How Do You Place Tube Skin TIs on Mixed-Flow Tubes?

The tube-skin TI placement note says to select a down-flow tube in a mixed-flow vertical arrangement. Here, mixed flow means that process fluid travels upward through some vertical tubes and downward through others. It does not mean turbulent flow, mixed-phase flow, or blending inside one tube. The upward-flow tubes are commonly called risers; the downward-flow tubes are downcomers.

Reject the usual quick interpretations

Do not install the thermocouple on the easiest tube to reach. Accessibility matters for installation and maintenance, but it does not override the specified flow-direction criterion. A sensor on an accessible riser answers a different measurement question than one on the required downcomer.

Do not treat mixed flow as a process-state description. A fluid can be single-phase or multiphase while the bundle still has risers and downcomers. Conversely, complicated internal flow does not prove that the external circuit contains both upward-flow and downward-flow tubes.

Do not infer flow direction from tube orientation alone. Every tube in the instruction is vertical. The distinction comes from the connected process circuit and actual direction of travel, not from whether the tube points up or down.

Finally, do not move an existing tube-skin point merely because its displayed temperature looks high or low. Sensor attachment, local heat flux, internal heat transfer, wiring, and input configuration can all change the indicated value.

Gate check: Proceed only after the team agrees that “mixed flow” describes different flow directions among tubes and that the required tube is a downcomer.

Trace the process circuit before choosing a tube

Use the current piping and equipment drawings, tube-circuit layout, fabrication records, and field connections to trace the fluid path. A header located above a tube does not by itself prove downward flow; interconnected passes, return bends, manifolds, and operating mode determine direction.

1. Identify the process inlet and outlet for the fired-heater or reboiler circuit.
2. Follow the connected path through each header, bend, or manifold shown on the circuit drawing.
3. Mark every vertical tube with an arrow showing process flow during the operating condition being monitored.
4. Classify tubes with upward arrows as risers and tubes with downward arrows as downcomers.
5. Compare the marked drawing with field routing and equipment identification before selecting a measurement tube.

If drawings disagree, stop selection work. Resolve the circuit from approved design records or a controlled engineering review; do not settle the disagreement by opening process connections or relying on surface temperature alone.

Gate check: The selected candidate must have a documented arrow from its upper connection toward its lower connection.

Select a representative down-flow tube

The instruction “a down-flow tube is selected” is a tube-selection rule. It does not say that every downcomer has the same metal temperature or that a downcomer is always the hottest tube. Tube-skin temperature responds to both external heating and internal cooling, so nearby tubes can differ because of heat-flux exposure, deposits, flow distribution, geometry, or sensor installation.

For a naphtha tower reboiler service, document the operating lineup used to classify direction. If alternate lineups can reverse flow, the measurement basis must state which lineup the installation represents.

Gate check: Record the selected tube, its down-flow path, the operating lineup, and why its heat exposure represents the intended monitoring duty.

Set the measurement location on the selected tube

Choosing the correct downcomer does not finish the placement. Engineering must also define the axial elevation and circumferential position. These determine what thermal condition the sensor observes.

At the tube wall, the indicated temperature reflects local heat input, conduction through the attachment, and cooling by the process fluid. A point facing stronger radiant exposure may read differently from one on a shielded side. A point near a bend, support, weld, or other discontinuity may not represent the straight heated length.

1. Use the approved heater or equipment drawing to identify the intended heated zone.
2. Mark the specified elevation on the chosen downcomer.
3. Establish the circumferential orientation from a fixed field reference, not a verbal description such as “front side.”
4. Check for interference from supports, clips, existing welds, insulation details, or adjacent instruments.
5. Record the final location before attachment so maintenance can reproduce it.

Do not choose an elevation or clock position from the quoted sentence alone; it supplies only the flow-direction choice. Obtain missing location details from the applicable project specification, drawing, sensor installation detail, or responsible engineering authority.

Gate check: The work package must show one unambiguous tube, elevation, and circumferential position.

Install the tube-skin thermocouple as a measurement system

A correctly chosen tube can still produce a bad temperature if the attachment or signal path is wrong. Use the approved attachment method and thermocouple construction for the service. Do not improvise attachment geometry, add an unreviewed weld, or substitute sensor materials because they are on the shelf.

Inspect the tube surface and installation area before work. Route leads to limit direct heating, mechanical strain, abrasion, and contact with hot or moving parts. Maintain thermocouple alloy continuity through extension wiring and terminations; an unintended metal junction creates another temperature-dependent voltage in the circuit.

At the instrument input, match the configured thermocouple type to the installed sensor and extension cable. Check polarity from the hot junction through every terminal. Confirm that cold-junction compensation belongs at the intended input device and that no unapproved signal conversion has been inserted.

Keep the permanent record tied to the physical installation: tube identity, flow direction, elevation, orientation, attachment detail, sensor type, cable route, terminal points, input channel, display tag, and alarm or trip destination where applicable.

Gate check: Pass the mechanical inspection, continuity and polarity checks, input-configuration review, and point-to-point documentation review before startup.

Prove the reading end to end

Verify the complete measurement chain rather than accepting a plausible display value. A plausible value can hide reversed polarity, the wrong input type, a crossed channel, poor attachment, or a display mapped to another sensor.

1. Confirm that the field label matches the selected down-flow tube and the approved installation drawing.
2. Verify continuity and insulation condition using procedures suitable for the installed instrument circuit.
3. Check terminal polarity and thermocouple or extension-wire type at each accessible connection.
4. Verify the input channel configuration and its mapping to the operator display, historian, alarms, and protective logic.
5. Apply an approved temperature or millivolt simulation at the proper test boundary and confirm the expected channel responds.
6. After startup, trend the point with process flow, firing duty, and comparable tube-skin measurements. Investigate abrupt offsets, impossible response direction, or a flat signal before relying on it for operating decisions.

Get production running only after the indicated point is proven to be the physical downcomer sensor. Then correct drawings, labels, and maintenance records while the trace is still clear.

Gate check: A controlled stimulus must reach the correct display and associated functions, and the live trend must respond credibly to operating changes.

Frequently Asked Questions