Frank Caprio is Hose Master’s corporate trainer, major market specialist, and dean of hose master university. Mr. Caprio has more than 35 years of experience in hydraulic, industrial and metal hose and expansion joint products and their applications. Frank is a member of NAHAD, AIST, ASME, and ASM International.
Q: What is an expansion joint and for what sort of fluid handling applications is an expansion joint required?
A: Expansion joints are flexible components installed in a piping system to compensate for various movements, misalignment, and other forces. Metal expansion joints accomplish this by incorporating one or more metal bellows into the unit. These bellows are formed by introducing convolutions into a metal tube, allowing the bellows (and the expansion joint) to deflect laterally, axially, or at an angle. Unlike flexible hoses, expansion joints are capable of elongating or compressing axially in response to changes in length of the adjoining piping system due to thermal expansion. Expansion joints are typically found in steam piping systems, chemical process lines, air systems, and as exhaust bellows on various engines used for transportation or power generation.
Q: Why is it important for an end-user to understand expansion, contraction, deflection and offset movements? How do these movements apply to expansion joints?
A: Expansion joints can be designed in almost limitless configurations, but there are very specific design formulae that must be used in order to derive the optimal design for any given application. The STAMPED acronym can be used to identify all of the application variables that will be factored into the design of the expansion joint: Size, Temperature, Application (how used, anticipated movements, cycle life expectancy, etc.), Media, Pressure, End fittings, and Dynamics (of flow). The expected movement(s) must be accurately defined, and it is important to specify whether these movements occur independently or concurrently, as different design calculations apply. The three types of movement to which an expansion joint can be exposed are: angular deflection, lateral offset (where the ends remain parallel as the movement occurs), and axial movement (compression and elongation). Exposure to torsional stresses should be avoided. The best design is one that satisfies the STAMPED variables at a competitive price. Improperly designed expansion joints are either over-engineered (and therefore overpriced), or they are under-engineered and may be vulnerable to premature failure from bellows squirm, corrosion, or metal fatigue (see photos)
Q: What are pipe-anchoring systems, and how do they relate to expansion joints?
|Overextension caused by installation in an unanchored piping section. Expansion joints are only one element of a well-designed piping system, where all components must work together to function properly.|
|Corrosion induced failure. Note the aggressive “knifeline” chemical attack at the heat-sensitized seam weld of the convoluted bellows.|
|Cross-section of over-compressed convolutions. The bellows was exposed to movements beyond the design limits, causing plastic deformation and subsequent failure due to metal fatigue.|
A: A well-designed piping system does not prevent movements from occurring, but instead controls and directs those movements to a flexible piping component, such as a pipe loop, a slip-joint, or an expansion joint. Before an expansion joint can be designed, it is imperative that the forces acting upon it are clearly defined. Pipe anchors and pipe guides serve to isolate sections of piping so that any movements directed to the expansion joint are controlled and predictable. There are many different styles and configurations of pipe anchors and guides, and they must be carefully designed to resist all of the forces acting on them.
These forces may include:
Spring force: The amount of force required to deflect the expansion joint, typically expressed in pounds per inch. These spring forces can be axial, lateral, or angular.
Pressure thrust: The amount of force exerted on the piping system when the expansion joint is pressurized and attempts to elongate. Typically a much higher force than spring force.
Pipe guide friction.
Media flow, wind loading, and other factors.
If the anchor point is not strong enough to restrain these forces, then the piping system (including the expansion joint) must be redesigned to prevent these forces from exceeding the limits of the anchors.
Q: When selecting expansion joints, what are some key considerations an end-user should make to ensure success?
