IES65979B2 - A steam curing apparatus - Google Patents

Abstract

The invention relates to an apparatus for curing plastics pipes and in particular crosslinked polyethylene plastics pipes. The apparatus comprises a steam curing unit the interior of which is lined with heat and condensation resistant panels; a steam generating unit and at least one heat and humidity resistant temperature monitoring device.

Description

A STEAM CURING APPARATUS Field of the Invention The present invention relates to an apparatus for curing plastics pipes, in particular crosslinked polyethylene plastics pipes. Polyethylene has been used in the manufacture of plastic pressure pipes for the last thirty years. The material has developed and been refined over that time to be used for many different purposes. One of the latest developments has been crosslinked polyethylene - a material with enough flexibility, strength and temperature resistance to be capable of carrying both hot and cold water within a building.

After producing the crosslinked polyethylene pipes by an extrusion process, the product then requires to be cured to complete the crosslinking process. - 2 Background of the Invention Conventionally steam curing of pipes has been carried out in small batches in tent-like structures with waterproof/heatproof tarpaulin covers. Small batches of pipes are cured at one time in order to ensure uniformity of temperature and humidity across all the pipes placed in the tent-like structures. The batches typically consist of up to a maximum of 250 kg of pipes.

. The problem with this prior art process is the difficulty in controlling the temperature and/or humidity within the tent. The control of temperature and humidity is an intrinsically important part of effecting the crosslinking process correctly. If the temperature/humidity are too low the crosslinking process only completes itself when the product is in situ which means that the pipe could actually shrink in length after being installed. On the other hand if the pipe was subjected to too much heat/humidity, the pipe would be crosslinked but its shape would be distorted (ovally) to such an extent that it would not be suitable for use as pipe. In addition to having these problems the tarpaulin method is not very energy efficient as there is a significant heat loss from the chamber.

Object of the Invention It is an object of the invention to overcome disadvantages of known steam-curing methods and provide a more efficient apparatus which allows an increase in throughput of the steam-curing process, both in terms of shorter curing time and increased batch size.

Summary of the Invention The present invention provides an apparatus for steam-curing 35 materials comprising: (a) a steam curing unit comprising walls, a floor portion, a roof portion and a door, the interior of said steam curing unit being lined with panels, said panels being heat and condensation resistant; - 3 (b) a steam generating unit and; (c) at least one heat and humidity resistant temperature monitoring device.

» Preferably, the lining panels are conventional stainless steel panels commonly used for refrigerated storage. The panels are suitably 4 fitted to the walls, floor, roof and door of the curing unit by means of expansion joints, which may suitably be metal. This prevents buckling of the panels when they expand in the high temperatures of the curing unit.

Preferably, gutter pipes are mounted below the expansion joints.

The gutter pipes may be made of stainless steel. Condensation droplets are most likely to form about a joint where they can be collected by the gutter pipes. A vent may be provided in the floor of the apparatus through which the gutter pipes pass, to allow the condensation droplets to be carried out of the apparatus. This prevents the droplets falling on the pipes being cured.

The steam generating unit preferably comprises a water supply, a plurality of heating elements and a steam pump. The unit may have three heating elements, one primary heating element and two elements which constitute a booster heating system. The booster heating system may be controlled by at least one thermocouple to switch the booster system on if the temperature in the steaming unit falls below a predetermined temperature.

The steam pump pumps steam around the curing unit.

A thermocouple suitable for use in the apparatus is preferably sheathed in stainless steel. Preferably, the thermocouple cables are fitted with an insulating polymer resin. The stainless steel will not corode in the humid conditions and so the readings are accurate and reliable throughout the life of the unit. Conventional thermocouples have a fibre insulator between the two conductors in the sheath which would become wet in the steaming unit and no longer act as an insulator.

The lining panels are preferably fitted to the walls, floor, door - 4 and roof of the steaming unit to define an insulation area therebetween, which may suitably be filled with an insulating material such as polyester fibre insulation.

The curing unit is suitably provided with a plurality of pipes which transport steam from the steam generating unit around the curing unit. The pipes are preferably perforated to allow emission of steam throughout its length.

Preferably, the steam curing unit has a capacity of 5 tonnes per cycle.

Description The apparatus will now be described in greater detail with reference to the accompanying drawings, in which Fig. 1 is a diagramatic representation of a steam-curing apparatus, the steam generation plant and the curing unit.

Fig. 2 is a plan view of the curing unit.

Fig. 3 is a side elevation of the curing unit.

Fig. 4 is a diagramatic representation of a regulatory valve fitted to the curing unit.

The steam-curing apparatus comprises a steaming unit (1) and a curing unit (2) as shown in Fig. 1.

The continuous steam generating unit (1) has three heating elements (3,4,5) which heat water stored in an insulated tank (6). Two of these elements (3,4) are used as booster heaters which operate in conjunction with the third primary heating element (5) while the unit (1) is heating up initially i.e. when a temperature of less than approximately 85°C exists in the steam curing unit (2). When the temperature in the curing unit (2) reaches approximately 85°C two of these three elements (3,4) are automatically switched off while the third element (5) continues to heat the water in order to produce - 5 steam. If the temperature in the curing unit (1) falls below 85°C the other two elements (3,4) are automatically switched back on in order to boost the temperature back up to 85°C.

The curing unit (2) comprises walls (7), a floor portion (8) and a roof portion (9), one of the walls being fitted with doors (10). The <* walls, roof, floor and doors are all manufactured from steel, metal, alloy or other rigid material.

