The dirty facts about railcar unloading.
Each year, tens of thousands of railcars move billions of pounds of plastic resin across the country. In this age of increasing quality consciousness, customers expect a high degree of purity and consistency in the materials delivered to their loading docks. Resin producers go to extremes to prevent contamination of the product shipped, but transportation and handling offer numerous possibilities for resin contamination. If that contamination ultimately shows up in a molded part, or a blocked orifice in a mold or die, the processor's first thought usually is to call the resin supplier. Then it may require a painstaking investigation to track down the source of the contamination. And all too frequently, the trail leads back to the point where material was unloaded from the railcar into the customer's plant.
Despite the enormous sums invested by resin producers in making and shipping high-quality products, improper handling of railcars can defeat their best efforts. Increased awareness of proper unloading procedures and cooperation with your resin supplier can reduce or even eliminate problems attributed to contamination.
WHERE THE DIRT COMES FROM
If a railcar compartment containing 45,000 lb of resin and 13 grains of sand (totaling less than 0.25 milligrams) were delivered to a molder of beverage closures, up to 20% of the hot-runner gates in a 64-cavity mold could become blocked over the course of a week.
Although the contamination level here was less than one part per billion, the mold must still be taken out of service and cleared in a mold shop. This costs valuable hours of production downtime. For a fiber, film or wire-insulation producer, much finer particulate contamination can cause problems of similar severity.
Contamination on this scale can come from the dirty shirt or coveralls of a worker reaching inside the railcar hopper to obtain a sample or from the sampling devices and containers he uses if they have not been properly cleaned and handled.
During hatch sampling, dirt in pockets, buttons, zipper tabs, belt buckles, smoking materials, hats, badges and pins are among the many things that can contaminate resin in a railcar. This is especially true when a sampler must reach several feet into the car because the lot has settled. Long-handled sampling devices can reduce the need to bend over, which tends to send objects tumbling from shirt pockets.
Larger objects such as bolts or tools are another potential source of contamination and can represent a danger to personnel and processing equipment. Extruder screws with barrier flights or Maddox mixing sections can trap particles larger than 0.020 in. Once trapped, these objects can slowly abrade the screw and barrel until the screw is pulled and the object removed. A wrench or bolt of sufficient size can become wedged between a screw flight and the hopper opening, causing broken screw stems or castings, damaged motors and drives, even fires. For this reason, installation of screens or magnetic devices designed to catch such objects before they enter the hopper is recommended.
When contamination does occur, pinpointing the source is critical to prevent it from recurring. Scientists have occasionally been called in to identify dead insects in resin supplies and help determine, based on their natural habitat, the most likely site of contamination. In another case, laboratory analysis of dirt found in resin sent overseas determined that the contamination had taken place in the country of destination, not before or during shipping.
KNOW YOUR RAILCARS
Making sure clean resin stays that way starts with the railcar itself. Proper handling procedures--both by the resin producer and the customer--can reduce or even eliminate contamination.
One reason resin suppliers lease or own fleets is to ensure that the railcars are dedicated to a specific resin and to control their care. Because supplier-owned fleets are dedicated to shipping one product, there is no need to be concerned about what else the car may have held during back-hauling, which may occur in common-carrier tank trucks, for example.
Reliable suppliers routinely inspect and repair railcars when they are returned. Himont U.S.A., for example, has an extensive inspection facility where mechanical components, ladders, walkways, caps, compartment linings, hatches and valves are examined and, if necessary, repaired. Every returning car is washed with purified water and dried with filtered air before it is reloaded.
During loading, Himont takes great pains to preserve the quality of the resin. Cars are loaded at stations protected from the weather. Discharge chutes or shrouds are placed directly into the hatch to reduce entry of dust or insects. Preshipment samples are taken with utmost care to avoid contamination. Hatches are sealed promptly to limit the risk of contamination.
Even the steel sides and cable locks of railcars do not enter 100% protection against contamination. Cars "breathe" when pressure, altitude and temperature change. USually, only moisture will enter under these conditions, but users of resins known to absorb moisture should have drying equipment.
When a railcar moves through standing water or heavy rain, water can accumulate in the discharfe tubes. When the discharge tube cap is removed, a brief but impressive stream of water may pour out. In this case, the product itself is rarely contaminated, but the supplier should be notified. After drying the tubes and purging 50 to 100 lb of resin, the compartment usually can be used safely. Before making the decision to use such compartments, however, a top sample should be taken to ensure that the compartment is dry.
Despite such threats as derailments, floods, vandalism and harsh weather, railcars are more secure against loss and contamination than any other method of shipment. Nevertheless, cars can be damaged in transit, possibly leading to contamination of the product inside, and should be inspected before unloading.
