Bloody Wonder vs Blood Baron


New member
DOUGLAS F. SWEET, PE., North American sales director for Nash Engineering Co. in Birmingham, Ala
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. LP 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
See more: Best Vacuums under $100.00

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 flatbox 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 flatbox system is upgraded to increase sheet dryness entering the couch and, although the flatbox 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 flatboxes.
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 flatbox vacuum pump, for example, is connected to the flatboxes--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.
For example, 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 reading 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 was 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 was measured. It was discovered that the inlet screens had been forgotten, and one particular screen was nearly blinded over. The cold temperature was due to the refrigeration effect from the rapidly expanding air across the plugged screen. The screen was cleaned off and the uhle box vacuum returned to satisfactory levels.
Uncovered barometric seal leg. Low vacuum can be caused when the vacuum level exceeds the limits of a vacuum pre-separator, seal leg, or 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 pre-separators operate with either a barometric seal leg (pipe) or with a low NPSH removal pump. The seal leg is the simplest design, but failure to correctly design, install, and operate this simple system will result in perpetual vacuum system problems, so a few basic engineering practices must be followed:
* 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 elevation for every 1 in. Hg of vacuum level in the separator. In addition to this conversion, it is necessary to add another 3 ft to 5 ft for friction and a safety factor.
* The bottom of the seal leg pipe should extend down into the seal rank to a point about 6 in. from the bottom.
* 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 pre-separator. Designing a seal tank with a volume equal to two times the seal pipe volume is sufficient. Some paper machines have been forced to operate at reduced vacuum levels due to poor system design and low installation levels of the pre-separators. Vacuum capacity and horsepower is then wasted when a vacuum in-bleed valve is required to limit vacuum levels.
Low seal water flow Many paper machines operate with liquid ring vacuum pumps, and significantly reduced seal water flow can result in lower pump capacity. Vacuum pumps require 10 psig to 15 psig seal water at a point measured upstream of an orifice. However, a plugged orifice may not let the proper seal water flow pass, even when the correct pressure is indicated. Remember: Pressure does not indicate flow! To obtain reasonable accuracy seal water pressure gauges should read 0 psig to 30 psig or 0 psig to 60 psig. A 0 psig to 100 psig gauge will not provide good accuracy around the desired level of 10 psig to 15 psig.
Note: Hot seal water-110[degrees]F to 120[degrees]F and higher-also causes reduced vacuum pump capacity. This is most often a system design issue and is not addressed here.
HIGH HORSEPOWER. How often do you get a call about the vacuum pump motor tripping out? It happens for a lot of reasons, and only occasionally does replacing the pump correct the problem. Again, to solve the problem, good troubleshooting skills must be employed and the questions of "what changed, and when did it change?" must be asked.
Two good staffing points for determining horse-power requirements are the operating conditions, or vacuum levels, and pump speed (rpm). These are discussed in the following sections, along with the pump horsepower overloading caused by water overloading, backpressure, and internal buildup.
Operating conditions. It is important to determine if the pump is operating at a point well above design vacuum levels. Remember, the vacuum gauge location must indicate actual pump vacuum levels. Also, the vacuum gauge must be accurate. The recommended gauge type is a 0 in- to-30 in. Hg vacuum only gauge, not a compound gauge that reads both vacuum and pressure, since getting a positive pressure in a vacuum line on a paper machine is practically impossible.
Also, discussing vacuum gauges and their typical condition often leads you to believe that "a good gauge is still in the box". Always be sure of the accuracy of vacuum readings. In addition, it is impossible to operate with a higher vacuum level at the paper machine than at the vacuum pump. If a pressure drop in the piping exists, the vacuum pump will be at the highest vacuum level.
After determining the vacuum level, compare it to typical operating conditions. Higher vacuum levels usually, but not always, cause higher horsepower requirements. Be sure the selected drive motor will allow the pump to operate at the full range of vacuum levels. Otherwise, a vacuum relief (in-bleed) valve will be required to limit operating vacuum levels.
