PIPE MANUFACTURE

The process of pipe manufacture at consists of a number of operations in sequence;
1. Fibre debagging,
2. Fibre preparation,
3. Asbestos cement slurry preparation,
4. Pipe forming,
5. Pipe curing,
6. Finishing and
7. Disposing of solid waste matter and effluent water.

Fibre debagging

Bagged fibre is transported to the fibre preparation unit, the Kollergang, by forklift trucks from the warehouse or the storage area just outside the fibre preparation unit. Different fibre grades are used for pipe manufacture in preset quantities per beater [1].

Debagging of the fibre and transfer to the fiberizing unit, the Kollergang, is done manually. At the entry to the discharge port of the Kollergang the bag is cut open and the fibre cake discharged into the Tumble Breaker (shredder) to separate the fibres. A suction tube connected to a vacuum pump is provided at the entry point to the Tumble Breaker to suck away chrysotile fibre strands so that they do not become air borne polluting the air. In addition the operator should wear a protective nose mask to avoid inhaling the asbestos fibres.
The plastic bag after debagging is deposited into a disposal bag for subsequent safe disposal in an approved landfill site.

Fibre Preparation

From the Tumble Breaker the fibres are introduced to the Milling machine. The purpose here is to open up the fibres and separate them. This increases their ability to act as reinforcing agents for cement. Milling of the individual fibre grades follows an automatically controlled and timed sequence.

From the Miller, the fibre is sent to the Hydropulper which works on the wetted fibre on a slow speed for 240 seconds and then on a faster speed for another 240 seconds before ejecting the churned wet fibre onto the Holding Tank. In the holding tank the wet fibre is continuously agitated to ensure continuous uniform density of the wetted fibre. Continuous agitation in the holding tank stops if fibre is not called for at least 2 hrs.

With the exception of fibre debagging the whole Kollergang fibre preparation unit is automated.

Asbestos-Cement Pipe Formation

The conventional Pipe Mazza machine is used for pipe manufacture. It consists of a series of two tanks called vats, each fitted with a single sieve cylinder and each filled with the asbestos-cement slurry prepared during the continuous process stage in the pipe plant.

As the sieve cylinders rotate in the slurry, a thin layer of wet asbestos-cement is continuously screened from the slurry and transferred to the machine felt (a rotating closed belt) to form a thin lamina. Pressing on the sieve cylinders and the felt are couch rollers, which serve not only as felt carriers but also to drain water from the asbestos-cement lamina.

The lamina is carried by the felt to a mandrel onto which it is continuously wound. When the layers of the lamina being wound around the mandrel reach the required thickness, the machine is manually stopped and the layer is cut parallel to the axis of the mandrel to remain with the wet pipe on the mandrel.

Upon removal of this pipe from the machine, a new mandrel is inserted, the machine then restarted and repeats the process. Different diameter mandrels are used to produce different diameter pipes as well as pipe joints.

The continuous process stage in slurry preparation up to pipe forming in the pipe plant consists of these processing workstations in sequence.

Continuous Process Slurry Preparation

This is done in the following processing and holding workstations.

i) Hydropulpers

These are the two slurry processing tanks in which a beater of slurry is made per given time through the appropriate mixing of fibre from the Kollergang, cement from the silos and water from the cones. The mix attendant is responsible for this workstation.

4000 liters of water are discharged into the hydropulper from the cone with the hydropulper agitator running at a low speed. 132.8kg or 76.8kg for pressure or sewer pipes respectively of fibre are then called into the hydropulper from the Kollergang with the agitator still on. 800kg of cement are then drawn from the cement silo into the hydropulper to complete raw material deposition. The agitator is then run on fast speed for 3 min before being switched to slow and mix transferred into the Stuff Chest.

ii) Stuff Chest

This is virtually a storage tank, which practically saves as the intermediary between initial slurry preparation in the Hydropulpers and mix dilution to get the required density in Mix Box. The Stuff Chest has a capacity enough to carry 3 beaters but for practical purposes it is used just to a maximum of 2 beaters at any given time, with a sensor, which detects the 2-beater level tripping the motors, which pump slurry from the Hydropulpers when the 2-beater level is reached.

