Trenchless maintenance methods help to repair or replace underground pipelines without digging up the entire pipe. They can be carried out in place at much lower cost and with much less impact on surrounding neighborhoods and the environment. But, how do you know when a pipe is nearing failure?
It's vital to be able to identify when a pipe is experiencing too much stress and is likely to fail. This enables corrective action to be taken before a pipe bursts causing product leakage with all the safety and environmental consequences of that failure. (Learn more about damaged pipe repair with Trenchless Rehabilitation Evaluation: How to Properly Inspect and Locate Damaged Pipelines.)
Hoop stress is one of the forces that an underground pipes experience that may lead to failure. It's primarily applicable to thin-walled cylinders like pipes where the wall thickness is much thinner than the radius of the pipe. Hoop stress is also known as circumferential stress.
It's a measure of the force working perpendicular to the radius of the pipe. If a pipe fails due to excessive hoop stress, it tears along the length of the pipe. This is different to longitudinal stress, which causes the pipe to snap in half.
Engineers are able to calculate the exact hoop stress magnitude based on a formula and compare this with the pipe specifications. When the actual hoop stress approaches the pipe specifications, a failure is imminent. A hoop stress calculation can be done as follows:
Hoop Stress = p x d / 2t
p = internal pressure in the pipe (psi)
d = internal diameter of pipe (in)
t = thickness of pipe
This simple equation can be used to calculate hoop stress in any pipe where the wall thickness is less than 1/20th of the internal diameter.
Now that we know what hoop stress is, how do we detect excessive hoop stress in a pipe’s wall before it's too late?
Actual Internal Pressure of the Pipe
Because the hoop stress equation is based on the internal pressure of the pipe, we need a pressure measurement on the pipeline. It is impossible to have pressure gauges mounted all along a pipe length especially since we are primarily talking about an underground installation. However, most pipelines have pressure gauges or transmitters at the pumping station and the destination point. (But how are preexisting pipes found? Learn how with An Intro to Underground Pipe Locators.)
Some may even have measurements at the point where the pipe goes underground and again when it emerges above ground. Based on the length of pipe and the pressure drop between the two points, you can perform a rough calculation of the pressure at any point in the line.
The section of piping exposed to the highest pressure is usually the discharge pipe from a pump as this is where the force is provided to move the product through the line. Monitoring this product pressure in the pipeline and comparing it to the pipe specifications will highlight when you have a problem.
The internal pressure of the pipe will give you information about the process conditions of the pipeline. It reveals when there is a blockage in the line or a shut-off valve that has been closed at the destination. These process disturbances cause a dangerous buildup of pressure in the line, but they don't reveal anything about the condition of the pipe.
To investigate whether cracks are forming in the pipe due to hoop stress, you need a more direct method to inspect the pipe wall.
Internal Pipe Inspection for Cracks Using Ultrasonic Inspection
The most common trenchless method for internal pipe inspections involves the use of PIGs (Pipe Inspection Gauges). Smart PIGs have sophisticated technology for helping engineers to inspect and evaluate defects in pipe walls. Ultrasonic inspection tools send an ultrasonic pulse at an angle to the pipeline wall. (Read more on pigging with How Pigging Works in Trenchless Repair and Rehabilitation.)
A transducer measures the return of the pulse, which is affected by the presence of cracks in the wall. From the cumulative analysis of all the measurements along the pipe wall, engineers can identify cracks and other defects, and how serious the problem is. The extent of damage and the impact on pipe integrity must be assessed in order to plan and execute an appropriate trenchless repair.
Minor Pressure Drop In Line
While no pipe failure is desirable, it's better to detect a small leak than to allow a pipe to rupture completely causing major environmental and safety risks. The same pressure drop technique we used to calculate the internal pipe pressure can help us to detect minor leaks.
When a pipe cracks and a small amount of product escapes continuously, it causes a pressure drop at that point in the line. This is because the flow downstream of the crack is lower than the upstream flow.
Using very accurate pressure measurement at the pumping station and destination point can reveal small changes in the pressure drop trend. Any noticeable change should be further investigated to prevent a small crack from becoming a major catastrophe.
What We've Learned
Hoop pipe stress is a normal tension exerted on the internal wall of a pipe due to the pressure of the product flowing inside. However, a pipe is only designed to handle a certain amount of stress. Overpressure will cause the pipe to crack and eventually fail.
Use these three effective ways to detect hoop stress in a pipe’s wall before it's too late. Preventing a pipe failure is always better than dealing with the environmental and safety consequences that inevitably happen when it bursts.