Spirax Sarco South Africa has completed the second batch of three 250 mm globe control valves for delivery to Norway.
The valves are designed for shipboard application where they are used to control the flow of steam at an inlet pressure of 25 bar and a temperature of 230°C. The outlet pressure can be as low as 4 bar and the maximum flow rate is 79 100 kg/hr.
The locally produced valves stand 1,7m tall and weigh over a ton
In order to cope with such a large pressure drop, the valves are fitted with specialised trim sets incorporating multilabyrinth energy dissipating stacks, which give bilinear system characteristics. The plugs and seats are hardened with Stellite 6 that is applied using a plasma arc process. This enables the valves to dissipate approximately 8250 kW of energy, by contrast a standard globe control valve would only be capable of dissipating around 1490 kW.
To achieve high reliability the valves are not fitted with pressure balanced trim, but rather relatively large actuators are used to guarantee a good shut-off which does not deteriorate as rapidly as if pressure balance seals were incorporated in the design.
The valves are therefore fitted with long stroke tandem piston actuators with integrated tank air fail systems. This allows a relatively compact design that achieves shut-off better than 0,01% of rated capacity, under test conditions. Prior to installation on the oil-tankers the valves were certified by Det Norske Veritas, the largest globe control valves manufactured in South Africa to be so certified.
Spirax Sarco says these valves typify the company’s commitment to design and build specialised valves that meet demanding client requirements and maximise productivity.
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