OTHER CONSIDERATIONS FOR TANK CLEANING
Fluid-driven machines generally cost less than motor-driven ones, although the cost depends upon the size of the machine. However, fluid-driven machines have many internal parts and disassembly, replacement and reassembly of worn parts can be time-consuming. In some cases, maintenance requires a special tool kit. Factory refurbishing usually is recommended based on hours of use.
Motor-driven machines require minimal maintenance and are serviced easily by the user. The motors are positioned outside the tank, ensuring long life and minimal exposure to harmful solvents.
Which is a better choice? Fluid-driven machines usually cost less. Motor-driven machines are less expensive to operate and maintain. The specifics of your operation such as water quality and hours of use will determine which is more cost-effective.
If you've determined that a motor-driven cleaner is your best choice, you must select between two types:
1. Machines with nozzles in a fixed position. These can be permanently installed or moved from vessel to vessel but the cleaning head is in a fixed position on the unit. Maximum operating pressure is 5,000 psi. Various types of motors are available and users specify extension length, flange size and a two- or four-nozzle hub.
2. Machines with retractable nozzles. These permanently installed units offer a higher level of automation (Figure 4). A pneumatic mechanism inserts and retracts the extension and the cleaning head. A control panel allows setting multiple stopping points between full insertion and full retraction to position the nozzles where more impact is required or to clean around obstructions. The control panel can be located away from the vessel for convenience or safety. Maximum operating pressure is 4,000 psi.
The properties of the residue or the cleaning agents and your desired level of automation will determine which option is best for your operation.
TANK CLEANING PERFORMANCE VALIDATION
| Acoustic Monitoring Device
Figure 5. Unit mounted on onside of vessel detects loss
of rotation and variations in rotation speed and fluid pressure.
Once you've selected and installed a new cleaning machine, how do you know if it's doing its job?
Visual inspection is one option. Monitor the machine to make sure it's working and inspect the inside of the vessel when the cleaning cycle is complete. Swab or riboflavin tests are common ways to verify cleanliness. Of course, the viability of these approaches depends on the size and location of your vessel.
Another option is using an acoustic monitoring device (Figure 5). A sensor mounted to the exterior of the tank "listens" to the performance of the cleaning equipment and identifies variations from a pre-determined baseline. It instantly can detect rotation failure and changes in rotation speed or spray pressure and can notify operators via audible or visual alarms. The monitoring device also transmits performance documentation for quality control and record keeping. It obviates visual monitoring and post-cleaning tests.
Six steps may provide significant benefits:
1. Reduce use of heated water. Hot water is costly but frequently is viewed as a necessary evil to remove some residues. However, increasing impact often can often get the job done and cut or eliminate the need for hot water.
2. Minimize "striping." Vessel cleaners provide 360° coverage. However, the solid stream sprays don't overlap as they rotate, so there's a small distance between the sprays and thus a so-called striping effect. The greater the distance the nozzles are from the vessel walls, the greater the distance between the sprays. The best way to minimize striping is to use a four-nozzle hub rather than the standard two-nozzle hub. This will cut striping in half.
3. Change spray head position. Use an adjustable ball fitting to clean vessels in sections. Clean the top half of the vessel, then lower the device and clean the bottom half of the vessel or change the angle to clean difficult locations.
4. Decrease the number of cleaning cycles. Simple adjustments to pressure and flow may enable a reduction in the number of cycles needed for thorough cleaning. To increase impact and cleaning efficiency it's far more effective to increase flow than pressure. Doubling flow rate boosts impact as much as 100%; doubling pressure only provides 40% more impact.
5. Recirculate. Do you spray and drain? Check into recycling your cleaning solution if you aren't using hazardous materials and your water is debris free.
6. Activate cleaning with the flip of a switch. Hard piping your vessel cleaner in place can save time and reduce labor costs. Consider permanently installing the device if the material or its temperature won't damage the cleaning equipment.
ANTHONY WOOD is a tank cleaning specialist at Spraying Systems Co., Wheaton, Ill. E-mail him at Anthony.firstname.lastname@example.org.