Modification Case Of Decoking Water Filter In Delayed Coking Unit

A customer’s 2.3Mt/a delayed coking device used the quartz sand decoking water filter element and the effect has not been ideal. In view of the problem of poor filtering effect and material running out of the filter after the long-term operation, the existing equipment was used to carry out technical transformation. 

After the transformation, the filter element of the equipment adopts a 316L wedge wire filter screen, and the coke powder filtration accuracy is 250 microns, which is much higher than the design requirement (300 microns).  

The effective filtration of decoking water is realized, the number of backwashing of the filter is reduced, and the clean decoking water after filtration and the industrial wind of the device are used to recoil and regenerate the wedge wire filter element. 

The equipment failure rate and maintenance costs of the hydraulic decoking system have also been greatly reduced, which fully meets the operational needs of downstream equipment without causing any new problems, and the overall equipment is operating well. 

Filter Operation Status Before Transformation 

The filter cavity is divided into upper and lower parts by the partition. 

A water filter nozzles are installed on the partition, and a certain height of pebbles and quartz sand is filled on it. 

The decoking water from the filter lift pump enters the upper cavity of the quartz sand from the inlet pipe, flows to the lower cavity of the filter through the quartz sand, pebbles, and the filter nozzles, and then flows out through the equipment outlet pipe。 

When the equipment is backflushing, the decoking water enters the lower cavity of the equipment from the outlet pipe. After the water Lateral Screens, pebbles, and quartz sand, the coke powder remaining in the filter material is washed out, flows out of the equipment through the internal pipeline of the equipment and the equipment inlet pipe, and is discharged to the coke storage tank through the sewage pipeline. 

The Equipment Has Exposed Two Main Problems In Long-Term Operation

  • The filtering effect is getting worse and worse. 
    The filter needs to be backwashed once in about 10 days. After 3 months of operation, it usually needs to be backwashed once every 4 to 5 days. After the equipment has been running for half a year, it is basically backwashed once a day, and the filtering effect is getting worse and worse. 
    After the equipment was used for 1 a, it basically did not play any role in filtering. 
  • The filter is out of material, causing serious damage to downstream equipment. 
    When the filter is working normally, a small amount of very fine sand particles will enter the downstream equipment with the decoking water. 
    In the later operation of the device, a large amount of quartz sand leaked due to the aging and damage of the water filter nozzles, and most of the quartz sand was brought into the downstream equipment through the high-level water tank. 
    In the end, nearly 10 t quartz sand was discharged from the bottom of the high-level water tank, and the equipment was cut off from there. 
    The equipment has been in operation for less than half a year, and a large amount of quartz sand has leaked, and the equipment has to be removed again. 

Problems And Effects Caused By Equipment Failure

The filter failure and the decoking water carrying quartz sand cause many problems to the operation of the decoking system equipment: 

  1. The sewage pipeline at the bottom of the high-level water tank is blocked. When a large amount of coke powder enters the downstream equipment, it will be deposited on the bottom of the high-level water tank, which will block the equipment’s sewage pipeline. It needs to be discharged every day to ensure that the sewage pipeline is unblocked.
  2. The inlet filter of the high-pressure water pump is blocked, causing the high-pressure water pump to evacuate. The failure of the filter caused a large amount of coke powder and quartz sand to enter the pump inlet filter. If it is not cleaned up in time, the inlet flow of the pump will be reduced and the pump will be emptied, which will not only cause great damage to the pump, but also seriously affect the decoking time. 
  3. The high-pressure water pump is severely worn, and the pump lift and efficiency decrease. The quartz sand carried in the decoking water causes abrasion inside the pump, which increases the gap between the pump port ring and the pump outlet pressure and the pump efficiency gradually decrease.  
  4. The high-pressure ball valve is severely worn and has a high failure rate. The decoking system has 6 sets of DN200 high-pressure ball valves and 1 set of DN25 high-pressure ball valves, and 3 to 4 sets are required to be repaired each year due to leakage. One of the most serious failures was the perforation of the DN25 high-pressure ball valve body due to the erosion of quartz sand. Since the start of the installation, 3 water motors and 1 coke cutter have been damaged due to internal leakage of the high-pressure ball valve.  
  5. The failure rate of the decoking control valve is high. The decoking control valve has a short service life, and it generally needs to be overhauled once every six months. Every time it is overhauled, it will be found that its valve seat is scoured and abraded.  
  6. The internal abrasion of the water motor is severe and the service life is short. When the equipment was in operation, the most serious water motor suddenly failed, causing stuck and buried drills. 
  7. Frequent failures of the focus cutter. The coke cutter cannot automatically change direction due to the deposition of coke powder and quartz sand. At the same time, the quartz sand will flush the nozzle, which will increase the nozzle diameter and reduce the pressure of the decoking water. 

