GB2040011A - Thermally insulated piping systems - Google Patents

Abstract

A piping system for district heating schemes etc. comprises a plurality of service pipes 9 for conveying fluids utilised in the scheme, installed within an outer conduit 1. Thermally and electrically insulating spacers 10 support the service pipes 9 in spaced relation to and within the outer conduit 1. Thermal insulation of the overall system is provided by evacuating the interior of the outer conduit surrounding the service pipes 9. Initial evacuation is performed by an injector 8. Thereafter the vacuum is maintained by a low capacity vacuum pump 4. Thermal insulating material may be applied to the pipes. To prevent corrosion, an inert purging gas is used prior to evacuation and cathodic protection can be applied to the conduit and service pipes. <IMAGE>

Description

SPECIFICATION Thermally insulated piping systems and method Insulated pipe lines are being used for district heating schemes, refinery plant services and crosscountry transmissions of fluids in above or below ground situations where thermally insulated piping systems and low heat losses are essential.

The satisfactory insulation of buried piping systems is in practice a problem. If a pipe carrying fluids below ground level needs to be thermally insulated, it is essential to prevent the penetration of ground water and harmful chemical or biological products to the insulation and to the pipe, and a vapour barrier if the fluid carried is cold e.g. liquid nitrogen.

Presently known factory made pre-insulated pipework systems fall into two main categories.

The first category for high temperature insulation (temperatures of the order of plus or minus 1300 to 3000C difference from the ambient) commonly comprise a steel service pipe thermally insulated with calcium silicate, mineral wool or other insulating materials and installed within a steel outer casing such that there is an air gap between the service pipe and its insulation and the outer casing. The disadvantages of these systems are that the systems have to be inspected, vented and drained regularly and it has been found it is difficult to ensure the insulation remains dry because condensation forms and water gathers at low points creating corrosion problems which lead to failure of the system. Heat losses tend to be high.

The second category of systems for low temperature insulation (temperatu res of the order of minus 2000C to plus 1 500C difference from the ambient) commonly include a metal or g.r.p. service pipe surrounded by polyurethane insulation within an outer case of polyethylene or PVC. The majority of the known systems in this category do not have an annular gap between the insulation and the outer case. It has been found that these systems are liable to fail due to the ingress of ground water which causes a rapid breakdown of the insulation followed by corrosion of the service pipe.

Many locations where it is desired to install underground insulated piping systems in the United Kingdom and other temperate countries (where water tables are high) pose considerable difficulty in excluding ingress of moisture to the insulation. A fault in an underground pipe or insulation system is usually difficult to detect and will normally go unnoticed until failure of the insulation and then the pipe occurs. Partial failure due to damp insulation results in continuous and expensive wastage of fuel and other severe penalties.

An object of the present invention is to provide an improved thermally insulated piping system which alleviates the problems mentioned above.

Afurtherobjectofthe invention is to provide a method of thermally insulating a piping system for conveying fluids.

The present invention provides a thermally insulated piping system which comprises at least one service pipe supported by thermally and electrically insulating spacers in spaced relation to and within an outer conduit, and means for evacuating the interior of the outer conduit surrounding the service pipe or pipes.

Preferably the spacers are designed to allow a flow of air within the interior of the conduit longitudinally of the spacers and also to allow lateral and longitudinal movement of the service pipe or pipes relative to each other and to the outer conduit due, for example, to expansion or contraction. Upon evacuating the interior of the outer conduit, the vacuum produced acts as a highly efficient thermal barrier to the transmission of heat between the service pipe or pipes and the outer conduit. A thermally insulating material can, if desired, be applied around the service pipes, provided that an evacuable space is maintained between the insulation and the outer conduit.

Where continuity of the service pipe-work is broken by valve chambers or other connections the outer conduit is preferably sealed to the service pipe or pipes by a flexible connecting means which is of low thermal conductivity and is electrically insulating, and the sections of the outer conduits surrounding the discontinuity are interconnected by small bore pipe-work to allow their interior volumes to communicate.

The present invention also provides a method of thermally insulating a piping system for conveying fluids and which comprises at least one service pipe supported by thermally and electrically insulating spacers in spaced relation to and within an outer conduit, said method comprising evacuating the interior of the outer conduit surrounding the service pipe or pipes.

Installation of a thermally insulated piping system according to the invention proceeds by running and connecting up the service pipes as required and connecting the outer conduit to the evacuating means. The evacuating means may include an injector and a vacuum pump. When the piping system is evacuated initially from ambient pressure, a comparatively large amount of gas has to be removed. An injector is a suitable device for this purpose. The evacuation is therefore preferably carried out in two stages, namely by first connecting the injector to the interior of the conduit to evacuate the conduit to a predetermined vacuum, and by thereafter disconnecting the injector and connecting a vacuum pump to the interior of the conduit.A small capacity vacuum pump is sufficient to maintain the vacuum in the piping system under normal conditions, and is economical both as to capital and running costs.

To prevent corrosion of a steel outer conduit and service pipes the conduit is preferably purged with an inert gas before the interior of the conduit is evacuated. A vacuum gauge which may incorporate an automatic failure device which gives an alarm if the vacuum falls at any time to below a predetermined level may be associated with the evacuating means.

An embodiment of the present invention, and the manner in which it may be performed, will now be described in detail by way of example and with reference to the accompanying drawing which is a longitudinal sectional view through an end section of a run of thermally insulated piping according to the invention.

