fluid will end up being sprayed. When tank cleaners are angled (as illustrated) we help to get more of this 90-degree concentrated cone onto the floor area.

Tank material of construction

Most smaller process vessels will be constructed from stainless steel. Stainless is a great material for keeping clean as it has a fine finish leaving little for residues to cling to. It is, of course precisely this reason why it is the material or choice for most hygienic tanks. When it comes to very large tanks, the cost of stainless steel may mean that other materials are used,and this may have a detrimental effect on cleaning.

When materials like concrete are used there is more opportunity for a residue to adhere to the tank wall. We see a similar effect occur in old metals tanks that have worn and slightly pitted surfaces. As such it is often necessary to compensate and adjust the tank cleaning head accordingly. Most rotary jet tank cleaners will be rated as having an effective cleaning jet length for any given pressure. The total reach of the jets will be much higher than this length,but the effective cleaning length is a judgment as to the maximum distance at which the cleaning jets will clean most residues. This judgment is normally based on experience and data from cleaning stainless steel tanks. So, when considering tanks made of material that are less conducive to easy cleaning we need to adjust the effective jet length. For concrete tanks we therefore tend to ensure the installed tank cleaners have an effective jet length some 33% - 50% higher that the figure it is rated for. So, for example, a tank cleaning head that has a 20 metres effective cleaning jet should be responsible for cleaning parts of the tank only 15 or so metres away from it.  

Conclusion

The cleaning of large tanks will not always be a simple case of scaling up the rules used for cleaning smaller tanks. Changes in the ratios of floor to overall surface area may necessitate changes to tank cleaning methodology. Similarly, different materials of construction may result in significant changes to the type of tank cleaner used. There are no hard and fast rules but, as with most things in life, experience really matters. It is, therefore, important to speak to a company that really understands large tank cleaning and who can bring to bear the tacit knowledge necessary to advise accordingly.

General rules

About SNP

SNP saves our customers time, money and hassle in improving their processes and products by helping them design or implement effective and efficient industrial spray systems and solutions.

SNP supplies spray nozzle solutions to industries worldwide, including the largest engineering, food processing, chemical and petrochemical companies, as well as a vast array of small and medium sized enterprises (SMEs). The company’s product range includes tank washing nozzles, hydraulic nozzles, air atomising nozzles, custom piping assemblies and spray accessories as well as disinfecting and washdown systems.

SNP is as skilled in advising design engineers at the beginning of projects as it is recommending and supplying replacement products that may be needed in a hurry. The company can also provide innovative custom solutions, including using non-standard materials. SNP holds ISO 9001 certification. SNP is an exclusive distributor for US spray nozzle manufacturer BETE, for pre-treatment specialist Uni-Spray, specialist paper industry nozzle manufacturer ML Gatewood and tank cleaning systems manufacturer Dasic. In addition, SNP supplies specialist spraying equipment from Guarany. Orders are handled quickly and efficiently from SNP’s own dedicated fulfilment facilities in the Midlands.
01273 400092 | info@spray-nozzle.co.uk | www.spray-nozzle.co.uk