_ Articles & Publications

Managing RT scope on a Process Plant Construction Project

Process Industry Projects, particularly Petrochemical ones, invariably face one bottleneck: Completion of NDT scope – Rather Radiography (RT) scope if one has to be very specific. Overruns in BOQs, manpower, equipment and machinery are always there and always get covered but the resultant increase in Radiography scope is the thing to look at – Within no time, we might be having a mountain to climb and because they are the last men standing, the Quality Control/Assurance guys face all the heat.

Here we are presenting some guidelines which may be adopted before radiography itself a critical path:

1. It is foremost important to have complete and authentic radiography scope in your hand. Total number of weld joints along with their details like size, thickness, service location must be made available. In ideal circumstances, an accurate welding BOQ would form the basis of this scope but always expect a few updates and revision to upset the applecart. Hence, constant follow-up will have to be ensured.

2. Every process plant has systems and sub-systems and all these have different priorities. Get the systems handover priority from the relevant team, usually the pre-commissioning or operations teams. This would enable you to prioritize radiography work.

3. Put in some effort to develop a system to track Welding and Radiography progress. Details may vary from project to project but once you have done that, dedicate one guy for to keep this system updated. A more thoughtful approach would involve work in close liaison with the execution/construction team so that all Welding progress translates into effective and efficient Radiography Progress.

4. Repair rate is critical as repairs invariably lead to time losses, repeat work, penalty shots and ultimately increased Radiography scope. Be proactive and keep welding quality in check. Consider this – If you have an original RT scope of 10,000 joints, there will be a difference of 1800 joints between a 2% repair rate and a 20% repair rate. Like Welding/RT progress, keep checks on welding quality as well – Firstly because that is your job as a QA/QC guy and secondly because it will save you from embarrassment. Calculate Repair Rate on a weekly basis and keep updating your remaining scope.

5. Developing the infrastructure for radiography is critical. A significant portion of weld joints will be shop welds and separate bunkers can easily ensure seamless, interruption-free RT of these joints. Better planning could increase the number of field welds and your life would be easier. Field welding is always more difficult and consequently has greater repair rate – Radiographing these joints would come at the expense of interrupting construction activities and would not make life easy for anyone.

6. Barricades and site evacuation are always a source of contention with the construction guys. Radioactive sources of lesser strength can reduce this considerably but will involve a trade-off between efficiency and time. Spending time on optimizing this area can save precious time.

7. RT time is priceless – Never should it go wasted. Radiography bottleneck concerns will grow as the project nears completion. By that time, other teams (Pre-commissioning, Operations, Construction) will have greater stakes and will pounce on any time window. In most of the cases, their parallel operation would always be possible and RT would automatically become a low-priority issue.

Operations and Pre-Commissioning teams loathe RT and Construction teams merely tolerate it because they have no choice. Make sure that you plan for those days when no help exists to avoid any last moment embarrassment.
Should things do not work out that way, be prepared to spend cash. Get the management to approve a few million dollars for more advanced NDE techniques but remember that in principle, all these techniques are being chosen merely because they do not interfere with the activities of other teams. Otherwise they carry scores of challenges of their own.
Making the right choice is important. However, it has a simple answer – Ultrasonic Testing (UT). Of course these are of various types but the biggest advantage they offer is exactly what we mentioned earlier: These techniques do not require evacuation of the area. At the end it boils down to the right type of Ultrasonic Testing Technique and in case you do not have unlimited reservoirs of cash (which is usually the case), the timing of mobilizing the Ultrasonic Testing teams at your site.

Time of Flight Diffraction (TOFD)
TOFD saves time. Full stop. This is its greatest attribute. It has its limitations though and wouldn’t work for pipes of diameters less than 6inches and thickness less than 12mm. However, if cleaned meticulously, joints of lesser thicknesses and sizes too can be scanned using this technique. However, it has major limitations, some of which are listed below:

• The technique does not work on pipe fittings.
• The minimum diameter of pipe-to-bend welds that may be scanned using this technique is 8inches.
• The technique requires at least 300mm of straight, clean surface on either side of the weld joint. Hence, any welds adjacent to supports or having less than 300mm of straight clean surface around the weld may not be tested.

1
Fig.1 TOFD scanning in progress


Whenever you begin liaison with any NDE Services Provider, ensure mentioning the acceptable code against which TOFD pass/fail criteria has to be performed.

