WO1988006835A1

WO1988006835A1 - Means in connection with an enclosed electrical apparatus

Info

Publication number: WO1988006835A1
Application number: PCT/NO1988/000017
Authority: WO
Inventors: Jan Grosch
Original assignee: Jan Grosch
Priority date: 1987-03-02
Filing date: 1988-03-02
Publication date: 1988-09-07
Also published as: EP0306510A1; NO870842D0; NO161100C; AU1366888A; NO161100B; NO870842L

Abstract

A means in connection with enclosed electrical apparatus for use in locations where the environmental factors may cause corrosion and there is an additional risk due to the occurrence of inflammable and explosive gas mixtures, comprises a gas conduit (5) integral with the electrical cable or cables (6, 6') connected to the apparatus. The gas conduit (5) has an internal diameter which is less than the quenching diameter of the gas mixtures in question. A pressure relieving nozzle (7) is provided on a free end of the gas conduit (5) extending into the enclosures (8) and may be adapted to function as an explosion-blocking device.

Description

Means in connection with an enclosed electrical apparatus

The present invention concerns means in connection with an enclosed electrical apparatus for use in locations where environmental factors may cause corrosion in the apparatus and there further is an additional risk due to possible occurrences of inflammable and explosive gas mixtures . More particularly the means comprises a gas pipe or gas conduit integrated in the electrical cable/cables which are connecte to the apparatus .

Such locations may for instance be offshore oil and gas installations as well as mines . In such conditions are the electrical apparatus not only exposed to corrosion, but re¬ present in themselves an element of danger as sparking may ignite possible occurring inflammable and explosive gas mixtures . These elements of danger one may try to avoid by enclosing the electrical apparatus, for instance electrical motors, in such a way that the apparatus are protected agains corrosion, and sparking that may lead to ignition of inflammable and explosive gas mixtures in the vicinity of the apparatus is avoided. Additionally the enclosure is made explosion-proof in the sense that it will withstand a sudden and violent increase of pressure inside the enclosure. Such pressure impulses are usually denoted as an explosion in the electrical apparatus, but is not seldom caused by a discharge in the electrical apparatus, which leads to a pressure increase in the surrounding atmosphere .

Regulations and requirements concerning such explosion- and fireproof enclosures for electrical apparatus in imperiled locations are found in CENELEC European Standard (EN) 50014 which discloses general requirements for the construction and testing of electrical apparatus for use in potentially explosive atmospheres . Among others are for instance specifie maximum experimental safe gaps for instance between flanges, for glands and gaskets etc.

Regulations for explosion-proof enclosures for electrical apparatus are further disclosed by EN 50018 wherein specifications are given for flameproof joints, connections and sealing gaskets, for instance for rotating electrical machinery as well as for breathing and draining devices . It is stated that such enclosures may be adapted to withstand an internal test pressure from 3,5 to 15 bar applied over a period between 10 and 60 s. An internal pressure impulse will during this period decay through connections and joints.

It is thus not possible to make an enclosure of this type completely tight. This may however, lead to corrosive media penetrating into the enclosure and causing corrosion of the electrical apparatus. In order to avoid this the enclosure is furnished with an internal protective atmosphere, for instance in form of dry air or an inert gas such as nitrogen. The protective gas may be delivered from a central location by a conduit to each separate enclosure. Instances of this are disclosed by DE-OS 3113000 and DE-PS 3126602. By further applying an overpressure to the protective gas such that the atmosphere inside the enclosure is at a higher pressure than the external atmospheric pressure, it is further protected against corrosive media such as for instance water, vapour and gas penetrating into the enclosure . Regulations for such pressurized enclosures are disclosed by EN 50016 where it is specified that the overpressure should be at least 0,5 mbar (50Pa) relative to the external pressure at every point within the enclosure and its associated ducts or joints where a leakage can occur. Further it is required that a protective gas used for purging and pressurization of the enclosure shall be non-flammable. As mentioned it is common to use dry air or an inert gas such as nitrogen.

As described above it is not possible to make explosion-proof enclosures completely tight and hence the protective gas leaks out. In order to maintain the minimum pressure there must hence be continously supplied gas via gas pipes or gas conduits adapted to that purpose. Preferably such gas conduits may be provided integral with the electrical cable/cables which lead to the enclosure of the electrical apparatus. An example of this is disclosed by DE-PS 3438522.