Two pumps are available to pump the slurry into the Stuff Chest each from one of the two Hydropulpers. An additional inlet comes from the Wet Waste Digester, which pumps its aqueous recycled slurry into the stuff chest.

iii) Mix / Dilution Box

This is an Automatic Mix Box, which has 1 slurry inlet pipe from the Stuff Chest, 3 water inlets - two of which are from each of the two Settling Cones and the other carrying backwater from the Vats and felt cleaning sprays. Slurry dilution in the Mix Box is automatic and occurs according to the control parameters set and inputted from the control panel by the pipe forming machine operator as to how the density of the mix should be. These control parameters control the valves, which regulate the inputting of the slurry from the stuff chest.

Mix from the Mix Box is pumped to two distribution boxes one for each of the two vats. Each of the distribution boxes empties to its Vat at 4 outlets to evenly distribute the mix across the Vat.

Pipe forming Mazza Machine

This machine consists of a series of two Vats, each fitted with a rotating sieve cylinder and filled with the asbestos-cement slurry mix pumped from the Mix Box and distributed into the Vats through the Distribution Boxes. As the sieve cylinder rotates in the slurry, a thin lamina of asbestos-cement is continuously screened from the slurry through perforation of water through the sieve and this lamina transferred to the rotating bottom felt of the machine.

Dewatering of the lamina on the machine is achieved through Couch Roller press - nipping the lamina and felt between itself and the Sieve Cylinder and through vacuum pumps, which suck water from the lamina.

Sieve Cylinder removes 50% of the water at pick-up, 5% removed through Couch Roller press nipping, 15% removed by vacuum boxes, 10% by anvil roller, with the remainder going to the heaters at the curing tunnel.

The continuously rotating felt transfers the lamina onto to a rotating mandrel held on the machine onto which the asbestos - cement lamina is wound to give a pipe of the required thickness. Different diameter mandrels are used to produce different diameter pipes and pipe couplings.

After forming the pipe on the mandrel, both are removed from the machine and replaced by a new mandrel all this being done through a combination of machine and manual effort, and the entire process is repeated.

Two essential stages in this machine in pipe formation are worth noting:

• Lamina Formation
  Formation of the lamina in the initial stages on the sieve cylinder is facilitated by hydrostatic pressure, generated by the differential height between the level of the slurry in the vat, on the outside of the sieve, and the level of the backwater inside the cylinder. For this lamina to be suitable for eventual lamination into the product at the mandrel, it must have a controlled thickness, which is continuously uniform in both the direction of the machine and across the width of the machine. This is fulfilled by having 4 slurry distribution points equally spread across each vat where this introduced slurry is further churned by hog shafts so as to introduce a slurry of uniform density to the sieve.
  
  This churning also fulfills the requirement that the slurry has a uniform distribution of fibre in the cement matrix, with as near random orientation as possible.
  
  The lamina must also be capable of being dewatered, to required levels in stages prior to formation into the product. Filterability of the asbestos-cement – water slurry also plays an important part in the formation of the lamina
• Pipe formation
  Lamina thickness delivered to the pipe roll-up stage affects the production rate. Although it is possible to increase the thickness of the lamina by increasing the solids content at the vat, there is a limit to the overall lamina thickness that can be used. This is primarily due to the undesirable loss of lamination reinforcement that occurs when a reduced number of thicker layers are used to build up the pipe.
  
  In pipe formation, the machine operator works using the Factory Manufacturing Parameters Document, which specifies for each pipe:
  1. Lamina thickness,
  2. A machine beam pressure,
  3. Wall thickness,
  4. Felt length in m/min

Felt length, lamina thickness and wall thickness, are displayed on the control panel so that the operator knows when to stop the machine on achievement of these set manufacturing parameters.

v) Calendering

Pressurized air is forced through the wet pipe to open it hence avoid having it stick onto the mandrel upon drying. The wet pipe still in its mandrel is subsequently rolled to further enhance this separation all-round the pipe and ensure production of a well-rounded pipe.

Non-sticking of the cured is also achieved through coating of the mandrels with unused oil just before infeed into the machine for pipe forming.

Pipe curing

The asbestos cement pipes after calendaring are pre-cured in a curing tunnel which is heated to accelerate the hardening process. To obtain maximum final strength it is necessary for the cement to undergo complete hydration. This is only possible if adequate water from the wetness of the pipe or from another source is in close contact with the cement in the pipe and the pipe is not allowed to dry out throughout all the curing stages in the Curing tunnel, the Curing floors and Curing dams.

i) Curing tunnel

This is where the pipes are pre-cured by introducing them to the curing tunnel whilst they are still wet immediately after forming and calendaring. They stay in the tunnel for 4 hrs under constant rotation every 20 minutes to interchangeably expose all surfaces to the heaters to avoid deformation. Four hours is enough for the pipes to considerably harden but avoid complete drying. Accelerated cooling in the tunnel is satisfactory provided

• The tunnel is maintained with a relative humidity of minimum 55%.
• The temp range in the tunnel is within 39 – 590 0C.
• The semi-cured pipe emerges with residual water content of 5%.

