VACUUM PUMPS AND SYSTEMS TRENDS AND OPERATIONS
By Pulp & Paper Canada
Paper machine vacuum systems have evolved with the papermaking processes. As with the modern paper machine itself, the vacuum system and its components are quite different than what was common just 20...
By Pulp & Paper Canada
Paper machine vacuum systems have evolved with the papermaking processes. As with the modern paper machine itself, the vacuum system and its components are quite different than what was common just 20 to 30 years ago.
However, many older paper machines are still producing and setting their own production records, as most have had one or more rebuilds over the years.
The older machine vacuum system consisted of several flatboxes, a suction couch, one or two suction presses and Vickery felt conditioners. Felt cleaning and dewatering progressed with the use of wringer rolls and then to uhle boxes. Typically, the uhle boxes would all be connected to a common vacuum header. The entire vacuum system required only three or four vacuum pumps. For the liquid ring pump systems, there was an abundance of fresh water to supply the seal water. Besides liquid ring vacuum pumps, some mills operated with rotary lobe pumps, some of which used fresh water injection for cooling. The centrifugal type vacuum pumps, or exhausters, gained and lost popularity during a period as liquid ring pumps became more efficient.
Vacuum systems evolution
Significant changes in the paper machine processes occurred through the 1980s. Twin wire formers became common on new machines and older mills were rebuilt to incorporate gap formers or top wire units. Press sections were now double felted with no-draw configurations, suction pick-up and various types of sheet transfer and suction boxes. Faster machine speeds required suction boxes ahead of press nips to minimize sheet fluttering. Highly loaded, long nip, presses were installed on board machines and new felt designs were developed to accommodate extreme loads and stresses of the new presses.
The effect on the vacuum system was that many would double in size after a rebuild. The new formers and presses had more vacuum points. New, multi-layer forming fabrics allowed higher vacuum levels due to the increased use of ceramic suction box covers. Higher machine speeds required additional suction roll (couch and press) vacuum capacities. New press felt designs resulted in heavier, stiffer felts, which required higher vacuum levels for acceptable cleaning and dewatering. Also, uhle box slot widths were increasing to accommodate faster machine speeds, while maintaining dwell times over the slots. The vacuum system was designed to connect each vacuum service to its own vacuum source. This eliminated vacuum level variations as grades changed and felts aged.
Vacuum pumps became larger in order to minimize the quantity. New vacuum pump designs allowed operation at only 30 to 50% of the seal water required in the mid 1950s. The pumps also became more efficient. Many vacuum pump systems were driven by large, slow speed synchronous motors and many paper machines required only two motors to drive a total of eight to 10 vacuum pumps. The synchronous motors required less space, were very efficient, reliable, and would allow power factor correction of the mill’s power supply. As individual vacuum pumps were added, they would be driven by induction motors and v-belts up to about 300 hp, and with gear reducers when larger motors were required. Although newer mills were being built with larger and deeper basements, more equipment was required for the newer machines. Large hydraulic systems for the press sections, chemical feed stations, press pulpers, additional centrifugal pumps and vacuum pumps would all compete for space in the wet end basements.
Water use became an issue in the 1970s. Mills closed up their water systems, which affected the supply of seal water for the vacuum system. Many mills recirculated the seal water and blended some fresh water to maintain temperature. Others cascaded the seal water to allow the cooler water to flow to the high vacuum pumps first, collect it and then pump it to the remaining vacuum pumps.
Vacuum systems of 2000 and beyond
The modern machine vacuum system is usually the largest process of the paper mill, with the exception of the paper machine itself. When observed with today’s 3D computer modeling, it can easily be seen that the system, consisting of eight, 12 or more pumps, will occupy several bays in the basement and require motors with 4000 to 8000 installed horsepower. The vacuum piping will usually be stainless steel, at least to the point of the vacuum separators. Piping diameters of 36 and even 42 in. are common on high-capacity services, like the suction couch and will require 1000 or more total feet of pipe to interconnect the entire system. The vacuum piping will compete for space with the machine drives and cantilever beams on the back side of the machine en route to the vacuum pumps. Pre-separation to remove white water, felt water and other contaminants is now more common to maintain water quality as most seal water is recovered for re-use or recirculation.
The trend of the highly loaded, long nip press has spread from just board grades to all paper grades. Felt designs to accommodate these presses have resulted in higher vacuum capacities and operating levels. Properly designed felt conditioning vacuum systems will have uhle boxes operating up to 15 to 20-in. Hg. New felts will operate at 10 to 12-in. Hg only for a few days until initial felt compaction and filling occurs. These changes to pressing along with the desire to achieve higher sheet consistencies off the former have led to most vacuum pumps operating at 15-in. Hg and above. Flatbox vacuum headers are quite often operating at 12 to 15-in. Hg on fine paper and newsprint. They may reach 16 to 18-in. Hg on board grades.
