U-Loop Expansion Joint

Frequently Asked Questions

Group 21 Created with Sketch.

Metraloop® Expansion Joint

  • The best way to absorb thermal growth and seismic movement.
  • Develops minimal anchor loads, resulting in minimal guiding and anchoring requirements
  • Capable of movement in all directions
  • Available in stock sizes up to 12”.  Custom models up to 84”
  • UL, FM, UPC, IAPMO and CSA listed models available
3D Drawings
Group 3 Copy 3 Created with Sketch.

VRF Metraloop

  • Meets demanding conditions of VRF systems
  • Rated for 700 psi @ 300° F
  • Stainless steel elbows with the Metraflex Exclusive Copper to Stainless Steel Conversion Fitting
Group 3 Copy 3 Created with Sketch.

UPC Listed Copper Metraloop

  • Protects potable water from thermal and seismic movement
  • Only UPC listed product of its type
  • NSF 61 & 372 Certified for potable water.
  • Available with Press Fit ends, Press Ready ends, and standard sweat ends
  • Develops minimal anchor loads, resulting in minimal guiding and anchoring requirements
Group 3 Copy 3 Created with Sketch.

Seismic BreakAway Hanger

  • Use when pipe hangers and seismic bracing do not allow full-rated movement
  • Ensures free movement of the seismic joint during seismic event
  • Can be reset after seismic event
  • Cable tether available when needed
  • Smart alternative solution to pipe hangers
Group 3 Copy 3 Created with Sketch.

Fireloop® Seismic Expansion Joint

  • X, Y, Z movement
  • Lower pressure drop
  • Install in piping crossing the seismic joint
  • Protects piping and building during quake
  • Pre-tested, pre-assembled
  • Only one support
  • Use with the Seismic BreakAway Hanger
3D Drawings
Group 3 Copy 3 Created with Sketch.

AGA/CSA Gas Metraloop

  • AGA / CSA listed Metraloop for combustible gas and liquid applications
  • Seismic protection for gas lines and anywhere a gas line expands/contracts
  • CSA / AGA listed and certified to meet UL 536
  • Capable of movement in all directions
  • Proudly made in the USA
3D Drawings
Group 3 Copy 3 Created with Sketch.

DWV Metraloop (PoopLoop II)

  • IAPMO listed Metraloop for DWV (Drain Waste Vent) applications
  • Seismic protection for DWV lines and anywhere else DWV lines expands/contracts
  • Floating flange design allows for proper pitch of piping
Group 3 Copy 3 Created with Sketch.

Dog Leg Expansion Joint

  • The one joint that accommodates pipe movement from two runs of pipe.
  • Utilized for both thermal and seismic applications
  • Capable of movement in all directions
  • Develops minimal anchor loads, resulting in minimal guiding and anchoring requirements
Group 3 Copy 3 Created with Sketch.

Copper Metraloop

  • Protects water systems during both thermal cycling and seismic movement
  • Develops minimal anchor loads, resulting in minimal guiding and anchoring requirements
  • Capable of movement in all directions
  • Available up to 4” diameter
3D Drawings
Group 3 Copy 3 Created with Sketch.

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 Metraloop®?

No, you cannot direct bury a MetraLoop®. The hose length is calculated based on the minimum bend radius of the hose, and it is assumed that the entire length of the hose will be able to bend. If a MetraLoop® is direct buried the hose will be point loaded and fail prematurely.

A vault should be used in place of direct burying MetraLoops®. 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 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.”  and “What are the anchor loads for a Metraloop.”

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


What are the anchor loads for a Metraloop®?

In a Metraloop® the sections of hose and braid are perpindicular to the pipe.  The braid restrains the hydrostatic end load.  The anchor load for a Metraloop is the lateral spring rate of the hose.

Metraloop Anchor Loads

Metraloop                         Anchor

Size                                      Load

2”                                         76 lbs.

2.5”                                     83 lbs.

3”                                         90 lbs.

4”                                         120 lbs.

5”                                         186 lbs.

6”                                         202 lbs.

8”                                         260 lbs.

10”                                       283 lbs.

12”                                       390 lbs.

How far away from the seismic separation can I place my Metraloop®?

The building codes have not kept up with the requirements to provide guidance for the installation of piping that cross a seismic separation in a building.  The only code that does address this is NFPA 13, however, unless specified otherwise, this is only mandatory fire sprinkler piping.


NFPA 13 specifies that the seismic joint (Metraloop) be installed 2 feet from the seismic separation.  Additionally, the bracing to secure the piping to structure must be installed no more than 6 feet from the seismic separation.


The parameters provided in NFPA 13 are very conservative.  The practical reality is that a seismic joint (Metraloop) will work just fine if installed farther than 2 feet from the seismic separation, likewise for the bracing being installed farther than 6 feet from the seismic separation.  If this is the case, attention should be paid to make sure the piping is properly supported.  Additionally, you should make sure that the piping configuration does not amplify the movement of the seismic joint, and if it does size the joint properly.

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.


Find Your Local Metraflex Representative.

Find a Rep
Rep icon Your Local Rep