Metal Expansion Joints

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Metragator Metal Expansion Joint

  • Externally Pressurized Metal Expansion Joint
  • Allows more movement than any other bellows joint.
  • Rugged design protects the bellows.
  • Standard models up to 12”.  Larger sizes available.
3D Drawings
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MNLC Bellows Expansion Joint

  • Metal Bellows Expansion Joint
  • Low corrugation stainless steel bellows
  • Available in 50 lb., 150 lb., and 300 lb. classes
  • Standard models up to 12” diameter, custom models available up to 60” diameter.
3D Drawings
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The MetraPak

  • Handles temperatures up to 875⁰ F.
  • Handles up to 500 PSI.
  • Handles 24” of pipe movement.
  • Standard models up to 24” diameter, custom models available up to 60” diameter.
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Exhaust Expansion Joints

  • Exhaust Bellows Expansion Joint.
  • Low corrugation stainless steel bellows
  • Standard models up to 24” diameter, custom models available up to 60” diameter.
  • Can be provided with flanges to mate to standard generator sets.
3D Drawings
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MC Metal Bellows Expansion Joint

  • Ring Controlled Bellows Expansion Joint
  • Hydrostatically formed stainless steel bellows
  • Available in sizes up to 54”
  • Available in 150 lb., and 300 lb. classes
3D Drawings
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MPTR Metal Expansion Joint

  • Metal Bellows Expansion Joint/Pump Connector
  • Low corrugation stainless steel bellows
  • Built in control rods
  • Features Vanstone flanges.
3D Drawings
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Seismic Gator Inline Seismic Expansion Joint

  • Hybrid gimbals / externally pressurized design.
  • Use for seismic applications when a Metraloop will not fit.
  • Available standard through 8” pipe size.  Consult Metraflex for larger sizes.
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What joint do I use for a building joint?

A building joint is installed in a structure so that static loads are not transferred from element of the structure to another due to movement.


When piping crosses a building joint, an expansion joint must be installed to prevent building movement from damaging the piping system.  The expansion joint must be able to handle movements in all directions (X, Y, and Z planes).  The ideal joint for this application is a MetraLoop.  If there is a space constraint the Seismic Gator can be used. Seismic Gators develop much higher anchor loads than Metraloops, see “How do I calculate in line bellows anchor loads”.


Although a building joint may resemble a seismic joint, it is not required to restrain the piping on each side of the building joint.  We recommend using slide guides on each side of the Metraloop.


In many cases the building joint expansion joint will be used to handle the thermal movement of the adjacent piping as well as the design movement of the build joint.  In this case the thermal movement and the building movements must be added together to properly size the expansion joint.

How to handle thermal contraction

Standard bellows expansion joints are designed to primarily handle compression resulting from pipe expansion.  In the case of a pipe contracting due to a chilled system media, the expansion joint should be pre compressed at the factory to allow the extension of the joint when the pipe contracts.

What expansion joint do I use on plastic pipe?

Metraloops are an ideal expansion joint for plastic pipe. This is due to the low loads the loop exerts on the pipe. Flanged ends are preferred to prevent cracking of thin wall threaded fittings.

An alternative solution to the Loop is the 711 Plus. The 711 Plus is a full face rubber expansion joint with control rods integrated into the flanges on the joint. The integrated control rods prevent excessive stress from being transferred from the expansion joint’s flanges to the plastic flanges.

Can I direct bury a Metragator?

Metragator – No, you cannot direct bury a Metragator. The outer coating on a Metragator is not suitable for direct burial. There is also a concern that debris would work it way into the housing of the joint and damage the bellows.

A vault should be used in place of direct burying Metragators. Metraflex does not currently offer any expansion joint capable of direct burial.

Do I need to worry about thermal expansion of natural gas lines?


If a gas line is run across the roof of a building, it will be subject to thermal loads due to the heating and cooling that occurs between day and night and change of seasons. Depending on the location of the installation, you can see a substantial change of temperature that would require additional flexibility.  The ideal product for this is the CSA / AGA Listed Gas Metraloop.

