TECHMAIL: Piping Flexibility Basics

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Piping Flexibility Basics

Too much flexibility can be as bad as too little By Liang-Chuan Peng, P.E., President, Peng Engineering, Houston, Texas; and Tsen-Loong Peng, P.E., Senior Engineer, The M. W. Kellogg Co., Houston, Texas;.

One of the major requirements in piping design is to provide adequate flexibility for absorbing the thermal expansion of the pipe.

With the downsizing of engineering staffs and the availability of computer programs to calculate pipe stress analyses, there is a need to review the principles.

This article discusses three basic areas of piping flexibility: forces and stresses created by thermal expansion, problems associated with too much flexibility and the significance of field-proven layouts.

Thermal expansion stresses

The job of checking the piping flexibility used to belong to very specialized engineers. Today, with the help of computers, engineers with little or no piping stress basics are doing the piping flexibility analysis.

Many of these design jobs are not done properly. It is important that engineers who are associated with the piping flexibility know some basic principles.

As the pipe temperature changes from the installation condition to the operating condition, it either expands or contracts. This is called thermal expansion. The expansion of the pipe has the potential of generating enormous force and stress in the system.

However, if the piping is flexible enough, the expansion can be absorbed without creating undue force or stress. Providing the proper flexibility is one of the major tasks in piping system design.

It is obvious that a shorter pipe requires less capital expenditure than a longer one. The shorter layout also reduces the operating cost due to a smaller pressure drop. However, the direct shortest layout generally is not acceptable for absorbing the thermal expansion.

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Engineers have a tendency to provide more flexibility than required. In fact, it is widely believed the more flexibility that is provided, the more conservative the design. This is actually a very serious misconception.

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With all the added elbows and pipes, the pressure drop of the system can be many times higher than originally estimated. Therefore, not only it is possible to create cavitation at the pump suction, but vaporization can actually take place in the middle horizontal section of the piping. This vaporization creates intermittent local vapor spaces along the pipe. This type of slug flow produces huge unbalanced flow forces on the elbows, causing the whole piping system to shake.

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Railroad-construction analogy

A railroad-construction analogy can be used to illustrate the principles of piping flexibility (Fig. A). Of the four possible methods of railroad construction, case a is constructed with the rail ends nicely fitting together without any gap. With this kind of neat construction, the rail expands, creating very high axial force when the weather gets hot. This force very often can push the joints out of alignment, making them hazardous for the trains to pass over.

In case b, a small gap is provided at each point joining two adjacent rails. When the weather gets hot, the rail will expand into the gap without creating any force. This will maintain the rail alignment and allow the train safe passage. However, because of the existence of the small gaps, slight bumpiness is expected.

Instead of a small gap, case c provides a large gap at each joint. This excessive flexibility, no doubt, provides enough room for the rails to expand to and maintains the alignment of the railroad. However, when the train passes over, it jumps and, if severe enough, derails.

This shows that the flexibility almost always comes with some operational difficulty. A sufficient gap is required, but an excessive gap can mean disaster.

In case d, the flexibility is completely eliminated by welding the rails together. This type of construction ensures a very smooth ride and is adopted by the high-speed railroads. Of course, one still has to be concerned about the thermal expansion and flexibility. However, if proper anchors and lateral restraints are installed, the railroad can be operated safely without any problem.

The remainder of this article is at
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It is well worth reading.

Paul Received on Fri Mar 09 14:40:00 2001