Dall Tube

      It is a modified version of venturi tube. It produces large differential pressure with low pressure less than the conventional venturi tube.

      It consists of a flanged spool piece body with a short, straight inlet section terminating in an abrupt decrease in diameter or inlet shoulder. This is followed by a conical restriction and a diverting outlet separated by a narrow annular gap. The high pressure tap is a hole drilled through the body tangent to the inlet shoulder. The low pressure tap is drilled through the body so as to connect with an annular slot in the throat.

      It is not suitable for measuring the flow of fluids containing solids which could settle out in the throat slot. The Dall tube is used for water, sewage, air and steam flow measurement. The Dall tubes are normally cast in gun metal. But for 450 mm and larger sizes, high grade cast iron is used. When it is required to protect the tube from corrosion, it may be lithcote lined.

Advantages

1. Low head loss

2. Short lying length

3. It is available in numerous materials of construction.

Disadvantages

1. Pressure difference is sensitive to up-stream disturbances.

2. More straight pipe required in the approach pipe length.

3. It is not considered for measuring flow of hot feed water.