Nonrecoverable pressure drop

The pressure drop calculated earlier does not represent the correct value. Downstream of the orifice/Venturi, the velocity gradually decreases, and some of the velocity head recovers. The actual pressure drop is the nonrecoverable pressure drop and is less than the pressure drop calculated earlier. The nonrecoverable pressure drop is calculated as  

---

A liquid is flowing through a 6-mm-diameter orifice with flange taps in a 50 NB sch. 40 pipe. The flow parameters are presented as follows. Estimate the maximum nonrecoverable pressure drop. 

Differential pressure = 848.5 kPa (from Equation 2.12) Nonrecoverable pressure drop = 835.4 kPa (from Equation 2.20)... 

Normally, the blowdown of the valve is set at 7 to 10% of the set pressure (except in the pilot-operated valve, in which it can be set as low as 3%), and based on the normal setting of blowdown, the mechanical design code restricts the inlet line nonrecoverable pressure drop to a maximum of 3%. The estimated pressure drop is only the frictional pressure drop and excludes any drop or gain due to change in velocity head. This means the velocity head chmge due to the reducer installed at the PRV inlet should be ignored in estimating the pressure drop. 

The flow contingency shall be the maximum contingency flow through the valve. This is the maximum capacity of the relief valve. For convenhonal and balanced-bellows PRVs, the nonrecoverable pressure drop shall not exceed 3% of the PRV set pressure. 

Nonrecoverable (actual) pressure drop, kPa Ap i Acceleration head, kPa... 

---