Why Did My Liner End Up In The Main?

Original Post from 2 years ago, but got the question again, so I’ve updated and republished it.

“I shot a liner today, thought I measured very carefully after measuring my camera marks, and cut my liner accordingly. My question is why did my liner that I carefully measured, end up 6″ into the main?”  

Well, there are many reasons, and let’s go through all of the them.

Question # 1.  How did you measure the liner?  If you answered that you went by the counter on the camera, your answer is wrong.  Camera counters tend to give false lengths for many reasons including, not setting the counter to 0 when starting, a counter that doesn’t register true lengths, and finally transposing the the actual measurement from 45′ 6″ to 46’5″.  We’ve seen all of the above reasons for going “long”.  But you say, “no I used tape to mark the exact distance!”

Question # 2.  Were there other pieces of tape on the cable?  If you don’t remove the old tape every time, you have no way of knowing for sure that the tape you used today is the tape you’re measuring from or from a previous job.  Again, you say, “no I used new tape and didn’t have any old tape on the cable.”

Question # 3.  Did you stretch the material when you pulled it off the roll or out of the box?  Ah ha, the answer.  No I just assumed that the length would be whatever came off the roll/box.  You are saying I should stretch the material once I’ve determined the length I need?  YES!  All material stretches and to get the relaxed material to the length, you should pull it tight before measuring. But don’t stop there.  One more question.

Question # 4. How much pressure did you use to install the liner?  The more pressure used in inverting the liner, the greater the stress on the liner and the more it will stretch.  What pressure did you use to invert the liner?  “I always use 25psi to invert, does it make any difference?”  Yes, the higher the pressure the more stretch.  The stretchy liners more than the stretch dampening liners, but all will stretch some and 25 psi is a lot of pressure.  If needed to get the liner in, that’s a consideration you need to make before lining, not trying to fix afterwards.  Turns, bends and offset joints may dictate higher pressure.  If you’re seeing those conditions on the pre-lining inspection, you may want to adjust to consider stretch.

Question # 5.  Which material did you use?  Many of the offerings we stock include stretchy material that miliner beyond host pipenimizes the wrinkling of the liner.  This material stretches length wise as well as around the diameter  of the pipe.  It does this to reduce material bunching at turns and bends, but in selecting this material, you will add the stretch factor that you need to address.  Mitigating it can be accomplished by pulling the liner material into the cal tube and shooting them together as the cal tube doesn’t stretch and will take the stretch forces away from the liner.

So we’ve identified the problem. Solving it is pretty self explanatory. Use the camera instead of transferring measurements. Clean old markings off your camera cable so there is no doubt which mark to use. Stretch the liner before cutting and verify by the markings and on the tube.  Add material for turns, bends etc. Subtract if extra pressure is needed in shooting a test pre-liner. Finally, if you choose a stretchy liner, don’t plan to hit a mark.

 

 

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