TECHNICAL WRITING SAMPLE:
CHEMICAL INDUSTRY

INTRODUCTION

Powder formulation is the process used when the Formulations Operator makes the dry Wessex. It involves, mixing together the active ingredient, known as the technical., with the inerts Chimerical 57 and Cab-O-Fair. The technical is taken from the tech hold bin where it is stored after the Wessex Damp End makes it. Screw conveyors take the technical to the primary blender for formulation. The inerts are dumped from bags at the inerts dump 'station on the first floor mezzanine. They are then pumped to the inerts blender on the fourth floor. From the inerts blender, a batch of inerts is released to the inerts weigh feeder. The inerts are charged from the weigh feeder to the primary blender., It is located below the feeder on the third floor. There, the technical and the inerts are blended together.

The formulation is then released to the sifter on the second floor mezzanine. The sifter divides up the materials according to their size. The fine powder goes directly to the secondary blender on the first floor mezzanine. Larger pieces of material are sent to the mill on the second floor for grinding and, then, on to the secondary blender. These two groups of product -materials are blended together inside the secondary blender. A third product size, those pieces too big to use in the final product, are removed from the formulation and are set aside as sifter rejects. The sifter rejects are used in making the liquid formulation.

After the good product is blended in the secondary blender, it is pumped up to a finished product bin. There are two finished product bins. The tops of the bins are visible on the outside balcony of the fourth floor.

Each of the blenders, tanks and bins has its own bin top dust collector. These dust collectors remove formulation materials from the air. The materials are then fed to the main dust collection system, the primary and secondary dust collectors. The material collected is used as rework. Such rework is added in small amounts during the dry formulation process.

The mill, the sifter and the dust collector blower are normally left on all the time. However, the F.O. starting a shift must inspect each of them and be sure they are operating normally. The Operator starting up also checks the amounts of technical and inerts available for formulation. Usually the last Operator on duty will leave enough for the new Operator to start right away.

Whenever there is not enough technical in the tech hold bin, the F.O. must wait until the Damp End has started up. If there is a high level in the tech hold bin, supervision may direct the Operators to pack out pure technical in 50 kilo drums, or the Damp End can be directed to shut down until the F.O. catches up.

SEQUENCERS

The F.O. oversees the formulation process at the F.O. control room on the second floor. (It is also called the "F.O. Shack".) At the control room panel board, the F.O. operates and monitors the two sequencers for the primary and secondary blenders. These sequencers regulate the operation of the two blenders and their related equipment. They control the order in which the formulation steps take place.

The mode of operation for both the primary blender sequencer and the secondary blender sequencer is controlled by one selector switch on the panel board. There are three modes: MANUAL, SEMI-AUTOMATIC and AUTOMATIC. In the MANUAL mode, each of the formulation steps must be manually advanced by the F.O. In the AUTOMATIC mode, the sequencers will start another batch when the previous batch is complete.

The usual mode for the F.O. is SEMI-AUTOMATIC: in SEMI-AUTOMATIC, the Operator starts the sequencers and they complete their steps automatically. When the sequences are completed, the Operator must push the START push button again in order for another batch to begin.

CHARGING INERTS

When additional inerts are dumped at the inerts dump station, they are measured out by the bag. The two types of inerts must be dumped in a certain ratio. The ratio changes every other time the materials are dumped. First, three bags of Chimerical 57 are dumped with three bags of Cab-O-Fair; then, four bags of Chimerical 57 are dumped with three bags of Cab-O-Fair. The F.O. keeps alternating between the two ratios as additional inerts are dumped. (An air hood must be worn during all dumping.)

At the inerts dump station, the Operator fills the inerts fluid flo pump. This is the pump used to take the inerts up to the inerts blender. It can be operated at either the inerts dump station on the first floor mezzanine or at the pump itself on the first floor.

The inerts fluid flo pump has a four-stage cycle: FILL, ACTIVATE, DISCHARGE and PURGE. With the pump set for FILL, the F.O. dumps the inerts. The Operator then pushes the ACTIVATE button. This causes the pressure to rise. The rest of the cycle is completed automatically. 'When the pressure has risen to a certain level, a valve opens, releasing the pressure and inerts and forcing them up to the inerts blender. When the pressure drops, the main discharge valve closes. A special purge valve opens to release the remaining inerts up to the inerts blender. This also clears any remaining inerts from the transfer line to the inerts blender. When the fluid flo pump returns to its FILL stage, it is ready to receive another batch of inerts.

As the inerts blender receives each batch, its agitator comes on and blends the inerts together until a timer cuts it off. (Note: Unlike the other agitators, the inerts blender agitator always operates at the same speed whenever it is running.)

WEIGHT CONTROLS

During the primary blender sequence, technical is charged from the tech hold bin, and inerts are charged from the inerts weigh feeder. The amounts of technical and inerts charged to the primary blender are regulated by digital weight settings on the panel board.