Pump speed. Be sure the actual pump speed is the same as what was originally intended for the installation. Sometimes, something as simple as the wrong motor getting installed-1,800 rpm vs. 1,200 rpm- can be the problem. This is more likely in a new system or after some motor maintenance.
Drive ratios of v-belts and gear reducers should be compared to the actual output speed or pump rpm. Drive manufacturers use the term "exact ratio" for determining the actual output, or driven speed. Also, with the newer, high efficiency motors, the full load speed is usually closer to the nominal rating of 1,200 rpm or 1,800 rpm. For example, selecting a drive based on 1,150 rpm (a common speed for older motors) and installing a new motor rated at 1,190 rpm would yield a 3.5% increase in pump speed, with a comparable increase in bhp.
Water overloading and air/water separation systems. Another common reason for high horsepower is severe water overloading. This can come from excess seal water or from the paper machine vacuum service (couch, uhle box, etc.) A liquid ring vacuum pump has a rating for a specific seal water flow and increasing this by even 25% or 50% does not typically cause a power problem. Flows that are two to three times the rated flow are most likely causing motors to overload, or belt drives to fail, Also, sudden slugs of water are problematic. These can be intermittent, causing difficult troubleshooting.
High seal water flows are caused by several reasons, including high seal water pressure, lack of orifices, and worn spray nozzles (if the pump has them) -or all of the above. Typical seal water pressure is 10 psig to 15 psig. Again, this pressure reading should be before the orifice and spray nozzle. As long as the orifices and spray nozzles are intact, seal water pressure can be up to 15 psig or 20 psig without difficulty. Beyond these pressures, excess water is only wasted and contributes to power problems.
Older vacuum systems are often found to have worn spray nozzles or nozzles that have been removed and replaced with a straight pipe. The nozzle functions as an orifice and more than 20 years of continuous flow will enlarge the nozzle and allow as much as two times the desired flow to pass.
Excessive flows, called carryover, from The paper machine are usually detectable and can be resolved. The easiest way to detect carryover is to look at the water discharging from The suspect vacuum pump, if the flow is visible. Cloudy water discharging from a vacuum pump using clear seal water is a good sign of carryover.
Many vacuum systems have vacuum pre-separators between the papermaking process and the vacuum pumps. The purpose of The separator is to remove water and contaminants from the air stream prior to the vacuum pump. These are common on flatboxes and uhle boxes. Sometimes, these are also found on the couch or other suction rolls.
Locations for pre-separators are determined by the type of suction device and machine speed. Any stationary vacuum or suction box, as opposed to a suction roll, should have a separator before The vacuum pump. These applications would include flatboxes, transfer boxes, pick-up shoes, and uhle boxes.
Suction rolls, especially a couch or suction drum roll, should have pre-separators on machine speeds below 1,000 fpm. At these speeds, The water removed under vacuum gets entrained into the roll and internal suction box, and this will flow to the vacuum pump. At higher speeds, the water slings out of the suction roll shell due to centrifugal force. Under some conditions, there can be significant flows of entrained water from suction rolls on twin wire formers at higher speeds.
With an understanding of The application of air/water pre-separation equipment, there must also be some knowledge of the proper piping methods and auxiliaries such as seal tanks and low NPSH removal pumps. Even though a separator exists, the separated water must exit the system through a barometric seal pipe or low NPSH pump. As discussed earlier, the seal pipe and seal tank can be used when There is sufficient elevation between the separator bottom and the liquid level in the seal tank. Vacuum systems with limited separator elevations may require a low NPSH pump. There is a significant amount of engineering applied to the design and installation of These systems, and this will not be covered here.
However, the point is that air/water separation systems between the paper machine and The vacuum pump are extremely important and affect the vacuum pump operation. Some mills are operated without any form of pre-separation after the uhle boxes on the felts. Although it is not desirable for the water, chemicals, fiber, and felt hairs to pass through The vacuum pump, many liquid ring pumps run for years under these conditions.

See more:
Last edited: