July/August 2011

ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

A globe valve has a pressure recovery factor of 0.83; a butterfly valve has a pressure recovery of 0.61; a ball valve has a pressure recovery factor of 0.63; and an eccentric plug valve a pressure recovery factor of 0.79.  Which valve will have the least overall pressure drop?

A. Globe

B. Butterfly

C. Ball

D. Eccentric plug

CAP answer

The correct answer is B, butterfly. Pressure recovery factor can be defined as "the increase in fluid static pressure that occurs as fluid moves through a valve from the vena contracta to the valve's outlet and downstream piping." [ISA-RP75.23-1995]

Mathematically, pressure recovery factor can be expressed as: Cf = FL = √[(P1-P2) / (P1-Pvc)]

Where,            

P1 = Upstream Pressure
P2 = Downstream Pressure
Pvc = Pressure, vena contracta

The pressure recovery factor is proportional to the square root of the overall pressure drop (P1-P2), so that the valve with the lowest recovery factor will have the lowest over pressure drop.