- Buyer's Guide
M any paper mills have lubrication and hydraulic systems that, it would seem, have been constructed over time by several committees with no common goals or objectives. Needless to say, these systems are far from state-of-the-art in design and construction. There are many reasons that systems can fall into this kind of condition, including poor OEM design, designs that failed to incorporate room for process or speed improvements, systems that were installed in locations that prohibit routine maintenance, lack of sensory feedback to control centers, etc. Often the problems are associated with mill capital restraints which force mill engineers to purchase inadequate systems.
The path to underperformance may be incremental. At installation, a system is robust enough to support the production requirement. The inadequacy becomes evident only during planning for mill capacity expansion. Engineers recognize that they must live with the existing hydraulic or lubrication system and begin to modify the original design to get the necessary output for the new capacity.
System circuitry and arrangement are often not given sufficient consideration during original design and purchase. There is a clear advantage to addressing changes that may be required to adapt the equipment for your specific mill environment, standards, store spares, predictive maintenance programs and finally, future growth requirements.
The following discussion addresses key considerations for lubrication and hydraulic systems, and circuit or design deficiencies typically encountered.
Cleaned, conditioned oil is the primary objective, and is the basis for good system performance and productive equipment operation. Reservoir design is the foundation for oil conditioning and integrity in both hydraulic and lubrication systems. How the reservoir contains, directs and separates the oil prior to the pump suction is key to the conditioning of the oil and system health. Contamination is a fact of life in mill systems. New reservoirs should be designed, and existing reservoirs should be modified for conditioning of return oil contamination.
Circulating Lube Oil Systems
Hot return oil, (what should be) oversized piping, debris, moisture and possibly an acidic atmosphere make stainless steel plumbing highly desirable in the paper mill environment. Otherwise, rust and scale create an ongoing oil cleanliness and oil quality battle.
In older systems, return traps such as roll filters and bags provided first-line separation and visual detection of debris at the reservoir. Improvements may include smaller inline nonrestrictive return/ water traps in systems with improved reservoir inlet trap designs. Further assistance to oil circuits at inline supply filtration, improved bearings and seals, consideration when using flushing agents, stainless steel plumbing and improved reservoir atmosphere ventilation add value with return trap protection.
Return Section Containment/Baffle Design
The proper design of the tank return section is a crucial factor in oil cleanliness. The return section of the reservoir should be designed to encourage water, air and heavy debris to separate from the oil. This is done by forcing the oil over a full-width disperser and baffle. The return section can also have an onboard kidney loop (filter) circuit or a dehydrating purifier for cleaning and drying the oil. This maximizes cleanup and retention time and minimizes the reservoir size when combined with additional overflow/underflow baffles, heaters and an air space evacuation/conditioning package.
Proper Headspace Conditioning
It is common in mills for oil lube reservoir temperature to be maintained at an elevated level to encourage water and air separation from the oil. The water vapor can then condense on the inside of the reservoir, promoting rusting in carbon steel tanks, after which it drops back into the oil. Alternatively, breather/blower packages draw filtered air into, across and out of the reservoir. Either refrigerant or descant dried air moved through a blower in the headspace helps dehydrate the oil without risk of condensation. An emission absorber/diffuser (demister) or a drop leg should be used to coalesce and scavenge any oil carried out in the air, especially where the air path is not preconditioned (chilled and dried). Reservoir air bladders or KleenVents are beneficial in particulate-laden environments. They work well when used in conjunction with off-loop oil drying conditioners.
Reservoir inspection hatches and roll filter covers must be properly sealed and should be secured with compression clamps to allow for easy access. Return, instrument and access ports should enter through the reservoir top to allow for proper sealing and service or upgrade access. Any reservoir port provided should have a locking isolation valve (with an external valve port plug for cleanliness) to provide multipoint accessibility should off-system service or oil access be required. All too often, one can find port plugs but no access valving on oil system reservoirs, rendering corrective measures useless until it can be taken down and drained to provision with valving.
Return line filtration should be generously sized, easily serviceable and located as far from the suction area of the tank as possible. Return filters can also serve as a diffuser, if properly sized, to deal with cold oil (viscous at startup), which carries high levels of entrained air that must be released prior to entering the suction chamber or a reservoir. Select a filter housing that has multiple ports, element serviceability with minimal oil loss, and no service exposure to settled upstream debris in the housing when elements are changed.
On hydraulic systems, commonly found round or bubble type clean-out covers are typically small and attach to a single center baffle. An elevated round opening severely limits access required to clean the reservoir interior. If used, the clean-out covers should seal against center baffles along with baffle modifications to maintain a true 180-degree flow path in the reservoir and maximize oil residence time. The preferred larger rectangular covers, when overlapping the reservoir bottom, allow no-lip wipe-out and greater access on each side of the baffle. It is suggested that any end cover, if bolted on, should bolt onto a pretapped window frame full pass welded to the reservoir.
Bolt-on or Hinged Tops
Bolt-on top units usually have most of the circuit components mounted on top. Bolts, pipe grommets and seal gaskets create places for water and dirt to enter the reservoir. Hinged top units are usually tall and narrow, making cleaning more difficult. With downcomer pipes installed and the lack of low point clean-out covers, proper cleaning is almost impossible. Top-latching components and seals typically breakdown over time, leading to the ingression of contaminants. As typically found in this design arrangement, fixed pump suction downcomer pipes can harm oil circuits, allowing debris or water to enter a system and displace the usable oil volume.
