Bridging occurs when stationary particles in the system trap more particles to form a bridge across the face of the channel. This rapidly leads to a complete blockage. The primary means for preventing bridging relies on adequate channel sizing. A channel with a diameter which is 10-20 times greater than the particle diameter is not susceptible to bridging. This minimum diameter rule applies to all line elements including valves, pumps and instruments. Small pumps, flow control valves and reduced bore instruments such as mass flow meters are generally unsuitable where solids are present.