Filter Modification

After the filter is cut off and out of service because the decoking water is not filtered, there are still many system problems, such as the high cleaning frequency of the high-pressure water pump inlet filter, the coke cutter often fails to automatically change direction, and the bottom of the high-level water tank deposits more coke powder, etc.  

Modification plan

Remove the packing, partitions and pipelines inside the equipment. Install 70 Φ133 × 550 mm wedge wire filters (material 316L) on the new partition again. This structure is conducive to the regeneration of the filter element.   

According to equipment requirements, the newly installed wedge wire filters has a filtration accuracy of 300 microns (the actual filtration accuracy is 250 microns). 

Due to the harsh working conditions, in order to ensure the safe operation of the downstream high-pressure water pump, the design flow rate of the filter element is 10 times the rated flow rate of the high-pressure water pump (285m3/h), which can handle 2850m3/h decoking water. 
The filter screen is installed under the partition, and the medium enters the bottom of the filter from the equipment inlet pipeline, and is filtered from the outside to the inside of the filter screen. Impurities block the outer surface of the filter element to form a filter cake, and the filtered decoked water enters the upper cavity and is led out from the drain pipe.  

On the upper manhole cover of the equipment, a new DN100 nozzle is added to introduce 0.6MPa industrial wind into the upper cavity of the equipment. 

When the equipment filter element is recoiled and regenerated, it first uses the filtered decoking water stored in the upper cavity of the equipment for recoil, and finally uses industrial air for backflushing. 

Due to the increase in equipment safety discharge, the original DN50 safety valve was changed to a DN100 caliber safety valve. 

The inlet and outlet pipelines of the modified equipment were changed from the DN150 caliber of the branch pipeline to the DN250 caliber that is the same as the inlet and outlet main pipes of the original three filters. 

Effect After Equipment Modification

After the transformation, the equipment filter element adopts a 316L wedge wire filter screen, and the coke powder filtration accuracy is 250 microns, which is much higher than the design requirement (300 microns), which fully meets the operation requirements of the following equipment. 

And it did not bring any new problems, the overall operation of the equipment was good, and equipment maintenance costs were also greatly reduced. 

Since the filtering area of the equipment is large enough, it only needs about 20 d to perform recoil once in actual operation. After recoil, the equipment is like new equipment. 

Since the filter reformation and operation, the hydraulic decoking system equipment has been running smoothly, and the failure rate has been greatly reduced. The advantages that have been shown so far are:  

  • The drainage pipeline of the high-level water tank is unblocked. 
  • The inlet filter of the high-pressure water pump is not cleaned up again.  
  • The failure of the decoking control valve has been significantly reduced, and the current service life has exceeded 12 months and no problems have been found. 
  • The failure rate of high-pressure ball valves has been significantly reduced. At present, on average, only two high-pressure ball valves need to be repaired a year. 
  • The service life of the water motor is significantly prolonged (about 9 months), and there is no phenomenon of stuck or buried drill. 
  • The focus cutter has no difficulty in automatic reversing. 

The decoking water filter in the hydraulic decoking system has not attracted enough attention in many delayed coking devices, and most of them have poor operating results. 

Even in some installations, the equipment has been in a state of removal for a long time, which directly affects the normal operation of the entire hydraulic decoking system equipment. 

As a result, equipment failures of the decoking system occur frequently, and equipment maintenance workload and repair costs remain high. 

The operation of the decoking water filter is one of the main reasons for the failure rate of the hydraulic decoking system equipment. Therefore, it must be paid close attention to in actual production and equipment management. 

The system operation must select the filter equipment suitable for the working condition.