Inspection of Heat Exchangers, either online or offline, that is, In-service inspection of Heat Exchangers, or Shop Inspection, or Turnaround Inspection of Heat Exchangers has its own specific procedures stated by Company and accordingly, the standard followed.
In brief, I will detail some of the inspection activities carried out for the HE inspection followed in common for the Oil and Gas Industries. Provided that, follow appropriate ITPs (Inspection Test Plan) relevant to your Scope and Standards laid by your Organisation.
Table of contents
What are Heat Exchangers?
As you know, HEX or the Heat Exchangers are devices that are usually on the upstream or downstream of equipment for efficient heat transfer from one medium to another. In short, utilizing the principle of conduction, convection, and radiation.
Moreover, the heat exchangers commonly used in plants are the Shell and tube type, plate type heat exchanger, or an Air-cooled type of heat exchanger.
In general, the heat exchangers heat or cool the fluids, hence also used as a condenser or reboiler for equipment.
How to inspect Heat Exchangers?
Heat Exchanger Inspection plays a vital role during an initial assembly construction, or In-service inspection, either online or offline.
Moreover, a heat exchanger external inspection is the most simple and effective visual inspection method applied in the industry. Beginning from the construction phase (visual incoming material inspection), to the commissioning (testing), and implementation (in-service) phase.
On the other hand, an internal inspection of heat exchangers involves tube inspection – cutting, rolling, dimensioning during construction or repair, and also the tube thickness measurements and hydro-test of the equipment to check the integrity as a whole as required by codes either online or offline.
Most external inspections are a cost-effective means, and repairs have proper evaluation before execution.
However, in certain cases, internal inspection becomes a requirement if there are any process considerations. Nevertheless, high costs are a concern.
In general, internal inspections are normally not a requirement in service, unless otherwise as stated by code, the company standards for a planned turnaround to verify the integrity of tubes for its intended operations without failures.
How to do an External Inspection of Heat Exchanger?
Inspection of heat exchangers is the most exhaustive topic, and applies different strategies as formulated by the respective reference standards as applicable. During a Exchanger Construction phase TEMA is the most followed standard by most industries. However, for Inservice inspection API 510 is most common. ASME Section VIII Div 1 and NBIC also plays a vital part for stringent Inspection requirements to Codes.
For example, a Pressure vessel inspection code API 510 states that
A visual inspection is performed from the outside of a pressure vessel to find conditions that could impact the vessel’s ability to maintain pressure integrity or conditions that compromise the integrity of the supporting structures (e.g.ladders, platforms, supports).
Ensure a thorough visual inspection of heat exchangers that are with insulation on the external, for CUI (corrosion under insulation), free from any sign of a leak, or mechanical damage.
Additionally, a partial or complete insulation removal is a mandate as per the procedure. Moreover, the exchanger inspection intervals are by the respective codes or an RBI Assessment.
Visually inspect all nozzle flanges on the gasket faces for damages and recommend repair or replacement.
Finally, recommend a NDT inspection for external joints (as per code or findings) on the welds, Girth joints, nozzles and attachments.
In short, the external inspection of exchangers involves inspecting the equipment visually for any abnormalities such as sign leaks, external damages, insulation damages, including supports, flange leaks, verification of process abnormalities, or an onstream inspection where applicable.
As per API 510, a visual inspection has to be at no more than five years or the nearest internal/on-stream inspection unless an RBI assessment evaluates it as not a requirement.
Credits: theengineerspost.com
What are the Internal Inspection of Heat Exchangers?
In contrast, the Internal Inspection of Exchangers have their plans after a proper evaluation of the requirement, based on a process issue, or an exchanger reliability concern.
Therefore, the outcome for internal inspection may be cleaning of the exchanger equipment, tube testing for integrity for safe operation, a repair requirement, or replacement of exchanger components like flange, nozzles, shell, baffles, bundle, tubes, or channels.
Furthermore, the steps to follow in common are as below, for exchanger inspection internally, and may vary based on service or industrial standard requirements.
Pre-inspection
Firstly, an internal equipment inspection is the best opportunity to analyze the equipment. Therefore, ensuring a proper inspection record is normally a mandate. Utilize a standard checklist that covers all details pertaining to equipment.
Existing coating conditions needs either internal and external, need to be on record, and appropriate actions taken. The topic on Blasting and coating inspections will be covered elsewhere in this site.
If it isn’t a scheduled inspection, identify the reason for the problem whether it is a suspected leak, pressure drop, or poor performance of the heat exchanger.
Fouling of Tubes
Next, look for debris or fouling of tubes after opening. Collect samples and photographs for evaluation. Then, look for damages to the channels or shell of equipment, the tube sheets, and the tubes. (pic credit: wikipedia.org)
Cleaning of Tubes
Afterward, recommend a proper cleaning method. Wherever applicable, the tubes have to be removed from the shell, and cleaned. Accordingly, a provide hydro blasting pressure for both the shell side and tube side as per procedure, ensuring to prevent tube damage due to cleaning.
Provided that, a chemical cleaning requirement should be evaluated with the process specialist.