There are two weight settings which control technical charging: the technical fast weight and the technical slow weight. These control the speed at which the technical is charged, as well as the amounts. Most of the technical is charged at a fast speed until the fast weight is reached. Once the fast weight has been satisfied, the last few pounds are charged slowly until the amount of technical added equals the slow weight setting on the panel board. Slow weight settings are used in order to assure an accurate cut-off.

For example, for a typical 1265-pound technical charge the fast weight might read 1255 and the slow weight, 1265. The first 1255 pounds would enter the primary blender at a fast speed. Then, the last ten pounds would charge slowly until the 1265-pound slow weight is satisfied.

Notice that the technical slow weight figure on the panel board is also the weight of the entire batch: the 1265 pounds includes both the fast and slow technical charges. Although only the last ten pounds are actually charged slowly, the slow weight setting shows the cumulative, final weight.

As the technical is charged to the primary blender, the primary blender scale weighs the amounts. The primary blender scale has a red digital read-out on the panel board. When the technical charge is complete, the inerts are charged to the primary blender from the inerts weigh feeder. Earlier in the sequence, before the technical has been charged, the inerts are prepared when the inerts blender releases a batch to the inerts weigh feeder.

An inerts weigh feeder scale registers the amount of material inside the feeder. The scale has a red digital read-out on the panel board. The inerts weigh feeder scale is used to show amounts subtracted from the inerts weigh feeder. For this reason, the read-out is a minus number. For example, after an 85-pound batch has been charged, the scale would read -85 pounds. To refill, the inerts blender would release another 85 pounds to the inerts weigh feeder, and the feeder scale would rise from -85 to 0.

The refilling and charging of the inerts are controlled by three panel board settings: the refill weight, the preset weight and the slow weight.

The refill weight is the cut-off point when the inerts weigh feeder is refilled with another batch. The inerts blender discharge valve releases the inerts batch to the inerts weigh feeder to prepare the feeder for the charge to the primary blender. As the refilling nears completion, the valve will start to close whenever the refill weight is reached on the inerts weigh feeder scale.

As mentioned earlier, in order to prepare an 85-pound batch of inerts, the inerts weigh feeder scale must rise from -85 pounds to 0. However, to assure an accurate cut-off, the inerts blender discharge valve must start to close before the scale goes all the way to 0.

Therefore, for an 85-pound batch, the refill weight might be set for -10. When the scale has risen from -85 to -10, the blender will start to close. While the valve is still closing, the additional 10 pounds needed will fall through to the inerts weigh feeder. This brings the weight of the batch up to the desired weight of 0. (Note: In actual practice the inerts weigh feeder scale may not read exactly 0 at the end of the refilling. However, later in the primary blender sequence, the scale is tared, that is, re-set for 0.)

The two other inerts settings, the preset weight and the slow weight, control the charge from the inerts weigh feeder to the primary blender. Like the technical, the inerts are charged at a fast rate until most of the batch is released; then, the last few pounds are charged slowly. The principal works the same as with the technical charge, but the panel board weight settings are set up somewhat differently.

The preset weight decides the total amount of inerts charged the primary blender. It is the combined weight of both the fast and slow inerts charges. If 85-pound batches are being charged, the preset weight is set for 85 pounds.

The slow weight is set on the panel board for the actual number of pounds which are charged at the slow speed. In the case of an 85-pound batch, the slow weight might be set for 5 pounds. This would cause the first 80 pounds to be charged at the fast speed and the last 5 pounds to be charged at the slow speed.

Notice that, unlike the technical, there is no "fast weight" setting, as such, for the inerts. The inerts panel board settings also differ from the technical in that the slow weight setting is only the amount of the actual slow charge. It is not the cumulative, final weight. (Recall that the slow weight number for the technical is the total of both the fast and slow charges.)

THE POWDER FORMULATION PROCESS

To begin the primary blender sequence, the F.O. selects the mode of operation and sets the sequencers' START-STOP switch to START position. These controls affect the operation of both the primary and secondary sequencers. In the MANUAL mode, the Operator will have to turn on each of the sequencers' steps individually in order to control the formulation process.

In the following summary of the formulation process, it is assumed that the F.O. has chosen either the AUTOMATIC or SEMI-AUTOMATIC mode. It is also assumed that each of the succeeding sequencers' steps are satisfied and that they proceed normally from step to step. However, the Operator must always be aware of the potential for problems and the fact that the sequencers can not advance automatically unless the preceding steps are completed.

As soon as the primary blender sequence begins, the sequencer refills the inerts weigh feeder. The inerts blender discharge valve releases a batch of inerts to the weigh feeder. When the refill weight is reached, the valve starts to close and the weigh feeder scale stops at, or near, 0. Since the technical is charged before the inerts, the inerts batch remains in its feeder until it is time for the inerts charge to the primary blender.

The primary blender agitator starts running at slow speed. Two technical screws charge technical to the primary blender at fast speed until the fast weight is reached on the primary blender scale.