A: The large number of variables to be simultaneously considered make proper expansion joint selection a difficult task. While using the STAMPED acronym helps to identify the variables that must be considered, it does not help the end-user to derive the data requested by the acronym. For example, the anticipated movements may be difficult to identify. There may only be vibration movement, such as is present in exhaust systems. Although the magnitude of movement is minute, the high frequency of the vibration imparts considerable stress to the bellows. Compare this to a steam piping system where a full cycle may only occur a few times per year, but the amount of movement is much larger. Each application requires a unique design to maximize performance at a competitive cost. Work with factory-trained professionals that can help to identify and measure all of these variables and can make recommendations as to the proper configuration of the piping system.
Q: How should an end-user approach the expansion joint design process? What are the high-level best practices you typically recommend?
A: There are two scenarios where expansion joint design typically occurs. The first is when a new piping system is being designed and built. These new systems are carefully planned by design engineering firms, and the complete piping system arrangement (including piping, anchors, guides and flexible components) is designed as a whole, often by collaborating with the component manufacturers. In this situation, the application requirements are clearly defined and the expected life of the components can be accurately predicted. The second scenario is when problems arise in an existing piping system. These problems may arise years after the initial construction of the piping system, and therefore the root cause may be difficult to identify. Was corrosion a factor? Did the metal bellows fatigue prematurely? Once the mode of failure has been identified, then solutions can be implemented. Some best practices to consider when evaluating an existing piping system include:
Evaluate the surrounding piping system: Is the system properly anchored and guided? Have there been piping modifications made? Have any anchor points been removed or repositioned? These can change the forces acting on the expansion joint.
Install the expansion joint near an anchor point: This keeps the anticipated piping movements controlled and predictable.
An internal liner may be required to prevent flow turbulence or abrasion: Expansion joints located within four pipe diameters of tees, elbows, valves, or any other source of turbulence may require flow liners to prevent resonant vibration from exerting excessive stress to the bellows. Liners may also be used to protect the flexible bellows element from abrasive media.
What is the maximum working pressure of the system? Metal expansion joints are generally recommended when operating pressures exceed 250 PSIG, as the metal bellows are better suited than rubber expansion joints for high-pressure service.
When measuring the length of an expansion joint currently in service, is the measurement taken in the “hot” or “cold” position (i.e. before or after thermal movement)? This is important to not only define the movement required, but it may also be used to facilitate installation.
Q: What are some common pitfalls you see end-users encountering in the specification, design and/or application of expansion joints? How can end-users best avoid such pitfalls?
A: Because expansion joints are highly engineered items, there are many ways to improperly select or design the unit. Here are some commonly overlooked items to consider:
Correctly identify all anticipated movements or offsets. One way to accurately define the thermal movement is to measure the difference between “hot” and “cold” positions when a shutdown occurs. Also, a laser level can be utilized to measure the offset of an expansion joint that is exposed to lateral misalignment.
Ensure the piping system is properly routed, anchored, or guided. Rely on assistance from trained professionals if you are not fluent in pipe design.
Accurately measure flow velocities. Get advice on whether or not a flow liner would help to prevent flow-induced vibration, which can accelerate metal fatigue. Since flow liners are often directional, make sure the expansion joint is installed in the proper flow direction to prevent damage to the liner.
Identify potential sources of external corrosion. Remember that the bellows material must be compatible with both the medium being conveyed and the surrounding environment, including cleaning chemicals, exhaust gases, etc.
Make sure the bellows convolutions are able to deflect as intended. Prevent any foreign matter from impacting into or onto the convolutions, which can limit their movement. Also, install outer covers on expansion joints if needed to protect them from external damage.
Do not assume that a stock item bought from a pipe supply house will function properly “because it fits”. Metal expansion joints can be made from a variety of alloys, using various strip thicknesses, numbers of plies, and with differing convolution geometries and hardware options. Changing any one of these variables will have a dramatic effect on the performance and life of the unit.
This is the second part of a two-part Q&A based on the content of the Hose Master University training, which focuses on applications and best practices for specifying flexible metal hose and expansion joints for fluid handling scenarios. Part I of this Q&A, “Getting a Handle on Metal Hose,” appeared in the March issue of Flow Control magazine (page 32).