CCT Installation Procedures 3 — Questions and Answers
Question 1: Why must impulse lines for differential pressure flow measurement be sloped continuously from the process taps to the transmitter with no low points?
- To prevent the impulse line material from work-hardening under vibration
- To ensure gas bubbles or condensate pockets cannot accumulate and create false differential pressure (Correct answer)
- To keep the fluid velocity in the impulse line below the erosion threshold
- To maintain equal electrical resistance in both legs of the circuit
Correct answer: To ensure gas bubbles or condensate pockets cannot accumulate and create false differential pressure
Trapped gas or liquid pockets in impulse lines create density-based head errors that offset the differential pressure reading.
Question 2: When installing a control valve, what is the standard recommended upstream and downstream straight pipe run to prevent cavitation-induced damage?
- 2D upstream and 2D downstream
- 10D upstream and 5D downstream (Correct answer)
- 5D upstream and 10D downstream
- 20D upstream and 20D downstream
Correct answer: 10D upstream and 5D downstream
The typical industry guideline is 10 pipe diameters (10D) upstream and 5 diameters (5D) downstream to allow the flow profile to stabilize before and recover after the valve.
Question 3: During the mechanical installation of an orifice plate, what common error will cause the measured differential pressure to be erroneously high?
- Reversing the orifice plate so the beveled edge faces upstream (Correct answer)
- Installing the orifice plate with the beveled edge facing downstream
- Using a plate with a bore diameter slightly larger than specified
- Failing to lubricate the flange bolts before torquing
Correct answer: Reversing the orifice plate so the beveled edge faces upstream
Reversing the orifice plate places the sharp edge downstream, changing the vena contracta location and producing a higher-than-expected differential pressure.
Question 4: What is the purpose of a manifold valve assembly (3-valve or 5-valve) used during transmitter installation?
- To provide secondary process containment in case of primary seal failure
- To allow the transmitter to be zeroed, calibrated, or removed without shutting down the process (Correct answer)
- To amplify the differential pressure signal before it reaches the transmitter
- To filter particulates from the process fluid before they reach the transmitter diaphragm
Correct answer: To allow the transmitter to be zeroed, calibrated, or removed without shutting down the process
The manifold isolates, equalizes, or vents the transmitter so it can be calibrated or maintained while the process remains pressurized.
Question 5: A gas chromatograph analyzer is being installed for a process sample system. What is the critical role of sample conditioning upstream of the analyzer?
- To add calibration gas to the sample stream on a timed schedule
- To bring the sample to the correct temperature, pressure, flow rate, and remove particulates and liquid dropouts (Correct answer)
- To convert the analog sample signal to a digital format for the DCS
- To store surplus sample gas for use during analyzer downtime
Correct answer: To bring the sample to the correct temperature, pressure, flow rate, and remove particulates and liquid dropouts
Sample conditioning ensures the analyzer receives a clean, representative sample within its specified operating envelope, preventing damage and measurement errors.
Question 6: When field-wiring a 4-20 mA two-wire transmitter, what is the correct way to verify polarity before energizing the loop?
- Use an oscilloscope to check for AC ripple on the terminals
- Use a continuity tester to check resistance from positive to negative
- Use a multimeter in DC voltage mode to confirm the supply positive connects to the transmitter positive terminal (Correct answer)
- Use a clamp meter around both wires simultaneously to check for differential current
Correct answer: Use a multimeter in DC voltage mode to confirm the supply positive connects to the transmitter positive terminal
Confirming DC voltage polarity before energizing prevents reverse-polarity damage to the transmitter's internal electronics.
Question 7: Which installation standard requires calibration records to be traceable back to a national or international standard?
- NFPA 70 (NEC)
- ISO/IEC 17025 (Correct answer)
- OSHA 29 CFR 1910
- API RP 500
Correct answer: ISO/IEC 17025
ISO/IEC 17025 is the international standard governing calibration laboratory competence, requiring traceability of measurement results to national or international measurement standards.
CCT Installation Procedures 3
Why must impulse lines for differential pressure flow measurement be sloped continuously from the process taps to the transmitter with no low points?