Bissell 9400 ProHeat Upright Wet/Dry Vacuum Review

A discussion of common problems in paper machine vacuum systems helps mills correct the actual problem, not just its symptom

ALL WET-LAID PROCESS PAPER MACHINES have vacuum systems, and these generally operate without frequent problems. However, when difficulties occur within the vacuum system, it is often difficult to identify the actual problem and its cause. Many times, the symptoms get treated, rather than the problem itself. The troubleshooting guide in this article should assist mill maintenance and production personnel in quickly determining the causes of vacuum system problems and identifying the differences between vacuum pump problems and those external to the pump. The article will cover the most commonly occurring vacuum system problems.

When troubleshooting vacuum systems, as with any other operating problem, good techniques must be applied and questions must be asked. These questions include, "What changed and when?" And "Is this a problem or a symptom?". The vacuum system problems discussed in this article include:

Low vacuum levels

High horsepower-motor trip outs

Hot pump operation

Pump vibration

LOW VACUUM LEVELS. Low vacuum levels are one of the more common problems/symptoms experienced on a paper machine. The word "symptom" is really emphasized here, because the difficulty is rarely caused by the vacuum pump alone. However, the vacuum pump often gets changed out, only to discover the problem still exists.

First, recognize that vacuum levels are a measurement of resistance to airflow, where the airflow is induced by the vacuum pumps and the resistance is the various dewatering processes and system piping. Changes in resistance to airflow are caused by various process variables, including sheet moisture, basis weight, refining, felt porosity, suction roll condition, machine geometry, and machine speed. The following sections describe the effects of sheet moisture and suction roll condition on vacuum levels.

In addition, the following sections discuss some typical causes of low vacuum levels, including: open valve in the vacuum line or header, plugged screens at the vacuum pump inlet, uncovered barometric seal leg from a pre-separator, or low seal water flow at the vacuum pump.

Sheet moisture. Typically, a drier sheet will allow more atmospheric air to flow through it. This is why flat box systems are designed for higher air flows on the last suction boxes nearest the couch. As the sheet gets drier, it requires more vacuum capacity to maintain the same vacuum level. Additionally, if higher vacuum levels are desired, even more vacuum capacity is needed.

Cases have been observed in which a flat box system is upgraded to increase sheet dryness entering the couch and, although the flat box goal is obtained, the couch vacuum level drops. This lower couch vacuum is due to the lowered resistance to airflow from the drier sheet, which is caused by improved dewatering from the flat boxes.

Suction roll condition. Several problems can occur due to the condition of the couch or other suction rolls. First, the internal seal strips must be functional so that the rolls seal properly against the internal surface of the shell. If the seal strips are binding in their holders, internal showers are not operational, or loading (pneumatic or other type) is not uniform, there will be an internal vacuum leak. This may be evident if the suction zone is covered with plastic while the paper machine is down and the suction roll vacuum pump is run. The vacuum level at the suction roll should be higher than normal. If not, you may be able to hear air leakage from within the roll.

Another vacuum related problem can occur when there are partially plugged holes in the shells of suction rolls. These plugged holes cause higher resistance to airflow and result in an unrealistically high vacuum level. Those mills that adjust refining based on couch vacuum may under or over refine due to couch roll problems.

Open vacuum line or header. Vacuum headers have a unique way of sprouting new pipe runs with isolation valves during weekend shifts. This apparently gets the papermaker through to the next shutdown when a pump can be changed or repaired. However, by that point, many of these new interconnections are forgotten and the original purpose becomes obscure. After a while, these valves are left open, making system analysis and troubleshooting almost impossible.

Always trace the vacuum piping to be sure the flat box vacuum pump, for example, is connected to the flat boxes-and nothing else. Also, be sure that valves leading to other vacuum pumps or paper machine vacuum points are closed. In cases where spectacle blanks are used to separate vacuum services and pumps, be sure these are intact. Many hours have been spent chasing vacuum losses only to find that a spectacle blank made of carbon steel has the lower third of its area completely corroded away.

Plugged screens at the vacuum pump inlet. Many mills elect to keep startup screens in place at the vacuum pump inlet. This is not a problem, as long as the screens are of a substantial material. Also, it is important to locate the pump vacuum gauge directly into the vacuum pump inlet, below the screen, since there is usually a tapped connection at that point. This allows you to observe any increase in the vacuum at the pump compared to the level at the paper machine. The screens may have a tendency to blind over with felt hairs, fiber, old lunch sacks, etc. all of which act as a throttling valve at the vacuum pump. This causes a high vacuum reading at the pump and a low vacuum reading at the process.