Determine the Problems with Level Transmitter and Gauge

In this biogas generation system, cow manure is used as a feedstock to produce methane gas (CH4), which is then used to fuel an engine to turn a generator and make electricity.

The waste heat from the engine is used to maintain the cascaded digesters (“reactors” R-101 and R-102) at optimal temperatures for anaerobic bacteria to digest the manure and produce biogas (approximately 105^oF):

Level Transmitter and Gauge

LIC-56 registers a manure level of 3 feet 10 inches, while the operator’s manual gauge reading is only 3 feet 7 inches. The calibrated range of LT-56 is 0 to 4 feet. Your first step is to measure current in the cable connecting LT-56 and LIC-56, and there your digital multimeter (DMM) registers 19.33 mA.

Based on this information, determine at least two potential problems in this system. Also, determine whether or not a hydrostatic (DP) level transmitter would be suitable for measuring manure level in R-102.

Answer:

The current value agrees with the indication at LIC-56, and so the problem is not with LIC-56. This leaves the transmitter (mis-calibrated, plugged impulse line), a change in process density (assuming a hydrostatic DP transmitter) or the operator’s manual measurement of manure level.

Hydrostatic (DP-based) level measurement is appropriate for R-102, assuming either remote seal(s) or purging. Otherwise, solids in the manure will surely plug up the transmitter’s impulse line(s) over time.

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Credits: Tony R. Kuphaldt