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Product Motion Questions
As piping or tubing heats up, it grows in length. Conversely as it cools it shrinks. This growth can affect the piping, and the mating equipment, in many ways. If the piping is allowed to free float it may damage equipment or structure.
What is axial motion?
Axial motion is the motion created by the pipe expanding or contracting. The pipe gets longer as it heats up and expands. The pipe gets shorter as it cools down and contracts.
What is lateral motion?
Lateral motion occurs in any of the take offs where pipe changes direction 90°. This could be a simple change in direction or a take-off from the mainline that is at an angle (often 90°) to the main pipe line that is growing or contracting.
What is angular rotation?
Angular rotation is motion that occurs when the take-off or the direction change is something other than 90°.
Where does vibration occur?
Vibration is commonly seen in piping systems at the pump and at rotating equipment.
What is torsion?
Torsion is motion that occurs around the centerline of an assembly.
In designing the piping system one must be aware of the stresses caused by motion and its effects on the system and mating equipment and/or structures. Typically the system will be designed so that anchors break up the system into segments and then pipe guides are used to control and/or direct motion(s) into specific directions. Once the direction and the amount of motion are determined by the design, a flexible piping product can be selected to alleviate the stress and absorb the motion so as to protect the system and mating equipment.
Flexible Product Questions
It is important to understand which flexible piping products are capable of taking which kind(s) of motion(s).
What are braided flexible hose assemblies?
Braided flexible hose assemblies are capable of absorbing pipe vibration, lateral offset and angular rotation. The amount of live flexible hose within the assembly determines the amount of lateral offset or rotation it is capable of absorbing. The amount required for a given offset depends on whether the offset (simple piping misalignment) or rotation is fixed, moving, moving to one side of the centerline or both, and the bend radius of the hose being used. Braided metal hose assemblies ARE NOT SUITABLE for axial motion. Compression is not allowed as the outer braid must remain taunt under system internal pressure to contain the hose. The taunt braid is what gives the assembly its pressure capability. On a side note: this is why you never want external control rods or devices on a braided hose assembly, the braid must be allowed to grow taunt under system pressure. They are not capable of extension simply because the braid will limit the length of the assembly. They are also not capable of torque.
Summary
Motions allowed: vibration, lateral, and/or angular rotation
Advantages: inexpensive, no maintenance, typical stock lengths on the shelf (longer lengths can be manufactured for greater motion) and exerts no pressure thrust on the system. All metal construction allows a multitude of end connection possibilities. If it can be welded you can weld it to metal hose.
Optional: External guard (for abrasion protection), interior liner if media is abrasive or flow is turbulent, exterior insulation wrap (hot media applications to protect from burns), interior liner recommended if flow rate exceeds 75 ft./sec. for a liquid or 150 ft./sec. for a gas when hose in straight configuration.
What are braided, flexible hose loops (aka: expansion loops)?
Braided flexible hose loops are designed to take motion in more than one plane. They are constructed of two or more flexible hose sections joined by elbows or return bends. Depending on the configuration of the loop it may also take motion in all three axis’s (X, Y and Z) and rotation about all three planes simultaneously as well. The same braided metal hose is used to give it the required flexibility. They are able to take large amounts of motion axially and laterally. As with all braided hose assembly the braid must remain taunt under system pressure in order to maintain its maximum pressure capability.
Summary
Motions allowed: vibration, lateral, axial and/or angular rotation
Advantages: inexpensive and much smaller foot print when compared to hard pipe loops and the required site labor, no maintenance, and exerts no pressure thrust on the system. All metal construction allows a multitude of end connection possibilities.
Optional: External guard (for abrasion protection), interior liner if media is abrasive or flow is turbulent, exterior insulation wrap (hot media applications to protect from burns), interior liner recommended if flow rate exceeds 75 ft./sec. for a liquid or 150 ft./sec. for a gas when hose in straight configuration
What are rubber expansion joints?
Rubber expansion joints are manufactured primarily by one of two methods; molded spheres and wrapped arch or spool type. Rubber is an excellent absorber of noise that is caused by vibration. In addition to vibration they are suitable for fair amounts of lateral, axial and angular motion. The sphere types are available as a single or double (for greater motion). The arch or spool type can be made with one or multiple arches for greater motion. Rubber is a maintenance item; it will dry out and crack over time requiring replacement. Drying out and cracking depends on many factors, including environment. Using exterior insulation on a rubber expansion joint is not recommended, specifically in hot applications. The insulation will contain the heat and accelerate the drying out of the rubber body. The heat of the piping system must be allowed to be released to atmosphere in order for the joint to breathe, dissipate heat and allow a decent life expectancy. The rubber body will lose heat at a greatly reduced rate when compared to a metal expansion joint, or the metal piping itself, making the use of exterior insulation unnecessary. They should not be installed in inaccessible locations where they cannot be periodically inspected.
Summary
Motions allowed: vibration, lateral, axial, angular rotation and/or torque
Advantages: molded sphere type is very inexpensive in the common EPDM or neoprene bodies. Flanges rotate for easy bolt hole alignment. The mating flange seals on the rubber face making it a wetted rubber connection. The flowing sphere body is self-cleaning. The arch, or multiple arches, style joint, is typically much more expensive than a sphere but alternate elastomers, and/or reducing styles can be produced.
Optional: unless properly anchored a limit device should be used to limit over extension from pipe movement and/or extension from joint pressure thrust. Limit devices can also be provided to restrict over extension in systems that run colder than they installed temperature. The spool type can be supplied with a filled arch(s), which reduces motion capability by 50% to keep media from collecting in the arch.
What are metal expansion joints?
Metal expansion joints can be provided in numerous designs: internally pressured, externally pressurized, dual tied, gimbal to name a few. Based on their designed they can take axial, lateral, and angular motion. The single bellows internally pressurized is the most common for fair amounts of axial motion. Where large axial motions or higher temperatures are involved an externally pressurized joint is typically supplied. Externally pressurized joints are not capable of lateral motion. In applications with large amounts of lateral motion a dual universal tied expansion joint is typically the best choice. All metal joints require the piping system to be properly anchored and guided. Many manufacturers stock the simple internally pressurized and externally pressurized joints. Other styles and/or custom design metal joints (depending on the complexity) can take several weeks for manufacture.
Summary
Motions allowed: vibration, lateral, axial, and angular rotation depending on the style of the expansion joint
Advantages: Standard designs and designs for a specific application. All metal construction means no maintenance.
Optional: even in properly anchored systems a limit device should be used to limit over compression from pipe movement and/or extension from joint pressure thrust. Internal liner(s) is recommended (not necessary with externally pressurized joints) for turbulent flow and/or high flow rates. External covers (or shroud) can be added to protect the joint from external damage and to protect personnel in the event of joint failure. Intermediate anchor base can be incorporated into most metal joints to reduce installation time and labor costs in the field.
