A paper machine is not like an injection moulding machine or a filling line that can pause and restart. The paper machine runs a continuous moving web of paper – at speeds between 200 and 1,200 metres per minute depending on the grade and machine type. Every control point on the machine – the headbox slice lip controlling basis weight, the suction press roll loading, the dryer section air supply, and the reel spool change actuators – is controlled by pneumatic actuators operating on instrument air at 5–7 bar. When instrument air pressure drops below the minimum operating pressure of these actuators, they lose position simultaneously. The headbox slice opens or closes unpredictably, the web basis weight goes off-specification, the press roll loading drops, and within seconds the wet paper web, which has very low strength, breaks. A web break on a paper machine running at 800 m/min means: threading the broke through the entire machine takes 20–60 minutes, all paper produced in the minutes before and after the break must be rejected, and the broken sheet material (broke) must be returned to the pulp chest and repulped. On a machine producing 200 tonnes of paper per day, one hour of web break downtime costs 8–10 tonnes of lost production. The compressed air system supplying paper machine instrument air must have a receiver tank sized for minimum 15–30 minutes of full demand at operating pressure – so that even if the compressor fails, the instrument air supply continues long enough for a controlled machine slowdown and safe stop.

The wet end of a paper machine – the section from the headbox to the couch roll – is an extremely humid environment. Steam, hot water spray showers, the forming fabric and wire carry and release large quantities of water continuously. The relative humidity at the wet end is effectively 100%. When the instrument air supply to wet end actuators and valve positioners contains moisture, the moisture enters an environment that is already saturated with steam and water. The consequences are accelerated corrosion of actuator internal components (stainless steel and brass fittings corrode faster in the wet end steam environment), control valve positioners that develop sticking and hunting behaviour as corrosion roughens the internal spool and bore, and differential pressure transmitters on the headbox and press section that give erratic readings when condensate forms in the instrument air impulse lines. A refrigeration air dryer is essential for paper machine instrument air. For critical control loops on high-speed machines, a desiccant air dryer providing a pressure dewpoint of -40°C is recommended to ensure no condensation even in the hot, humid wet end environment.

Paper mill effluent contains dissolved cellulose fibres, lignin compounds, starch (used in paper sizing and coating), paper coating pigments and process chemicals. This combination gives paper mill effluent a Biological Oxygen Demand (BOD) of 500–2,000 mg/L – three to ten times higher than the effluent from a typical chemical plant or food factory. The biological treatment stage of the ETP must supply oxygen at a proportionally higher rate to sustain the activated sludge bacteria that break down this high-BOD load. This means the Roots Blower for ETP aeration at a paper mill must be sized for much higher airflow than a blower of the same rating installed at a chemical or food plant ETP. It also means that if the ETP aeration blower fails at a paper mill, the biological treatment process begins to fail much faster – because the high BOD quickly depletes all dissolved oxygen in the aeration basin, and the bacteria begin to die within hours. Restoring biological treatment after a complete die-off can take weeks and requires re-seeding of activated sludge. RR Enterprise is the only local Roots Blower manufacturer in the Vapi-Silvassa belt and provides emergency replacement and on-site repair for paper mill ETP blowers.