Paper Drying System comprises of following controls
Steam Pressure/Flow Control
Steam Pressure/Flow Control to individual dryers and drying groups. Maintaining the precise steam pressure and temperature in every dryer is the basic objective of the paper drying system. A higher than desired pressure or lesser pressure is just not fine.
The variance in basis weight, moisture levels, minute variations in fibre consistency, the fibre composition including additives etc make it important to have a pressure control that is quick in response, accurate, safe and reliable.
Differential Pressure Control
Differential Pressure Control required to maintain the desired differential pressure which will completely evacuate the condensate from each dryer without causing excessive blow-through. Higher Dp results in excess blow-through and lesser Dp results in condensate flooding in the dryer. The Dp control is most critical considering the dynamic nature of the operation. Dp control also includes control of Vent steam during paper break conditions.
Separator Vessel Level Control
Separator Vessel Level Control for each separator vessel. This control too is complex as a slack response can result in wet steam entering into the succeeding cascade groups or in the other scenario live steam blowing through the separator.
Split Range Control System
Split Range Control System is required in Thermocompressor based system to ensure that TC acts as the primary control. This is a critical control as multiple valves need to be operated as per a complex algorithm with single pressure input.
Heat Exchange Temperature Control
Heat Exchange Temperature Control is required to maintain the desired vacuum is required to maintain the desired vacuum in the low pressure group – usually the first group.
Paper break control
An essential operational and safety control whose function is to evacuate condensate from the dryers and maintain the dryers in a warm condition during paper breaks.
All these controls need rugged, reliable and state of the art process controllers. One could go in for individual controllers, PLCs or a complete DCS system. Each of these needs to be suitably selected and engineered into the paper drying system with complete integration taking into account control valve trim and its Cv / Kv values, the transmitters and also the process.