Internal Inspection of Exchanger Post-Cleaning
In general, visually inspect the complete exchanger internally on shell, partition plates, nozzle welds, all welding joints, and the tube sheet for signs of leaks, cracks, fatigue, warpage, corrosion, or mechanical failure.
Ensure to document existing Ultrasonic thickness measurement on the shell, channels and nozzles. Plus, record the Vernier scale measurements on tubes. Also, a complete visual inspection on flange face, and the shell and piping flanges.
Moreover, a DPT (Dye penetrant test) or a MPI (Magnetic Particle Inspection) technique is a mandate as per the recommended code (eg., ASME BPVC or API) on welds, partition plates, tube sheets, tube to tube sheet welds, and any weld repairs where applicable.
Furthermore, the Tube bundles, wherever applicable, have to be taken out and inspected for fouling, damage, erosion and integrity of bundle assembly (tie-rod, baffles, welds).
Additionally, the recommended NDT techniques by Code to ASME Section V Procedures shall follow to verify indications for acceptability to code. Mostly, the Dye Penetrant Tests, Ultrasonic Test, Advanced NDT techniques, or a Radiography Test.
Plus, consider relevant advanced nonvisual NDT inspection techniques (credits: iare.ac.in) for tube inspection based on the material of construction of tubes, like
- IRIS Internal Rotary Inspection System
- MFL Magnetic Flux Leakage
- ECT Eddy Current Testing
- RFT Remote Field Testing
- NFT Near Field Testing
- TECA Tangential Eddy Current Array
- AE Acoustic Emission Testing
Nevertheless, the inspection coverage requirements will be by an RBI Assessment or an authorized Inspection Specialist. Nevertheless, the wall thickness in a corrosion service as per API 510 is
Additionally, a radiography test may be a need for joints, and locations where accessibility permits as per the procedure.
Finally, evaluate a hydrostatic test as per Specialist recommendation to identify the equipment tube-tube sheet, and shell integrity. Furthermore, a hydrostatic test may be from the shell side, tube side, and finally on the box up as per the procedure requirements.
Provided that, Tube plugs may have to be installed as per TEMA requirements during fabrication, or as per API Standards for thin down tubes or hydrostatic test leak tubes.
Firstly, whenever a tube plug installation is done, initial step is to vent the tube (tube puncture). Also called the positive venting of tube, as it releases trapped gases or chemicals, especially in the case of thinned down tubes.
Furthermore, tube plugs can be a ferrule taper plug, taper plug or a mechanical plug. Defective tubes that are under consideration for replacement, and currently not feasible for removal, have to be plugged with plugs from approved material verified by PMI (Process Material Identification) analysis.
In general, the Tubular Exchangers Manufacturers Association (TEMA) allows plugging of tubes based on feasibility, and under certain conditions where manufacturers may plug either a maximum of 1% of the tubes or 2 tubes without prior agreement.
Industry-approved heat exchanger tube plugging depends on various factors. Mostly more than 30% of plug tubes are a cause of efficiency drop.
However, while in service, defective heat exchanger tubes are mostly plugged to a maximum of 10% in general during internal inspection. However, this procedure is mostly not for a U-tube exchanger or a multi pass exchanger.
Additional tube plug installation has to be done after evaluating with concerned process specialist, thus ensuring thermal efficiency is normal.
Accordingly, the proper plug hammering technique on tubes, or a seal weld for plug installation for weld tubes, are put in record as per code requirements.
Additionally, ASME PCC-2 Article 2.14 lists the requirements for exchanger repairs and inspection requirements.
What are the defect identification capabilities of Advanced NDT techniques?
Firstly, each ndt technique has its limitations and depends on multiple criteria like service, material, accessebility, thickness range, and more.
Please refer the below table for best understanding of NDT inspection using Advanced Techniques.
Reference Standards
- API Standard 660 – Shell-and-Tube Heat Exchangers
- ASME Section VIII, Division I – Rules for Construction of Pressure Vessels Division 1
- API 661 – Petroleum, Petrochemical, and Natural Gas Industries Air-Cooled Heat Exchangers
- TEMA, Class “R”— for the severe requirements of petroleum processing (and usually including most large-scale processing applications).
- API 662 – PLATE HEAT EXCHANGERS FOR GENERAL REFINERY SERVICES
- NBIC – National Board Inspection Code
- ASME Sec V – Nondestructive Examination
- API 632 – WINTERIZATION OF AIR-COOLED HEAT EXCHANGERS
- ASME PCC-2, Article 3.12 for guidance on the inspection and repair of shell and tube heat exchangers
- API 572, Annex A for more information on the inspection of several types of heat exchangers
Wrapping Up
Internal and external inspections of heat exchangers are a critical part of any heat exchanger operation. They are not always required but they are always in many situations important as per industrial standards.
We have found that they are a great tool to help you understand what’s happening in your regular inspection of heat exchangers. Therefore, as a specialist, You should be doing internal and external inspections at every opportunity.
As a general disclaimer, please note that all inspections stated above need do not tend to relate with your your industry, process or service. So, please refer to your company norms, a standard approved procedure or codes applicable to your industry.
Happy to have your thoughts on the article.
