Why Pulp Mill and Paper Machine have to be different?
Not only VFD but control systems should also be different for Paper Machine and Pulp Mill?
The pulp and paper industry is an energy-intensive industry, and thus it is highly significant to understand when/where/why & How much energy is to be distributed and what is type of energy required. The classification is not only into the heads, of Heat and Electricity but it goes deep down by defining what pressure and temperature of steam are required and is it saturated and superheated. Similar is the case for electricity; nowadays a single manufacturing unit considers using electricity at different voltages. This shift has been observed because of increasing awareness about the efficiency of systems, reliability of equipment, and dependability of control systems involved.
The energy consumption happens at every stage in a pulp and paper unit from the yard of raw material to the dispatch center. The unit is thus usually divided into sub-sections, most commonly: Pulp Mill, Paper Machine, Finishing Section, Utilities (Which includes Vacuum, water supplies, and electrical control systems), Boiler, and Power Plant (Power Plant is not always present), Effluent Treatment System. The two areas of major focus are Pulp Mill and Paper Machine as far as Electricity is concerned.
What happens in a Pulp Mill?
Pulp Mill is the most important section of a paper-making unit which defines what grade of paper is being manufactured, in what quantity, and what are going to be the properties of end product. That's very similar to a household kitchen where the taste of food depends on the method of preparation and ingredients used. In every process, there is a unit operation hidden and that means a physical or chemical change is happening. The need for energy in such cases is inevitable and thus electrical consumption comes into play. Most of the processes are driven by induction motors these days and it is the necessity of the process to decide when a motor shall operate, at what speed, how frequently, and what shall be the direction of rotation. There is a piece of equipment coupled to every motor and the working behavior of the equipment changes with speed.
Thus, the process is majorly dependent on use of VFDs or AC Drives or Variable Frequency Drives. Process parameters are the key factors defining the usage and control topologies. The major equipment as per nomenclature is:
Equipment behavior is very important, as well as it is significant to observe the installation conditions and operational habits. It is not always possible to have dedicated centralized control rooms or panels rooms that have air-conditioned dust free environment. Particularly, in case of retrofits or brown field projects where old installations are being modified a lot of challenges related to panel sizing, mounting size and directions, panel cooling, dust conditions, cable length, control methodology and power quality. Let us understand the impact of all these.
Firstly, most of the applications related to pulp mills are standalone applications. By that we understand that the VFD is driving an individual equipment which does not corelate to the equipment or stage ahead of behind it. However, automation of process with help of level transmitters on stock chests, pressure transmitters on pumps, and consistency transmitters at various places and Storal Regulator (oSR or freeness) Transmitter makes sure that process is run smoothly ensuring pulp quality and availability at all times. Process can be integrated with flow meters and load cells to generate consumption reports of material which can be co-related to the energy consumption in collaboration with steam flow meters and energy meters.
What covers Paper Machine Section?
Any Paper Machine has few sub-blocks listed as:
Approach Flow system - ensures that post pulp mill the pulp is diluted so that sand separation and screening can be done while maintaining a constant pressure and consistency input to the headbox for formation.
Vacuum Section - Generates vacuum by suction of air from forming fabric and felt fabrics
Paper Machine – the process of formation of pulp web, drainage, water suction, press section, and heat drying.
Headbox - Component of machine which ensures constant distribution of pulp across the width of the machine, and the jet wire ration is maintained to ensure constant grammage of the paper web.
Size Press Kitchen - preparation zone for sizing chemicals and starch solutions which is to be coated on the paper web at size press section.
Steam and condensate
The Paper machine has several sections irrespective of the grade of paper and each section has to move in synchronization with the previous and next stage so that the web formed is transferred continuously till the final wrapping. Vacuum defines how water is removed and what kind of drag loads are to be fed to forming section motors and press section motors.
On basis of the moisture contents, the drying system draws steam and the condensed steam is taken back to the boiler area. The steam input and condensate pumping is also a part of automation and it works to control the flows as per the demand of the paper machine.
The components of paper mill which include the finishing section covering slitter rewinder and sheet cutters; Boiler, Water circulation systems, and effluent treatment systems are all separate processes that can be automated as per the site requirements and can be clubbed with integrated control using a central DCS. Advantages of a central DCS system is one point monitoring and complete MIS generation as and when required.
Now the application of pulp mill is to operate loads like pulper, refiners, thickeners, dispersers, and other equipment which are usually the ancillary process pumps that are majorly governed by speed and do not have constant loads. There is equipment which have high starting loads or torques such as refiner and on the other end equipment like pumps have a standard operation curve that is known before installation. On the other hand paper machine VFDs have to operate in sync with each other (irrespective of the communication protocol selected) and they have to deliver high starting torques to drive the heavy weight of equipment coupled to them, and the resistance faced by vacuum and fabric installations. Even during the run paper machine VFDs need to have a controlled behavior to ensure zero paper breakage and proper quality wrapping at the end.
This is the biggest difference and for that the selection of VFDs have to consider factors like overloadability, ramp up/down time range, repeatability, starting back during ramp down without variation (also called last speed take up) and plenty of other factors.
We thus recommend to always allow your automation solution providers to decide and select the system during proposal stage as the vendor only will be the service provider for smooth functioning of the Paper Machine and Pulp Mill, from amongst a large number of options available.