Cold Climate Design

Protect your pipe from ground freeze.

By Robert Leonard

Throughout the more elevated regions of North America, particularly in Canada and the northern United States, winter temperatures drop below the freezing point, and the experience of driving over potholes is not easily forgotten. It occurs mainly during the early part of winter or in the spring when the ground thaws.

Potholes are the result of an action called “frost heaves.” Water infiltrates a roadway bed, accumulates and, through the action of freezing and thawing, breaks away the road surface, creating a pothole.

Frost heaves occur when ice expands, as it first forms, while the temperature of water drops to the freezing mark. When water turns to ice, it expands by nine percent. It then contracts as the temperature continues to drop and expands again, when ice warms to the freezing point. It’s near the freezing point when damage is done.

Protecting Piping in the Frost Zone
Frost can penetrate to depths of three feet or more. This is known as the “frost zone.” Since fuel handling regulations allow the piping to be shallow buried within the frost zone, precautions must be taken in the service station layout design phase to ensure that the piping and its connections do not become damaged due to frost-induced ground movement (frost heaves). Concrete pads at vehicle fill points limit the ground movement mostly, but not outside the concrete apron, between there and the tank pit.

Tank pit with FRP Dog Leg Piping (plan view)

FRP Tank Siphon Arrangement (Dog Leg Method)

To understand the effects of frost heave on pipe, picture an upward or sideways force bending a length of pipe in the middle. The pipe length will shorten in its “chord” length because the ends are not fixed. If you clamp the ends and apply sufficient force to bend the pipe in the middle, significant force will be required to restrain the ends from moving inward, and the pipe would have to stretch.

You want to create a situation where the pipe can be allowed to bend from such a force, while permitting one of the ends to move, allowing the chord length to shorten without putting undue stress on itself or its end connections. This can be accomplished within the frost-free zone of the storage tank area.

FRP Tank Siphon Arrangement (Dog Leg Method)

Dog Leg Piping
Storage tanks are situated in a “free-from-frost” zone, buried and anchored to concrete pads. The piping located in that zone is also unencumbered by frost, and it is thus able, within limits, to flex, bend and move linearly without excess stress loading, due to the flexibility of a dog leg piping arrangement.

The piping connected to the storage tanks is routed through the tank pit area, utilizing two lengths of pipe joined by a 90-degree elbow (the dog leg), with each length a minimum of four-feet long. The piping continues on and enters the piping trench, then onward to the dispenser boxes. The piping trench is within the frost zone. The piping will lock into the trench and will move with the trench should a frost-heave occur. As the chord length of the pipe shortens or lengthens, the pipe will pull/push on the dog leg portion, which absorbs the stress load through its flexible action.

This same approach applies to lines that are routed outside the tank pit area, such as vents and remote fill lines. Siphons between tanks, although not affected by frost, should also use dog legs due to the possibility of tank roll of one storage tank to the other.

FRP Tank with Sump Showing FRP Dog Leg Piping

Design Considerations
Sound engineering has been used to determine the four-foot dog leg length for fiberglass pipe sizes two-inch to six-inch single and double wall diameters, as well as for newer two-inch to four-inch coaxial pipe diameters. The application of Euler’s & Johnson’s column and bending formulas and other engineering principles have been applied to standardize the API recommendation of four-foot minimum lengths. The negligible thermal expansion and ease of maintaining uniform slopes have resulted in the use of fiberglass piping in buried petroleum applications. These design considerations have been adapted as law by various government fuel safety jurisdictions, and are common practice.

In summary, the correct use of four-foot dog legs in buried fiberglass pipe installations will eliminate the risk of damage due to frost heaves. Safe, trouble-free installations depend on proper design and the diligence of the installing contractor to learn proper installation techniques and practice them on site.

   Current Issue   |   Past Issues   |   Search   |   Contact Us   |   Send Files    |   Advertising   |   www.pei.org

The PEI Journal • First Quarter 2008 • Volume 2, No. 1 • Contents are Copyright © Data Key Communications, Inc.
All rights reserved. • Nothing may be reproduced in whole or part without written permission of the publisher.