Root Cause Analysis For Product
By Shireesh Swami
One of the clients was encountering issues pertaining to the premature failures of both the product receiver (PR) filters in Polypropylene Plant. In the PP plant, resin and cycle gas from product discharge is separated in product receiver where gases are filtered through pulsejet type bag filtering system and sent to vent recovery system for monomer recovery. The conveying gas is passed through the filters to remove fine particles (99.99% removal efficiency for particle size of 2 microns & above) of resin from stream going to vent recovery system. The product receivers are equipped with filter bags having blowback system to dislodge the collected dust over the filter bags.
The objective of the study was to;
- Analyze the product receiver filter system and identify the probable causes for the premature failure of the filters.
- Recommend measures to improve the performance of the PR filters.
In PP plant, downstream of reactor consist of two product discharge systems which use dense phase conveying for product transfer. The PR filters are installed in the top portion of product receiver vessels. The resin from reactor product discharge system (PDS) is conveyed to two receivers by means of conveying gas, where resin is separated from conveying gas through these filters. The clean gas from PR filter is sent to vent recovery system. The process gas from vent recovery system is used for the blowback.
During the operation it was noticed that after cleaning and replacement of filter bags, the differential pressure across the filters gradually increase over a period of time (approximately 2 months) leading to the frequent plant shutdown and production losses in the plant. Following major issues were observed at various instances of bag filter failure;
- Accumulation of product above the tube-sheet and subsequent carryover to the guard filter at suction of vent recovery compressor.
- Cracks in the tube-sheet welds
- Bulging of tube-sheet
- Damage to the bag & bag holders
- Filter cages lifting
Analysis & Short Term Recommendations
The overall process parameters, design & construction of the product receiver system were studied, analysed and carried out the detailed hydraulic calculations. The study report recommends incorporation of certain improvements for immediate execution within the existing installation which would address the present issues with minimal commercial implications and relatively lower shut down time.
- It was observed that allowable pressure drop limit is exceeding with the existing line size. In order to meet the pressure drop criteria it was recommended to change the existing line size. The pressure control valve was suggested to be resized and suggested to remove flow orifice installed in the line to quickly replenish the accumulator with the pulse jet gas for the next pulse.
- Suggested to increase the pulse jet gas volume by installing additional header of the same size as that of the existing header above the existing header
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to act as accumulator.
- Recommended to increase the tube sheet thickness (Bag holding Sheet) to approximately 8 mm from the existing 6mm to take care of the higher differential pressure.
- Instead of present fillet weld between the two different thicknesses of tube sheet and tube sheet ring, full penetration butt weld was suggested. With this the stress concentration shall be minimized and the weld failure is avoided due to cyclic loading.
- Longterm Recommendations
The study report recommends incorporation of future or long term improvements within the existing installation for reliable and sustainable operation without down time of the product filter and hence improved production.
- Consistently high differential pressure could be an indicator of insufficient air to cloth ratio. The designed air to cloth ratio was on higher side, although in many applications it may still work as desired if the blowback system is operating efficiently. The pleated filters being one of the recent developments in the filtration technology, can offer 2 to 3 times the filtration area with respect to the conventional filter media. This significant increase in the available filtration area can be gained by retrofitting existing conventional filter bags with pleated bag filters. It will reduce the system’s differential pressure and air to cloth ratio thereby improving airflow, reducing the number of filter cleaning cycles and improving the dust collector system’s overall performance.
- Upgrading the conventional baffle plate design to ladder vane type baffle plates for effective flow distribution and even grain loading to the filter elements. The bag filter operating at higher dust loads and inlet velocities tend to damage the filter bags due to direct impingement. In order to regulate or effectively distribute the flow, either inlet hopper can be enlarged or baffle plates can be introduced at the inlet of the hopper. The inlet baffle plates effectively regulate the high inlet velocity of the bag filters. The ladder vane type baffle plates or distribution screens have been suggested for bag filters with high dust loads or inlet velocity. The ladder vane baffle has several openings that create a more uniform gas flow velocity profile in the filters, reducing turbulence and uneven grain loading. By allowing better distribution of the inlet air, the baffle minimizes dust re-entrainment and reduces the amount of material carried to the filter surfaces.
Client approached TCE for retrofitting solutions of the product receivers as they were frequently failing (once in approximately every 2 months) leading to significant production loss of around 2 weeks until the product receivers are retrofitted and put back in service. TCE carried out the detailed root cause analysis and site visit to identify the probable causes of the failures. It was observed that there were shortcomings on process design of the product filtration system and the problems were faced ever since the plant commissioning. These short comings have been duly addressed and recommendations on short term and long term solutions have been communicated to client for further implementation.