525
6:00 am
September 1, 2007
Print Friendly

Storage Preservation Strategies

0907_equipment_reliability_img1In their quest for improved machinery reliability, many companies have rightly turned their attention to such opportunities as synthetic hydrocarbon lubricants, dry sump oil-mist lubrication and automated grease lubrication systems. To protect equipment bearings, Best-of-Class companies have taken steps to install the most advanced bearing housing protector seals. These rotating labyrinth seals are configured so as not to allow O-rings to contact the sharp edges of an O-ring groove. In some applications, Best-of-Class companies also use face-type, magnetically-closed housing seals. These are of great importance at facilities wanting to fully protect gearboxes in harsh surroundings (think: cooling tower fans), or in process plants where environmentally friendly closed-loop oil mist lubrication systems serve centrifugal pumps. All of these lube-related protective measures represent tangible, cost-justified steps in the direction of extending uptime and reducing failure risk. Nevertheless, despite the lubricant used and how a bearing housing design protects the lubricant against intrusion of external contaminants, if storage protection is ineffective—or lacking altogether—we still find dirty bearings. Frequently, dirt contamination begins with the way lubricants are transferred from storage to the equipment.

Use the right transfer method
0907_equipment_reliability_img2Studies by one of the world’s most competent and experienced bearing manufacturers, SKF, have shown the exponential decrease in bearing life due to what some—erroneously—consider “minor” contamination. All too often, this contamination comes from oil transfer containers with open spouts, missing filler caps and rust and caked-on dirt. Where dirty containers (see left portion of Fig. 1) and unacceptable work practices are still the norm, even the best lubricants and most advantageous bearing housing seals will be of no help in attaining high equipment reliability. Hence, the replacement of questionable transfer containers with rustproof, suitably proportioned and purposefully designed oil transfer tools (right portion of Fig. 1) should be a priority issue for modern industrial plants. A quick-action push-pull valve incorporated in the spout allows for the adjustment of oil flow to the particular task demanded. The lid and spout arrangement of purposefully designed containers keeps oil in, and contaminants out. It has been shown that payback periods often can be measured in days [Ref. 1].

That said, the cost-effectiveness of these lubricant containers is quite selfevident. Responsible reliability professionals consider them essential lubrication management tools. Indeed, it makes much economic sense to first ensure lube oil cleanliness before contemplating any of the other, more glamorous, high-tech approaches to optimized lubrication.

Beyond proper lubricant transfer methods, though, there also is the issue of storage adequacy that affects fitness for use of lubricated components. The first and foremost of these are rolling elements—older terminology: “anti-friction” —bearings.

Proper storage of bearings 
Spare parts protection should be among the priorities for sound asset management. Proper storage of parts—and lubrication while they are being stored—may vary depending on component and configuration. Most rolling element bearings can be stored in their original packages for several years, but the storage facility and mode of storage must be correct. There are four general requirements that must be observed:

  • The relative humidity in the storeroom should not exceed 60%.
  • The temperature should be stable within reasonable limits, although no quantitative numbers are available. Aiming for a range between 0 and 40 C (32 and 104 F) and not allowing the temperatures to fluctuate more than 10 degrees C (18 degrees F) per hour seems reasonable here.
  • Bearings must be laid down flat on the storage shelves. The loads acting on the rolling elements are now evenly distributed whereas, with “on edge” or standing storage, much of the load would act on just one or two of the bearing’s rolling elements. Moreover, the weight of the rings and rolling elements in the standing position might cause permanent deformation because the rings are relatively thin-walled. Think of an apple pie—it would not make sense to store it on edge.
  • Sealed or shielded bearings may have been pre-filled with grease whose lubricating properties are adversely affected by long-term storage. For these bearings, assume a twoyear shelf life, unless the grease (or bearing) supplier will certify a higher (or lower!) number that differs from the two-year rule. This issue then implies that reliability- focused users would refuse to purchase “surplus bearings.” More often than not, cheap surplus bearings are ones that someone else has discarded because of uncertain age, unwise storage method and unknown provenance. Since cheap surplus bearings are unsuitable for use in machines at reliability-focused facilities, they should be relegated to duties such as paperweights, doorstops and boat anchors.

