Choosing the Right Assembly: Single Union, Double Union, and Multi-Piece Ball Valves

The ball valve is celebrated in fluid control for its quick, quarter-turn operation and reliability as an on/off isolation device A Guide to Ball Valves: How They Work, Their Types, and Why They Dominate the Industry. However, not all ball valves are built the same way. The construction of the valve body—specifically whether it is a single-piece, two-piece, three-piece, or a union design—has enormous implications for its installation, serviceability, and suitability for various applications.

This guide focuses on the three most common multi-piece designs, highlighting how their assembly simplifies critical maintenance tasks.

1. The Single Union Ball Valve

A Single Union Ball Valve features one union end connection and one fixed end connection (either threaded or socket fusion).

Design and Function:

• Assembly: It consists of a main valve body, the ball, the stem, and one union nut.
• Maintenance: The single union allows the entire valve body to be disconnected from one side of the pipeline simply by unscrewing the union nut. This is an improvement over single-piece valves, which typically require cutting the pipe for removal.
• Applications: Commonly used where the valve needs maintenance or replacement, but only one side needs to remain connected, such as diverting flow or on a dedicated branch line UPVC Single Union Ball Valve.

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2. The Double Union (True Union) Ball Valve: The Service Champion

The Double Union Ball Valve, often called a True Union Ball Valve, is the gold standard for maintenance-friendly installations.

Design and Function:

• Assembly: Features two union ends and a central body piece that houses the ball and seats.
• Maintenance: The valve's center section (the entire working mechanism) can be removed from the pipeline for full inspection, repair, or replacement without disturbing permanent pipe connections. This reduces downtime and simplifies seal/seat replacement, extending valve lifespan.
• Applications: Essential for corrosive media or high-cycle applications where maintenance is predictable and frequent, such as in Chemical Processing and Wastewater Treatment. See our UPVC True Union Ball Valve with Double Unions here.

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3. The Three-Piece Ball Valve: Metal's Repair Advantage

While plastic union valves dominate plumbing and lower-pressure industrial uses, metal valves—particularly the Three-Piece Ball Valve—offer the same maintenance benefits for high-pressure, high-temperature fluid control.

Design and Function:

• Assembly: Built using three body pieces bolted together: a main center body and two end caps.
• Maintenance: Like the True Union, the three-piece valve allows in-line repairability. Removing body bolts (or flange bolts) lets the main center section be lifted out while end connections remain in place. Ideal for seal and seat replacement.
• Applications: Crucial for industries requiring robust construction with easy maintenance access. Common in Power Generation and Oil & Gas with Stainless Steel Three-Piece Ball Valves here.

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Final Selection: Prioritizing Serviceability

When selecting a ball valve, the decision often comes down to budget and service requirements.

In highly cycling or abrasive environments, the savings from reduced downtime and simplified maintenance justify the investment in a True Union or Three-Piece design. For long-term performance, always select valves in line with recognized benchmarks like ASME B16.34 standard, covering materials, pressure ratings, and testing.

Routine inspections and maintenance—facilitated by these designs—are the most effective way to ensure valve reliability and extend service life.