Union Flexible Metal Hose: Sizes, Connections, and Selection Guide

A union flexible metal hose is a metal hose assembly designed to connect piping or equipment while accommodating movement, vibration, or installation misalignment. “Union” usually describes a detachable fitting connection, but the exact end style and dimensions can vary by supplier. To select the right assembly, confirm the nominal hose size, connection standard, pressure and temperature requirements, fluid compatibility, and the movement the hose must handle. This guide explains how to compare those details and prepare a clear request for quotation.

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Who This Guide Is For

This guide is for engineers, maintenance teams, procurement professionals, and distributors sourcing flexible metal hose assemblies for industrial or commercial systems. It is especially useful when a drawing or specification refers to a “union” connection without fully defining the fitting type. Since product terminology is not always used consistently, buyers should confirm the connection details with the supplier before ordering. The examples below are for selection planning, not a substitute for a manufacturer’s drawing or technical confirmation.

What a Union Flexible Metal Hose Is

A flexible metal hose assembly typically combines a flexible metal hose with end fittings attached to suit the installation. Many designs use a corrugated metal hose to provide flexibility, with an external braid used in some configurations to help contain pressure-related forces. A union fitting allows a connection to be assembled or separated using a mating union component, which can be helpful where routine removal or maintenance is expected. The assembly’s actual capabilities depend on its materials, construction, dimensions, and end connections.

How the Union Connection Works

“Union” is a broad purchasing term rather than a complete connection specification. It may refer to a union nut and mating seat, a threaded union, or another separable fitting arrangement, depending on the product and market. These styles are not automatically interchangeable, even when their nominal sizes appear similar. Ask for the fitting drawing, thread or sealing details, and the required mating part before approving an order.

Sizes, Materials, and Key Specifications

Hose size is commonly stated as a nominal diameter, such as DN or NPS, but nominal size does not necessarily equal the hose’s measured inside or outside diameter. As representative size equivalents, DN15 is commonly associated with NPS 1/2, DN25 with NPS 1, and DN50 with NPS 2. Treat these as nominal references and check the supplier’s dimensional drawing for the actual assembly. Also specify the required overall length and how that length should be measured, such as face-to-face between fittings.

Materials and Construction Options

Material selection should account for the conveyed medium, operating temperature, environment, and cleaning or maintenance conditions. Stainless steel is a common choice for metal hose components, but the appropriate grade must be confirmed against the fluid and service conditions. Depending on the application, an assembly may include a corrugated hose, braid, and metal end fittings, with the specific construction selected for the required duty. Do not assume that a material suitable for the hose is automatically suitable for every fitting or seal in the assembly.

Specifications to Confirm

• Nominal size: State DN or NPS and request the actual hose and fitting dimensions.
• Length: Provide the required installed length and measurement method.
• Connections: Identify the union style, thread or seat, mating component, and any required orientation.
• Service conditions: Give the medium, operating pressure, temperature range, and any pressure fluctuations.
• Movement: Describe whether the hose must absorb vibration, axial movement, lateral movement, or repeated flexing.
• Environment: Note external exposure, corrosion concerns, cleaning requirements, and installation constraints.

Match the Hose to the Application

Flexible metal hose assemblies are used where a rigid connection would make installation difficult or fail to accommodate expected movement. Potential applications include equipment connections, piping transitions, and systems subject to vibration, provided the assembly is selected for the specific service. A flexible hose should not be treated as a universal substitute for rigid piping or as a support for misaligned equipment. The installation layout and operating conditions determine whether a particular assembly is appropriate.

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Plan for Movement and Routing

Describe the movement in practical terms rather than simply asking for a “flexible” hose. For example, a preliminary layout might show 25 mm of expected lateral movement and 100 mm of available clearance; these are illustrative project inputs, not product ratings. The supplier needs enough information to assess the routing, bend geometry, length, and fitting orientation. If the hose will move repeatedly, identify the movement direction and expected operating pattern so the design can be reviewed accordingly.

A Practical Selection Framework

1. Define the service: Record the fluid or gas, pressure, temperature, and operating conditions.
2. Identify the connection: Provide the mating equipment details, thread information, union type, and sealing arrangement.
3. Confirm the size and length: State the nominal size and required face-to-face or overall dimension.
4. Describe movement and routing: Include a sketch or photograph showing direction of movement, available space, and connection orientation.
5. Request technical confirmation: Ask the supplier to verify the proposed construction and provide a drawing for approval before production.

For example, a request might specify a DN25 (NPS 1) assembly with a 2 m face-to-face length, a stated union connection at each end, and defined operating conditions. The 2 m length is only an example of how to communicate a requirement; it does not indicate a standard size or recommended design. Include the actual mating details and project requirements rather than relying on nominal size alone. Where the system has a formal engineering specification, use it as the controlling document.

Pricing, MOQ, and Lead Time

Price and availability depend on the selected hose construction, materials, nominal size, length, fittings, and any required documentation or packaging. A standard configuration may be easier to quote than a nonstandard combination, but availability and minimum order quantity must be confirmed for the specific request. Lead time can also depend on component sourcing, production scheduling, and drawing approval. To support a useful quotation, provide a complete specification and target quantity at the beginning of the discussion.

Common Buying Mistakes

• Ordering by size alone: Two assemblies with the same nominal size may have different end connections or dimensions.
• Using “union” without a drawing: The term may not uniquely identify the nut, seat, thread, or sealing method.
• Ignoring movement direction: A hose routed for one type of movement may not suit another.
• Assuming all metal hose is interchangeable: Materials and construction must be checked against the actual service.
• Approving production without checking the interface: Confirm mating dimensions and assembly length before release.

Supplier Evaluation Checklist

Choose a supplier that can review the application details and clearly document the proposed assembly. Ask whether the quotation identifies the hose size, materials, connection description, length measurement, and any assumptions that still require confirmation. Request a drawing or dimensional information so your engineering team can check fit and mating compatibility. Also clarify quotation validity, minimum order quantity, lead time, packaging, and what documentation can be supplied for the order.

At XINCHI, we support B2B buyers by reviewing the requested size, union connection, application conditions, and assembly details before quotation. Share a drawing, an existing sample specification, or the mating equipment information where available. We can then discuss a suitable configuration and identify any details that need confirmation before an order is placed. Final selection should be based on the approved specification and the requirements of your system.

Key Takeaways and Next Steps

The right union flexible metal hose is defined by more than nominal size: the connection interface, materials, operating conditions, length, and movement all matter. Treat DN and NPS references as starting points, and verify actual dimensions and union details against a drawing. Before requesting a quote, prepare the service conditions, connection information, installation length, movement description, and required quantity. Send those details to XINCHI for a configuration review and quotation, and confirm the final drawing before production.