Mitech recently subjected its Valvtechnologies' ball valves to various test factors to determine if there could be a cost saving in the manufacture of the instrument as well as carrying out test comparisons with a competing 80 mm plug valve.
All secondary air pump discharge valves are fitted with limit switches that enable Mitech to monitor every cycle. When a valve is changed, a counter is manually reset to zero on the DCS control system. The VT valves are switched by a double acting actuator with an air supply of 550 kPa. Both valves were tested in the same process conditions. The medium consisted of 60% slurry and 40% secondary spillage and a copper spent solution with a pH of 2,2. The pressure drop over an open or closed valve was 380 kPa and 1050 kPa respectively, while the flow rate through the valve was 180 m3/h. The temperature of the solution was 60°C and the velocity through the valve 20 m/sec.
The results proved that the main advantage of the VT valve is the fact that its Belleville spring washer exerts a pressure of 450 kPa on the ball at all times, preventing any solution from entering the seals around the shaft. As it is a full bore valve, there was no obstruction of the flow, as in the case of the opposition's plug valve. The face-to-face of both valves are the same. A disadvantage of the VT valve is that it is twice the weight of the plug valve, thus an increase in the safety factor is required, but as the frequency is so low, the risk is reduced.
Mitech's final results indicated that the other plug valves lasted anything between 12 000, 18 000, to 24 000 cycles, with a maximum of 28 000 cycles completed, although maintenance costs were high due to repeated valve failure. The VT valve in comparison, is currently on 98 000 completed cycles operating at maximum efficiency.
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