German based resin producer, Ticona, recently required some outside the box thinking to overcome a recurring control valve failure. The situation could not be tolerated for long, but the harsh conditions in fibre enforced plastic production meant that abrasion and corrosion occurred on the body, tapered seat and plug after only a short operating time, leading to total premature failure.
Lift valves of the type used until the 1980s could not be considered because of the emissions regulations, which forced chief planning officer, Thomas Labahn, to take a long hard look at the problem. A ball sector valve that he saw during a plant tour of valve manufacturer Schubert & Salzer caught his eye. “As soon as they explained the control valve to me, a light went on in my head,” is how he describes his Eureka moment.
Ball sector valves
Ticona conducted a test of the ball sector valve. “The very first attempt was a success. The valve lasted nearly 10 months, almost three times as long as before,” explains Labahn. The trick: the valve is made from standard material (stainless steel) and only has hard chrome-plated (stellited) ball sector surfaces. This is the reason why it is significantly less expensive than the previous valve made from Hastelloy. Furthermore, the damaged valve can be repaired and even costs a good deal less to buy than does the plug valve made from Hastelloy. Considering the numerous control functions in which these valves are used at Ticona under extremely challenging conditions, the saving potential is quite considerable.
A small actuator is sufficient
After 10 months in operation, only the holder of the seat ring had come loose, and the manufacturer is already working on a solution to this problem with an improved variant. “We found there was abrasion on the parts that were not stellated but came into contact with the medium,” describes Labahn. As far as the client is concerned, the reason for the longer service life is principally to do with the geometry: “The media flows through the ball sector element in a totally different way than in the plug valve,” explains Labahn. In future, completely stellated valves are expected to offer even greater improvements.
Other important aspects in favour of using ball sector valves are their comparably light weight and relatively low actuator thrusts. The friction against the stuffing-box seal in plug valves means they have a high break loose torque and this explains why they need powerful actuator thrusts. The torque with the ball sector valve, on the other hand, is relatively low and the actuators can be correspondingly smaller. A single action Wirematic actuator with a safety position is used on the valves.
Furthermore, the valve does not get blocked because the entire pipe cross section is left unobstructed when the ball sector is fully open. Ticona is also happy when it comes to the control response: “In our case, the control accuracy that has been achieved is comparable to that of the plug valve we used to use,” reports Labahn. “We are planning to test using the ball sector valve as a manual valve as well soon, says operations assistant Volker Gatzert. They anticipate this will result in greater ease of movement at the same time as saving space and weight.
Principle of the ball sector valve
The ball sector valve consists of a hemispherical shell – the ball sector – that is held securely in the valve body using two large bearing stems. Part of the shell is used for isolating, while the other has a hole with a diameter corresponding to about 80% of the nominal size of the valve. The ball sector is turned through about 90° from fully open to fully closed. The shape of the opening cross section changes from completely round to elliptical in this case. This practically round shape for the flow opening reduces the risk of blockage when small control settings are made.
Ball sector valves are used as isolating and control valves for fluids, gases, turbid media and sludge, especially in the pulp and paper and chemical industry. Abrasive slurries of iron ore, coal, lime and fly ash are also controlled with this type of valve.
For more information contact Rowan Blomquist, Macsteel Fluid Control, +27 (0)31 581 7803, [email protected], www.macsteel.co.za
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