This Week’s Subject: Cold Flow Stabilization

Question: Hi Joe – When I’m using squeeze-off tools to stop gas flow in plastic gas mains, my supervisor tells me to allow for cold flow stabilization. Why do this?

Answer: Your supervisor is right on! Cold flow stabilization is simply the relaxation of the polyethylene (PE) pipe, due to the stress of the squeeze-off procedure. For an effective squeeze-off, it’s important to ensure relaxation allowance once the mechanical or hydraulic squeeze tool fully compresses the pipe. This is accomplished by allowing 1 minute per inch of the pipe diameter being squeezed. Butthe time should be doubled for temperatures below 32°F.

Examples: 4 minutes are allowed for 4-inch-diameter pipe at 80°F, and 8 minutes are allowed for 4-inch-diameter pipe at 20°F.

Tip: Hydraulic squeeze tools involve a specific gauge pressure that’s attained before allowing for cold flow stabilization. This gauge pressure will drop during cold flow approximately 150 to 200 psi, requiring re-pumping to the original pressure for a completed squeeze-off.

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