English
Created on 07.25

The Main Components of a Ball Valve Assembly

July 25, 2026
The main components of a ball valve assembly include the valve body, ball, stem, seats, and seals.
These parts work together to control fluid flow and create a leak-tight seal when the valve closes. You can also find additional components like the bonnet, gland nut, and actuators in many assemblies.
What Is a Ball Valve Assembly?
A ball valve assembly is a complete unit comprising all the parts needed for quarter-turn flow control. The assembly includes the valve body, internal ball, stem, seats, seals, and operating mechanism. When the handle rotates 90 degrees, it turns the ball inside the valve body to either allow or block fluid flow through the system.
Main Ball Valve Assembly Parts
The ball valve assembly comprises several critical components that work together to power its mechanism.
Valve Body
The valve body is the main housing that contains all internal components.
Ball valve bodies come in three configurations: one-piece, two piece ball valve assembly, and three-piece designs. One-piece bodies are economical for standard applications. Two-piece housings are the most popular choice for general industrial use. Three-piece bodies allow easy removal of the center section for cleaning and maintenance without disconnecting the entire valve from the pipeline.
Ball
The hollow, perforated ball is the central component that controls flow. When aligned with the inlet and outlet ports, fluid passes through the hole in the ball's center.
Rotating the ball 90 degrees positions the solid side across the flow path, blocking fluid passage.
Stainless steel and chrome-plated brass are common materials for the ball.
Stem
The stem connects the ball to the handle or actuator. It transmits rotational force from the operating mechanism to the ball. Blowout-proof stems prevent stem ejection in high-pressure applications. The stem passes through the valve body and is sealed to prevent external leakage.
Seats and Seals
Seats create the sealing surface on either side of the ball.
When the valve closes, the ball presses against these seats to prevent leakage.
PTFE is the most common seat material for standard applications. Applications with high pressure, high temperature, or corrosive chemicals use Viton, FKM, Nitrile, or PEEK materials. O-rings, backup rings, and chevron packing provide additional sealing at various points in the assembly.
Ports
Ports are the openings where fluid enters and exits the valve.
Standard ball valves have two ports: an inlet and an outlet.
The ports connect to the piping system through various methods:
• NPT threaded connections
• Welded joints
• Flanged connections
• Socket weld fittings
Handle or Actuator
Manual ball valves use a quarter-turn handle for operation. A 90-degree turn of the handle completely opens or closes the valve. Lockable handles prevent tampering or accidental operation. Automated valves use actuators mounted to the stem. Actuator types include:
• Electric actuators
• Pneumatic actuators
• Hydraulic actuators
Bonnet
The bonnet covers the top of the valve body and secures the stem assembly.
It provides access to internal components for maintenance. The bonnet also protects the stem and packing from external damage. The bonnet bolts to the valve body and compresses the packing material around the stem.
Gland Nut and Packing
The gland nut compresses packing material around the stem to create a seal and prevent external leakage.
The packing material sits in the stuffing box between the stem and valve body. Graphite packing is common for high-temperature applications, while PTFE packing works well for standard conditions.
Thrust Washers
Thrust washers reduce friction between the ball and valve body during operation. These washers reduce operating torque and extend component lifespan. They handle the axial forces generated when the ball rotates under pressure.
Ball Valve Assembly Drawing: Reading the Components
A ball valve assembly drawing provides a cross-sectional view of the valve, showing how each component fits together.

These technical drawings use numbered callouts to identify parts like the body, ball, stem, seats, bonnet, and seals. The drawing also shows dimensions, material specifications, and the proper orientation of sealing surfaces.
Key elements in assembly drawings:
• Component identification: Numbers correspond to each part in the bill of materials
• Cross-sectional views: Show internal arrangement and alignment of parts
• Dimensional data: Critical measurements for proper installation and maintenance
• Material callouts: Specify materials for pressure-containing components
Every starry ball valve assembly undergoes full component inspection before shipment, ensuring accurate alignment and superior sealing performance.
API6D Casting Trunnion Ball Valve
Size Range:
 2″ ~ 48″ (DN50 ~ DN1200)
Pressure Rating:
Class 150LB ~ 2500LB
Body Material:
Carbon Steel (A105, WCB), Stainless Steel (SS304, SS316)
Design Standard:
API6D, ASME B16.34
Two Piece Ball Valve Assembly vs Other Designs
The two piece ball valve assembly is the most versatile design for industrial applications. It splits the body into two sections bolted together, allowing access to internal components without removing the valve from the line. One-piece valves offer the lowest cost but require complete removal for internal service. Three-piece valves provide the easiest maintenance access but at a higher initial cost.
With automated assembly lines and traceable production records, STARRY guarantees consistent quality across every valve batch.
Starry manufactures ball valve assemblies with quality-tested components for demanding industrial applications. Browse our ball valve collection to find the right assembly for your system requirements.
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Conclusion
Ball valve assembly parts work together to provide reliable quarter-turn flow control. A ball valve assembly drawing identifies component positions and specifications during maintenance.
The two piece ball valve assembly design offers the best balance of cost and serviceability for most industrial applications.

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