Tank cleaning is a dirty business
It might take a year, maybe three or even five years, but at some stage in their operating life storage tanks will need to be cleaned.
Vertical storage tanks, particularly those holding crude oil, heavy fuel oil, slops oil or catalytic cracker residue, will accumulate a collection of stones, grit, sludge and mechanical items swept down the pipeline or from the tanker making a sea delivery.
Tank cleaning is neither an easy, nor an inexpensive operation, but it is necessary. The contents of the tank have to be reduced to a minimum level, new entries might have to be formed, a secondary storage facility made available to receive the residual product, services and utilities provided close to the tank, and if physical entry is required it must be gas free.
Oil Pollution Environmental Control (OPEC), one of the best ways to extend the time period between cleaning is to install a submerged rotary jet (SRJ) mixer system as a permanent fixture. A similar unit can also be used as a temporary device fitted to the storage tank to enable it to be cleaned at any time. As a permanent feature of the tank’s maintenance system, the SRJ mixer is mounted in the centre of the tank, and used on an intermittent basis to control the formation of sludge – how frequently will depend upon the condition and characteristics of the oil, and the frequency of loading and discharge. The unit can direct a single jet, or two opposing jets of oil horizontally across the tank while it slowly rotates at a programmed rate. This action cuts into the sludge that has settled on the tank floor, re-suspending and homogenising the hydrocarbons that have become trapped in the sludge. About nine years ago OPEC was involved with a project for the Formosa Petrochemical Chemical (FPC) company. At stake was the potential to supply 28 centre mounted SRJ mixer systems for installation in vertical floatingroof storage tanks of 87 metres diameter, and a capacity of up to 700,000 barrels of crude oil. FPC undertook a detailed study of available equipment, including the use of side entry propeller mixers (SEPM), and compared the benefits and disadvantages of employing each of the systems commercially available.
Ilsley says: ‘The most notable reason that FPC gave for choosing to go with the SRJ mixer technology was its ability to eliminate the buildup of sludge in the tanks, thereby maximising the amount of storage capacity available at all times.’