Next, the primary blender agitator stops and the screws feed additional technical at slow speed until the slow weight (the final, cumulative weight of the technical) is reached on the primary blender scale.

During the slow technical charge, it may be necessary for the F.O. to operate the tech hold bin unloader. The tech hold bin is normally kept at a relatively low level, and the unloader is used whenever the bin is nearly empty. The tech hold bin unloader is a rotary arm inside the tech hold bin which assists the removal of technical and keeps the material in the bin at an even level. The F.O. must be sure the unloader is turned off once the slow weight is satisfied.

When the technical charge is completed, the primary blender sequencer prepares for the inerts charge. The inert weigh feeder scale is tared to 0. The primary blender agitator starts running at a slow speed.

The fast inerts charge begins. The inerts weigh feeder charges the inerts at fast speed until the amount of inerts charged is the difference between the preset weight (the total weight of the inerts charge) and the slow weight (the amount to be charged at slow speed). For an 85-pound inerts charge (with a preset weight of 85 pounds and a slow weight of 5 pounds) the fast charge ends when 80 pounds of inerts have been charged to the primary blender.

With the fast charge completed, the inerts weigh feeder switches to slow speed and adds the slow weight to the primary blender.

At this point, the primary blender scale should show the total of the combined technical and inerts charges. The inerts weigh feeder scale will show the amount of inerts subtracted from the inerts weigh feeder (In out example, -85 pounds).

As the primary blender sequencer advances, the primary blender agitator switches from slow to fast speed. This is to assure that the technical and premix materials are thoroughly blended together. When a timer times out, the fast blend ends, and the agitator changes to slow speed.

The primary blender sequencer checks the secondary blender sequencer and makes sure the secondary blender is ready to receive another batch. When the secondary blender is ready (in its HOME position), the primary blender bottom valve opens, and its rotary valve starts turning. The primary blender agitator continues at slow speed.

This releases the materials to the sifter. The bottom valve allows the formulation to drop from the-primary blender. Then the materials pass through the rotary valve.

The primary blender rotary valve controls the overall rate of the formulation process. This is because it regulates how fast the materials can be discharged from the primary blender. It is contained inside the chute between the primary blender bottom valve and the sifter. The rotary valve turns around much like a steamboat paddle. The formulation can only pass through the valve when it is turning. It scoops up the materials arriving from the blender and drops them on the other side toward the sifter. The speed at which the valve turns can be adjusted by the Operator when directed by supervision.

Inside the sifter are two sifter screens which divide up the materials. The sifting is helped by a two-horsepower motor which shakes the screens, and by a number of bouncing balls kept between the first and second screens.

The first screen stops the sifter rejects. These pieces of material are too big to use in the powder formulation. They drop down a chute into a sifter rejects drum kept by the mill on the second floor.

The second sifter screen holds back the good product which made it through the first screen but can not pass the second screen. These materials are dropped down a chute to the mill. Inside the mill they are ground down to product specifications. The mill contains a rotor with a large number of blades. A mill screen holds the product inside the mill until it is ground down to the right size. After grinding, the materials drop into the secondary blender.

Meanwhile, the fine pieces of product--those that manage to pass through both sifter screens--go directly into the secondary blender.

It takes about thirty minutes for the formulation to pass from the primary blender to the secondary blender. Inside the secondary blender, the two types of good product--those materials which passed straight through the sifter and those materials which had to be run through the mill--are blended together to become final product. As this happens, the secondary blender sequencer stays in its HOME step 0 while its agitator runs at slow speed.

When the primary blender has released all its materials, its bottom valve closes, and its rotary valve stops turning. The primary blender scale is tared to 0, and the secondary blender sequencer starts to advance. A batch counter on the panel board advances by one digit.

As the secondary blender sequence gets underway, the secondary blender agitator runs at fast speed for about two minutes. When a timer times out, the agitator switches back to slow speed.

The secondary blender releases the materials to the finished product fluid flo pump. Like the inerts fluid flo pump, the product fluid flo pump has a four stage cycle of FILL, ACTIVATE, DISCHARGE and PURGE. The product fluid flo pump has a fill valve that must be open in order to receive materials from the secondary blender. Assuming this is the case, the secondary blender agitator returns to its fast speed.

Located between the secondary blender and the fluid flo pump fill valve is the secondary blender bottom valve. This valve opens and releases the product from the secondary blender to the fluid flo pump. When the secondary blender scale drops to 0, the fluid flo pump fill valve closes, but the bottom valve remains open.

The pump completes the cycle bringing the batch to a finished product bin.

There are two finished product bins. They are located on and below the outside of the fourth floor. Each of the bins holds about 15 batches. A selector switch in the F.O. shack decides the bin used.

When the product fluid flo pump is empty (after the PURGE stage), the pump fill valve opens and the secondary blender bottom valve closes.

The preceding discussion is intended to give the trainee an overview of what is happening during the formulation process. The F.O. must also learn the specific step numbers for the primary and secondary sequences and what happens during each step number. These steps are listed and described in the two sections that follow.

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