Due to an accident the gas conduit may be ruptured or broken in one or more places and explosive and inflammable gas mixtures may penetrate into the conduit and enter the enclosure where the protective gas after such an accident quickly will leak out. The incoming gas mixture may then be ignited by a spark from the electrical apparatus in the enclosure and the explosion or fire may propagate back via th gas conduit and ignite possible volumes of dangerous gas mixtures at the point of rupture . Further may a gas fire or gas explosion at the point of rupture propagate into the enclosure and cause great damage. A possibility of preventing this may be obtained by providing a pressure and temperature sensitive sensors at a regular intervals in the gas conduit i order to actuate connected valves in the gas conduits closing off the parts of the conduits being damaged and thus prevent the further propagation of the fire and/or the explosion. This solution is however very equipment-demanding and expensive, as the gas conduit system in a location of the above-mentioned type, with a number of enclosed electrical apparatus, can be very extensive.

The purpose of the present invention is thus to provide a solution to these problems . This is achieved with means according to the invention by giving the gas pipes or gas conduits an internal diameter which is less than the quenching diameter of the said gas mixture/mixtures, a free end of the gas pipe or gas conduit extending into the enclosure being provided with an explosion-blocking device. Further^■ features of the means according to the invention are disclosed by the appended dependent claims .

By quenching diameter is here understood a characteristic of a gas mixture, the quenching diameter expressing the minimum diameter of a tube through which a flame in a stationary gas mixture can propagate indefinitely. Under these conditions the flame speed settles down to a value which is approximately equal to the standard burning velocity. The quenching dia ter does not depend on the material in the tube wall, although a strong material of course is necessary. For practical purposes the importance of the quenching diameter is that it represents the upper limit of the aperture diameter in a flame arrester which insures quenching of the flame. Beyond this limit the flame cannot be quenched regardless of the length of the arrester. It is then assumed that the gas mixture is at standard atmospheric pressure and temperature. As gas mixtures in conduits and pipes usually are non-stationary the explosion flames here propagates much faster than the standard burning velocit . In such conditions must the apertures of a local flame arrester in reality be less than the quenching diameter, for instance at most one half of this.

The ef ectiveness of the flame arrester increases with its length. The present invention employs gas- conduits which throughout their length have an internal diameter which is less than, and preferable less than 0,5 times the quenching diamter, which effectively will prevent the propagation of fire even in non-stationary gas mixtures .

Values of the quenching diameter of gas mixtures which typically occur in locations of the above-mentioned type are given in the following table. In this connection one should note that the quenching diameter varies with the mixture ratio of the gas , and the values in the table are minimum values for the different gas mixtures at atmospheric temperature and pressure. Most saturated hydrocarbons and solvent vapours have a quenching diameter in air which are close to that of propane, but faster-burning gases and vapours have smaller quenching diameters T a b l e

Standard burning velocities and quenching diameters of some gas mixtures

Standard burning Quenching diameter

Gas mixture velocity (m/s) (mm)

Methane/air 0,37 3,68

Propane/air 0,46 2,67

Butane/air 0,40 2,79

Hexane/air 0,40 3,05

Ethylene/air 0,70 1,91

+

Town gas/air 1,13* 2,03

Acetylene/air 1,77 0,79

Hydrogen/air 3,35 0,86

Propane/oxygen 3,96 0,38

Acetylene/oxygen 11,28 0,13

Hydrogen/oxygen 11,89 0,30

*Town gas containing 63% H- (Source: Health and Safety Town gas containing 51% H_ Executive. Flame arresters and explosion reliefs. HS(G)1 London 1980, HMSO)

The quenching diameter must not be confused with the maximum experimental safe gap as these are specified by EN 50014 and for practically purposes disclosed in EN 50018, subsection 4, table 1-4.

It will be understood that the internal diameter of the gas conduits must be adapted to the quenching diameters of the gas mixtures which may occur in the gas conduit's environment

As mentioned above may explosive and inflammable gas mixtures penetrate the enclosures if the pressurized protective gas di appears. An internal explosion is then a normal event. Also due to discharges, for instance because of isolation faults i the electrical apparatus, a violent pressure increase may tak place inside the enclosure. The enclosures are according to the present regulations constructed for such events. However, in order to prevent that such an internal pressure impulse propagates from the enclosure and into the gas conduit there is provided a pressure-relieving nozzle on the end of the gas conduit inside the enclosure.

