Troubleshooting paper machine vacuum systems: part I: low vacuum levels can be caused by roll problems, vacuum line disturbances, and plugged pump screens.
All wet-laid-process paper machines have vacuum systems, and generally, these operate without frequent problems. However, when difficulties occur within the vacuum system, it may be difficult to identify the problem or cause. Often the symptoms--rather than the root cause--get treated. This troubleshooting guide can be used to assist mill maintenance and production personnel in quickly determining the causes of vacuum system problems and in identifying the differences between vacuum pump problems and problems external to the pump(s).
Certain system problems occur most frequently, so these issues will be covered. When troubleshooting vacuum systems or any operating problem, good techniques must be applied and appropriate questions should be asked. Key questions include "What changed, and when?" and "Is this a problem or a symptom?"
This article covers the problem of low vacuum levels. Future articles will discuss high-horsepower motor trip-outs, hot pump operation, and pump vibration.
Low vacuum levels are one of the more common problems/symptoms experienced on a paper machine. The word symptom is emphasized here, because the difficulty is rarely caused only by the vacuum pump. However, in an attempt to solve the problem, a mechanic often changes out the pump, only to discover that the problem still exists.
Vacuum level is a measurement of resistance to airflow, where the airflow is induced by the vacuum pump(s) and the resistance is produced by 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.
Typically a drier sheet allows more atmospheric air to flow through it. This is why flatbox systems are designed for higher air flows on the last suction boxes, or those nearest the couch. As the sheet gets drier, more vacuum capacity is required to maintain the same vacuum level. Additionally, if higher vacuum levels are desired, even more vacuum capacity is needed. Cases have been observed where a flatbox system is upgraded to increase the dryness of the sheet entering the couch and, although the flatbox goal is obtained, the couch vacuum level has dropped. This lower vacuum level is due to the lowered resistance to airflow (the drier sheet) caused by improved dewatering by the flatboxes.
Several problems can occur due to the condition of the couch or other suction rolls. First, the internal seal strips must fit 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 types) is riot uniform, there will be an internal vacuum leak. This leak may be observed if the suction zone is covered with plastic (while the machine is down) and the suction roll vacuum pump is run. The vacuum level at the suction roll should be higher than normal. If it is not, you may be able to hear air leakage from within the roll. Another vacuum-related problem could occur due to partially plugged holes in the shell of suction rolls. The plugged boles cause higher resistance to airflow and result in an unrealistically higher vacuum level. Mills where adjustments in refining are made based on couch vacuum may under- or over-refine due to couch roll problems.
Vacuum piping and pump screens
Other typical causes of low vacuum levels include an open valve in the vacuum line or header, plugged screens at the vacuum pump inlet, an uncovered barometric seal leg from a preseparator or low seal-water flow at the vacuum pump. Vacuum headers have a unique way of sprouting new pipe runs with isolation valves during weekend shifts. These repairs often get the papermaker to the next shutdown so that a pump can be changed or repaired. However, many of these interconnections get forgotten, and their original purpose becomes obscure. Over time, these valves are left open, making system analysis and troubleshooting almost impossible.
Trace the vacuum piping to be sure, for example, that the flatbox vacuum pump is connected to the flatboxes and nothing else. Also, be certain 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 that these are intact.
Many mills elect to keep the 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 install the pump vacuum gauge directly into the vacuum pump inlet, below the screen; there is usually a tapped connection there. With this setup, one can observe any increase in vacuum at the pump when compared to the level at the paper machine.
Pump screens have a tendency to plug with felt hairs, fiber, lunch sacks, etc.; all of these act as throttling valves at the vacuum pump. The result is a high vacuum reading at the pump and a low reading at the process. One mill was ready to replace a uhle box because of what appeared to be poor sealing of the uhle box cover and inadequate end deckles. The "symptom" was a vacuum pressure of only 6 in. Hg at the uhle box. There was no vacuum gauge at the vacuum pump, but the cast iron inlet flange and proximity beneath the flange were sweating and about 40[degrees]F colder than the ambient temperature. A gauge was installed in the pump inlet, on the pump side of the screens, and 23 in. Hg vacuum was measured. The inlet screens had almost completely plugged. The cold temperature was due to the refrigeration effect from the rapidly expanding air moving across the plugged screen. When the screen was cleaned, the uhle box vacuum returned to satisfactory levels.
Low vacuum can be caused when the vacuum level exceeds the limits of a vacuum preseparator/seal leg/seal tank system. High vacuum levels can draw all the water out of the seal tank, leaving the seal piping open to the atmosphere. Vacuum preseparators operate either with a barometric seal leg (pipe) or with a low-NPSH [AUTHOR: PLS. CLARIFY "NPSII."] removal pump. The seal leg is the simplest design, but failure to design, install, and operate this simple system correctly will result in perpetual vacuum system problems.
A few basic engineering practices must be followed. First, the distance (elevation) between the bottom of the vacuum separator and the liquid level of the seal tank must be sufficient to overcome the vacuum level. There must be 1.13 ft of pipe elevation fur every 1 in. Hg of vacuum level in the separator. In addition to this convention, it is necessary to add another 3-5 ft of elevation to account for pipe friction and to add a safety factor. Second, the bottom of the seal-leg pipe should extend down into the seal tank to a point about 6 in. from the bottom. Finally, the volume of the seal tank must be sufficient to allow the seal piping to fill with water when under vacuum and before there is water flow from the preseparator.
Designing a seal tank with a volume equal to two times the seal-pipe volume is typical and sufficient. Seal-tank volume may be reduced by using a seal leg with a smaller diameter than that of the water outlet connection in the separator. For example, a separator with a 10-in.-diam water discharge connection can generally be reduced to a 6-in.-diam drop leg after 2-3 pipe diameters below the discharge opening. Because the volume in the barometric leg will be lifted the same height at a given vacuum level regardless of pipe size, the volume is reduced in the smaller pipe. Some paper machines have been forced to operate at reduced vacuum levels due to poor system design, low installation levels of the preseparators, or insufficient seal-tank volume for the barometric leg(s). Vacuum capacity and horsepower are then wasted when a vacuum in-bleed valve is required to limit vacuum levels.
A final item that may cause low vacuum levels is low vacuum pump seal-water flow. Many paper machines operate with liquid-ring vacuum pumps, and significantly reduced seal-water flow will result in lower pump capacity. (Note, hot seal water, 110-120[degrees]F and higher, also causes reduced vacuum pump capacity. This is more often a system design issue and is not addressed here.) Liquid-ring vacuum pumps typically require 10-15 psig seal-water pressure at a point measured upstream of an orifice. However, a plugged orifice may not let the proper seal-water flow pass, although a correct pressure may be indicated. Remember: Pressure does not indicate flow! Seal-water pressure gauges should read 0-30 or 0-60 psig to provide reasonable accuracy. A 0-100-psig gauge will not give sufficient accuracy at the desired level of 10-15 psig.
Taking a methodical approach pays dividends when troubleshooting paper machine vacuum systems. Part II of this series will discuss high-horsepower motor trip-outs and their effects on vacuum systems.
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|Title Annotation:||Vaccum Systems|
|Author:||Dunn, Dan R.|
|Publication:||Solutions - for People, Processes and Paper|
|Date:||Sep 1, 2001|
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