A ball valve seals because of contact pressure between the ball and the seats. One of the principal challenges in designing a ball valve is determining how to generate sufficient force to create this contact pressure. If the force is too little at any point in the pressure range, the valve will leak. If it is too great, the valve will be difficult to actuate and its cycle life may be compromised.
Live-loading is one means of applying force. Live-loading refers to a spring that fits somewhere between the end screw and the seat. Usually, the spring pushes against a seat carrier, which is a device that holds and positions the seat for ideal contact with the ball (Figure 3).
Pressure Helps to provide Tight Sealing of Subsea Wellhead Valves
Live-loading is especially important for three-way valves where the sealing force is provided solely by the upstream seat. Another source of sealing force is system pressure itself. Valves that are not live-loaded may rely entirely on system pressure to generate the force between the ball and the downstream seat.
In this case, sealing force may be adequate at the upper pressure range, but inadequate in the lower pressure range. Once the seat has become compacted – or ‘taken a set’ – at high pressure, resealing at lower pressures may be difficult. The seat material may not have enough memory to return to its original shape. Therefore, with low system pressure and no springs, there may not be enough force to make the seal.
Cranking it Up: Sometimes, Brute Force Is Just Too Much
Under such circumstances, a technician may crank the end screws tighter to prevent leakage. This action may correct the immediate problem but with an unintended result: Actuation will be difficult, especially at high pressure. As a result, to actuate the valve at high pressure, the technician may need to use a ‘cheater bar,’ a bar that extends the handle’s length enabling greater leverage.
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