The core design of a Manual Ball Valve dictates its optimal use as an On/Off Isolation Valve—specifically for Fully Open and Fully Closed service. Its central component, the spherical ball, contains a bored port. When the valve is fully open, this port aligns perfectly with the pipeline's bore, offering minimal flow resistance and near-straight-through flow conditions. This characteristic of "quick actuation and low pressure drop" is the ball valve's primary advantage.
However, once the valve is placed in a throttling position (partially open), the valve's performance and longevity are severely compromised. In professional terminology, standard ball valves are designated for Isolation Service, not Control Service.
When a ball valve is used for throttling, the drastic reduction in flow area creates a Venturi Effect between the ball and the seat. This causes the fluid velocity to become extremely high, accompanied by a sharp drop in static pressure.
Cavitation: If the medium is liquid, the local pressure can fall below the fluid's vapor pressure, leading to the instantaneous formation of vapor bubbles. As the fluid accelerates and pressure recovers downstream, these bubbles violently collapse (implode). This collapse generates intense local pressure shockwaves, causing severe pitting damage (pitting) to the ball surface and the downstream seat.
Erosion: In media containing solid particles, slurries, or high-velocity liquids, the abrasive wear on the valve's internal parts is devastating. Particles impact the exposed edge of the ball and the valve seat at extremely high speeds, causing rapid material wear, compromising the sealing surface, and leading to internal leakage.
An ideal flow regulating valve should possess an Equal Percentage or Linear flow characteristic curve, which ensures stable and precise flow regulation across various degrees of opening.
Standard ball valves, however, exhibit a highly non-linear flow characteristic curve:
90% to 100% Open: The flow rate change is minimal; the "effective regulating range" is very narrow.
0% to 30% Open: A minuscule change in valve opening results in a huge, uncontrolled, jump in flow rate.
This non-linearity means that operators or automatic control systems cannot achieve precise flow setting through small valve adjustments. In the low-opening range, the valve is excessively sensitive to changes, leading to flow fluctuations and system oscillation, significantly compromising the stability of process control.
Ball valves typically employ soft seats (e.g., PTFE, RPTFE) to achieve excellent Bubble-Tight Shutoff (zero leakage) performance.
Under throttling conditions, the high-velocity flow is forced through the narrow gap between the ball and the seat, creating a powerful "Jetting" effect. This jet directly attacks the soft seat material, causing the soft seats to quickly experience erosion, deformation, or even material peeling.
Once the seating surface is damaged, the ball valve's reliance on zero leakage is permanently compromised. Even when fully closed, the media will leak through the damaged passages, which is unacceptable for systems requiring strict isolation (e.g., Double Block and Bleed systems). Frequent throttling operations drastically shorten the ball valve's service life, increasing maintenance costs and downtime.
When operated long-term in a partially open position, especially with media containing impurities or viscous substances, the non-exposed portion of the ball surface (the sealing area that contacts the seat) is prone to accumulating solid particles or coke deposits.
When an operator attempts to close or open the ball valve from the throttling position, these accumulated deposits can become lodged between the ball and the seat, leading to:
A significant increase in Operating Torque, making manual actuation difficult or requiring an oversized actuator.
The deposits grinding the ball and seat surfaces during the cycling process, causing further sealing damage, potentially resulting in the valve seizing or becoming locked.
For applications requiring precise and durable throttling, the industry typically utilizes purpose-built Control Valves:
Globe Valves or Angle Valves: Their internal flow geometry is specifically designed to withstand high differential pressure throttling. Fluid impact is directed toward the easily replaceable trim (plug and seat).
V-Port Ball Valves: While retaining the ball structure, these valves feature a V-shaped or parabolic opening cut into the ball or seat. This design effectively improves the flow characteristics, allowing for better regulation precision even at low openings, making them an ideal choice for moderate throttling services.
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