Blog February 19, 2014

Methods of tank container surface grinding – Part One

By Nicola Byers

Each series of tank containers is built with certain calculated strengths. The strength of each tank is, amongst other factors, related to its minimum calculated shell thickness. The thickness we know is the minimal calculated thickness plus a safety margin and this information can be found on the data plate of each tank. The difference between the data plate and the actual thickness of the shell is the margin that can be reduced without any real risk.

With transport laws, the total weight of the combination (truck/chassis/container/load) has a limit. This means that if manufacturers are able to decrease the weight of the truck, chassis or container, the weight of the load can increase. More product per movement means higher revenue and profits. Thinner plates on a tank make it cheaper and lighter. Therefore the margins between actual and data plate minimums are decreasing. Correct and precise measuring and grinding then becomes increasingly important.
Current Methods of grinding:

1. The flap disk treatment (angular grinder)

This treatment is most widely used but unfortunately the worse of all. It is strongly advised not to use this process due to the following reasons:

  • Risk of high voltage machinery (Medium)

In most countries it is forbidden to use high voltage machinery as there is a risk of damaged wires causing a short-circuit resulting in electrocution. (This is not the case if an air supplied grinder is used)

  • Waste of abrasive (High)

The abrasive has a tough polyester backing preventing the abrasive pellets doing their job as soon the disk starts to “wear out”. Due to the manufacturing process the disk is expensive compared to other abrasives made from sheet.

  • Risk on surface damage (Very high)

As the rpm of the angular grinder is very high and the flap disk is hard with very tough edges, damage is prominent.
If the disk is positioned incorrectly, (bear in mind the tank is round and the operator moves his arms in a circle, it immediately results in a deep circular groove. Very high pressure has the same result.

  • Stock/product removal (Modest if used the proper way)

The contact area of a flap disk is small so the pressure on the abrasive may not be sufficient to remove a high level of stock/product. Increasing pressure may increase stock/product removal but this also increases the risk of damage. Therefore many movements have to be made, increasing repair time.

  • Surface quality after treatment (Bad to poor)

Grooves of treatment form obstructions to the product as it runs along the surface. It is simply impossible to keep constant contact pressure, positioning and speed of the tool along the surface whilst following the whole circumference. The end result will consequently be very irregular.


2. Manual Grinding with a flexible disk

This process in not commonly used for the grinding of stainless steel tank containers, but for product removal. However it is still used on occasion but has some serious economic and technical disadvantages. Therefore it is not advised for corrosion / pitting repair.

  • Risk of high voltage machinery – same as method 1.
  • Stock/Product removal (Little to modest)

As the disk itself and the supporting rubber disk are very flexible, any extra pressure on the tool will result in an increased contact area because the disks will bend. Bending of the disk can easily be limited by using a tougher support disk. The tougher the support the less it bends and more stock/product is removed.

  • Risk of surface damage – (Very likely)

As mentioned, a tougher disk will give more stock/product removal. Simultaneously the risk on surface damage increases if it is wrongly positioned.

  • Waste of abrasive (High)

As only the outer edges of the disk are used (depending on pressure, positioning and toughness of the support disk) the inner radius of the disk is not used at all. At least half of the abrasive is wasted.

  • Surface quality after treatment (Poor)

Same as method 1.


3. Manual wide belt grinding

In relation to surface quality, this method would be one of the best ways to manually grind the tank. However, this type of machine is really designed for calendaring (a way of smoothing) and not for stock removal from stainless steel.

Therefore, it is not advised for corrosion/pitting repair.

  • Risk of high voltage machinery

Same as method 1.

  • Stock/product removal (Low)

The 10cm wideness of the belt enlarges the contact area and with that reduces the contact pressure per cm2. This consequently results in very low stock removal. Increasing the pressure on the tool results in high friction and the machine can move out of control.

Explanation of the case:
The contact area reached with such a tool is about 15cm2 (depending on pressure). Having the average need of about 0,5 to 1 kg per cm2 (depending on necessary stock/product removal), the pressure on the tool should constantly be 7.5 to 15kg. Physically it is impossible for the employee to push and control that force on the tool for 8 hours a day on a circular surface and simultaneously move it in a straight line along the circumference with same speed. As the tool cannot be kept vertical beside the body, it is the equivalent power needed to move a bucket of water up, down and around out of reach! As mentioned, the machine is designed for calendaring, a process in which low pressure is a must. Changing belts on this machine is a challenge and takes a long time, therefore resulting in a longer repair time.

  • Risk of surface damage (Low).

With normal use it is not possible to position this machine the wrong way. It is a non-aggressive way of treatment. In the worst case scenario the machine can leave some curved grooves instead of straight vertical grooves.

  • Waste of abrasive (Average to high)

The belt is used over its full width. In that sense there is no waste. However with the sharpness of the grit (pellet), there is! Most abrasives remain sharp to the end due to the pellet breaking under pressure. As proper contact pressure cannot be reached with this tool, the pellet will not break but it will become blunt due to friction and rubbing. The pellet is not used to the end.

  • Surface quality after treatment (Proper)

The surface quality is good as the grooves are in line (even if they are not straight) with the way product runs out and damages are also rare. Variations in stock/product removal are caused by a difference in speed to the contact area along the circumference. It can be low as this way of grinding is not aggressive.