Flexible Metal Hose

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Can I use a straight hose for a seismic application?

No, a straight hose will not be able to provide the degrees of movement in the X, Y, and Z planes. See the illustration below.  A single section of hose can move laterally if one end is allowed to be drawn in.  A single section of hose cannot handle axial movement without damage to the hose.

Flexible Hose Assembly

A properly functioning seismic joint requires two sections of hose working together.  The two sections can be parallel as in a Metraloop, or perpendicular as in a Dog Leg, or at a right angle as in a V – Loop.

Do I need control rods on a hose and braid pump connector?

No, The braid acts as the control rods. The rods would be redundant and might limit the vibration isolation ability of the hose and braid pump connector.

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.

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