CQM’s Automatic Tube Cleaning system (ATCS) can be a safer and cost effective alternative to manual cleaning. Read further to understand the achievements.
Introduction:
The proper performance of shell and tube type heat exchangers within a process can affect the cost of the final product, or even the production rate. Unfortunately, heat exchangers are prone to fouling, its nature depending on the fluids flowing within and over the tubes: and the reduction in heat transfer that results almost invariably has an impact on product cost. To reduce this impact, heat exchanger performance should be continually monitored and the heat exchanger cleaned at intervals that are determined by the increase in DP and reduced DT across exchangers.
Exchanger tube cleanliness is historically difficult to achieve using traditional methods including water jetting often requiring multiple passes, thereby extending the duration of a shutdown unnecessarily and often production going into critical path.
The principal types of fouling encountered in process heat exchangers include:
• Particulate fouling
• Corrosion fouling
• Biological fouling
• Crystallization fouling
• Chemical reaction fouling
The injection water heaters (E-415A, B, C, D & E) of the Cairn Mangala Process terminal acts as back bone for heating the injection water required in downhole injection both as Power fluid and injection water for well water lift purposes.
In Cairn’s case, it was observed that more than one mechanism found to be dominant amongst others and principally it is the oil, salts and calcium carbonate in the produced water. From the historical data it is evident that fouling increase overtime, the trajectory is very acute leading to very high DP resulting in initiation of frequent cleaning. The exchangers E-415 A/B & C though are designed to accommodate the fouling resistance basis the
original known water composition, but the current changes has resulted in increased rate of fouling and intolerable leading to choking of tubes, requiring frequent cleaning and equipment downtime.
CQM through the studies and field trials conducted over 24 months have finally arrived at the perfect ball geometry and material that suits the process medium. CQM as per original proposal commissioned ATCS on the exchangers E-415-A, B & C on trial basis to overcome the requirement for manual cleaning. The system was implemented in March 2013 and after brief operation the system was put on standby due to initial inconsistency,
requiring further investigation.
Post the initial trials conducted in 2013 CQM continued with the design and trial at lab scale using new ball geometry and alternate materials to check suitability with process medium i.e. Oily water in this case.
After repeated trials, CQM have finally developed a more robust solution with ball made out of Silicone and perfect ball geometry. This was implemented in the recent trials starting 14th Jan’15 and found to be operating successfully.
The current balls have yielded exemplary results comparable with the new exchanger both in maintaining low DP and delivering higher heat transfer DT consistently at varying flows.
The implementation of ATCS in the long run is will offer advantages in;
1. Reducing energy consumption by improving heat transfer efficiency
2. Reducing costs associated with manual cleaning of the heat exchanger
3. Reduce downtime and improve availability
4. Help increase production by meeting the water injection targets
5. Reduce operations and maintenance costs
6. Protect and extend the life of heat exchanger
7. Reduce environmental impact
Challenges:
The following challenges were being faced by Cairn that are addressed with implementation of ATCS;
- Heavy fouling and repeated tube cleaning
- Increased cleaning frequency
- Severe time constraints leading to loss in water injection and consequential effect on oil production
Progress and achievements to date:
CQM’s online automatic tube cleaning system (ATCS), implemented on Water injection heaters (Shell & tube exchangers) E-415-A, B & C in March 2013. Subsequent to several attempts to optimise the design, finally on 14th January the system was once again commissioned and field trial undertaken under the supervision of Cairn. The current skid arrangement has common pump and control unit across all three exchangers. The balls are introduced into the system along with process medium at regular intervals, monitored by the inbuilt PLC system. The ATCS skid comes with its own pump that helps in fluidising the balls stored in the ball collector and for injection into the process stream.
ATCS operates on injection cycle, collection and restoration cycle. The overall time duration of operation falls at 20 minutes and repeats itself in regular period interval across all three exchangers namely E-415A/B 7 C. The material selection for the balls was an extended process where in CQM had trial tested various materials before concluding the suitability of Silicone as best suited for the process medium.
ATCS was commissioned on unit E-415B on 14th Jan’15 followed by E-415C on 28th Jan’15. ATCS has been in operation since the aforesaid dates and found to be performing well and has been maintaining DP and DT in line with the original design conditions.
ATCS has been instrumental in achieving and maintaining higher DT and lower DP across exchangers E-415 B/C thereby reducing the need for intervention and consistently delivering higher outlet water temperature required to maintain operational efficiency of the plant. The data was gathered pre and post implementation of ATCS and the same is tabulated in the chart above and below for ready reference.
From the chart it is evident that temperature post ATCS implementation has been in the range of +18 to 21℃ at varying flow rates. The temperature in comparison with the exchanger without ATCS shows a low temperature difference in the range of 7 to 8℃, same is exhibited in the case of Exchangers E-415 D/E, which is visible from the data gathered at site.
When fouling occurs, the temperature of the heating fluid must rise if the same amount of heat is to be transferred through the tubes. This temperature rise must be associated either with an increase in the total energy input to the process or a reduction in production rate, both of which represent a cost incurred due to fouling. Clearly, in order to make economic decisions, the costs must be quantified at a series of points in time and, preferably in relation to the fouling resistance.
Currently Cairn uses DP as criteria to make the decision for cleaning of the exchanger. The time duration for cleaning of exchangers vary depending on the extent of cleaning required. The data shows that the system has been taken offline for duration anywhere between 5 to 7 days in a stretch to achieve a totally clean exchanger.
The time interval for cleaning of exchangers is not uniform because of the varying flowrates and hence cannot be treated with certainty. The same goes to the downtime required to achieve a totally clean exchanger.
It is observed that periodical cleaning using mechanical means such as water jetting or rodding techniques to break the hard scales could possibly reduce the life of the exchanger tubes leading to tube failures and blanking of tubes or replacement of bundle over time. Both the above scenarios are expensive considering the reduced availability of exchanger for process heating and or replacement.
The economics of tube cleaning at regular intervals compounded with reduced availability and consequential loss of production is expensive, therefore the ATCS is considered as the best alternative in this case because there is no costly downtime and or mechanical cleaning of exchangers.
Additionally the ATCS needs minimum to no intervention and automated system which doesn’t interfere with the process.
The methods involved in working out an economic analysis of the ATCS implementation is a time function and given that lack of production data, the summary below is an estimate and the same need to be validated.
The above analysis only considers two key factors and there are many more including savings on account of reduced thermal input, etc. which is not considered here.
Conclusion:
The ATCS from CQM offers a great technical, operation and commercial advantage and therefore the implementation of device across other exchangers of similar service is recommended.
The current ATCS equipment supplied to Cairn for Field trials and for validating the concept and are not built for rigorous service and dose not confirm the explosion proof standards required at site and we recommend changing the same at the earliest after implementation of the same on newer units.
