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September/October 2011

ISA Certified Control Systems Technician (CCST) Program

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

A fluid is flowing through a 10-inch diameter pipe at a velocity of 6 feet/sec. When the pipe reduces to an 8-inch diameter, and all other flowing parameters remain the same, the fluid velocity becomes ________ feet/sec.

A. 2.550

B. 6.075

C. 9.375

D. 12.75

CCST answer

The “constant” between the run of pipe that has a 10-inch diameter and the run of pipe with a diameter of 8 inches is that the flow rate (and other fluid properties) is the same in both pipe lengths.

Flow through a round pipe can be expressed as:

Q = Velocity (ft/sec) x Area of Pipe (ft²)

Since flow is constant between the two pipe sizes, we can set:

Velocity₁  x  Area₁  =  Velocity₂  x  Area₂ ,

where the subscript = 1 for the initial conditions (10-inch pipe) and subscript = 2 for the final conditions (8-inch pipe).

Solving for Velocity₂:

Velocity₂   =  Velocity₁  x  Area₁ / Area₂

For the 10-inch pipe, Area (in ft²) = pi x D² / 4 = 0.5454 ft².  Don’t forget to divide 10 inches by 12 to get Diameter in feet before “squaring.”

For the 8-inch pipe, Area (in ft²) = pi x D² / 4 = 0.3490 ft²              

Substituting these values in the equation for Velocity₂ above:

Velocity₂   =  Velocity₁  x  Area₁ / Area₂

Velocity₂   =  6.0 ft/sec  x 0.5454 / 0.3490=  9.375 ft/sec

The correct answer is C. 

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press