Pipe Extraction

After pre-curing in the curing tunnel pipes are extracted from their mandrels using the Extractor machine, which firmly holds the pipe using pneumatic pressure and pulls out the mandrel to remain with the pipe. The pipes are then transferred through a combination of chain conveyor system. The Extractor can handle a maximum of 3 diameter 75 150mm pipes per given time, 2 diameter 175 to 250mm pipes one time and only one diameter 300 to 900mm per given time.

Dents on the mandrel, caused by a light Jack Hammer, the machine that destructively knocks stuck pipes from the mandrels. Shot-blasting is used to correct this problem and also to roughen mandrel surfaces which have smoothed out due to repeated use, which is an undesirable situation as this does not favour firm holding of lamina onto the mandrel at pipe forming.

ii) Curing floors

After accelerated hardening in the curing tunnel, the pipes go to the curing floors where they stay for at least 2 hrs covered in polythene plastic to avoid accelerated uneven cooling of the pipe which might crack it.

iii) Curing dams

From the floors the pipes are then submerged in water dams for 9-10 days for small diameter pipes of range 50mm to 300mm, and 14 days for larger diameter pipes from diameter 350mm upwards.

Sheeting Production Process

Most chrysotyle manufacturing is based on the Hatschek process. This is a wet process in which the fibre is added to a mixture of cement and water and then moulded, cured and hardened to form the finished product.This mixture can be moulded to various corrugations ie. The endurite, traffor tile, curved sheets, pantile, slates and various accessories. - Because it is a wet manufacturing process which prevents chrysotyle from becoming airborne, the application of basic controls can ensure a safe work environment.

DISTRIBUTION PROCESS (1)

Effective distribution is a critical aspect at Turnall to ensure that the product reaches the customer’s destination on time all the time. As part of our customer service excellency, customer delight is guaranteed through on time deliveries. Follow-up checks are also done as after sales service. About 80% of our products are distributed to the end user through stockists while the rest goes to the export market and a small proportion of less than 10% are cash sales.

STOCKISTS

We have about 50 stockists (distributors) although on average 30% of these are usually very active. Stockists are our main debtors and they buy on account or credit having gone through the credit application process. In order to become a stockist , the prospective client has

These stockists enjoy trade discount and settle their accounts on 30 days, currently it is 15 days. Trade terms are reviewed regularly in line with economic conditions.

Some stockists have a branch network countrywide e.g. PG, Redstar, Farm and City and Mashco. PG has over 35 branches evenly spread across the country. Redstar also have a significant branch network.

Other active customers have one or two branches located strategically in main urban centres for instance Union Hardware, SM Lurie, N. Richards, Building Supplies First Seller and Hardware Hyper. Below is a list of some of our top customers (stockists):

• PG (Countrywide)
• Redstar (Countrywide)
• Union Hardware (Harare)
• Supernet (Harare)
• Hardware Hyper (Harare)
• SM Lurie (Masvingo)
• K & P (Harare)
• Building Suppliers (Bulawayo)
• Halsteds (Harare, Bulawayo and Mutare)
• Farm and City (Countrywide, not in Matebeleland)
• UBM (Harare, Midlands, Mutare and Bulawayo)
• First Seller (Harare)
• Enjoy Sales (Masvingo)
• N. Richards (Masvingo)
• Mascho (Manicaland , Mashonaland)

Stockists are well represented in the country’s provinces or our sales territories which are covered and serviced by Sales Representatives namely; Harare East, Harare West, Matebeleland North and South, Masvingo, Midlands, Mashonaland East, Mashonaland West, Mashonaland Central and Manicaland.

LOGISTICS

Product is distributed to stockists by road through transporters (haulage trucks) which carry a normal load of 28t. Transporters engaged to ferry consignment should have a suitable fleet and have sufficient insurance cover for goods in transit.

Since Turnall does not have own fleet, we engage outside transporters. The Distribution Section in the Sales Department is responsible for sourcing and managing transporters.

Some stockists, however, collect using their own trucks or transporters and specify this on their purchase orders.

Customers are responsible for freight charges and whenever we deliver, we charge the freight on the invoice to recover the cost as we in turn pay the transporter.