GB2243475A - Gas and smoke alarm systems - Google Patents

Abstract

Existing multi-tube aspirated toxic gas and smoke alarm systems have either the ability to detect an occurrence quickly, but not to locate the source, or to locate the source but in the worst case only after an extended time interval with a probability of missing transient events. This invention, based on a novel rotary air selector valve 37, enables rapid detection of an event followed by rapid location of the source and also gives the ability to capture transient events and to distinguish between transient and continuing events. In operation, air from different zones is directed from tubes 22-28 through the valve to a common gas or smoke detector 32. Upon detection of gas or smoke, an alarm is energized and the valve is operated to successively direct air from each tube to the detector so as to enable location of the zone giving rise to the gas or smoke. <IMAGE>

Description

IMi-ROVEDE2iTW TO GAS AN7D SIiiOKE ALAitt: CYSTa A class of gas and smoke alarm systems exists known as aspirated systems, in which air is drawn from a multiplicity of locations, known generally as zones, to a central detector by means of pipes or tubes and an air-pump or fan, the air samples being processed by an analyser system to detect the presence of traces of toxic or dangerous gases or smoke. Two types are currently in widespread use. The first ty2e mixes the flows through the pipes and passes this total flow, or a fraction of it, through the detector, giving an alarm signal when the proportion of unwanted gas or smoke reaches a predetermined level.This ty e has the advantage of giving the earliest possible indication of the presence of the gas or smoke, but is unable to locate its exact position if, as is usual, it originates in only one zone. The second tyie purges and samIles the pipes sequentially in a continuous manner until it discovers the presence of the gas or smoke at alarm level. This type can clearly indicate the exact zone or zones which are affected, but suffers from the disadvantage that, in the worst case, considerable time may elapse before the alarm is given if the sequential sampling has just passed the affected tube prior to the occurrence, in which case it must await the next sampling period.Another serious disadvantage of this second type is that it is insensitive to a transient occurrence, e.g. a puff of smoke which may indicate the start of more serious activity, because by the time the next purge/sampling period arrives the smoke puff may have dispersed and become undetectable by dilution.

It is to overcome these known but accepted defects that the system which is the subject of this application has been devised.

/The The improved detection strategy is based on a novel sampling valve to be used in conjunction with an aEprotriate detector or analyser and which is capable of being switched to different operating modes by a control system to enable measurements to be made in a manner and sequence which circumvents the aforementioned disadvantages of the systems in current use.

The sampling valve consists of a stator to which the sampling pipes are brought and arranged in a circular ring. The outlets are coupled to a rotor so shaped and positioned that any one pipe may be selected and its output conveyed to a detector or the output of all pipes combined and so conveyed, or a combination of these two.

The following is a description, by way of example, of a preferred method of constructing the valve. Fig.l shows an exploded view of the valve. The stator plate 1 has a circular array of stub tubes 2 pressed into holes equally spaced on a pitch circle p;ith its centre round a bearing sleeve 3. The pipes being sampled are connected to the tubes 2. A circular rotor 5 rotates in conjunction with the stator plate 1 with the tube 6 acting as a bearing in the sleeve 3. Sampling hole 7 surrounded by a T ; sealing ring 8 is on the same pitch circle as tubes 2 and when positioned over one of the tubes is connected to it pneumatically by the sealing ring 8.Hole 7 is connected pneumatically to tube 6 by a passage 9 drilled into the connecting pillar 10, which is lowered from the contacting surfaces in order to clear the stator exhaust tubes 4. These tubes have approximately the same combined area as the array of inlet tubes 2 minus one tube, and they are connected to the remaining tubes, other than that selected, by means of the rotor cavity 11. A PTFE sealing ring 12 contained in a groove on the periphery of the rotor prevents the ingress of air into the cavity 11 when suction is in progress. It also rovides a a low-friction face bearing for the rotor and is oposed by another similar ring inserted into the stator. This gives the advantages of low friction and wear and easily replaceable wearing surfaces.The two STFE sealing rings 8 and 12 are backed by elastomer 0ring to give a perfect seal and spring contact and the grooves are sized so that seal 8 makes contact with the stator face slightly before seal 12 so that any slight irregularity on the surface will give preference to seal 8 which is the more important. Any slight leakage iast seal 12 is of no importance as it is being vacuumed to waste. Similarly a leak past seal 8 will not result in contamination of the gas sample but only a slight loss of gas to waste because of the negative pressure in the rotor cavity.

Rotation of the rotor 5 may be effected by means of a gear 13 fixed to its rear surface or by other suitable means. If a gear is used it is convenient to drive this by a motor 14 or motor-gearbox unit on the output shaft of which is mounted a Amnion 15 engaging gear 13. The motor is mounted on a rear stator plate 16 which is bolted parallel to the front stator 1 with spacers 17, the rotor 5 being sandwiched between them.

A shaft 18 fixed to the rotor and gear 13 ' protrudes through a hole in the centre of the rear stator Flate and carries a shaft encoder or switch 19 which has the same number of binary coded positions as there are tube inlets on the stator and enables the rotor to be positioned accurately over each of the inlet tubes by the motor acting as a positioning servo by wellknown means. A compression washer 20 positioned on shaft 18 between the rotor and rear stator keeps the rotor and seal assembly in close face to face contact with the stator 1.

/Sealing Sealing if required between the rotor shaft 6 and bearing/ output tube 3 can be by means of an 0-ring or a face seal similar to 8.

