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Our Vacuum gas oil hydrotreater unit is operating at 125 KBPSD. We are processing feed, HAGO, HVGO. LVGO from crude unit and HCGO from Coker unit.We are having 24 filter for removing contaminants including one backwash filter.Our filter dp during steady state operation is 0.7 to 1.0 bar. We often face problem of high pressure drop of 3.5 to 4 barduring crude unit HVGO pump changeover or during taking of HVGO exchanger in line.In crude unit there are 2 HVGO pump one running and other standby with total flow rated capacity of 1770 M3/Hr.Running capacity 1650 including pump around,IR and product HVGO to our unit.Normally we consume approx 440 to 470 M3/Hr HVGO during steady state. I am not able to find out the root cause for high dp across feed filter during such activity in crude. This results in throughput reduction in ZVGOHT unit. Please suggest the possible cause for increase in filter dp
 
Answers
13/12/2011 A: keith bowers, B and B Consulting, kebowers47@gmail.com
You did not mention whether the High DP is transient, or persistent after the crude unit changes. IF the high DP is persistent, requiring change out of the filters, there are two potential causes.
1) The pump returned to service has accumulated fouling deposits and that 'slug of dirt' reaches your unit and plugs the filter, or;
2) During the operation changes, there is a momentary surge in flow rate that collapses the filter cartridges.
12/12/2011 A: Eric Vetters, ProCorr Consulting Services, ewvetters@yahoo.com
Filter plugging could be from particulates in the feed such as coke particles in the HCGO or corrosion products like iron sulfide in any of the feeds. It is also possible to create asphaltene instability and precipitate asphaltenes with some feedstock combinations. Asphaltenes will be present in HVGO as part of the entrained heavy ends not scrubbed out in the wash oil bed.
I would run filterable solids tests on all the feed streams to see if there are any streams that are high in solids. I would also analyze the deposits from the filters to see if they are mostly inorganic or whether they are mainly asphaltenes.
If the foulant is mostly asphaltenes, you may need to do some blending studies to see if there are particular blend ratios that cause asphaltenes in the feed to destabilize.
12/12/2011 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
I can think of two possible reasons:
1. When the city does anything to disrupt the water supply system in our section of the city, when the work is completed, every faucet in every house experiences rust colored water and plugged screens for several hours before returning to normal. The cause, of course, is dislodged. iron oxide scale and any foreign material that may have entered the system when open. In your case, the pump suction piping arrangement may be such that solids may slowly collect in the suction piping of the spare pump and ultimately be trapped in the feed filter after switching pumps.
2. It could be that just the act of switching the pumps could dislodge loose scale in the system by the shock wave created by the switching. Some people conduct the switching operation in a deliberate stepwise procedure so as to maintain a constant flow rate without “bumps”.