Accurate vacuum and pressure gauges are essential on paper machines

Having visited almost 350 paper mills to optimize, troubleshoot and enhance the operation of paper machine vacuum systems, it is clear that many perceived process or equipment problems are no more than instrumentation issues.

Reliable, accurate instrumentation is essential for all processes in a paper mill.Benefits include the ability to maintain and optimize efficient paper/board/ pulp/tissue production while minimizing the use of various utilities (water and compressed air) and operation with the lowest possible energy consumption. Where automated process control is used, installing proper pressure and vacuum gauges provides low-cost redundancy and can show when transmitters and control data have lost accuracy.

What is a “proper” gauge? For pulp and paper systems, they should have a stainless steel case with brass or stainless internals and be liquid filled. Gauges should be chosen for the correct scale range and units of measurement. For example, when measuring 5-10 psi, a 0-100psi scale should not be used. Or, when measuring vacuum, be consistent with units of measurement. Choose inches of mercury ( Hg), or kPa, and be sure all gauges and transmitters have the same units.Also, avoid using a compound gauge (pressure and vacuum) in a vacuum system.

The use of correctly applied gauges — and even enhanced gauges with colored dials indicating ideal operating pressures — have frequently shown where the process can be improved and energy saved. Examples of these specialty gauges include:

• Vacuum pump seal water
• Suction-roll loading tube air pressure
• Suction-roll seal strip lubrication water
• Wire or forming fabric high-pressure water (needle showers)
• Press felt high-pressure water (needle showers)
• Uhle box lubrication shower water.

Note that all but one of these are for regulating water pressureand each has a direct energy cost associated with pumping water. For the fabric cleaning and Uhle box lube showers, the water is heated also with an energy cost. Water supply pressure is only 10 psi for the vacuum pump seal water but total flow in one system can be 500 to 2,000 gpm. Needle showers are operated at 200 to 400 psi but these are at low flow rates where a 0.040-in.-diameter nozzle passes only 0.5 gpm at 200 psi.

Minimizing and controlling suction-roll loadingtube air pressure also conserves energy. However, this is not regulating the flow rate of the air. The loading tubes exert a force against sealing strips which press against the inside diameter of the drilled suction-roll shell to seal the vacuum zone within the suction roll. Excess pressure does not add to sealing but does add to the drive horsepower because the sealing strips also act as a brake against the roll. Therefore, maintaining a low pressure of around 5 psi — and knowing that this is important — is essential to minimizing suction- roll drive-horsepower and maximizing seal-strip life.

These are important processes related to papermaking. Color coding with green/yellow/red on the dial provides quick indication of the efficiency of process control. Once installed, operators gain knowledge of the desired pressures as well as those to avoid.

Doug Sweet & Associates, Inc. www.dougsweet.net

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