October 2008

Documenting skills is value-add

ISA certification provides an objective, third-party assessment, and confirmation of a person's skills. InTech covers two certification areas in this monthly Certification department.

ISA Certified Automation Professional (CAP) program

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

The following question comes from the CAP study guide, Performance Domain III, System Design, Design, specify, and procure the hardware/software used in the system

CAP question

Which of the following control valve body styles is MOST likely to develop blockage in a paper pulp slurry service?

A. Pinch

B. Characterized ball

C. Butterfly

D. Globe

CAP answer

The correct answer is D, globe.

A globe valve consists of a plug on the end of a stem that moves the plug onto a seat. Because of the body design of this valve, cavities in the body can easily fill with materials and develop blockage. Ball-, butterfly-, and pinch valve body style have straight-through flow, and they are less likely to develop blockage.

Reference: Bela Liptak, Instrument Engineers' Handbook - Process Control, Volume 2, CRC Press, and ISA Press, 2001

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 differential pressure (d/p) cell is mounted 3 feet below the zero level in a tank. For a true level reading, the transmitter output must be:

A. Calibrated for elevated zero

B. Calibrated for span

C. Calibrated for true zero

D. Calibrated for suppressed zero

CCST answer

If the d/p cell is not located at an elevation that corresponds to 0% level in the tank, we must calibrate it to account for the difference in elevation.

This calibration adjustment is "zero elevation" when the cell is located above the lower tap; it is "zero suppression" when the cell is located below the lower tap.

To calculate the amount of elevation or suppression, we must measure the distance between the zero reference level and the high-pressure port of the d/p cell. Taking into account that 27.68" H2O = one (1) psig, calibrate the transmitter for the distance measured in the direction necessary to result in an accurate output.

When the d/p cell is at the same elevation as the zero level in the tank, calibrate the transmitter for "true zero."

The correct answer is D, calibrated for suppressed zero.

Reference: Mike Cable, Calibration: A Technician's Guide, ISA Press 2005

Nicholas Sheble (nsheble@isa.org) writes and edits Certification Review.