Valves, Actuators & Pump Control


The Stafsjö HX story – greater demand calls for action

February 2021 Valves, Actuators & Pump Control

In 2011, Stafsjö received a request from a well-known OEM supplier to the pulp and paper industry. The supplier wanted a fast and technically reliable valve capable of operating within a broad range of pressures, pulp concentrations and temperatures. The development team at Stafsjö got to work and the story of the 20 bar rated HX began.

Over the last few years, the production volume at pulp and paper mills across the globe has increased, with new mills being built capable of achieving new record production volumes, and old plants being converted or upgraded. As a knife gate valve supplier, Stafsjö supplies a major portion of the equipment, focusing on product solutions to realise efficiency and production goals.

Stafsjö worked in close collaboration with its customer to develop a prototype and ran a large number of in-house tests. Test units were then provided to the supplier for more advanced endurance tests at its own facilities.

All the prototype tests proved successful and Stafsjö has 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 and now the valve is also used within other areas and sectors that were not initially anticipated. But it is within the pulp and paper sector that 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 the content is sent onward to a blow tank. During blowdown, the process equipment is subjected to high-velocity flow, pressure, abrasion and extreme temperatures, 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, the valves are locked open. When a digester needs to be shut down for maintenance or inspection, the HX valve is closed and locked, allowing that particular digester to be depressurised and the blow valve removed, or blowdown piping disengaged, while the rest of the digesters continue to process pulp as normal.

Stafsjö’s HX provides 100% bidirectional isolation, to prevent any back flow from reaching the shutdown area. Downtime is minimised, 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, with the MV series held as a standard stock item up to DN300.

For more information contact Desmond Delport, Valve & Automation, +27 11 397 2833, desmond.delport@valve.co.za, www.valve.co.za


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