With any type of lining system there is always the risk of damage to the liner due to objects falling in the tank. Even though drop in liners can be easily repaired it makes sense to try to avoid this issue in the first place.
One solution that a drop in liner allows for is the addition of a rigid sheet of plastic to be installed on top of the bottom of the liner. This rigid shield can protect the liner from damage due to any dropping parts.
Once the tank liner is installed the rigid shield can be put into place. Naturally, it begins with cutting the rigid sheets to the correct size needed. Along the perimeter of the shield a series of holes (1 in. diam. typically) are cut into the sheet. These holes allow a tab of flexible PVC to pass through the sheets. The other end of the tab is welded to the liner itself. On the loose end of the tab a hole is punched to allow a ½ in. diameter PVC pipe to pass through it. The pipe acts as a pin to hold the sheet into place.
Because the shield material (PVC or polypropylene) will float in water, or any denser solution, the shield will rise and create a hydrolytic cushion between the shield and the liner. Any item striking the bottom of the tank will have to displace the solution between the shield and liner which greatly reduces the impact on the bottom of the liner. The depth of this cushion can be adjusted by lengthening or shortening the tab length.
Not only has this type of liner shield proven to be the most effective, it can be easily removed for tank cleaning and is often reusable when the liner needs to be replaced.