September 19, 2017
The efficiency and availability of any wash plant is affected by poor or non-existent maintenance practices. Maintenance programs should cover safety, lubrication, and wear parts. Types of maintenance include preventive, reactionary, and time based; logic dictates preventive is always better. The first, most basic part of any maintenance plan should be to thoroughly read and understand equipment manuals. These manuals are designed to ensure equipment longevity and efficiency.
Start with safety
The number one priority in any maintenance task is safety. Do a review of conditions surrounding the maintenance: What tools are required? What man power? Will work platforms/man lifts be required? If you are cutting into equipment for access or replacement, evaluate if flammable materials such as rubber lining or flammable fluids are present. Please, never heat an item to assist with dismantling without reference to the manufacturer — a recent fatality demonstrates the absolute need for this.
As part of routine maintenance, check guards and warning decals. Guards that may be seen as ‘inconvenient’ find their way to the ground because bolts have been lost or an operator could not be bothered to replace them. However, guards should be replaced and warning decals cleaned or replaced. Used equipment should always be checked for the integrity of all safety items before use.
Avoid unintentional damage
Some equipment, especially vibratory, requires re-torqueing of bolts. This is a critical step; if missed, it can cause significant damage to equipment and probable loss of warranty. Check whether you need a torque-multiplier if the torque is higher than available tooling. Also, check what specification, such as lubricated or dry threads, is used.
While lubrication schedules vary, check the quantity and type of grease/oil required. Determine answers to questions such as: is replacement of lubricant needed following a fixed time after start-up? Is the period based on environmental conditions? A seemingly obvious (but not always heeded) guideline is that Zerk fittings should be cleaned before engaging the gun and dirt cleaned around oil filling points. Inspect spent lubricants for contamination. In some cases, analysis should be carried out for tracing specific metals associated with certain types of failure.
Don’t be too quick to employ a program of preventive maintenance where teardown and reassembly is carried out ‘just to check’ — certain failure curves have the highest incidents of failure immediately after initial start-up. Sometimes, the old adage “if it ain’t broke, don’t fix it” can be the best advice. This can be the case with pump glands, for example.
So, what equipment-specific things can we watch for in a typical wash plant?
Scrubbers/trommels — Apart from lubrication, mounting alignment is very important to prevent permanent damage to tires and drives. Checking screen media condition is an obvious maintenance activity, but often forgotten are the internal lifters and advancing/retarding paddles, which can significantly affect performance if their height is below a certain critical point. Material will not rotate and interact effectively. Saving a few dollars by letting the lifter go for a month or so can cost much more in lost/non-spec production.
Screw-shaft based equipment — Wash plant equipment such as log washers, blade-mills, aggregate conditioners, coarse material washers, and fine material washers all have a common weakness — the lower bearing. There are various designs — from the complex ‘submerged’ bearing to the simple outboard pillow-block — and each claims an advantage, but all require lubrication. Paddles and flights need periodic inspection. Any time metal paddles are used, take care, as worn edges can be razor sharp. Wear and fastenings are important to check, as is checking their connection point to the shaft. Check the shaft for run-out and any deflection at the shaft ends, as eccentric motion can cause premature failure of bearings and even the shaft. Shaft straightening should be carried out by experienced personnel only.
Pumps — These are the heart of a wet process. The correct selection of the materials of construction is critical to long-term, cost-effective service life. There are three key areas in any pump: wet end, gland, and bearings. It is important, particularly with open-vane style impellers, to check the clearance between the impeller and the suction side liner. Too wide a gap leads to bypass inefficiency and, in slurry pumps, accelerated wear. Impeller wear should be monitored, as reduction in performance is most likely due to impeller wear.
Pump glands seal the pressure generated inside the pump wet end from the outside. They come in various forms, including the industry standard packed gland, as well as proprietary glands and mechanical seals. Typically, they are ‘set-and-forget’ and last a full season, but it is critical that they are set up properly in the first place. This is one area of the pump where regular teardown and reassembly as ‘preventive’ is likely to cause more problems, as initial set up takes finesse, and glands have a higher ‘start-up failure’ rate. Always check the quality, pressure, and flow of any lubrication water.
