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), Maintenanceof Instruments and Systems, 2nd Edition (2005), ISA Press