Patent ID: 7469717

Claim:
A fail-safe shutter for use with a cross-stack monitor capable of monitoring the opacity of stack gases, comprising: (i) a body member having an aperture through which, in use, an associated optical monitoring instrument is adapted to sight the stack gases to be monitored; (ii) a resilient gasket surrounding said aperture; (iii) a linearly movable gate having opposed side edges, a leading end and a trailing end, with said side edges loosely engaged by guidance means; (iv) said gate, being linearly moveable between a retracted, open position in which said aperture is exposed, and a closed position in which said aperture is both closed-off, and scaled, by a portion of said gate engaging said resilient gasket to protect components of both said shutter and said optical monitoring instrument from said stack gases; (v) a tail, extending from said trailing end and also having opposed side edges, with one side of said edges being provided with a toothed rack; (vi) means to bias said gate toward said closed position; (vii) an electric motor, a gearbox and a magnetic clutch, with a pinion connected to said magnetic clutch and in mesh with said toothed rack; (viii) a roller opposite said pinion engaging said other side edge of said tail to ensure engagement of said pinion with teeth of said rack; (ix) said clutch, when energised, connecting said gate to said motor via said gearbox, and said motor, when energised, serving to drive said gate via said gearbox and said energised clutch to said open position, with attainment of said open position de-energising said motor only, and with said bias means being insufficient to overcome the mechanical advantage of said gearbox, whereby said gate is retained in said open position until such time as said clutch is de-energised either by a power failure, or by purge gas sensing means detecting absence of purge gas, or by other means; and (x) control circuitry to maintain and/or interrupt and/or return electrical supply to said clutch, and/or to said electric motor.