2
Fig.2 Principle of TOFD technique

For American Codes, the most suitable acceptance standard for high pressure piping is ASME B31 code case 181. You may for the API 1104 for lower thicknesses. That, however, is a bit stringent.
TOFD has no value if Calibration blocks meeting the applicable code’s requirements are not available. Arrange for them beforehand so that it saves precious man-hours when inspection teams arrive. We know of cases when days were lost merely because Calibration Blocks could not be arranged in time. TOFD calibration blocks, as per ASME Sec. V, are of plain steel plates.
Since TOFD has that “clean” 300mm requirement on both sides of the weld, make sure that joint preparation (for TOFD) is cleaned properly. Once again, to reiterate, this is its primary purpose: 300mm of clean, even surface, free of any weld arc strike marks or paint on both sides of the weld joint. Remember that paint removal is very important as paint will affect the ultrasonic waves being used in the process. For cleaning, it is advisable to go for emery paper instead of grinding to avoid metal loss.


Pulse-Echo Ultrasonic Inspection
TOFD’s limitations have been mentioned earlier. Pulse-Echo can help us test joints which are beyond TOFD and can help us scan joints that have access to them from one side only. Customized, dedicated software for analyzing and interpreting Pusle-Echo data in all three views (top, end and side) are available and are far more effective relative to manual interpretation with no record available.
Unlike TOFD, Pulse-Echo has no blues with diameters and thickness. You can perform it on joints of small diameter and low thicknesses. A consolidated procedure can be developed which, like TOFD, will require calibration blocks after which inspection can be proceeded with seamlessly. Remember that Pulse-Echo’s limitations are only related to time involved in preparing a weld joint for scan and then actually scanning it.

3
Fig.3 Automated Pulse –Echo in progress


Calibration Blocks for Pulse-Echo are made from cut pieces of pipes to be scanned. Notches are to be made on the pipe inner and outer surfaces. Originally, piping diameter and thickness had to be the same as mentioned in ASME Section V. However, a recent development against ASME code case 2638 has allowed for +/- 25% tolerance in thickness – This should please both the execution staff and the working inspectors and will go a long way in reducing the effort required to prepare calibration blocks.

4
Fig.4 ASME Piping Calibration Blocks

Like TOFD, Pulse-Echo also requires clean, free space on both sides of the joint. Unlike Pulse-Echo, this is not about having 300mm on both sides but having 3.5 times the thickness of the weld joint on either side. The centre line of the weld joint has to be marked for reference using a centre punch. Capping has to be chipped off if enough of space is not available for scanning in order to cover the whole weld.
Data interpretation though takes a bit longer than TOFD. If following American codes, the usual acceptance criteria are the ones mentioned in ASME B31.3. In certain cases, Phased Array Ultrasonic Testing may be used as an alternative to Pulse-Echo.


Close Proximity Radiography
Close Proximity Radiography should not have been here in the first place. It is after all radiography and does require barricades. However, the combined use of specially designed shielding material with low strength sources, usually Selenium-75, ensures that this cordoning off is as low as 2m in most cases.
Like UT, it has its limitations with the low strength source ensuring that thicknesses higher than 35mm are beyond its range. However, some firms have developed shielding material for Iridium-192 as well which lets them scan joints of thickness up to 65mm. Unlike UT though, Close Proximity RT has no issues with shapes and locations – it just requires cooled, completed joints and can be a valuable resource when time is of paramount importance. Most critically, it is much cheaper than Ultrasonic Testing techniques.
A careful analysis shows that in crunch situations, it offers a valuable middle ground between radiography and UT and if managed properly, a combination of these techniques can easily ensure that most joints can be scanned without interfering with construction progress.

�����Summarily, it can be said that the key to managing your NDT scope is having complete command of the scope and progress based on which plans should be prioritized. There will always be bottlenecks but an intelligent selection of various techniques can change a grave situation to a manageable one. Such a selection would see engaging the right technique being employed at the right time.

 

       
 
Kamran Wasti
Working as a Project Engineer in Engro Fertilizers Pakistan on a new Fertilizer Project since 2007. Earlier on, he has been working with Sanden, Pakistan from 2006 to 2007, with Johnson & Philips from 2005 to 2006 and with DDFC, Pakistan in 2004.

 

 
Ashfaq Anwer
Working as an Inspection & Reliability Engineer in Engro Fertilizers Pakistan on a new Fertilizer Project since 2007. Earlier on, he had been working as a Project Engineer with Fauji Fertilizer Company Limited, Pakistan since from 2005 to 2007.

 

       
 
All rights reserved to thePetroStreet (c)