Materials Handling in Power Plants

Materials handling in nuclear power plants is different from that in thermal and hydro power plants. Handling of conventional equipment such as pumps, motors, generators, etc. is similar to that in conventional power plants if these are not irradiated. Handling of radioactive materials, which are present in nuclear power plants, requires special care.

Materials handling system adopted in any power plant depends upon its nature and capacity viz. whether it is nuclear, hydro, thermal or captive power plant and how much is its capacity. The materials handling system installed in a thermal power is more exhaustive as it should be suitable to handle large quantities of coal, and ash generated. For a typical 500 MW unit, which is common at present in our country, the ash generated is of the order of 3000 T per day. To handle such quantities of coal and ash on a daily basis, requires comprehensive and reliable materials handling system.

The failure of materials handling system in a thermal power plant even for a day may lead to accumulation of large quantity of ash and the unit may have to be shut down.

A typical power plant will have motors, generators, pumps, etc. weighing from a few kilograms to thousands of kilograms. The materials weighing up to a few kilograms can be handled manually. The heavier ones require sophisticated materials handling systems. Materials handling may be required on intermittent or continuous basis as coal or ash handling in a power plant. The equipment required for handling will be forklifts, cranes, conveyors, etc. Sometimes we may require a materials handling equipment to handle equipment installed in a special location.

Bridge or travelling cranes is commonly used for handling heavy equipment in a power plant. These cranes are installed at the time of construction itself. The hoist of the crane can be moved in all the three x, y and z directions. . An unloading bay is generally provided where bridge crane is installed. The unloading bay is required to facilitate loading and unloading of equipment from transporting equipment such as trailer or truck. The capacity of crane should be decided depending upon the maximum load to be handled.

Materials Handling in Thermal Power Plants
In a modern thermal power plant, we have motors, generators, pumps,, etc. of various ratings and require a materials handling system to handle these equipment during operation and maintenance. Also a thermal power plant handles large quantities of bulk materials – the largest being coal and ash. A typical 500 MW unit requires handling approx. 6000 T of coal and 3000 T of ash on daily basis.

The ash content is quite high, around 40%, in the coal available in the country. You can imagine the complexity of coal and ash handling in a thermal power plant. Gradual deterioration in the quality of coal is also responsible to some extent in requirement of handling such large quantities of coal and ash. A typical power plant is highly capital intensive. And requires investment of the order of Rs.3 to 4 crores per MW of installed capacity. Hence, a reliable and efficient and cost-effective bulk material handling system is required.

Coal Handling
Basically, coal handling system consists of a hopper for receiving coal from rail wagons or trucks. Feeder controls flow of coal from the hopper and a conveyor or elevator raises the coal to the top of the storage bunker. Another conveyor delivers the coal to the furnace. Apron feeder is commonly used.

A hopper, usually underground, is used to receive rail wagon car or truck discharge. In cases where outdoor storage areas are required, a reclaiming hopper is also provided for a cable drag scrapper or bull dozer to reclaim coal.

A grating is usually provided over the hopper. This serves to protect the equipment, and the size of the grating opening determines the maximum lump size the system can handle. For truck hoppers or reclaiming hoppers, the grating is also usually designed to support the weight of the truck or the bulldozer.

Skit hoists are suspended at each end of the wire rope, which is wound around a motor driven driving drum situated at about the coal delivery head. When one skip at ground level is being loaded, the other will be at the top delivering the coal to the belt or bucket conveyor, etc. On the winding being reversed, the loaded bucket is lifted up, and the empty one returned to the ground level for loading. The buckets are provided with shoes which slide on guide rails; the rails being slightly inclined to ensure continuous contact between slide and rail at all times.

Ash Handling
Ash produced as a result of burning of coal is identified as fly ash and bottom ash, 20% being bottom ash. The ash handling system may incorporate combinations of hydraulic, pneumatic and mechanical systems for handling bottom ash and fly ash from the boiler. Continuous development work on ash and dust handling systems has been aimed at improving equipment reliability, reduced manpower, lower maintenance cost, and operational efficiency by way of low water and power consumption. The ash handling system can be broadly divided into three subsystems i.e. Bottom Ash System, Fly Ash System and Slurry Pumping System.

Slurry Disposal System
Slurry pump can be horizontal or vertical. As compared to clear water pumping, there are many additional requirements and fire cautions for pumping ash slurry. The major requirements are:

1. We should select optimum velocity so that power consumption is minimum and at the same time avoids settling and choking of pipelines.
2. Head flow characteristics and tip speed of slurry pumps should be selected to avoid wear and tear of pump internals.
3. Use of variable speed fluid coupling to make the system amenable to changes in duty point requirements, likely extension of disposal distance in future, reducing power consumption and control surges during starting and stopping.
4. Conducting surge analysis study and providing suitable appurtenances for preventing surges in long distance pipelines. These appurtenances have to be suitably designed for handling slurry.
5. Lining of pipelines for combating abrasion and/or corrosion e.g. cast basalt lining.

Materials Handling in Nuclear Power Plants:
Nuclear Power plants do not have any transportation problems. It is because of less requirement of fuel. Hence large storage facilities are also not required.

Materials handling in clear power plants is different from that in thermal and hydro power plants. Handling of conventional equipment like pumps, motors, generators, etc. is similar to that in conventional power plants if these are not irradiated. Handling of radioactive materials, which are present in nuclear power plants, requires special care. It is so as radiation from radioactive materials is harmful and may lead to diseases such as cancer, etc. if the person is exposed for a long time.

Therefore, exposure is to be limited to save the personnel from the harmful effects of radiation. In case it is not possible, the remotely controlled materials handling is used. If the materials handling is not urgent, it is postponed until the radiation level comes down to an acceptable level.

This may vary from a few days to years depending upon the radiation intensity and type of material to be handled. There are other methods also to bring down the radiation level down. These methods are separation of radioactive waste into high, medium and low level wastes. High level waste is to be stored for a long time to bring down its radioactivity to acceptable level. Medium and low level wastes are chemically treated to bring the radiation to a low level before disposing them off.