pH Monitoring System Triggers an Alarm

This pH monitoring system triggers an alarm if the pH value of the process water in the neutralization tank drifts past either of two thresholds (trip) values:

pH Monitoring System

pH Monitoring System Triggers an Alarm

Answer the following questions about this pH alarm system:

1. If a wire breaks loose at TB56-4, creating an “open” fault in the loop circuit, determine what will happen at the alarm unit (AAH, AAL-41) and also where you would expect to measure voltage in the loop circuit and where you would expect to measure no voltage in the loop circuit.

2. If breaker #25 in panel L4 suddenly trips, what will happen in this system? Will an operator still be able to read the pH value of the water in the neutralization tank?

3. If a fire breaks out near the conduit through which cable 52 runs, causing the plastic insulation around the conductors of cable 52 to melt and consequently causing those conductors to short together, what will happen in this system?

Where would you expect to measure voltage in the loop circuit, and where would you expect to measure no voltage in the loop circuit?

Where would you expect to measure current in the loop circuit, and where would you expect to measure no current in the loop circuit?

4. Calculate the loop current value when the pH measures 6.8 inside the neutralization tank.

5. For those who have studied pH measurement, explain why pH “neutralization” is an important control process in the industry.

6. How can we tell from this diagram whether the 4-20 mA output of transmitter AIT-41 is active or passive (i.e. sourcing or sinking)?

Answers:

If a wire breaks loose at TB56-4, creating an “open” fault in the loop circuit, determine what will happen at the alarm unit (AAH, AAL-41) and also where you would expect to measure voltage in the loop circuit and where you would expect to measure no voltage in the loop circuit. The AAL would trip (but not the AAH), and we would expect to measure voltage between the wires of cable 52 but not between the wires of cable 30.

If a fire breaks out near the conduit through which cable 52 runs, causing the conductors inside cable 52 to short together, what will happen in this system? Where would you expect to measure voltage in the loop circuit, and where would you expect to measure no voltage in the loop circuit? Where would you expect to measure current in the loop circuit, and where would you expect to measure no current in the loop circuit?

The AAL would trip (but not the AAH), and we would expect to measure no voltage anywhere in the loop circuit. However, we would still have current at the terminals of the AIT-41 transmitter (although no current to the right of the short).

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Credits: Tony R. Kuphaldt

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pH Monitoring System Triggers an Alarm

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