In 2011 Stafsjö received a request from a worldwide known OEM supplier based in the Pulp and Paper Industry. They wanted a technically fast and reliable valve capable of operating within a broad range of pressures, pulp concentrations and temperatures. The development team at Stafsjö got down to work and the story of the 20 bar/300 psi rated HX began.
We have experienced over the last few years that the production volume at pulp and paper mills across the globe is ever increasing, new mills are being built capable of achieving new record production volumes and old plants are being converted or upgraded. As a knife gate valve supplier Stafsjö supplies a major portion of the equipment, focusing on product solutions to realize new efficiency and production goals.
Stafsjö worked in close collaboration with its customers to develop a prototype and ran a large number of in-house tests. Test units were then supplied to OEM suppliers’ for more advance endurance independent tests at their own test facilities.
All the prototype tests proved successful. Stafsjö have since supplied high pressure HX valves in sizes ranging from DN 200 all the way up to DN 900 in stainless steel and also in high alloy materials such as SMO 254, titanium and duplex. The potential area of use has increased over time. Now the valve is also used within other areas and sectors that Stafsjö had not initially anticipated. But it is within the pulp and paper sector where the application scope of HX is at its greatest – now and most likely also in the future.
The Stafsjö HX knife gate valve ensures batch digester isolation during pulp stock production
In chemical pulping, some fibre lines operate with a continuous cooking process while others operate in batch cooking. In the batch cooking process, the wood chips and cooking liquor are heated with steam under high pressure. Once the cooking process is completed, the digester is emptied through a blow valve at the digester bottom, and onward to a blow tank. During blowdown, the process equipment is subjected to high velocity, pressure, abrasion and temperature, as well as severe vibration. The batch digesters discharge to a common header, which is connected to a blow tank further downstream. When one digester is out of service for blow valve repair or other maintenance, the chemical pulp mill normally shuts down all digesters in series to ensure the work can be carried out safely. This results in a major loss of pulp production.
Stafsjö’s severe service HX valve has successfully been installed as a maintenance block for complete isolation in batch digester clusters. The HX valves are placed downstream of the blow valve, but before the common pipe or header leading to the blow tank. During regular batch cooking processes, Stafsjö’s HX valves are locked open. When a digester needs to be shut down for maintenance or inspection, the HX is closed and locked, allowing that particular digester to be depressurized and the blow valve removed, or blow down piping disengaged, while the rest of the digesters continues to process pulp as normal. Stafsjö’s HX provides 100 % bi-directional isolation, to prevent any back flow reaching the shut-down area. Downtime is minimized, maintenance work can be performed safely and the production levels can remain steady because the other digesters can continue to feed the mill with pulp stock while the one digester is being maintained. The Stafsjö HX valve is built to withstand the unique pressure, temperature, chemical, abrasion and vibration extremes associated with this application.
Stafsjö knife gate valves have been represented on an exclusive basis in Southern Africa for more than 25 years. The MV series is a standard stock item up to DN300 / 12”.