0907_equipment_reliability_img3Protecting “inactive” machinery 
Machinery in storage must be protected from the elements. Painting, plating, sheltering, use of corrosion-resistant materials of construction and many other means are available to achieve the desired protection [Refs. 1, 2 & 3]. Although the protection of bearing housings is of primary importance in most fluid machines, a similar set of protection requirements applies to both “about-to-becommissioned” and “temporarily deactivated” equipment. The storage method discussed here refers to that “inactive machinery” category. The means or procedures chosen for the preservation or corrosion inhibiting of fully assembled, but inactive fluid machines will logically depend on the type of equipment, expected length of inactivity, geographic and environmental factors and the amount of time allocated to restore the equipment to service.

The basic and primary requirement of storage preservation is exclusion of water from metal parts that would form corrosion products—that means rust. These corrosion products could then find their way into bearings and seals. A secondary requirement might be the exclusion of sand or similar abrasives from close-tolerance bearing or sealing surfaces. All or any of the chosen storage preservation strategies must aim to satisfy these requirements.

Machinery preservation during pre-erection storage or long-term deactivation (mothballing) will have an effect on machinery infant mortality at the startup of a plant or process unit. Many times, machinery arrives at the plant site long before it is ready to be installed at its permanent location. Unless the equipment is properly preserved, scheduled commissioning dates may be jeopardized, or the risk of failure is greatly increased.

0907_equipment_reliability_img4Long-term storage preservation by nitrogen purging is well known in the industry. Generally, this method of excluding moisture is used for small components, such as hydraulic governors or large components, such as turbomachinery rotors kept in metal containers. Nitrogen consumption is governed by the rate of outward leakage of this inert gas and may be kept at a low, highly economical rate if the container is tightly sealed. Alternatively, the container could be furnished with an orificed vent to promote through-flow of nitrogen at very low pressure. This is called “nitrogen sweep” or “nitrogen blanketing.” Whenever the preservation of field-installed inactive pumps and their drivers is the primary objective, simply providing a moderate-cost oil mist environment will prove highly effective. Such oil mist preservation systems have contributed substantially to the flawless commissioning and operation of equipment in Best-of-Class or Best Practices plants. While it is, of course, feasible, applying a nitrogen purge will incur higher costs.

Oil mist preservation 

General setup… 

0907_equipment_reliability_img5An oil mist console like the one normally used to lubricate rotating equipment will be used to generate a preserving mist. A large and a small console are shown in Fig. 2. Since none of the equipment is rotating, a basic unit without all the supervisory alarms and back-ups often will suffice. It is recommended that air and oil heaters be used to ensure mixing effectiveness and maintaining the correct air/oil ratio. These heaters are mandatory if ambient temperatures during the period of storage drop below 50 F (10 C). Typical R&O (rust and oxidation inhibited) turbine oils (ISO Grade 32) can be used in the mist generator lube reservoir to provide oil mist at an approximate header pressure of 20″ of water column (~5 kPa).

A large oil mist console can serve hundreds of machines laid up in a temporary outdoor storage yard. One such location, often inundated by rain, is shown in Fig. 3. A storage yard in an arid part of the world will look no different (Fig. 4). A pipe header runs the length of the storage yard. Mounted along the way and at the top of the header are a number of manifolds (Fig. 5) into which reclassifiers are screwed. Plastic tubing connects the point of oil mist application at the machine to the reclassifiers. This is illustrated in Fig. 6, where plastic tubing leads to application points on a small turbine that is part of a lube oil skid. Note that even the oil reservoir is blanketed by oil mist.

It pays, however, to remember that actual on-site installed, spare or standby pumps and motors are being protected by oil mist. Oil mist both lubricates running equipment and protects non-running machines. This lubrication mode would gain even greater acceptance if these dual capabilities were being mentioned more often.

0907_equipment_reliability_img56Getting back to temporary storage, some owners have occasionally elected to construct covered temporary storage yards. Storage under cover, though, is probably more for the benefit and convenience of inspection personnel—it is neither required nor cost-justified for equipment protection. Needless to say, storage in a warehouse also is feasible. Fig. 7 depicts oil mist applied to the equipment inside a half-open crate located indoors.

