Orifice permanent head loss

Dali Flow Tube - The advantage is this type of flowmeter is that it has a permanent head loss of only 5 % of the measured pressure differential. This is the lowest pressure drop of all orifice meter designs. Flow ratios as high as 1 10 (e.g., 1.0 to 10 kg/s) can be measured within + 2% of actual flow. Dali flow mbes are available in different materials and diameters up to 1500 mm. 

Power input per unit mass of the system is equal to the rate of energy dissipation per unit mass of the liquid and it is estimated by considering the permanent pressure head loss across the orifice. The rate of energy dissipation due to eddy losses is the product of the head loss and the volumetric flow rate. Frictional pressure drop at downstream of the orifice can be calculated as,... 

For ordinary installations in a process plant the venturi meter has several disadvantages. It occupies considerable space and is expensive. Also, the throat diameter is fixed so that if the flow-rate range is changed considerably, inaccurate pressure differences may result. The orifice meter overcomes these objections but at the price of a much larger permanent head or power loss. 

After passage through the orifice, flow disturbance during retardation causes the dissipation of most of the excess kinetic energy as heat. The permanent loss of head is therefore high, increasing as the ratio of flo/fli falls, ultimately reaching the differential head produced within the meter. 

Prepare recovery. Because of the large friction losses from the eddies generated by the reexpanding jet below the vena contracta, the pressure recovery in an orifice meter is poor. The resulting power loss is one disadvantage of the orifice meter. The fraction of the orifice differential that is permanently lost depends on the value of P, and the relationship between the fractional loss and p is shown in Fig. 8.20. For a value of p of 0.5, the lost head is about 73 percent of the orifice differential. 

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