In particular, bits of the compartment lining material (typically epox) can be dislodged by derailments, sideswipes, use of improper vibrators, denting from a sledgehammer, careless use of vacuum wands, and similar accidents. If these bits end up in the resin, the distinctive color of the liner material aids in detection and identification of the contaminant.
TAKE CARE IN UNLOADING
IF te railcar is not damaged, passes the initial inspection--including a careful examination of valves and unloading tubes--and its cable-lock hatch seals are intact, the processor can safely assume the contents are in the same condition as when the resin was loaded. A car with one or more broken seals may have been tampered with an should be carefully checked and sampled.
It is good practice to record the time, date and seal number before cutting the seal. New seals should be placed on an open hatch if the car is not in asecure, fenced location and more than a little time passes between opening and unloading.
Open railcar hatches and outlets are a major cause of water, dirt and insect contamination. Hatches should never be allowed to stand open.
Before sampling or unloading a railcar, there are several safety issues that require attention:
* The car should be immobile--chocked firmly and grounded.
* All personnel in the vicinity should wear safety glasses at all times. Hard hats and hearing protection should also be worn when required by the operation.
* The top of a railcar is far enough above the ground for a fall to cause injury. A safety line or temporary railing should be used to prevent falls.
* Always use cutters designed for removing cable locks (normally used to close car hatches). Other devices greatly increase the effort, the hazard to personnel and the danger of damaging the hatches or caps.
* Beware of electric shocks during sampling. Most plastics are excellent insulators and can build up an enormous static charge during loading and transportation. While the shock is not usually dangerous by itself, it can cause involuntary movement or possibly a fall.
Consult with you resin supplier or safety professional for a complete list of safety recommendations.
Once you have taken these precautions, you can start the unloading process. To prevent sever damage to the railcar, always open at least one hatch to the compartment being unloaded. Fit the top hatch and outlet with a weather-resistant filter assembly to prevent pulling insects, dust or rain into the resin. A filter should also be placed over the end of the valve tube not attached to the unloading hose.
If unloading does not take place over a concrete pad or other easily cleaned surface, place plywood sheets or other material under the discharge tubes to catch spilled or purged material.
Initially open the valve slightly, gradually opening it further during unloading until fully open to ensure complete removal. Should the valve be frozen, a3ft length of pipe may be slipped over the valve handles to increase leverage. One molder repeatedly used the coupler on the those to bang open valves. This practice was quickly halted when aluminum dimples that had been chipped from the coupling were found to be clogging mold gates.
Flexible stainless-steel hose is recommended for unloading, but aluminum or plastic hose may be used if regularly inspected. When not in use, all hoses shoould be capped to prevent entry of foreign material, including rodents that might consider an unattended hose a ready-mode burrow.
Carefully attach a flexible hose to the discharge tube, ensuring that the flow will be in the direction of the hose spirals. A hose that is installed backward may shed pieces of metal into the resin flow. Bends in the hose should be as gradual as possible. In summer, unloading hoses should be shaded and protected from other sources of heat.
To avoid development of resin forms such as "snake skins," "angel hair, " or excessive dust, conveying conditions should be carefully controlled. Velocities should be kept below 5500 ft/min (lower in extreme heat or cold) by adjusting air flow. Gauges and controls should be used to maintain suitable conveying conditions and ensure a relatively high concentration of resin in the conveying system.
When a compartment is empty, shut off the conveying system and close the open hatch. Close and lock both outlet caps in place. Again, always remember to shut off the conveying system before closing the hatch, because the compartment can be crushed from the vacuum.
Prepare to empty the next compartment in a similar manner. A checklist, such as the accompanying example, can help ensure that unloading is performed according to standard operating procedures.
Once a car is empty, it should be closed, with all caps returned to their position and hatches dogged. When a car is returned to the supplier, residual resin may shake out, polluting the railroad bed and endangering wildlife, unless valves are closed and capped.
After unloading, be sure to clean up spilled resin from concrete pads or plywood sheets for recycling. Lightweight plastic pellets are easily transported by running water. Poor unloading practices quickly become apparent in drains or nearby bodies of water. Purged or spilled resin should be recycled, saving money and protecting the environment.
|Printer friendly Cite/link Email Feedback|
|Title Annotation:||plastic resin contamination|
|Author:||Miller, Richard C.|
|Date:||Dec 1, 1990|
|Previous Article:||Thermoforming wakes up to advanced technologies.|
|Next Article:||Studies show strengths, weaknesses of HDPE and coex recycle.|