ISA Certified Control Systems Technician (CCST) program

CCST

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 transmitter connected across an orifice plate in gas service by flange taps and sensing (impulse) lines will probably experience abnormal readings if:

A. sense lines are of equal height and length
B. meter is mounted below process line
C. meter is mounted above process line
D. sense lines are condensate free

CCST Answer

The correct answer is B, “meter is mounted below process line." In gas service, the meter/transmitter should be mounted above the process line so that any condensables will fall out of the sensing (impulse) lines by gravity. Mounting of the meter/transmitter below the process line will likely allow condensables to accumulate in the sensing lines, thereby negatively affecting the pressure reading.

As long as there is no condensate in the impulse lines, the two sensing lines can be of slightly differing lengths. Normally, the two impulse lines are run in the same path from process taps to meter/transmitter, so the lengths are generally the same.

Impulse lines that are too long (>50 feet) can introduce measurement error due to pipe friction. If long runs are required, larger diameter tubing is often required to preserve measurement accuracy. 

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