The doors (10) allow access of the product to the curing unit and when these doors (10) are closed the steam curing unit is completely sealed.

The interior of the curing unit (2) is lined with stainless steel lining panels (11). These panels (11) are fitted to the interior of the curing unit (2) so as to define an area (12) between the lining panels (11) and the outer walls (7) which may be filled with a fibre insulation, preferably a polyester fibre insulation (approximately 10 cm thick). These lining panels (11) are fitted to the interior of the outer walls (7) so as to provide a continuous surface within the curing unit, while metal expansion joints (13) are fitted between the panels to allow for metal expansion at the high temperatures of the unit.

Stainless steel gutter pipes (14) are mounted under these expansion joints (13) to allow any condensation droplets which are more likely to occur at a joint (13) to gather. The guttering pipes (14) are interconnected and fed to a drain outlet (17) in the floor (8) via a down-pipe (18), rather than falling on to the product being cured in the unit.

The curing unit (2) is further provided with a system of steam outlet pipes (15) which distribute the steam supplied by the steam generating unit (1) uniformly throughout the curing unit (2). These steam outlet pipes (15) are fitted between the sub-floor formed by the lining panels (11) and the main floor (8). Steam outlet pipes may also be provided between the lining panels (11) and the outer walls (7) and/or the outer walls (7) and/or the roof portion (9). Steam outlet pipes may also be provided between the lining panels (11) and the outer walls (7) and/or the outer walls (7) and/or the roof portion (9). The pipes(15) distribute the steam uniformly from this position into the - 6 curing unit (2). The lining panels for the floor (8) is inclined so as to provide a run-off facility for any condensed vapour which may then be drained.

In order to prevent unwanted pressure build up within the curing unit (2) due to the generation of vapours within the unit a pressure release valve (19) is fitted to the unit. This pressure relief valve (19) may be set to automatically open once a certain pressure within the unit is reached and then automatically close once the excess pressure has been released. It may be desirable however to have this relief valve (19) partially open at all times so as to maintain a constant pressure within the unit (2) by releasing a preset amount of pressure relative to the added pressure being generated by the supply of steam from the steam generating unit (1).

The temperature and humidity within the steam curing unit (2) is continuously monitored by a series of thermocouple devices (20).

These devices are positioned so as to measure the air temperature within the curing unit (2). The information supplied by these thermocouples (20) is continuously monitored and if the temperature registered by these thermocouples falls below a set temperature, ideally about 85°C, then the booster heaters (3,4) are switched on so as to heat the temperature within the curing unit (2) back to above 85°C. These thermocouple devices (20) are adapted to withstand the conditions of temperature and humidity within the curing unit while still providing an accurate measurement of the temperature in the unit. The thermocouples (20) therefore should not corrode nor should the insulation used between the two conductors of the thermocouple become a conductor itself when exposed to humid conditions.

Use of Steam Curing Apparatus In use the quantity of plastics to be cured is placed within the unit. The steam curing unit is then heated to approximately the required temperature. Then the quantity of plastics to be cured is then placed within the unit. One method for placing products, particularly high-density, crosslinked, polyethylene pipes is to use a stainless steel trolley or 'bogey* which is loaded outside the unit and then wheeled into place when fully stacked with the product to be - 7 cured. The arrangement of the product on these trolleys and in particular the arrangement of the product within the unit is important. In particular care must be taken to ensure that the steam jets which supply steam from the floor of the unit are in no way impeded and thus a uniform distribution of steam is maintained and to further ensure that the product itself is not distorted in shape by the way in which it is arranged.

The product is then subjected to the required amount of exposure to the environment within the steam-curing unit.

Example Using the apparatus of the present invention it is possible to cure 5 tonne batches of product in three and a half days. The prior art process allows the curing of 250 kg batches in four days. Thus in one year 22.5 tonnes of product could be cured. With the increased efficiency of the apparatus of the present invention even on an average weekly load of 8 tonnes (less than full capacity) 416 tonnes of product can be cured, representing an eighteenfold increase in thoughput. Operating under full capacity the apparatus of the invention allows a 23 fold increase in throughput. tonnes of 6m lengths and 2 tonnes of coils were cured simultaneously. The coils can be either 100m in length for the 15mm diameter size or 50m in length for the 22mm and 28mm diameter sizes.

Normally lengths and coils of the same diameter size are cured together which means that the most effective batch sizes are: Pipe Diameter Batch Size 15mm 22mm 28mm 7500 lengths and 300 coils 4000 lengths and 315 coils 2500 lengths and 200 coils Different combinations may be cured together but this may reduce the efficiency of the unit.

Claims (5)

1. An apparatus for steam curing materials comprising: 5 (a) a steam curing unit comprising walls, a floor portion, a roof portion and a door, the interior of said steam curing unit being lined with panels, said panels being heat and condensation resistant; 10 (b) a steam generating unit and; (c) at least one heat and humidity resistant temperature monitoring device. 15

2. An apparatus according to Claim 1 wherein the lining panels fitted to the interior of the curing unit are provided with expansion joints.

3. An apparatus according to any one of Claims 1 or 2, wherein the steam generating unit comprises a water supply, a plurality of heating 20 elements and a steam pump, preferably the heating elements comprising one primary heating element and two elements which constitute a booster heating system.

4. An apparatus according to any one of Claims 1 to 4 wherein said 25 temperature monitoring device is a thermocouple preferably sheathed in stainless steel.

5. An apparatus for steam curing materials substantially as herein described with reference to any one of Figures 1 to 4 .