Mind Piping System Mechanics

Ensure your layout can handle anticipated stresses.

By Amin Almasi, rotating equipment consultant

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Piping systems — pipes, fittings, valves and other items — not only must convey fluids (liquids and gases) from one location to another but also must cope with mechanical stresses. So, it is essential to check a system’s mechanical behavior under regular loads (internal pressure, thermal stresses, dynamic forces, etc.) as well as under occasional and intermittent loading cases such as special vibration or pulsation. This evaluation usually relies upon commonly used rules and guidelines or involves review by an expert; sometimes specialized software performs the piping stress analysis. Typically, pipe stress engineers verify that the routing, nozzle loads, hangers and supports are appropriate and adequate to ensure allowable pipe stress isn’t exceeded during situations such as sustained operations, pressure testing, etc.

Localized stresses in piping systems and their supports demand attention because they can lead to different types of failures. Such high stresses in steel structures and piping components can arise, for example, from sharp corners in the design or inclusions in a material.

Another area of concern is operating temperature range; piping and its supports for high or very low operating temperatures require special designs. High temperature poses issues of strength of materials, thermal expansion and thermal stress. Low temperature brings its own set of rules and guides; for instance, most ordinary steels become more brittle as the temperature decreases from normal operating conditions. So, it’s necessary to know the temperature distribution for these applications and select materials accordingly.

Piping Design And Layout

An elbow provides a change in direction in a piping system. This adds pressure losses to the system due to impact, friction and re-acceleration. As fluid enters the inlet of an elbow, it typically continues moving straight ahead to the first (or primary) impact zone; the fluid then is deflected at an angle toward the outlet of the elbow. Many different factors, such as the elbow design and the fluid’s characteristics and velocity, determine the deflection angle. In many designs, the fluid will hit one or more secondary impact zones before exiting the elbow.

Elbows and bends are available in a variety of angles and types. For instance, 90° elbows come in short and long radius versions. Short radius elbows have a center-to-face dimension of 1 × diameter and typically are used in tight areas where clearance or space is an issue. Long radius elbows have a center-to-face dimension of 1.5 × diameter; they are the more common type and are used when space is available and flow is more critical.

Reducers provide a change in pipe diameter. They are either concentric (Figure 1) or eccentric. Concentric reducers retain the existing pipe centerline, while eccentric ones shift the centerline. Eccentric reducers are useful, for example, to maintain elevation bottom-of-piping (BOP) in a piping system or with flat-side-up (FSU) in a pump suction to avoid problems such as gas pockets.

The design of piping branch connections is a critical task; poor arrangements have caused numerous failures. Coming up with a proper design requires a great deal of effort; many issues, such as fluid dynamics, mechanical robustness and localized stresses, come into play. The general rule (with some exceptions) is to use a top-side branch connection when the fluid is a gas, and a usually a bottom-side branch connection when it is a liquid. However, many factors, including application and fluid details, influence the selection. For example, a low temperature service (whether liquid or gas) typically should have a top-side branch connection to cope with the possibility of ice formation within the pipe during normal operation; the ice, which would flow at the bottom of the pipe, could block a bottom-side connection.

Fluid hammer is an important consideration for many piping system designs. When the flow through a system is suddenly halted at one point, because of a valve closure, machinery trip (such as a pump trip) or another reason, the fluid in the remainder of the system doesn’t stop instantaneously. As fluid continues to flow into the area of stoppage (upstream of the valve or machinery), it compresses, causing a high pressure situation at that point. Likewise, on the other side of the restriction, the fluid moves away from the stoppage point, creating a low pressure (vacuum) situation at that location. The fluid at the next elbow or closure along the piping system is still at the original operating pressure, resulting in an unbalanced pressure force acting on the valve seat, the elbow or the stoppage location. The fluid continues to flow, compressing (or decompressing) fluid further away from the point of flow stoppage, thus causing the leading edge of the pressure pulse to move through the piping. As the pulse passes the first elbow, the pressure now is equalized at each end of the pipe run, leading to a more-or-less balanced pressure load on the first piping leg. However, the unbalanced pressure now has shifted to the second leg. The unbalanced pressure load will continue to rise and fall in sequential legs as the pressure pulse travels back to the source (or forward to the sink). The ramp-up time of the profile roughly coincides with the elapsed time from full flow to low flow, such as the closing time of the valve or trip time of the machinery. Because the leading edge of the pressure pulse shouldn’t change as the pulse travels through the system, the ramp-down time is more or less the same; the duration of the load from initiation through the beginning of the ramp-down approximately equals the time required for the pressure pulse to travel the length of the piping leg. Piping design must consider these issues as well as other operating parameters such as how fast a change (such as closing of a valve) could be.

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Comments

  • Good article, one aspect that could be mentioned is how good layout and detailing can be most cost effective, by minimising the number of welds and fittings within a system.

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