Experiments on affects of heating on Schedule 40 PVC Pipe

Introduction - The strength of PVC pipe is reduced significantly with heating. Depending on the diameter, normal PVC burst pressure can be above 1000psi. However heating the pipe to only 100c (the boiling point of water) will make it so soft you can easily squeeze it with your hand. There has been some discussion as to how long it takes the heat from a firing motor that uses PVC pipe as its structure to reach the outside of the pipe and reduce its strength. The theory is that the PVC will ablate and that it also does not conduct heat very well so that the outside of the pipe remains cool and therefore retains much of its strength for some time after the motor has fired. This page reviews a simple experiment I undertook to understand how long it takes to heat the pipe from the inside out.

The Experiment Setup - The setup consists of a stand to hold a length of pipe horizontally at such a height that a propane torch flame can be introduced into one end. A weight is hung on the end of the pipe that protrudes some distance from the edge of the stand so as to allow a bending point when the pipe gets soft enough to bend. In order to measure the time reasonably accurately a sheet of metal was held between the propane nozzle and the entrance to the pipe to protect it until we began timing. In this experiment the pipe was 1in ID schedule 40 gray electrical conduit measuring 16in long. Approximately 10in extended beyond the stand with a 5lb weight hung at approximately 9.5in from the stand. The pipe has also been constricted at the exit to about .75in ID to help keep the hot gases in the pipe. Although this experiment does not reproduce exactly the conditions inside a motor (especially the high pressure), it was felt that since propane has a burning temperature of 2000c compared to a typical sugar motor of approximately 1350c, it would serve as a good conservative analogy.

Conclusion - It took a surprisingly long time for the pipe to lose strength. The flame was introduced at approximately 1 second into the video and heat waves can be seen to emanate from the exit almost immediately. Approximately 10 to 12 seconds later flame and smoke begins to appear from the burning and ablating of the inside of the pipe. The flame and smoke increases until approximately 25 seconds after the experiment started, the pipe finally begins to bend and the experiment is stopped. Even at a conservative starting point of when the PVC is seen to actually begin to burn (at the 10 second mark) we still have approximately 15 seconds before it begins to lose strength. This is well beyond the typical short burn of 1 to 3 seconds of most small to medium sugar based motors.

CAUTION - Since we had heavy rains, this experiment was conducted inside a garage with both front and rear doors open for ventilation. Since burning PVC produces poisonous gases it is recommended to be conducted outdoors with the experimenters well away from fumes and smoke.

View the experiment video    2.7 Mbyte WMV file.