Non-standard referencing in tube inspection causes confusion during repair (Plug tube)

Non-standard referencing in tube inspection causes confusion during repair (Plug tube)

#heatexchanger #tubeinspection #heatexchangertube #typeofheatexchanger #referencetestside #testside

Key Learning Point:

“Clearly referencing the test side in accordance with international standards is essential in the inspection of heat exchanger tubes. The test side serves as the primary reference point for verifying the location of the inspected tubes and is critical for accurately identifying the exact positions of tubes that require repair”

Background:

          Internal tube inspection of heat exchangers using tube inspection methods is often carried out by multiple companies. Each company may adopt different approaches to identifying the test side, as heat exchangers come in various types and are installed in different orientations depending on the location. Furthermore, there is no centralized authority that enforces a standardized method for test side identification, resulting in companies reporting based on their own interpretations.

This inconsistency can lead to confusion among personnel from other departments or organizations, potentially causing misinterpretation of inspection results. Such discrepancies can affect both the accuracy and credibility of the data.

          Reference test side or referencing the position in tube inspection refers to the designation of a “reference side” used as the starting point for inspecting tubes within a heat exchanger using various techniques, such as Eddy current testing (ECT), Remote field testing (RFT), Internal rotating inspection system (IRIS), or Remote visual inspection (RVI). Clearly defining the test side is essential, as it helps reduce confusion in identifying tube positions, ensures accurate communication among departments such as inspection, maintenance, and engineering, and helps prevent errors in tube marking or incorrect plugging.

Case example 1: Vertical stationary fixed tube heat exchanger

Team A conducted a tube inspection on a vertically installed stationary fixed tube heat exchanger and reported the inspection results with the test side referenced as the “Top tube sheet”. The inspection found severe damage in the first three rows, which required urgent repair. However, when the maintenance team proceeded with the repair work, they were unable to identify the damaged tubes due to the unclear test side reference. It was not specified which side was considered the first row. As a result, the maintenance team requested confirmation of the exact damaged tube positions from team A, who performed the inspection.

This case led to delays in confirming the repair locations and posed a risk of incorrect repairs if the test side reference was misunderstood.

On the image, confusion may arise because both the first and last rows of the tube sheet contain an equal number of tubes.
Without a clearly defined reference direction, the interpretation could be ambiguous from either side, potentially leading to incorrect tube repair.

Case example 2: Floating head heat exchanger – orientation change causing confusion

A vertically installed floating head heat exchanger was removed and laid horizontally for inspection. Team A referenced the test side as the “Inlet side”, assuming it was the front. One tube with external mechanical damage was found but not marked on-site.

When the maintenance team attempted repairs, they couldn’t locate the damaged tube due to unclear test side reference. The drawing only labeled “Top” and “Bottom tube sheet,” with no indication of inlet or outlet. This caused confusion and delay.

This case emphasizes the need for consistent and clearly defined test side referencing between inspection and maintenance teams.

The image shows a heat exchanger where the first row of the tube sheet was not rotated to the top, and the test side was referenced as the Inlet. However, the test side should have been referenced according to the drawing,

which is the top tube sheet as per the reference drawing.

Case example 3: Horizontal heat exchanger and issues with test side referencing based on inspection orientation

Team A was assigned to inspect a horizontal heat exchanger installed in a production unit. The equipment was originally installed with the front facing north and the back facing south. The team started the inspection from the south side and referenced the test side as the “South side”, based on the actual on-site installation. After detecting severe damage, the inspector requested an expanded inspection. However, since team A had other inspection duties, team B was assigned to continue the inspection. Meanwhile, the heat exchanger was removed for external tube cleaning and relocated to the cleaning yard. At the new location, the equipment was positioned with the front facing west and the back facing east. This change caused confusion in confirming the correct test side, leading to delays in continuing the inspection.

The image shows a horizontally installed heat exchanger originally oriented north-south at the site. The equipment was later removed and relocated to the cleaning yard, where it was positioned west-east. This example demonstrates that referencing the test side based on cardinal directions should be avoided when inspecting heat exchanger tubes.

Technical Detail:

Heat exchangers are classified into three main types according to the design standards of the Tubular exchanger manufacturers association (TEMA)

1.      Stationary fixed tube heat exchanger

2.      U-tube heat exchanger

3.      Floating head heat exchanger

A vertical heat exchanger is not a new type of heat exchanger, but rather an installation orientation, which can apply to any of the three main types above.

A flowchart of different types of heat exchangers according to TEMA (Tubular exchanger manufacturers association) standards, which are widely used in the design of shell and tube heat exchangers in the industry.

1.      Stationary fixed tube heat exchanger

Key features of the equipment:

• Both ends of the tubes are permanently attached to the tube sheet.
• The shell cannot be removed without damaging the tube sheet.

 

For reference test side:
Test side referencing should align with the flow direction on the tube side of the equipment, as indicated in the drawing.