What is the difference between expansion joints that comply with ASTM F-1120 VS ASTM F-2934.

ASTM F-1120 is often used to specify expansion joints because for many years it was the only ASTM standard addressing metal expansion joints.  The specification ASTM F-1120 was developed in the “Ships and Marine Technology” technical committee of ASTM. The intent of this specification is for it to apply to expansion joints that are installed in Naval ships.  There are several requirements in F-1120 that are not necessary for a typical HVAC system, such as 100% radiographic inspection and manufactured with an ASME Section VIII stamp.  (ASME Section VIII (8) establishes rules of safety governing the design, fabrication, and inspection of boilers and pressure vessels, and nuclear power plant components during construction).  This would be what you would want if you were building an aircraft carrier.

ASTM F-2934 that was developed for products intended for HVAC applications.

Metraflex offers expansion joints that meet ASTM F-2934 or ASTM F-1120.

What if my pressure requirements exceed the rating the flexible connector’s submittal?

For higher pressure applications, we can add a second layer of braid (double braided) to the flexible connector. This second layer of braid adds extra tensile strength to the connector, allowing it to have a higher-pressure rating than the standard option.  There are also higher-pressure hoses available as well.

For high pressure applications, please contact The Metraflex Company.

When do I need to add a liner to a hose or expansion joint?

There are three reasons you would want to specify including a liner for a hopes or expansion joint.

1. For internally pressurized bellows the requirement for needing a liner is spelled out in EJMA (Expansion Joint Manufacturers Association) per the table below.

For a hose the requirement for needing a liner is more straight forward.  Any liquid application of 25 feet per second or gas application over 75 feet per second should have a liner.

2. Any applications that are erosive or have particulate matter that may damage the hose.

3. For plumbing applications where the pipe may need to be rodded to clear an obstruction. An example of this would be the DWV Metraloop.

Please note: Externally pressurized expansion joints by design have a built-in liner.  Examples of externally pressurized expansion joints include Metragator, HP, and HPFF.

What kind of liner should I use?

This will depend on the type of product.


For internally pressurized joints, a solid liner is used that will not interfere with the bellows as shown below.  This type of liner can either be permanently welded in place or slipped into place.


Note.  Liners for internally pressurized bellows are flow dependent.

For hose products an interlock hose is used.  This will match the bend radius of the corrugated hose.

Do I really need anchors for this expansion joint?

Yes you do. See, “How do I calculate in line bellows anchor loads.”

How do I calculate in line bellows anchor loads?

It is common mistake to underestimate the anchor loads developed by an inline bellows joint.  This is the case for both internally pressurized and externally pressurized joints.  For inline bellows joint, the anchor load can be calculated by adding the three major loads together.


Pressure Thrust  Pressure X effective area (Use the highest pressure possible, often the test pressure). The effective area of a bellows is often overlooked.  The effective area can be found by calculating the area of the “mean” diameter of the bellows.


Deflection Load Published Spring Rate X movement of the joint. The deflection load is the force it takes to bend the stainless steel bellows


Frictional Resistance   Total weight of pipe, media, insulation X .3 coefficient. The frictional resistance is the force it takes to overcome the friction of any hangers and guides in the piping system


For the effective area and spring rates vary from joint to joint.  You can find the data of each joint with the bellow links.



HP Compensator 3” Axial

HP Compensator 2” Axial 

HPFF compensator 2” Axial

HPFF compensator 3” Axial

How many anchors do I need for my thermal expansion application?

The rules are:

1. To divide up the piping system into the largest sections a single joint can handle

2. Place anchors between the joints.

3. Review that the anchor locations are suable for the installation of an anchor. In line bellows expansion joints can develop high anchor loads.  See How do I calculate in line bellows anchor loads?

4. Only install one expansion joint between anchors.


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