The plastic ball valve is one of the most popular quart […]
The plastic ball valve is one of the most popular quarter-turn valves in the world market today, with multiple types available. However, all ball valves operate in the same fashion: a sphere opens and closes, allowing flow through the valve.
There are four key components of ball valves which we will examine:
The Floating Principle
Design and Testing
The Floating Principle in Ball Valves
Historically, engineers designed ball valves to have soft seats made from plastic-based materials to form a tight seal against a floating ball. The ball valve floating principle means three things occur inside the valve:
The ball floats freely and does not attach to the stem.
Under pressure, the ball should move into the downstream seat and create a preset suitable to seal against a soft seat material.
The seat will perform its best seal at the highest operating pressure.
In designing a ball valve, one of the most important aspects is to dimensionally control all the metal parts, surface finishes and tolerances in relation to one another. Furthermore, the ball must be perfectly round and have the preset suitable to seal against the soft Teflon materials, or the harder seats like reinforced PTFE, peek or metal.
The secret of any seal design is to get local deformation (that means at the point of contact the seat yields) and fill in voids between the metal ball and the soft seat. Without this local yielding, you cannot get bubble-tight or vacuum performance.
Design & Testing of Ball Valves
Most floating design ball valves go through the hydro shell test which is 1.5 times the ANSI rated pressure. The valve soft seat test is at either 80 or 100psi in the closed position.
(Note: It is important not to test the ball valve higher than the operating pressure in the closed position. For system pressure tests higher than expected operating pressures, it is important to leave the ball valves in the open position.)
Most all smaller sizes (1/4” – 3”) floating ball valves with either weld-end or threaded-end connections have pressure ratings of WOG (water, oil or gas) or MAWP (maximum allowable working pressure). The MAWP or WOG pressure ratings are considered a do-not-exceed pressure at normal ambient temperatures.
In most soft-seated valves, there is a pressure-temperature curve, maximum pressure at ambient and maximum pressure at the highest temperature rating of the seat. The stress levels of the valve seat and its ability to thermal cycle over these limits determine this. The valve seat must be strong enough and thick enough to take fluid dynamic effects without folding over or deforming.
Each valve seat design varies per its applications, material strength, and physical compounds. The pressure-temperature curves and the torque curves vary with the seat materials and design of the individual valves.
Bidirectional Sealing in Ball Valves
The spring-loaded seats on the upstream and downstream sides allow the valve to seal in two directions. Therefore, installation of the valve is possible in any flow direction during.
Trunnion ball valves suite bidirectional sealing with both upstream and downstream flow applications best.
If pressure comes from upstream flow direction, it could be possibly cause valve seat damage. Unsupported Teflon seats can cold flow and be destroyed under high velocity fluid effects. They are primarily vulnerable to erosion or solids in the flow stream. It is always a good idea to provide all pressure and flow information for high pressure (2000+ psi) reverse flow directions when floating ball valves are required.
A ball valve design where the body is held in-line with the stem supporting the ball is a trunnion ball valve. In this configuration, the centerline of the ball spherical surface and centerline of the stem diameter are identical. Under operating conditions, the ball does not move from this position or “float” up or downstream.
Typically, a surface in the body which carries the pressure end loads supports the ball. There are seats on each side of the ball that are spring loaded into the ball making it a true bidirectional sealing valve.
Testing the trunnion design at full rated pressure in either flow direction is possible without any soft seat damage.