Globe Valve Internals: Plug, Contoured, and Characterized Disc Designs

Globe valves are the workhorses of throttling and flow control in industrial piping systems. Unlike gate valves or ball valves designed primarily for on-off service, globe valves are engineered to regulate flow across a wide range of positions from fully open to fully closed. The relationship between valve position and flow rate, called the flow characteristic, is determined largely by the shape of the closure element (plug or disc) and the geometry of the seat through which flow passes. Understanding the different internal disc and plug designs enables proper selection for specific flow control requirements.

Flat Disc Design

The simplest globe valve disc is a flat disc that seats against a flat seat ring. As the disc lifts from the seat, an annular gap opens between the disc face and the seat bore, allowing fluid to flow. The flow area increases approximately linearly with stem travel near the seat, but the flow characteristic is not precisely linear across the full travel range. Flat disc designs are simple, inexpensive, and easy to regrind when seats become damaged. They are used for general-purpose throttling in moderate service conditions. The flat disc provides good shutoff performance since the entire flat face seats against the seat ring with even contact, making it forgiving of minor surface imperfections.

• Flat disc: simple design, easy to regrind, good shutoff, approximate linear flow near seat

• Needle plug: long tapered needle for precise throttling of small flows at high differential

• Contoured plug: shaped body to achieve specific linear or equal percentage flow characteristic

• Parabolic plug: smooth curved profile for equal percentage characteristic control valves

• V-port plug: slotted disc for characterized flow in smaller control valve sizes

Contoured and Parabolic Plugs

Control valves for process applications use precisely shaped plug or disc profiles to achieve specific flow characteristics. An equal percentage characteristic is the most common requirement for modulating control: the flow increases by an equal percentage for each equal increment of stem travel, meaning the valve is very sensitive (changes flow greatly) near full open and less sensitive near the closed position. This characteristic compensates for the non-linear installed gain that results from the pressure drop across the valve changing as flow changes in most piping systems. The parabolic plug profile is the classic shape for achieving equal percentage characteristic in globe-style control valves and is used in most single-port control valve designs.

Cage and Characterized Trim

Cage-style globe valve internals use a cylindrical cage with precisely machined ports or windows through which flow passes rather than a conventional plug-seat geometry. As the plug travels up and down within the cage, the flow area is determined by how much of the cage port area is uncovered. The size, shape, and distribution of ports in the cage determine the flow characteristic. Cage trim offers several advantages: the cage guides the plug throughout its travel, reducing vibration and wear; cage ports can be shaped to achieve any desired flow characteristic by varying port geometry; anti-cavitation and noise-reduction cage designs use multiple small ports or labyrinth flow passages to reduce pressure drop per stage and control cavitation and noise; and cages are replaceable trim components that allow characteristic changes without replacing the valve body.