There is a school of thought that feels a double block function can be achieved by implementing a double piston effect seat component. A double piston effect seat will seal upstream on a ball and also downstream on a ball if the upstream seat begins to leak.
This is achieved by a force which is created by the line pressure acting on the two areas created by the difference between the inside diameter of the O ring and the seat to ball contact diameter.
Because the sealing forces are proportional to the areas, a large cross section O ring needs to be used. There are a large number of companies that limit the maximum section of the O ring because of the higher possibility of explosive decompression occurring whilst in service.
To minimise the risk of explosive decompression it is suggested by seal manufacturers that the smallest cross sectional O ring should be used in a groove which provides a minimum of 85% fill.
This then causes a second problem for double piston effect seals. Because the O ring slides a considerable distance, depending on the direction of operation, between the body and the seat, a groove fill of this proportion cannot be achieved.
There are other considerations to take into account when using double piston effect seats. Because the double piston effect seats seal the ball in both directions simultaneously, the pressure in the ball cavity cannot escape which could lead to an increase in torque and possible lock-up of the valve.
If the pressure in the ball cavity increases due to thermal expansion then the pressure cannot escape. As the temperature increases the pressure also increases and the material strength of the valve and bolting decreases. With the mechanical characteristics of the valve and media moving in an opposite direction, there could be a potential for the body or bolting to fail.
So whilst double piston effect seats do provide a cheap means to provide both upstream and downstream sealing, to perform a true double block function we have to utilize two independent isolating balls in series.
Tags: Ball Valves, oliver twinsafe, oliver valves, oliver valvetek
Posted in: Ball Valves