The principal advantages of this concept in comparison with alternative methods using arrangements of discrete valves are: (i) simplicity and small size substantially independent of number of inlets.

(ii) a standard unit of, say, 16 inlets may be used for any number of positions up to 16 by blocking the unused inlets and ignoring their selection without significant cost penalty.

(iii) all unselected inlets are continuously ---urged so that any position may be selected and the analysis performed immediately.

(iv) perfection of sealing of the rotor/Ftator combination is not of prime importance because small leakages flow into the waste vacuum and no contamination of the analysis sample takes place.

(v) wiring and control simplicity. A conventional 16-way selector requires a minimum of 17 wires and 16 relays or triacs and has no means of reporting a malfunction, e.g. a sticking valve. This new conceit requires only 5 wires for full remote control and confirmation of 16 positions.

/(vi) (vi) unlike arrays of solenoid valves in which cost is pro.ortional to size, it is easy and convenient to use larger bore sampling pipes than the usual 6mm.

Increasing the size to l0mm greatly reduces the airflow resistance and enables the use of a centrifugal fan for purging rather than a positive displacement pump with the advantage of lower cost, longer life and lower noise level.

To illustrate the principles involved in the invention the arrangements of the valve and other components in a fire detector system with seven aspirated zones are shown in Fig.2.

The Eelector output 31 of the 'eight-way valve 37 is connected to the detector 32. This detector may be, by way of exam le, of the ionizing or optical types well known in the fire detection industry, which produce an electrical output proportional to the smoke content of the air throughput. The urge outlets of valve 37 correspond to tubes 4 in Fig.l but are shorn for clarity as two tubes 29 and 30 in Fig.2. Outlet 29 is looted back into inlet 21 of the eight inlet tubes. The other seven inlets 22 - 28 are each connected by tubes of aF ro riate length to desired sampling zones where the presence of smoke requires to be detected.Outlet 30 is taken via an electromagnetic normally-open shut-off valve 33 to the inlet of air pump 36 which may be a positive-displacement or centrifugal type depending on the bore of zone sampling tubes used. The outset of the detector 32 is similarly connected to the inlet of pump 36. A relief valve 34, which comprises a shut-off valve similar to 33 but normallyclosed, with an air restrictor 35 on its inlet is interposed between valve 33 and pump 36 so that when 33 is closed to shut off the flow from outlet 30, valve 34 simultaneously opens to /keep keep the flow through the pump, and hence the other pressure and flow conditions in the system, constant. The restrictor 35 is adjusted to do this.

In normal operation the rotor of valve 37 is positioned with its sampling outlet over inlet 21 and valve 33 is olen and the pump draws air through all the sampling tubes 22 - 28. part of the total flow passes through the detector 32 and part is bypassed through valve 33. Immediately smoke is detected in 32 the alarm is given and valves 33 and 34 are activated, closing off the flow from outlet 30. Valve 37 commences sampling the individual inlets 22 - 28, dwelling on each for a period just sufficiently long to allow the detector to clear and redetect the presence of smoke on the individual inlets. In -ractice this takes less than 5 seconds ner zone. During this scanning process there is flow only in the tube which is being sampled through the detector.All the other tubes are static and reserve the conditions at the instant of initial detection and alarm. Thus even a short puff of smoke lasting only 3 seconds can be detected and reliably located.

I;hen the initial scan has been completed and the event(s) in the zone(s) located, valve 33 may be reopened and the scan repeated after a short delay to purge the zone tubes. This will indicate whether the incident(s) were transient or continuing if smoke continues to be detected.

The operation of the system is controlled by a microprocessor- based data acquisition and control system. For the basic system as described the inputs to the control system are: (a) output from shaft encoder 38 indicating valve position (b) output from smoke detector 32 indicating level of smoke content /and and the outputs are : - (c) drive signal to servomotor 39 to achieve the desired position (d) signal to open/close valves 33 and 34 (e) appropriate alarm and zone indication signals The control sequences are software based and function in the manner described in the preceding paragraphs according to well- knoan microprocessor technology.

The system as described may be extended to particle detection and counting as used to control clean environments by using a suitable detector in place of 32.

To give an indication of the level of imrovement, a seven zone system as described avith sampling zone tubes 100 metres long can detect the onset of a smoke puff lasting only three seconds in under 20 seconds, and locate it to one of the seven zones, in the worst case in a further 25 seconds. This contrasts with a similar initial detection time for the existing summed-floal types with no zone location, or in continuously-scanning types, a worst-case alarm and zone location time of three minutes, with a high probability of missing transient events.

Claims (4)

1. CLAIM

   !;hat we claim is:1. A multi-tube aspirated gas or smoke detection and location system in which the normal mode is aspiration through all tubes simultaneously, the flow being channelled wholly or partially through an analyser or detector, the operation of which gives a signal. suitable for an alarm, stows the flow in the tubes and switches the system to sampling the contents of each individual tube in sequence through the detector to locate the zone or zones in which the incident originated.
2. 2. A system as described in claim 1 in which the aspiration in the tubes is restarted after the initial individual sampling/locating scan and after an interval sufficiently long to purge the tubes a further individual sampling scan is carried out to determine whether the incident(s) was/were transient or continuing.
3. 3. A multi-tube aspirated gas or smoke detection and location system as described in claims 1 and 2 in which the flow switching needed is provided by an automated rotary valve as described in the first part of this specification.
4. 4. A gas control valve as described which enables the aspiration to be switched from all the zone tubes simultaneously to individual tubes selectively with the option of purging or aspirating the remaining tubes not so selected.