Circulating Lube and Hydraulic Circuits
Low-Watt Density Dry-Well Heaters
Low-watt density dry-well design heaters, especially for high viscosity oils, are often important to maintain oil integrity and mill safety. High-watt density heaters (steam or electric) can damage the oil and its additives (cracking, sludge, oxidation, etc.), especially when water, debris, acids or air are present. Low-watt density dry-well designs employ surface temperatures that will not harm the oil or its additives, but can easily maintain the reservoir temperature, even while the pumps are not running. Dry wells allow element servicing or upgrading without disturbing the reservoir oil volume. A flexible element design works well in areas with restricted access. Solid-state thermostat control (SCR) with a high-limit safety is recommended for maximum element life, energy conservation and safety.
Offline (Kidney Loop) Filtering
Valved access ports (such as quick-connect couplings) should be located on opposing sides of the reservoir, at the low point in the return area, to allow the use of a vacuum dehydration purifier, flushing filtration skid or kidney loop filtration. These ports should be oversized (two inches or more) to allow for alternate filtration measures. A contained return section will help isolate and restrict the mobility of ingressed water or wear debris before the rest of the system can be damaged. Low-volume off-loop or kidney loop filtration systems protect the oil, pumps and pressure filters, and premature plugging can indicate an ongoing problem. Kidney loop filtration has been used effectively in many applications such as chip truck dumps, turbine systems, lowerators, vacuum pumps and gear boxes.
Lube Pump Selection Tips
Due to the multitude of oil weights used in lube systems throughout the mill, a prudent selection process needs to be established to meet the pumping needs with a minimum of different pump/motor arrangements. Most applications fit into the performance range of rotary screw pumps. At 1800 rpm, these pumps can handle a wide viscosity range, maintain proper internal sealing and provide a wide range in pumping capacity.
Pump Connection Plumbing
When possible, use SAE flanged ports. These allow for easy maintenance and leak-free connections. Hoses, rather than hard piping, should be used between the pump ports and system piping to reduce vibration and mechanical stresses on the system. Over time, pump manufacturers have changed the pump dimensions for new models. Hosing allows for minimal pump change-out time, access for troubleshooting and minimum disturbance of hard plumbing. Suction ports on reservoirs should be oversized. Most manufacturers size the pump ports based on either 1200 rpm or a light viscosity oil which may vary considerably from actual operating speeds and fluid viscosity.
Hydraulic Pump Selection Tips
Pressure compensated variable volume pumps respond to the system requirements of flow and pressure. This design will provide a long life and energy efficiency. These pumps can use a number of control devices to match output to your system’s needs. As with any hydraulic pump, check the manufacturer’s requirements for minimum oil viscosity, minimum and maximum operating pressure and inlet conditions. When possible, C-face mount for alignment integrity.
On both hydraulic and lube oil systems, most suppliers typically size filter housings near their upper volume (flow rate) limit. Filter housings should be selected with ample flow capacity at the required micron filtration, oil viscosity and any potential operation surge. This will provide proper filtration/component protection under various circumstances, such as component wear, varying oil temperature and changes in oil flow rates and pressure. For hydraulic circuits, filter housings are sized by the information in a product brochure. This practice, if used in lube system design, will result in undersized housings, poor element service life, excessive flows though the element and operating expenses higher than expected. Lube oil, given higher viscosity ranges, may be 25 times heavier at startup, and seven times heavier oil viscosity and any potential surge condition.
Housing design has been a problem in bearing circulating lube oil systems where water gets past bearing seals and back to the lube system reservoir. Water can attack the uncoated or nonalloy filter housing components. One trend is toward coreless design filter housings. The cores in the housings are typically stainless steel with taper-fit sealing surfaces. The filter elements are composite capped (plastic), with a spiral nonwoven fabric and bonded pleat support for cold oil surge and surface area integrity. Where there is no metal in the element, all its parts are impervious to moisture. Synthetic fiber construction can improve filter performance, DP and flow throughput.
Oil Level Gauge and Inline Sight Glass
External oil level sight gauges show reservoir oil level without lifting an access lid or cover, exposing the reservoir oil to contamination. In older systems, internal tank top corrosion can be dislodged by the activity around an access hatch. Inline sight glasses or gauges are valuable diagnostic tools that should be located in bearing outlet pipes, warm-up loop plumbing, pressure control returns and gravity return piping. Inline sight glass provides visual diagnostics for oil condition, flow verification and air entrainment.
Lubricant Condition, Pressure, Level and Temperature Indication
With more equipment operation tied to distributor control systems (DCS), low-voltage devices for oil pressure, temperature and level are replacing 110V/60Hz contact switches. System pressures, temperatures and filter differential pressures can be monitored and trended easily in the DCS. Low-voltage sensors, such as thermistors, are used on bearing ports to indicate lube flow in intermittent applications and in oil reservoirs to sense moisture phase detection. Inline sight/flow meters feature oil flow sensors that send a low-voltage signal to the DCS for processing.
Oil Sample and Proactive Maintenance Devices
Proactive maintenance monitoring and controlling root causes such as contamination are essential to minimize machinery expenses and downtime. Located at various points in the system plumbing should be oil sample devices such as mini-checks (minimess), which feature an internal safety check which must be depressed by a matching sample tube assembly or S/S ball valves with a stainless ball passage that rides on a Teflon seat. These will allow operations multitask accessibility to track the following: oil condition, system temperature and system pressure using devices such as field analysis kits, photomicrographs, portable laser particle counters, test kits and viscosity instruments.