The use of the high horsepower, low speed synchronous motors has not been as common recently. Many systems will use large induction motors with gear reducers driving two or three pumps each. This will separate the entire vacuum system into as many as four drive systems. Individual vacuum pumps reach approximately 25 000 cfm, which is 250% of the largest capacity pump operating 25 years ago.
Another difference of the modern vacuum system is the use of a fully recirculated seal water system. Although the newer design pumps require less seal water, the total flow for a large machine can be 1000 galpmin, or more. The use of cooling towers has long been a common method to remove the heat from the seal water. Typically, the seal water will increase 25 to 30F as it picks up heat of compression and condensing of vapor within the pump. Splash fill cooling towers have proven to be preferred for this application. Some systems will also incorporate a gravity type filter, ahead of the tower, to remove fibre, which may have carried into the system. Cooling tower systems have performed well on paper machine vacuum systems. However, they do require attention. Water quality can be controlled through a combination of blowdown (purge), chemical treatment, filtration and maintaining pre-separation processes.
Materials of construction of today’s vacuum pumps have changed to incorporate stainless steel and various internal coatings. This is a departure from the previously all cast iron pumps. Since seal water isn’t becoming any cleaner, vacuum pumps are more likely to be supplied with stainless steel bodies or possibly an epoxy or rubber coating. Some models are also available with stainless rotors. A few mills have opted to install all stainless steel pumps, which is an expensive up-front decision. Again, proper pre-separation, good system design and maintaining seal water quality will aid in extending vacuum pump life in modern systems.
Modern vacuum pump designs have incorporated the capabilities of internal inspections. The manufacturers offer a variety of inspection ports, properly located to allow good visual observations of the critical areas of the vacuum pump internals. Many mills have adopted excellent preventive maintenance practices that include inspections of vacuum pumps’ interior, every year or at least every other year. The visual inspections sometimes require a fibre optic device to allow complete observation. These inspections provide quality informa
tion, which is superior to data collected from a performance test. Although a performance test will provide information on the current capacity of a vacuum pump, the inspection will tell why the pump may be performing below design capacity.
This discussion about the newest paper machines and their associated vacuum systems does not eliminate existing mills from upgrading and enjoying the same benefits. Many vacuum systems are good candidates for upgrades. These changes can usually be justified based on vacuum system horsepower reduction, reduced maintenance expenditures and lower fresh water usage. Additionally, the vacuum system modernization may include adding incremental vacuum capacity to specific areas, such as the couch or uhle boxes.
The vacuum system upgrade should begin with an analysis of the existing vacuum pump condition compared to “new pump” performance. This establishes available vacuum capacity, in cubic feet or cubic meters per minute, for the various vacuum services. The vacuum capacity requirement for the vacuum services then needs to be established. This is determined from the machine builders and TAPPI factors. TAPPI factors are widely used as a guide for calculating vacuum capacity requirements, for most vacuum services, based on paper and board grades and production speeds. The vacuum system “survey” as it is commonly referred to, can be performed by representatives of the vacuum pump suppliers and/or paper mill consulting firms. Some paper companies have personnel in corporate engineering departments who regularly handle vacuum system surveys.
Recommendations from the survey may include replacement of older, less efficient pumps. Also, multiple, smaller pumps can be replaced with single, larger pumps, resulting in significant space savings. Beyond the replacement of vacuum pumps, repairs of some pumps could be identified. The cause of vacuum pump wear, or lost performance, may be identified and solutions given to prevent reoccurrence. As mentioned before, the cost of the vacuum system improvements are often recovered through paper machine performance gains and/or savings in maintenance costs, fresh water reductions and horsepower savings.
The machine vacuum system is often out-of-sight and out-of-mind and, therefore, neglected. Its effect on paper machine performance is sometimes not fully understood and as long as the pumps keep running, they tend to be forgotten. The evolution of the paper machine and associated vacuum requirements have led to much larger vacuum systems. These systems require larger pumps, additional power and more space. Modern vacuum pump designs have features minimizing the effect of the newer requirements and can be beneficial in upgrading older systems. Most often, these vacuum system upgrades will provide positive, measurable results and will return savings to the mill. The savings will assist the mill in remaining cost effective and competitive as we progress into the 21st century.Douglas Sweet is North American Sales Director, Paperworkx, Charlotte, NC, a division of the Nash Engineering Company, Trumbull, CT.