Abstract: Changes in the flow-rates of each one of a plurality of fluid streams which pass from respective conduits (13) to a common conduit (12) are monitored by taking a representative sample of fluid from a sampling point (16) of the common conduit, determining the change in composition of at least key components of the sample relative to a preceding sample obtained in the same manner, and computing from the change in composition the composition of the fluid stream whose flow-rate has changed, so that the fluid stream can be identified by reference to a library or database of fluid stream compositions. When the fluid streams are gases (e.g. gases to be flared at an oil refinery or chemical process plant), each sample may be analyzed by a gas chromatograph (18) at predetermined intervals and the fluid stream whose flow-rate has changed may be identified automatically by comparison with a database or library (22) of fluid streams. The thus identified fluid stream may be recorded and/displayed (19).

Abstract: Fluid flow in parallel-connected conduits is regulated by sensing the pressures and/or temperatures at the junctions of the conduits, and regulating the supply of power to a fluid propelling means operable in one of the conduits in accordance with a signal which is representative of the sensed pressure drop and/or temperature drop between the junctions. In a furnace installation having an air preheater (14) in a duct (12) and a parallel-connected by-pass duct (18), the pressure difference and/or temperature difference between the junctions (19, 20) of the ducts (12, 18) is/are employed to generate a signal which regulates the supply of power to an induced draft fan (17) in the air preheater duct (12). The damper (21) normally provided in the by-pass duct (18) may be removed or set at its open position.