Instrument Installation and Mounting
Mounting decides the measurement
A correct instrument mounted badly produces wrong data. Start at the process tap: it must be in a representative location, not a dead leg or the bottom of a gas line, with a root valve, and the impulse line must slope so gas vents back to the process and liquid drains back. Liquid lines slope down toward the transmitter; gas lines slope up; steam uses a condensate pot or pigtail so the transmitter always sees a water leg. Slope is typically at least 1 in per foot.
Differential-pressure transmitters create head error whenever they sit at a different elevation than the taps. A wet leg must stay full; a drained leg shifts zero. At commissioning you enter the bias as zero elevation or suppression (a positive or negative LRV) and record it.
Flow elements need straight run: an orifice plate is commonly 5 pipe diameters upstream and 2 downstream, though specifications vary and vortex and magnetic meters need more. Tap orientation differs for liquid, gas, and steam. Thermowells need immersion of roughly 2 to 3 inches, or about 10 pipe diameters upstream and 5 downstream of a disturbance, plus enough length to clear conduction error.
Other selection points
Ambient temperature and sun, vibration, corrosive atmosphere, access for calibration, and a visible tag. Level instruments need bridle and nozzle elevations matching the taps; radar or ultrasonic needs clear space.
Rule of thumb
If the transmitter sits 30 in below the lower tap in a fluid of SG 1.0, the empty reading is 30 inH2O high, so the LRV becomes +30 inH2O (zero suppression).
Traps
Wrong slope direction, forgotten head correction, short thermowells, and ignored straight-run.