Even in the case of a conventionally designed, live-loaded ball valve, there may be issues with the seal at the lower end of the pressure range (<5,000 psig). The valve may create good sealing force initially because of the force delivered by the springs. But as the system pressure overcomes the springs – and collapses them – the sealing force may not be sufficient. When system pressure and the springs are arranged to act in series – one behind the other – there is no net sealing benefit at certain points in the pressure range (Figure 3).
Combined Forces Provide Tight Sealing of Ball Valves...
By contrast, in the FKB series mediumpressure valve, these two forces – system pressure and the springs – act on separate points on the seat carrier. Therefore, both contribute to the overall net sealing force. In (Figure 4), system pressure on the upstream side acts on one step in the seat carrier, while the springs act on a second step. Together, they create a combined force adequate for sealing across the entire pressure range.
...Across the Whole Pressure Range
This type of live-loading is termed direct loading. An additional design challenge for mediumpressure valves concerns handle orientation. In some cases, adjacent components may prevent the quarter-turn handle from being fully actuated. The challenge is to create a handle that can be repositioned in any one of four quadrants so a technician can reposition it without having to remove the valve and modify the valve supports.
This goal is achieved by introducing a stop plate between the handle and the valve body. The stop plate may be repositioned, shifting the on and off position by 90-degree increments (Figure 5).
Final Tests: Better Safe Than Sorry
On the two skids that AGI fabricated, there were a total of 36 valves and 452 mediumpressure tube fittings or end connections for use with SAF 2507. AGI checked all fittings with the handheld Swagelok gauge. Capital Valve & Fitting, the Swagelok authorised sales and service centre in Baton Rouge, La., also checked the fittings, and found them all to be pulled up and tightened properly...
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