Restraining techniques ensure that piping stays connected and helps prevent breaks that can shut down wastewater systems. Mechanical joint piping connections such as 45s, 90s, end caps and hydrant connections are common points of restraint as are connections in lift stations and treatment plants. Restraint of inline piping is so widely used that it has become standard procedure among city or project engineers. Instead of using restraints for specific applications, some engineers use them throughout force main wastewater systems. However, adding restraints in this way can increase project costs significantly. There are standards to follow and techniques to apply for restraining pipe connections in specific conditions and environments.
Although engineers typically decide when and how to restrain pipes, utility managers should be part of the process, since it is up to them to make sure the system works properly and to make repairs once it is completed. The first things to examine are factors that may pose a risk of pipe movement and separation.
The nature of the piping system
In a pressurized buried pipeline such as a wastewater force main, axial thrust forces act on the pipe based on changes in fluid velocity, pipe size, demand and pipeline direction. This generally happens at fittings such as plugs, caps, valves, tees, bends or reducers. Such hotspots categorically need to be restrained.<?p>
Earthquakes and ground movement can cause connection failures, beam or shear breaks, and cracks along the length of a pipe. A region’s geographic trends greatly affect how much the ground moves. Some regions regularly see movement, while others are relatively stable.
California, for example, experiences dramatic earthquakes, and it is no surprise that a high level of restraint is used on sewer pipes in many areas along the West Coast. Other areas on major fault lines incur ground movement that, although less dramatic, can still stress pipelines.
The New Madrid Fault Line is notable since it affects more than 15 million people in Alabama, Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri and Tennessee. Along the Ramapo Fault, which runs about 70 miles through New Jersey, New York and Pennsylvania, the ground movement has stressed piping systems even without full-blown earthquakes. In addition, scientists have warned of earthquake risk from hydraulic fracturing operations.
Ground movement can also be caused by seasonal weather changes, such as freeze-thaw cycles in winter and spring. Ground movement also can accompany extreme weather changes.
Other ground stresses
Pipe also can be affected by the ground in various other ways. The amount of ledge or rock in the ground can harm piping. In the presence of a high level of ledge, even slight movements caused by traffic or weather can cause piping to break. Swampy areas with moist and spongy ground that moves easily can also stress pipes.
Environments that lead to pipe uncoupling include tidal areas, bridge crossings and pipes running underwater. For these circumstances, it is best to consult with engineers on how to evaluate the risks.
Pipe Restraining Techniques
Several techniques are available for restraining pipes, each with advantages and disadvantages in cost, time and labor.
Until relatively recently, rodding was the most used restraining technique. Thrust rods are usually all-thread rods with washers and bolts that dog-ear into connections for restraint. Some installers even use rodding for flanged connections. The main drawback of rodding is that it is costly in time and materials.
Thrust blocks Thrust blocks are engineered concrete blocks placed at either end of a line of pipes or beside a joint to prevent pipes from pulling out. Whereas rodding strings pipes together so they stay connected, thrust blocks provide a solid mooring at the end or at a bend to prevent movement. Thrust blocks are typically made of concrete, but makeshift versions are made from steel posts, pressure-treated wood posts or bags of ready-made concrete.
The materials are inexpensive, but it takes time to construct the blocks, pour the concrete, and wait for it to cure. The wastewater system must be turned off to ensure that concrete cures properly before connecting the pipe. Additional costs are incurred in the time it takes to complete the job. In some cases, there is not enough space for thrust blocks, such as where utility lines are close by.
Concrete can also be used to restrain pipes by pouring it on the connection itself. While this can be effective, repairing the connection in the future can be tricky. At the very least, a pipe must be surrounded by plastic before the concrete is poured — otherwise, the entire pipe and connection must be cut out and replaced when repairs are needed.
Mechanical restraints and sleeves
Mechanical restraints and sleeves involve connecting a sleeve using multiple lugs. Several such products are on the market, and they are effective for joining pipes. However, the process is time-consuming. The larger the mechanical restraint device, the more bolts there are to tighten. This technique is particularly effective for large-diameter pipes that need significant reinforcement to stay connected.
The biggest drawback to mechanical restraint and sleeves is their high labor cost; the material cost of the lugs is also substantial. In addition, when using a product with lugs, the gripping mechanism creates stress points on the pipe. It can also take crews a long time to connect the lugs to the sleeves and so ensure that the lugs are tightened properly.
Coupling-restraint products use a mechanism to grip the pipes to restrain them. They are effective and relatively low-cost. Offered in a wide range of diameters, coupling restraints can be used on several types of metal and plastic pipes where utility lines either cross or run parallel to water and waste-water pipes. These close pipe-to-pipe settings make it difficult or impossible to install thrust blocks and rodding.
The coupling restraints also offer continuous dynamic deflection, meaning the pipe can flex within the coupling to maintain a strong connection while preventing pipe pullout. This can substantially reduce breaks, given that ground movement is a key cause for pipes pulling out.
The restraint’s chain of gripping teeth applies counter-pressure that pre-vents axial pipe motion. In addition, a progressive hydraulically assisted gas-ket self-inflates using existing water pressure. As water pressure rises in the pipe, water enters the gasket, which self-inflates and allows for dynamic deflection of the pipes while maintaining a secure seal.
To Restrain or Not to Restrain
Some circumstances require pipes to be restrained. These include waste-water lift stations, wells, piping in water and wastewater treatment plants, and industrial applications that involve hydrants and valves. In all these situations, water flow can fluctuate and create stresses in daily use.
The question of whether to restrain does not always have an obvious answer. Restraint makes the system stronger, but if the risk is relatively small, it might not be worth the cost. It is worthwhile to evaluate the areas of the system that need to be restrained and which restraint technique will be the most appropriate.
By looking at all factors, engineers and system operators can determine the costs and benefits of each solution before deciding what kind of restraint is optimal.
Author:Cristi Bruns is HYMAX training and technical field support manager for Mueller Water Products.
Click here to read the article on pg 36 of Treatment Plant Operator