Bearings in slurry pumps mostly have B10 lives in excess of 50,000 hours and should, if properly lubricated, provide years of service before failure. However, if the pump is cavitating (poor design) or running on ‘snore’ (low sump level causing vortexing), bearing and gland life can diminish to weeks, so maintaining make-up water valves is instrumental in assuring pump life. Failure of ‘wet-end’ bearings can also be caused by gland failure. V-belt tension can factor into ‘drive-end’ bearing and motor bearing failure, so belt tension should be routinely checked.
Hydrocyclones — These have no moving parts, but there are two failure modes, delamination and wear, that affect performance. Liners coming loose (due to wear or adhesion) cause disruption of flow inside the unit, and poor performance with misplaced particles is the result. Checking the internal lining of a cyclone should be done at least seasonally; this is particularly important when dealing with siphon (vacuum)-assisted cyclones/separators. The apex (spigot) is the fastest wearing component; they are sized based on mass-flow, so, as they wear, the underflow becomes more dilute and more fines will bypass into that stream affecting the wash-out of deleterious fines. If your product is getting dirtier, check the apex. Worn vortex finders can allow coarse fractions to short-circuit to the overflow and fill the ponds or contaminate the next stage of production. Worst case is wear can cause the pipe to detach from the plate, causing significant damage.
Separators or ‘siphon-assisted’ cyclones can be affected by holes worn in the overflow piping, which defeats control, and flushing will occur. Unfortunately, symptoms such as uncontrolled flushing are insidious and hard to trace. Close annual inspection for thin pipe sections should be carried out.
Dewatering screens — Dewatering screens typically have a very long service life, but there are some areas to watch. Side plate cracks (as with sizing screens) can occur. If noticed, trace the crack to its end with dye penetrant and drill a 1/8-inch to 3/16-inch hole. This ususally takes care of the problem, and the side-plate will last the life of the screen. Cracks in cross members or the bridge should be referred to the manufacturer immediately, as should any bolt breakage. The problem with cracking in dewatering screens is frequently the support structure rather than machine integrity. Lubrication is a general maintenance activity, but take extra care as some vibrator motor manufacturers use specialized grease that, if not used, voids the warranty.
Often, motor wiring gets draped across from the motor to the side of the screen and vibrates against adjacent surfaces — this is a very dangerous condition. Check this and replace the wiring as needed. Always check direction of motor rotation on restarting after electrical work.
It is not uncommon to see sideways tracking of material on a lightly loaded screen, this is typically not of concern, but springs or rubber buffers should be checked once a month. At rest, measure the height of each buffer and confirm with the O&M manual that it is within tolerance. The screen should sit square; any sideways deformation at rest should be investigated, as should sideways motion greater than 1/16-inch during operation. Record and refer this to the manufacturer. If the screen has an unusual action (non-linear), check the motor counterweights for settings — all four locations should be consistent — and confirm the motor direction of rotation. Where mechanical exciters are used, check oil levels at recommended intervals. Once a year, check the oil for contamination, as annual checks can reveal increasing wear. As with the vibratory motors, confirm counterweight settings and inserts for consistency.
Effluent treatment — Thickeners typically require little maintenance, but the large planetary gearboxes and the associated hydraulic power units do require routine attention. Presses are really a story for another day.
Remote condition monitoring is gaining more attention. Measuring power-draw, heat, vibration, pressure, and using belt scales can provide important insights into plant operation and needed maintenance. Subtle vibrations can prelude noise, which is when most operators will react (too late). Most importantly, in some circumstances, it can also prevent serious accidents.
As a final word, remember nothing will make up for inappropriate (often cheapest) selection of equipment. Equipment that is too small, over-rated, and thus overstressed will always have compromised life no matter how good the maintenance.