Use the following keywords:

• Inlet – the side where the fluid enters the tube side of the equipment.
• Outlet – the side where the fluid exits the tube side of the equipment.
• In-Outlet – the side where the fluid both enters and exits the tube side of the equipment.
• Reverse – the side where the fluid returns or flows back into the tube side of the equipment

2.      U-tube heat exchanger

Key features of the equipment:

• The tubes are bent into U-shape with a tube sheet on only one side.
• Both ends of the tube are connected on the same side.
• The shell side serves as the flow path for the other fluid.

For reference test side:

This type of equipment has only one tube sheet, as all operations can be performed from a single side. The top side can be identified by observing the lifting lug on the tube sheet.

Use the following keyword:

• Tube sheet – Specify the test side as "Tube sheet," with the first row considered as the top side, identified by the lifting lug at the top

3.      Floating Head Heat Exchanger

Key features of the equipment:

• One end of the head is “floating” or movable to accommodate tube expansion.
• The entire tube bundle can be removed.

 

For reference test side:
The reference should be based on the flow direction entering the tube side of the equipment. The top side can be identified by observing the lifting lug.

Use the following keywords:

• Stationary – the side where the fluid enters and exits the tube side of the equipment
• Floating head – the side where the fluid returns or flows back into the tube side of the equipment

Vertical heat exchanger

           A vertical heat exchanger is not a new type of heat exchanger, but rather an installation orientation in which the tube bundle and shell are aligned vertically. This configuration can be applied to all three main types: Fixed tube sheet, U-tube, and Floating head.

For reference test side: Reference should be made based on the drawing, and the orientation must be clearly specified.

Observe the first row in the drawing and identify which direction it faces according to the actual on-site installation. Be sure to include this referenced direction in the report. Use the following keywords:

·     Top tube sheet reference drawing – when the equipment is removed from the shell and laid horizontally

·     Bottom tube sheet reference drawing – when the equipment is removed from the shell and laid horizontally

·     Top view with north side – when the first row in the drawing faces north, based on on-site installation

·     Top view with east side – when the first row in the drawing faces east, based on on-site installation

·     Top view with south side – when the first row in the drawing faces south, based on on-site installation

·     Top view with west side – when the first row in the drawing faces west, based on on-site installation

The image shows an example of referencing the test side in the case where the first row in the drawing is oriented toward the north, as installed on-site.

Lesson learned:

“Clearly referencing the test side during heat exchanger inspections and ensuring a mutual understanding among all parties is essential, as this information is required in critical steps such as inspection planning, execution, result analysis, and maintenance.
If the test side is not clearly or correctly identified, it can lead to incorrect tube location identification, maintenance delays, or even disruption of the production process”

The impact of unclear or non-standard identification of the test side can lead to misunderstandings by those who use the information

-         It can lead to incorrect reporting of the inspected tube locations.
Since both sides of the tube sheet are mirror images of each other, identifying the wrong side can result in reported tube positions that do not match the actual inspected locations.

-         It can cause maintenance or tube plugging to be done in the wrong location.
If the test side is misunderstood and maintenance or tube plugging is performed on a tube without a defect; the actual defective tube may be left unrepaired. This poses a risk of future leakage or damage during operation.

-         It reduces the credibility of the inspection report and the inspector.
Such an error is considered critical in NDT and inspection work. It may lead to the client rejecting the inspection results, requiring a full reinspection, which increases both cost and downtime.

Preventive measures:

-         Provide training for operators and establish a standardized method for referencing the test side.

-         Verify that the tube sheet from drawing matches the actual tube sheet at the worksite before starting the inspection.

-         Have a previous inspection report, referring to the latest report to ensure consistency and continuity of inspection data.

-         Ensure that the supervisor or personnel responsible confirms the correct test side before starting the work and consistently reference the test side in the report using clearly defined keywords for the test side options.

-         Include photos or drawings in the report to indicate the test side and clearly mark the side where the inspection.

Keywords used for internationally standardized test side referencing of horizontal heat exchangers

Stationary fixed tube heat exchanger

-         Inlet – the side where the fluid enters the tube side of the equipment.

-         Outlet – the side where the fluid exits the tube side of the equipment.

-         In-Outlet – the side where the fluid both enters and exits the tube side of the equipment.

-         Reverse – the side where the fluid returns or flows back into the tube side of the equipment

U-tube heat exchanger

-         Tube sheet – Specify the test side with the first row considered as the top side, identified by the lifting lug at the top

Floating head heat exchanger

-         Stationary – the side where the fluid enters and exits the tube side of the equipment

-         Floating head – the side where the fluid returns or flows back into the tube side of the equipment

Keyword used for internationally standardized test side referencing of vertical heat exchanger

-         Top tube sheet reference drawing – when the equipment is removed from the shell and laid horizontally

-         Bottom tube sheet reference drawing – when the equipment is removed from the shell and laid horizontally

-         Top view with north side – when the first row in the drawing faces north, based on on-site installation

-         Top view with east side – when the first row in the drawing faces east, based on on-site installation

-         Top view with south side – when the first row in the drawing faces south, based on on-site installation

-         Top view with west side – when the first row in the drawing faces west, based on on-site installation