March/April 2010

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 gauge pressure transmitter that measures the pressure in a 150 # high pressure steam header is mounted 6 feet below the center line of the header.  The tap for the impulse line connects to the top of the header and rises 2 feet above the header center line, extends horizontally for 3 feet, and then drops down to the transmitter. In order to read the pressure in the steam header correctly, the transmitter output must be:

A. Calibrated for suppressed zero, the suppression equal to 8 feet of liquid head pressure

B. Calibrated for suppressed zero, the suppression equal to 6 feet of liquid head pressure

C. Calibrated for elevated zero, the elevation equal to 8 feet of liquid pressure

D. Calibrated for true zero

 I1

CCST answer

The correct answer is A, calibrated for suppressed zero with the suppression equal to the 8 feet of liquid height that is in the impulse line.

At zero gauge pressure in the steam line, you are essentially suppressing (pushing) the transmitter output back down to zero after the system reaches equilibrium with the 8 feet of impulse line full of liquid. The transmitter will read that impulse line liquid head pressure plus any pressure exerted on top of the liquid. 6 feet of liquid head suppression is incorrect as the impulse arrangement will cause 8 feet of liquid head to accumulate.  Elevated zero is incorrect as that adjustment is used to adjust for negative pressure offsets resulting from the transmitter being mounted above the zero reference point (high pressure tap point) and where a liquid in the impulse line or a capillary system would exert a negative pressure.

Reference: Thomas A. Hughes; Measurement and Control Basics, 4th Edition, ISA Press, 2007.