Most cement plants have a bank of storage silos to hold finished cement in inventory until required for packaging or bulk shipment. Storage silos allow cement plants to stockpile inventory until needed. Buildup on the vessel walls, however, can rob plants of the storage systems in which they have invested. Buildups slow material flow and decrease the “live” capacity of the vessel.

Overcoming these flow problems and recovering storage capacity may require silo cleaning. Given a number of ways to accomplish this, one is always advised to use an informed, safe approach.

Safety

First, do not try to clean a vessel from below. Poking from the bottom up inside the silo with a pole or pipe is asking for a large chunk of material to land on the operator's head — or the equally unsatisfactory alternative: burial under a mass of material that has suddenly broken free from the wall. To protect both plant personnel and the structure, the safest method is to clean down from the access opening(s) at the top of the vessel. That opening, however, is not to be used for putting people down into the silo, which likely would constitute a violation of the confined space entry rules. Under any circumstances, such a practice constitutes a significant safety hazard.

When working on top of a vessel, employees will need fall protection systems, including safety belts and restraining harnesses as required by OSHA. Typically, plant personnel won't be as comfortable at heights as they are on the ground. They won't have experience with fall protection systems and working at these heights. They may take risks or “shortcuts” that lead to accidents. Thus, for various reasons, calling in a specialist contractor experienced with the nature and conditions of the job is recommended.

Equipment for cleaning

A cleaning method is needed that can work inside the vessel — from the top — as it is controlled by personnel on the outside of the structure. Though manufacturers sell silo cleaning equipment, a single-plant operation where regular use of this equipment is not anticipated would probably find employing a service firm that uses this type of equipment more economical.

Several types of equipment can be used for silo cleaning. One of these operates like an industrial-strength “weed whip,” rotating a set of “flails” against the material in the vessel. The cleaning head is typically inserted through the access port down into the vessel on a pivoting arm. The arm is usually 8 to 28 ft (2.4 to 8.5 meters), allowing the cleaning of a radius of 56 ft (from a straight line down from the access port). This equipment is usually air powered by a compressor, with typical air consumption of 100 cfm at 85 psi (2830 l/min at 5.9 bar).

Precision

Any clean-out activity must be carefully controlled to avoid damage to the inner wall, which can reduce flow and cause continuing problems. Accordingly, arming an employee with a pickax and sending him into the vessel in a bosun's seat would be inadvisable. Such a method is extremely unsafe for the employee and risks damage to the equipment.

Equipment should have precision controls to maneuver the whip head and hold it in a cleaning position. A cleaning mechanism that “bounces around” from side to side in the vessel won't clean effectively; the cleaning head is not focused on the material and runs the risk of damaging the bin walls or bottom.

Successful cleaning also depends on an operator who has the skill to use the equipment properly. The operator should be thoroughly trained and, better yet, experienced in the procedure.

Whips and chains

The silo cleaning equipment uses its rotating “whips” to cut the material off the walls. A key to success is matching the flails to the material. Flails of a rubber-covered nylon rope, for example, have proved to work well in coal. These flails can be equipped with “knuckles” to improve the cleaning effect and extend whip life. The knuckles are most often constructed of urethane to provide impact without damage or sparks.

Steel chain is commonly used for portland cement or any compacted material where there is no risk of explosion. Nonsparking brass chain is effective for compacted materials where the risk of fire or explosion is present.

Strategy and tactics

Before the cleaning process is initiated, a path for loosened material to leave the vessel must be secured, and the discharge opening must be clear. A transport mechanism at the bottom — a conveyor, a truck, or a loader — is required to avoid buildup below the discharge and blockage of the opening as large quantities of material are removed.

In cleaning, the operator starts at the bottom and progresses upward. Wall accumulations are undercut until they fall by their own weight. Cleaning from the top would cause the removed material to fall on top of the lower accumulation with no place to go until the entire mass is cut away; when the entire section falls, then, the risk of damage to the bottom of the vessel or discharge is considerable.

This method may entail the occasional burial of the whip head. Generally, the falling material will push the suspended cleaning head out of its way. If, however, the cleaning head does become buried, the operator must wait for the material to feed itself out of the vessel, uncovering the whip head from the bottom.

No flow

If the material is hung up or bridged to a “no-flow” condition, the service provider will typically use a bin drill to open a 12-in. (305-mm)-diam hole through the accumulation. A channel is thus created through which the loosened material flows, and the silo cleaning crew then uses the bin whip to finish the cleaning.

If there are large chunks of material blocking the cone at the bottom of the vessel, a different solution is required. Under such circumstances, the crew drills into the chunks and uses a blast to shatter the lumps into smaller pieces that can be worked out of the vessel discharge. The preferred blasting method involves a reaction of CO2 and an igniter. The resulting blast is not an explosion, so there is no risk of fire or secondary explosion.

Avoiding emergencies

If a vessel is choked, that is, still running but nearly closed down, it will most likely get worse. As material falls through the vessel, it will build up on the accumulations, gradually restricting the flow path until blockage is total. Consequently, as soon as a partial blockage is noticed, scheduling a cleaning from a service is recommended. Time is then available to work the cleaning into the schedule of the plant and the cleaning contractor. Early intervention will also minimize mobilization charges and the cost of emergency plant outages. More will be paid for emergency silo-cleaning service — “hurry up and do it because my plant is shut down” — than for a routine procedure performed during a scheduled outage.

Benefits

Cleaning out finished cement storage can bring several benefits to a cement plant, including the recovery of material and storage capacity — both operating expenses previously paid for, but long considered missing and “off the books.” At one cement plant, the cleaning crew cleared a silo in two weeks that had been out of use for 35 years, adding 3,500 tons of “live” storage capacity to the company.

At another plant, the cleaning crew was able to remove enough buildup — written off over the years — to cover the cost of the cleaning through the sale of the material. In addition to making money on the sale of recovered product, storage capacity was regained. An investment rather than an expense, the timely cleaning of storage structures can readily provide economic benefit to an operation.

Costs

Because most cleaning projects are bid on a “time and materials” basis, a firm estimate cannot be easily provided. The length of time required for cleaning varies widely depending on the amount and characteristics of material in the vessel. Furthermore, some of those factors cannot be determined until the cleaning has begun. An efficient silo-cleaning crew can clean the walls in a 30- ∞ 60-ft (9- ∞ 18-meter) vessel and remove up to 150 tons of material in a day. (Working time excludes travel and setup.) Mobilization charges may be reduced by scheduling a cleaning in conjunction with other plants in the area.

Key to productivity

Hung up, clogged, or slow running silos will interfere with the efficiency and profitability of a plant. Keep your materials flowing freely by removing buildup from silo walls regularly, effectively, and safely.

Jim Stuckey is the Silo Cleaning Manager for Martin Engineering, Neponset, Ill., 309-594-2662, ext. 408