An example, as well as a few applications of the means according to the invention will be described in more detail below with reference to the accompanying drawing.

Fig. 1 shows schematically the conduit layout for a single circuit.

Fig. 2 shows the entry of the gas conduit into an enclosure.

Figs. 3, 4 and 5 show various embodiments of a pressure re¬ lieving nozzle.

In Fig. 1 an air compressor is designated 1 and 2 designates a container for gas which via valves 3 or 4 are conveyed through the gas conduit 5. The gas conduit 5 is integral with the electrical cable 6 and ends in an explosion-blocking device 7 in the enclosure 8. The explosion blocking device 7 may for instance be a pressure relieving nozzle.- The gas conduit 5 continues through the cable 6 ' which leads from the enclosure 8 to the following enclosure 8 ' shown to the right in Fig. 1.

In Fig. 2 there is shown in greater detail how the cable 6, 6' is led in and out of the enclosure 8, the free ends of the gas conduits 5 from the incoming and outgoing cable 6, 6' here respectively being connected to each other via a con¬ nector piece 10. The nozzle 7 may be a separate piece of tubing with suitable length and diameter. The electrical wires in the cable 6, 6' are designated by 9.

Fig. 3 shows an explosion blocking embodiment of the nozzle 7, wherein the diameter of the bore m is less than the internal diameter of the gas conduit 5, while the lenght 1 c be 6 mm or more.

In Fig. 4 there is shown a different embodiment of the nozzle 7 with a length of 6 mm or more and a substantially larger bore, but the nozzle aperture is reduced by inserting e.g. instance a metal wire 11 which gives the nozzle the correct flow section. A further embodiment of the nozzle is shown in Fig. 5, where the nozzle proper consists of a very thin piece of tube 12 leading from the gas conduit 5 and into the volume of the enclosure. Other solutions in order to provide a pressure relieving nozzle are also possible, the bore of the nozzle for instance being a screw-like channel. A pressure impulse in the gas inside the enclosure caused by the temperature increase by a possible explosion will then be relieved by the gas flowing through the screw-like aperture of the nozzle. It shall be understood that the embodiment of the explosion-blocking nozzle may be based on the regulations as for instance disclosed by EN50018 which is based on specified maximum experimental safe gaps.

The gas conduit 5 in the cable 6 will under normal circum¬ stances continuously convey gas, e.g. dry air or an inert gas such as nitrogen, to the enclosures in order to maintain the stipulated overpressure in the protective atmosphere of the enclosure. The gas conduit 5 may in special circumstances also be used for conveying fire-extinguishing gas from a central location for extinguishing fire in e.g. an electrical motor mounted in the enclosure. The gas conduit 5 may further be used for conveying a test gas to the enclosure in connection with testing the ability to withstand explosions and fire. In the same way the gas conduit may be used for conveying purging gas to the enclosure. If measuring equipmen are provided in the enclosure, for instance in the form of sensors or detectors , the gas conduit 5 may also be used for conveying test gas to the enclosure for testing the equipment

Thus the use of the means according to the present invention provides a gas conduit which simply and quickly can be fitted with costs being far less than other measures that provide a degree of protection which in reality is far inferior to that which may be obtained by employing the knowledge of the quenching diameter of gas mixtures .

Claims

PATENT CLAIMS 9

1. Means in connection with enclosed electrical apparatus for use in locations where environmental factors may cause corrosion in the apparatus and there further is an additiona risk due to the possible occurrence of inflammable and explosive gas mixtures, the means comprising a gas pipe or g conduit integral to the electrical cable or cables which are connected to the apparatus and where the gas conduit preferably is adapted for conveying gas under pressure, characterized in that the gas pipe or gas conduit has an internal diameter which is less than the quenching diameter of the said gas mixture or mixtures concerned.

2. Means according to claim 1, characterized in that the internal diameter is adapted to be between 0,13 and 3,68 mm depending on the gas mixture in question.

3. Means according to claim 1, characterized in that the gas pipe or gas conduit has an internal diameter which is at most one half the quenching diameter of the said gas mixture or mixtures in question.

4. Means according to claim 1, characterized in that a pressure relieving nozzle is provided on a free end of the gas pipe or gas conduit extending into the enclosure.

5. Means according to claim 4, characterized in that the pressure relieving nozzle is adapted to function as an ex¬ plosion-blocking device.