When cleaning atmospheric storage tanks or pressurized vessels, operators look to the contractor to maximize safety by eliminating or reducing personnel exposure and minimize downtime and costs. This means understanding operational practices and schedule/cost implications and requires engineering that includes:
- Waste characterization — Solids, sludge, liquid or all three.
- Analysis of physical characteristics — Where there is limited historical data of the tank’s condition or contents, a representative content sample should be taken from the tank congruent to how it will be found. For example, slop systems may have tankage into which a variety of materials have been transferred. Alternating aqueous versus hydrocarbon-based materials can stratify, making contact and removal difficult. If mixed materials are present, it is best to identify this condition and provide samples of each layer, unmixed, if possible.
For reactive products, including toxic materials, pyrophoric or reactive sludge (acidic or alkali), neutralization is warranted.
- Development of cleaning procedures — Equipment, chemicals and flow paths to ensure minimal personnel exposure or damage to client equipment.
Tanks with prior success using a particular practice are more easily addressed. For tanks converted to different storage or those not used for bulk hydrocarbons, procedures are more complex. Even tanks that hold clean products, including those with reactive components, can be challenging with respect to the methods and media used.
In mechanical cleaning, where pumps or jetting systems impart energy into the tank internals to disrupt and fluidize contamination for removal, use of the right nozzles allow fluidization to eliminate or dramatically reduce tank entry time to manually remove any remaining contamination.
Chemical decontamination is achieved by circulating a specific solution to remove, neutralize or deactivate the contaminants of concern within storage vessels.
A mix of these two methods may also be used, with mechanical hydraulics supplemented by chemical additives (such as viscosity modifiers) to enhance the effective-ness of a cutter stock or water in cases where residual materials are soluble in an aqueous solution. Chemical decontamination often precedes or follows mechanical removal based on the product and residual contamination in the tank.
With reactive products, an effective neutralizing agent, if applied to assure full contact with the contamination, can allow the product to be converted and removed, though the method of doing so may vary. For example, sulfuric acid used at industrial facilities for pH control on cooling water systems or similar processes is often stored in a horizontal drum on saddles to facilitate easy storage and access. Refineries that use sulfuric acid for alkylation purposes as a catalyst store the material in large field tanks of conventional fixed-roof design. Chemically, the approach to cleaning these tanks is similar; however, the application method varies dramatically. Horizontal drums have many accessible nozzles with which to establish circulation of the neutralizing agents, whereas large diameter tanks offer manways and a nozzle supplying transfer pump if it can be isolated and accessed for cleaning.
Regardless of cleaning needs, finding the right contractor means requiring proof sources and credentials specific to the type of tank and product to be serviced and considering only those with knowledgeable engineers and planners. Minimizing potential downtime even more can be achieved by considering a contractor that also offers inspection and repair services.
For more information, contact Lee Coll at (225) 921-1598, email lcoll@matrixservice.com or visit www.matrixservice.com.
