The present invention relates to a method for measuring particle size distribution of a flowing material that comprises solid particles in a carrier fluid, which may be liquid and/or gas.
Finnish patent application No. 903351 discloses a method for measuring particle size distribution of a bulk material, employing a particle size detector that comprises a stationary measurement member and a movable measurement member that is displaced cyclically back and forth in relation to the stationary measurement member. As the movable measurement member moves toward the stationary measurement member, a sample of the flowing material is captured between the measurement members and interferes with movement of the movable measurement member, and the minimum distance of the movable measurement member from the stationary measurement member is representative of the size of the particles in the sample. When the movable measurement member is displaced away from the stationary measurement member, the sample is no longer trapped between the measurement members and the bulk material flows between the measurement members. During the next cycle of movement, the movement of the movable measurement member toward the stationary measurement member might be interrupted by a sample containing particles that are larger or smaller than those of the sample which interrupted its movement on the previous cycle. By recording the minimum distance of the movable measurement member from the stationary measurement member on each cycle of movement, information regarding the particle size distribution in the bulk material is obtained.
From time to time it is necessary to wash the particle size detector, particularly the measurement members, in order to remove residues of the bulk material that remain on the measuring members and affect subsequent measurements. For this purpose, the particle size detector is located in a measuring channel and a by-pass channel is connected in parallel with the measuring channel. A bulk material feeding pipe is connected to a deflectable nozzle for guiding the bulk material flow either to the measuring channel or to the by-pass channel. The nozzle directs the flow of the bulk material under measurement to the measuring channel when the particle size distribution is being measured and to the by-pass channel when the measuring channel is being washed.
A control chamber is divided into two compartments by means of a displaceable actuator provided in the control chamber, and the displaceable actuator is connected to the deflectable nozzle by means of a spring-loaded coupling rod. One of the compartments of the chamber is connected to a washing water feed pipe for feeding washing water to the measuring channel. When the particle size detector is to be washed, a valve that connects the washing water feed pipe to a supply of water under pressure is opened. The pressure of the water supply is communicated to the control chamber, so that the displaceable actuator and the coupling rod are displaced against the resistance of the spring and the nozzle is deflected. The bulk material flow is directed to the by-pass channel instead of the measuring channel. Further, washing water is supplied to the measuring channel for cleaning the particle size detector. In order to interrupt the washing of the particle size detector, the valve of the washing water feed pipe is closed. The spring returns the displaceable actuator to its previous position and the nozzle again directs the bulk material flow to the measuring channel.
According to Finnish patent application No. 903351, the same particle size detector is used for both coarse and fine particles, which means that the detector is easily susceptible to mechanical wear. Likewise, the measuring accuracy of the particle size detector is reduced by the fact that the same detector is used for materials with different solid contents.