Referring to the drawing, the system shown comprises a steel outer conduit 1 the interior of which is connected through piping 2 to a differential vacuum gauge 3 and a vacuum pump 4. A valve 5 in the piping 2 enables the vacuum pump 4 to be isolated from the interior of the conduit 1. The piping 2 also has a tee-junction 6 to which is connected a valve 7 and an injector 8.

One or more service pipes 9 run longitudinally within the outer conduit 1 and are supported therein at longitudinally spaced intervals by thermally and electrically insulating spacers 10. To allow for expansion of the service pipes 9 relative to the conduit 1, the spacers 10 are designed to allow the service pipes 9 lateral and longitudinal movement required to accommodate the anticipated expansion and contraction in service, and are also designed so that they do not seal off the interior of the conduit 1 into separate compartments but allow a flow of air longitudinally within the conduit 1 past the spacers 10. Athermally insulating material such as calcium silicate or mineral wool (not shown) can be applied to the exterior of the service pipes 9 without interfering with the flow of air longitudinally within the conduit 1.

Where access to the service pipes 4 is required, e.g. within a boiler house or at a service point, the outer conduit 1 is terminated and the end thereof is sealed to the service pipes 4 in a thermally and electrically insulating manner. As shown at the left-hand side of the drawing, such sealing may be performed by connecting a bellows 11 between each service pipe 9 and the casing, the bellows being made of a thermally and electrically insulating material such as polytetrafluoroethylene. The bellows 11 serve the dual purpose of sealing the interior of the conduit 1 and allowing for service pipe expansion and contraction.When the piping system continues after such a discontinuity, the respective outer conduits 1 are preferably interconnected by small bore piping 12 containing an isolating valve 13 which allows each section of the piping system to be isolated fortesting, and a drain valve 14.

The injector 8 is used for initial evacuation of the system from ambient pressure when a comparatively large amount of air has to be removed. To carry out the initial evacuation, the valves 13 and 7 are opened and valves 5 and 14 closed. The injector 8 is then started up (it can be driven by a portable compressor not shown) and the interior of the conduit evacuated through the injector until the gauge 3 shows a predetermined level of vacuum is attained. Then the vacuum pump 4 is started up, valve 5 is opened and valve 7 closed and the injector 8 stopped. An injector can pull a vacuum of 25 cm of mercury. The vacuum created by pump 3 maintains a satisfactory degree of insulation of the service pipes 4 within the conduit 1, and need be only large enough for this purpose, thus economising in cost.

Such a vacuum pump is much smaller than the pump that would be needed if the pump were to be used for the initial evacuation. As mentioned above, to prevent corrosion of the steel conduit 1 its interior is preferably purged with an inert gas prior to evacuation. The purging step can if desired be repeated from time-to-time.

An alarm system may be incorporated in the vacuum gauge 3 to give warning of a system failure should the vacuum at any time fall below a predetermined level. If such a failure occurs due to a system leak, the compressor can be brought back, and connected through the injector to pressurise the interior of the conduit to show up the leak. After the leak has been repaired, the system is then reevacuated.

Cathodic protection can be applied to the conduit and service pipes.

The invention can be applied to existing piping systems having an air gap between the service pipe(s) and the outer casing.

Claims (15)

1. A thermally insulated piping system which comprises at least one service pipe supported by thermally and electrically insulating spacers in spaced relation to and within an outer conduit, and means for evacuating the interior of the outer conduit surrounding the service pipe or pipes.

2. A system according to Claim 1, wherein the spacers allow a longitudinal flow of gas within the conduit.

3. A system according to Claim 1 or Claim 2, wherein the spacers are such as to permit lateral and longitudinal movement of the service pipe or pipes relative to each other and to the outer conduit.

4. A system according to any one of Claims 1 to 3, wherein the evacuating means comprises an injector for initial evacuation of the interior of the conduit to a predetermined vacuum pressure, and a vacuum pump for maintaining a vacuum in the interior of the conduit.

5. A system according to Claim 4, wherein the evacuating means includes a vacuum gauge.

6. A system according to Claim 5, wherein the vacuum gauge incorporates means for giving an alarm if the vacuum falls below a predetermined level.

7. A system according to any one of Claims 1 to 5, wherein the or each service pipe is connected to the outer conduit at the ends thereof by flexible connecting means which is of low thermal conductivity and is electrically insulating.

8. A system according to Claim 7, wherein the flexible connecting means is a bellows.

9. A system according to Claim 8, wherein the bellows is polytetrafluoroethylene.

10. A system according to any one of Claims 7 to 9, which system includes a plurality of discrete sections of outer conduit and the interiors of said sections are interconnected by piping including an isolating valve and a drain valve.

11. A method of thermally insulating a piping system for conveying fluids and which comprises at least one service pipe supported by thermally and electrically insulating spacers in spaced relation to and within an outer conduit, said method comprising evacuating the interior of the outer conduit surrounding the service pipe or pipes.

12. A method according to Claim 11, wherein the evacuation of the interior of the outer conduit from ambient pressure is performed in two stages, name ly by connecting thereto an injector to evacuate the interior to a predetermined vacuum, and by thereafter disconnecting the injector and connecting a vacuum pump to the interior of the conduit.

13. A method according to Claim 11 or Claim 12, wherein the outer conduit is purged with an inert gas before evacuation is commenced.

14. Athermally insulated piping system substantially as hereinbefore described with reference to the accompanying drawings.

15. A method of thermally insulating a piping system for conveying fluids, according to Claim 11 and substantially as hereinbefore described.