Water and Effluent Treatment Magazine
Issue link: https://fhpublishing.uberflip.com/i/503268
20 WET NEWS MAY 2015 Storm tanks can deal with excess water during storms but their odour pollution is a problem. Is impingement cleaning the answer? The challenge: Whiffy storm tanks INSIGHT TAnkS & STorAge The big tanker cleaners will have 14, 16 or even 20mm nozzles that can throw a cleaning jet over 25m THE CONCEPT • The big tanker cleaners have 14, 16 or 20mm nozzles that can throw a cleaning jet over 25m, which means a cleaning radius of 25m • raw oil is put through high-powered rotary jet cleaning systems to dislodge residue and clean the tanker • rotary jets spin through a set cleaning cycle directed to each part of the tank NEED TO KNOW 1 A 50m diameter tank could be cleaned with one machine 2 Storm tanks need to have enough residue removed to prevent odour problems 3 Machines have an exposed gear system that can become gummed up with algae, silt or smaller debris THE VERDICT The redeployment of tanker cleaning technology to solve the problems of cleaning storm tanks is logical. The level of cleaning required in storm tanks is very similar to that of tankers because they do not need to be spotless. A s climate change drives more extreme weather the occurrences of wastewater and sewage systems being unable to cope with excess water is increasing. As a result, storm overfill tanks are being used more frequently as is the need to fill and empty them more regularly. It is unsurprising that more storm tanks are being commissioned into the UK's water system and the situation is acknowledged as a current "hot topic" of focus for the Environment Agency (EA) with attention being directed at reducing odour pollution. This means storm tanks are being used more heavily at the same time as the EA is insisting that any foul smelling residues need to be cleaned more thoroughly. Current methods of cleaning include: • Emptying large buckets of water into the tank from one end, creating a wave to remove debris from the bottom • Eductor / swirl systems, which use a large educator nozzle to create a whirlpool as the tanks empty to dislodge debris • Manual cleaning, where operators enter the tank with a pressure washer • Impingement cleaners, where powerful cleaning jets target each part of the tank Impingement cleaning is the most water efficient method of cleaning a tank. Although not new, this technology has been deployed for cleaning all types of tanks and process vessels for decades. Most large chemical, petrochemical and food pro- cessing plants will have impingement tank cleaners as part of their vessel cleaning sys- tems. This technology has been used with mixed results for cleaning storm tanks but oˆen runs into problems for the fol- lowing three reasons: • Scale. The size of tanks in the chemical and food industries are smaller than storm tanks • Environment. Unlike the impingement cleaners deployed in the process industries, storm A solution: Tanker cleaners give slick polish The rugged design of oil tanker cleaners make them ideal for cleaning storm tanks. I n shipping, cleaning oil tankers is essential - crude oil forms thick sticky layers of waxy residue over time, impeding the tanker's carrying capacity. The problem is that oil residue is very difficult to clean and using water to clean tankers creates a massive amount of oil / water mix that needs to be filtered and disposed of. Also, water does not clean oil residue off very well. The solution is to use crude oil as the cleaning fluid. Raw oil is put through high-powered rotary jet cleaning systems to dislodge residue and thoroughly clean the tanker. The tank is mostly emptied and the remaining oil sludge / mud is circulated through the tank cleaning system. The very large solids removed are filtered out but the rest of the sludge is passed through the impingement tank cleaner. These cleaners are more powerful than their counterparts deployed, for example, in the food industry, and they need to remain in place unused for months oˆen submerged in the crude oil in the tank. This type of cleaning technology could be applied to the challenges of cleaning storm tanks. The principles of impingement jet cleaning are exactly the same as those in the food, pharmaceutical and chemical industries. Rotary jets spin through a set cleaning cycle directed to each part of the tank, blasting away residue as the jet "explodes" outwards on impact. However, there are key differences in the design of oil tanker cleaners compared to process tank cleaners, including the presence of a sealed greased gearbox within the machine. Process industry tank cleaners are normally lubricated by the cleaning fluid with an exposed gear system. This is sensible in hygienic applications where contamination from lubricants is a considerable concern. Contamination conserns in the oil tanker cleaning applications are less important so a sealed lubricated gear box can be used, allowing far more particulate to pass through the cleaner with little or no risk of clogging it. Bacteria When designing rotary tank cleaners used in process vessels the focus is on creating a hygienic and self-cleaning machine. It needs to be polished and free from any traps and crevices where contaminants or bacteria occur. This inevitably means some sacrifices need to be made in ruggedness. In con- trast ship tank cleaners have none of these concerns and so have been designed to be extraordinarily tough, and this rugged design is perfect for use in cleaning storm tanks. The big tanker cleaners will have 14, 16 or even 20mm nozzles that can throw a cleaning jet over 25m, which means a cleaning radius of 25m, so a 50m diameter tank could be cleaned with one machine. A crude oil tanker does not need to be spotless – removing the vast majority of residue is sufficient. In contrast chemical and food tanks must be completely cleaned, so the patterns for process tank cleaning are tighter with more rotations and more water. The level of cleaning required in storm tanks is very similar to that of tankers because they do not need to be spotless. They just need to have enough residue removed to prevent odour problems. The redeployment of tanker cleaning technology to solve the problems of cleaning storm tanks is logical. The environments they need to operate in are probably tougher than anything they will be exposed to in storm tanks. n Ivan Zytynski is marketing manager at BETE tank cleaning systems are not situated in a clean factory or maintained by a team of process engineers. Instead they are leˆ in a cold, dirty environments exposed to the elements which are largely unmanned most of the year • Cleaning fluid quality. A reliable supply of clean filtered water is normally assured whereas storm tank cleaning systems will oˆen operate on final effluent with variable particulate content Simply taking a process tank cleaner out of its 'normal' envi- ronment and deploying it in a storm tank will not work. Some models on the market will have sufficient reach and power to clean large storm tanks but the tough environment and poor quality cleaning fluid quickly results in the machines clogging and failing. Typically these machines have an exposed gear system that can become gummed up with algae, silt or smaller debris which can find its way into the supply water. On the one hand we have a technology that can deliver more effective cleaning for less water and less cost, but conversely there are reliability issues when it is deployed.