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The use of Clarkson Knifegate valves in the Bayer alumina process has been a success story over the last decade. This use of this product has changed the way alumina plants isolate piping from the use of manual blind flanges to an air actuated, 1" per second closing, zero downstream leakage solution which is designed to handle the toughest abrasive slurry applications. The concept of the Clarkson valve is centered on the use of steel reinforced, totally encapsulated, high temperature EPDM elastomer sleeves which are under a calculated degree of compression. A compressed elastomer is the most abrasion resistant material for use in tough slurry applications. The slight venturi shape of the profile of the sleeves actually accelerate the slurry through the fully open pipe sized opening, which offers no resistance to flow nor exposed edges for the suspended particles to come into direct contact. There are literally thousands of Clarkson knife gate valves installed in the alumina business worldwide and this number grows on a weekly basis.  One of the keys to the success of this design is a feature that must be explained to a first time observer of the unit in action.The opposing sleeves are situated such that the "nose" (or leading edges) are in contact with each other when the valve is in the open position. The compressed sleeves form an actual "pressure vessel" which contains the slurry and keeps the metal body of the valve and the entire gate from contact with the abrasive media. When the valve is actuated, the knife literally parts the two sleeves and travels completely through the 6 o'clock position and blocks any flow downstream, effectively acting like a blind flange. There is absolutely no possibility of the valve leaking downstream with the gate in the closed position. It is during this parting of the sleeves when the valve is actuated that the unique purging feature is encountered. This is an engineered feature of the valve and is the difference between Clarkson and competing knife gate valves. As the sleeves are forced apart to allow the gate to travel up or down and the valve is under line pressure, a small amount of material is forced out of the bottom of the valve. This material is expelled through a specially designed cavity in the bottom of the valve in order to prevent scale from forming which would ultimately block the gate from traveling all the way through the sleeves and isolating the downstream piping. In the alumina process, the slurry quickly cools and forms an extremely hard rock like material that will render a lesser valve useless in a very short time. The material expelled from the valve can be piped away with the use of a bolt on adapter plate if the valve is placed over a walkway or heavily traveled corridor. There is a maintenance requirement for this valve, just like any other piece of equipment in this abrasive environment. The heart of the Clarkson valve is the elastomer sleeves. Just as a car must eventually replace the tires after a certain number of miles, the sleeves must be changed after a certain number of valve cycles. It has been determined that Clarkson valves should have the sleeves replaced after a period of 2-2.5 years of service in the precipitation area, based on how frequently the valves are opened and closed. The sleeves are in extremely tight contact with the gate as it moves up and down, and eventually the elastomer will wear away. This wear can be minimized by using Dow 111 or equivalent silicone based lubricant pumped into the grease fittings on the body of the valve. However, the valve cannot be expected to work correctly without some sort of maintenance interval. The operations and maintenance guide for the Clarkson KGA valve states on page 3 that the valve should be lubricated every 100 cycles for valves 3" to 10" and every 50 cycles for 12" and larger. While this is a very conservative approach, it is important to give the valves at least a shot of silicone based grease at least once every couple of months. This would greatly increase the life of the sleeves and prolong the required maintenance schedule for the replacement of the elastomer components. Maintenance The KGA may require occasional gate lubrication depending on the application. A silicone-based lubricant is recommended and may be applied using the fittings provided in the housing chest. (See Appendix A for approved lubrication) The gate may also be lubricated by spraying directly with a silicone lubricant to prevent scale build up. Sleeve wear can be minimized if the valve gate is scraped or wiped clean occasionally. The application of lubricant should include liberally coating the entire seal end of the sleeve. A thin film should also be applied to the outer diameter of the sleeve. Apply a thin film of lubricant to both faces of the knife gate from the knife edge upward to the center of the gate. Periodic re-lubrication using the fittings on the housing should minimize any operational problems in normal use. As a minimum the Clarkson KGA valves should be lubricated every 100 strokes for 3" - 10" sizes and every 50 strokes for 12" - 54" sizes. An individual application may require more or less frequent lubrication depending on the process and chemistry. If a valve cycles very infrequently, less than once per month, lubrication prior to each stroke is recommended. The unique feature of the valve is that a replacement of the two sleeves and two retainer flanges will bring the valve back to like-new performance. In a plant with over 500 installed valves, there must be regularly scheduled workers dedicated to rebuilding these valves. Without a program in place, the valves will continue to wear while in place and housekeeping issues will ultimately result. As previously mentioned, the valve is designed to purge a small amount of slurry from the bottom of the valve as it is cycled. Once the sleeves start to wear, the amount of purged material will slowly increase. This is an indication that the sleeves are nearing their useful life and the valve should be taken out of service for a sleeve/retainer flange change and a previously rebuilt valve put into its place to allow for minimum interruption of the process. If the sleeves are left to completely wear out, the valve will ultimately start to steadily leak through the bottom. However, the valve will still prevent downstream flow, even with worn sleeves. The safety feature of pipe isolation is not compromised. The use of Clarkson valves in the Bayer alumina process is an investment in the most advanced isolation valve available on the market. While there are less expensive options, there is no other valve that will outlast and outperform this equipment if a few simple maintenance functions are performed on a regular basis.  Donovan Summers-Sales Engineer Kingston, Jamaica Phone: 876-944-0191 email: dsummers@dsjamaica.com for more information. |