Estimating the carbon foot print savings achieved by moving from less efficient methods of tank cleaning to more modern ones is quite difficult. However, an approximation of the levels of saving can be made if we use overall cost as proxy measure for environmental impact. This method is not perfect but in the case of tank cleaning it is more or less true that if one reduces cost then sustainability will be increased.
Cost saving calculations
Firstly, the overall water consumption needs to be calculated for each stage of the clean.
Any chemicals used in the caustic stages need to be costed in as well.
The cost of heating the water needs to be calculated. This is straightforward enough using the energy cost to the business in Kwh multiplied by the Kwh needed to raise the water to the desired temperature. This can be modified downwards if heat recovered from another part of the factory is used. So, if 50% of the heat is recovered the overall cost can be halved.
Next the cost of water treatment and disposal needs to be calculated. This will vary greatly depending on the COD and BOD load of the waste water. When water recovery CIP systems are used an estimate of the percentage of recovered water should be made to reduce this cost. So, if only 25% of the overall water used is dumped then this cost can be similarly reduced. This “recovery percentage” can, of course, also be applied to the cost of the initial water and, when caustics are recycled, the cost of chemicals.
Finally, the energy costs required to pump the fluids through each stage of the clean need to be calculated. This can be derived from the wattages of the pump and the time taken. In practice, however, this has a very small effect on overall cost when compared to the other costs. A 2.5Kw pump running for an hour’s cleaning cycle only has an energy cost of 35p (at 14p/ Kwh) this is likely less than 5% of the overall cost of cleaning. Any changes in pump duty to a higher pressure, but lower flow rate, are likely to balance out meaning changes in pumping costs will be negligible when compared to the other factors.
When all this is added up together, we get an estimate of the cost of each cleaning cycle. This cost is a reasonable proxy measure for environmental impact.
Consider the following example
The current cleaning cycle is performed by and SVSTW 3/4” threaded spray ball running at 314 litres per minute at 2 bar. The cleaning cycle consists of a pre rinse for 10 minutes a caustic clean of 40 minutes and a final rinse of 10 minutes. The caustic clean is at 80 degrees (60 above ambient). It is estimated that 75% of water can be recovered and reused and 50% of the heating can be delivered by heat recovery. The cost of energy is estimated at 14p per Kwh. Fresh water is costed at £1.3 per m3 and water treatment at £3 per m3.
This is then compared to a rotary jet cleaner running at 8 bar pressure for example to Orbitor 2 with 6mm nozzles. This has a cleaning cycle of 19.5 minutes and a flow of 140 litres per minute. For pre rinse and rinse cycles a half cleaning cycle can be used on these tank cleaners as complete wetting is still achieved in this time.
SVSTW Spray ball running at 2 bar pressure
| Cycle | Cycle Time (min) | Flow rate (l/min) | Cost Of Water | Cost Of Waste Water | Cost of energy KWhr (£) | oC heated | % of heat from heat recovery | % of water recovered | Total cost/m3 water used | Total cost / cycle |
| Pre rinse | 10 | 314 | £1.30 | £3.00 | £0.14 | 0 | 50% | 75% | £1.08 | £3.38 |
| Caustic wash | 40 | 314 | £1.30 | £3.00 | £0.14 | 60 | 50% | 75% | £5.98 | £75.05 |
| Rinse | 10 | 314 | £1.30 | £3.00 | £0.14 | 0 | 50% | 75% | £1.08 | £3.38 |
| Total Cost Per Cycle | £81.80 |
Orbitor 2 jet cleaner running at 8 bar pressure
| Cycle | Cycle Time (min) | Flow rate (l/min) | Cost Of Water | Cost Of Waste Water | Cost of energy KWhr (£) | oC heated | % of heat from heat recovery | % of water recovered | Total cost/m3 water used | Total cost / cycle |
| Pre rinse | 10 | 140 | £1.30 | £3.00 | £0.14 | 0 | 50% | 75% | £1.08 | £1.51 |
| Caustic wash | 20 | 140 | £1.30 | £3.00 | £0.14 | 60 | 50% | 75% | £5.98 | £16.73 |
| Rinse | 10 | 140 | £1.30 | £3.00 | £0.14 | 0 | 50% | 75% | £1.08 | £1.51 |
| Total Cost Per Cycle | £19.74 |
As we can see from the above data the cost of each cleaning cycle is reduced by over a factor of 4. One can always argue about the amount of heat and water recovered or the raw cost of power and water, but the basic principle remains solid, the fewer m3 of water used, treated and heated in a cleaning cycle, the lower the environmental impact.
The cost of changing
There will be some initial costs incurred. Spray balls and spinners generally run at around 2 bar pressure. Rotary jet cleaners will operate best at between 8 and 10 bar, so there may be a need to upgrade the CIP pump when swapping over. In terms of flow rates, the rotary jet systems will normally be significantly lower, so the overall cost of pumping (even at the higher pressure) is not going to change that much but there may be a capital expenditure to change the pump to one with a higher maximum pressure.
Rotary jet cleaners themselves are more expensive than spray balls or spinners so that also needs to be considered, again this is a one-off capital expenditure.
Pipework is unlikely to need to be changed. The overall flow rates will generally be lower so the pipework that feeds the existing spray balls or spinners will in almost all cases be enough for the new duty.
Payback
Normally the capex needed to swap over can be paid for within a matter of months. Consider the example above. If we assume a capex of £5000 for a new higher-pressure pump and £3000 to cover the new tank cleaner and fittings, we then have an outlay of £8000 for the new systems. Let’s add on £2000 for installation costs for a round £10,000 of capex to swap over to new rotary jet cleaners. If the tank is cleaned once per day, we see a pay back within 161 days so under 6 months. Obviously for sites with multiple tanks all fed by the same CIP system the payback will be much quicker.
Conclusions
Improving the water efficiency of tank cleaning operations can contribute towards an organisation achieving its sustainability targets. In some industries, like dairy, this contribution can be very large whereas in others it is more modest. Spray balls and spinning nozzles are still very commonly deployed which means there is a great opportunity for engineers to meet those sustainability targets. This is a quick and painless win when trying to reduce the environmental footprint of an organisation. The really good news is that the costs of swapping can be quickly paid back and meeting the sustainability targets can also keep the bean counters happy. A win-win situation.