Storage site preparation… A few common-sense considerations will assist in defining long-term storage measures either indoors or outdoors:

  1. Choose a site that has good drainage and is located out of the main stream of traffic. This will reduce possible mechanical damage from trucks, forklifts, cherry pickers and automobiles. A covered fenced storage area is preferred for the convenience of personnel, but it is not needed by the stored pumps, electric motors and other equipment.
  2. Position stored equipment on cribbing (pallets) if the storage site has not been paved or concreted. Arrange the equipment in an orderly fashion with access for lifting equipment.
  3. Install temporary overhead supports for the oil mist headers as per Figs. 2 and 3. Piping for the oil mist headers should be Schedule 40 screwed galvanized steel with minimum size of 1½”. All piping should be blown clean with steam prior to assembly to remove dirt and metal chips. All screwed joints are to be coated with Teflon sealant (no Teflon tape) prior to assembly to prevent oil mist leakage.
  4. Install laterals (½” min.) from top of mist header at each piece of equipment. Attach a distribution manifold to the header or to each lateral. Each distributor block typically has eight connection points in which to attach the ½” tubing. This should be sufficient to provide mist to most driver and pump combinations.

 

Pump and driver storage preparations… 
A typical connection sequence would include:

  1. Connect ½” plastic or copper tubing from distributor block to reclassifier fitting attached to pump bearing housings. (See Fig. 3).
  2. Connect ½” plastic or copper tubing from distributor block to reclassifier fittings located in pump and turbine suction flanges. If wooden, plastic or metal flanges are used, drill ½” hole through the suction flange protector to permit the insertion of the reclassifier fitting. Once reclassifier fitting and tubing are inserted in the suction flange, seal the hole with duct tape. This prevents moisture and dirt from entering the mechanical seal and wetted area of the pump by maintaining a positive pressure of oil mist. No vent holes are required because of normal leakage around flange protectors.
  3. Electric motors modified for oil mist lubrication should be stored with oil mist flowing through the smallest size reclassifier attached at each bearing cavity. Electric motor hookups are very similar to those shown in this article’s various illustrations. (Note oil mist venting from the steam turbine governor in the foreground of Fig. 3).
  4. Coat all exposed machine surfaces with an asphalt-based preservative purchased from a reputable lubricant supplier. Re-coat exposed machine surfaces every six months if needed. The preservative may be applied by either spray or brush.
  5. Rotate pump and driver shafts ¼ revolution each month to prevent brinnelling of anti-friction bearings and bowed shafts.

Oil mist generator maintenance

  1. Check weekly to ensure that air supply is dry.
  2. Refill mist generator oil reservoir weekly.
  3. Perform weekly checks of air and oil heaters on mist generator.
  4. Check oil mist header pressure daily. (Verify ~ 20″ of H2O.)

To re-emphasize 
Equipment preserved by oil mist blanketing can be stored for years with minimum maintenance and cost. The photograph in Fig. 7, dating from the early 2000s, shows the thoughtfulness and professionalism that have brought us to modern oil mist preservation. It is assumed that the internal surfaces of equipment stored in boxes and wooden crates will have been coated with a light film of preservative oil. For both indoor and outdoor storage, the various equipment-internal volumes are kept at slightly more than atmospheric (ambient) pressure. Oil mist through-flow is being achieved by providing a small vent at the bottom of the equipment casings blanketed with this oil mist environment.

Whenever possible, the equipment purchase documents should state that oil mist will be used as a long-term storage means. This might allow equipment vendors to select or predefine the most convenient oil mist inlet and vent locations.

Two final points deserve to be reemphasized:

  • Many bearings fail because unclean containers contaminate the lube oil as it is being transferred from storage drums to pump bearing housings. Reliability- focused equipment users will only use properly designed plastic containers for their lube replenishing and oil transfer tasks. Each of these containers will cost only a fraction of the cost of a single bearing failure. This is one product for which the payback has occasionally been measured in mere days.
  • Some plants make bearing procurement and storage decisions only on the basis of initial cost and schedule. This is inconsistent with a reliability focus. Proper storage and asset preservation are of great importance to plant reliability and profitability. Neglecting these issues is certain to deprive a facility of ranking among Best-of-Class producers.

References

  1. Bloch, Heinz P. & Alan Budris, Pump User’s Handbook: Life Extension, 2nd Edition, Fairmont Publishing Co., Lilburn, GA, 2006
  2. Bloch, Heinz P. & Abdus Shammim, Oil Mist Lubrication: Practical Applications, Fairmont Publishing Co., Lilburn, GA, 1998
  3. Bloch, Heinz P., Practical Lubrication for Industrial Facilities, Fairmont Publishing Co., Lilburn, GA, 2000


Navigation