The number of slots per foot of pipe is determined by calculating the number required to achieve the required percentage of open area of the pipe's outer surface.

After selecting the open area, the slot width and the diameter of the liner, the number of slots per foot of liner can be determined from the following formula or read from the table below.

where

N = Required slots/foot (If N<32 round up to the nearest multiple of 4)
D = Outside diameter of the liner, inches
C = Required open area, percent
W = Selected slot width, inches
L = Length of the slot, inches

For straight slots, the slot length at the ID of the liner is used. This gives a conservative calculation of open area. The slot length and open area at the OD is 25% larger.

For seamed keystone slots, the slot length at the OD of the liner is used. This also provides a conservative calculation of open area because the seaming results in an open area at the ID of the liner that is usually 40% larger than that at the OD.