Q & A > Question Details
We are facing plugging of 350 distillate pump strainers in the VDU (lube oil refinery). The 350 distillate is the middle cut of the distillation column.
Since Sunday, the pump strainer has been cleaned more than 5 times due to coke deposition. All operating parameters have been compared to the PFD and no deviation has been observed except that the pumparound flow above the 350 distillate bed is less than the required.
Also this problem started after a revamp on our vacuum distillation column in 2013. Since then there has been severe coking in this pump and it requires monthly cleaning. But as of now the problem has become severe and the strainer is getting plugged in a matter of hours.
Your advice/insight on troubleshooting the problem will be appreciated.

Additional info:
We have sent the samples for analysis and are waiting for the results.
The heater skin temperatures were running high, and we adjusted it.
However, we are still seeing coke in the strainer. Although the frequency of plugging is reduced.
How can I increase the flow on the 1st pumpdown. The flow of the 2nd and 3rd pumpdown is above 50 m3/h but the 1st pumpdown is less than 35 m3/h.
05/01/2017 A: Sudhakara Babu Marpudi, Dangote Oil Refinery Company, m_sudhakarababu@yahoo.com
Reduced pump around flow is the possible root cause of this problem. The reduced flow may be due to the corrosion product entry into the pump around distributor and consequent partial choking of pump around spray nozzles. Spray patterns get changed due to choked spray nozzles and will lead to dry spots in the packed beds. Dry spots initiate coking process in the bed and lead to the vicious circle of nozzle choking and further coking. This problem will be severe with piped pump around distributors compared to channel / gutter distributors. Unfortunately the remedy for this lies in complete replacement of packed bed material.
05/01/2017 A: NS Murthy, Suez, murthy.ns.ext@suez.com
In addition to the points given by our other refining professionals, pl look into the following in that zone.
1. C factor and Wetting Velocity in the packing / trays
2. Pr drop across this zone.
3. Temp profile with changes after compensating for pr variation.
4. Potential increase in entrainment
5. Do the analysis of muck getting collected at pump strainer. Check for H/C atomic ratio to give some more insight on type of fouling.
Good luck.
04/01/2017 A: Hubert Mueller, Bayernoil, guhumueller@web.de
You did not mention the type of internals in your column, but I assume you have packings installed and the pumparound return is via spraynozzles.
If coking takes place in the packing then you most probably have an uneven distribution of the pumparound caused by partially or completely blocked nozzles. And this leads to dry parts in the packing, where coking happens.
To verify this I think you have only two possibilities: a radiographic scan and/or a shutdown of the unit and cleaning of the packing (if possible, better replace it) piping and each individual nozzle.
During the shutdown and opening of the column you have to be extremely careful to avoid autoignition of the coke in the presence of oxygen.
04/01/2017 A: Prakash Pimparkar, Environmental Consulting Services , prakash.pimparkar@yahoo.in
The problem could be in the VDU fired heaters, have you changed your throughput? This could be due to heater skin temperature increase or hot spots. Check the heater operating conditions.
04/01/2017 A: Krishna Rao Pulugurti, Retired/Consultant, pkrao2012@yahoo.com
There may be cracking taking place. Check Bromine Number of the 350 distillate. If it is more than 1, expect cracking.