It is possible that LVGO packing is mechanically damaged and you're getting partial channeling of upflowing vapors, resulting in higher DP across the bed. Depending on column configuration, this may result in increased end-point of LVGO due to heavier hydrocarbons entrainment. If there is a fractionation bed between LVGO and HVGO, it will be much difficult to diagnose these kind of problems.

Have any changes been made (crude type, VDU operating parameters etc.) before you noticed this behaviour?

Petro,

Vacuum column dP increased from normal 12mmHg to 25mmHg, it should cause significant vacuum residue yield increase, but you didn’t mentioned that. Does it happen?
If not, check dP measurement.
If yes, did dP increased during period of time or suddenly?

Regards,

Milutin

Let me describe this problem :

Increment in LVGO bed Delta P is overa period of time and not sudden like in Jan-06 it was 2 mmHg than slowly it was increasing order and it went up to 12 mmHg in October 06 with sharp peak and than again come down & now again with in one week beofore ,it has started increment up to 12 mmHg.

We suspect the following can be the reasons :

1.NH4CL
2.FECL3
3.Waxy mist/slopy/tary material
4.Coking.

Now when we have seen the LVGO Pump strainer it was having waxy material.But after analyzing it has found most of chlorides in to it.The following is the analysis of LVGO pump strainer deposits :

Chlorides,wt%    14.61
NH3,wt%     4.12
Sulphates,wt%    0.25
Phosphates,wt%    0.004
Sulphides,wt%    0.02

The material from the LVGO pump we have found sticky material which mets around 100 C.,Currently LVGO temp. we have increased up to 120 C and Detla P across LVGO bed seems marginal reduction.VR yield is higher by ~0.75 wt %.Presence of chlorides is confirmed (NH4Cl or FeCl3 is very difficult to confirm but chloride is there).

Vaccum column o/h temp. generally it remains around 40 C.While LVGO Draw temp. generally remains between 80 to 100 C. LVGO PA Flow generally remains 400 m3/hr.From October 06 to April 06 LVGO draw temp. was round about between 60 to 80 which was earlier between 80 to 100 C.

When in October 06 abnormal pick observed in Delta P of LVGO Bed at that time,PA Flow of LVGO was lower i.e. around 180-200 C at the sharp peak of 12 mmhg delta p of LVGO bed in October 06.PA Flow & DeltaP of LVGO bed was contradictory peaks,than again deltap of LVGO bed normalized.
In Jan-06 we have observed thin layer of coke in RCO & VR pump from which point slowly Delta P of LVGO bed started increasing.

We are also suspecting about the waxy mist entrainment from lower section of VDU column ,may be due to distributor tray damage /choking and entrainment of waxy mist up to LVGO bed.We have also observed waxy mist in the ejector condenser which led to increse in pressure of VDU column.To reduce the top pressure from oct06 onwards we have reduced LVGO draw temp. as i mentioned above.

SO,Now the question is the probable causes are :

NH4CL or Fecl3 or Waxy mist entrainment or Distributor spray nozzle malfunction,damage,choking or any other mistry ?

We have got a LVGO bed,LVGO circulation bed,HVGO bed and slop wax bed.

Request your expertise and throw some lights on this.

Once again thanks in advance to all of you.!!!

I wouldn't suspect vacuum condenser fouling, since you experienced LVGO packing increase in deltaP only (if I understood well). If condensers are fouled, it is easy to detect such phenomenon by touching the external surface of the condenser. Any subcooling areas and temperature gradients around heat exchanger are evidences of waxy deposits or vacuum system in-leakage, resulting in higher head demand for free condensate draining. We had similar problems in our VDU two years ago. This phenomenon results in higher column overhead pressure, not in DP rise in specific column areas.

Now, focusing on your LVGO pump deposit: are you injecting neutralizing amine or ammonia in the LVGO pumparound circuit? Amines are something that certainly must be avoided in vacuuum distillation units. If dozing NH3, please check the operation of upstream atmospheric distillation plant. How much chlorides you have in desalted crude?
If crystalization of waxy materials inside LVGO packing bed is taking place (which I never encountered so far), you may try to rise tower overhead temperature and, consequently, LVGO draw-off temperature. This should promote melting of hydrocarbons with high pour point inside the column. Also, if your LVGO pumparound temperature is too low (water trim-coolers?), you can cut the cooling water flow and see the results.

Of course, any of previously mentioned mechanical issues remains suspicious. To perform this kind of investigation, you can cut several holes in column insulation around the LVGO packing (at the height approx. 10cm above LVGO bed) and measure the temperature profile with IR thermo-gun. If maldistribution is an issue, you'll see very different (external) temperatures around the packing. At last, column gamma scanning can give you an answer what is exactly the problem, concerning mechanical integrity of column internals.

Regards

We have observed waxy mist material in ejector condenser also.Means fouling is suspected in ejector condenser too.No spray nozzles seems choked in LVGO Circulation bed and same with the HVGO bed as per back pressure of distributor spray nozzle.We are using sour water containing 80 ppm NH3 with high TDS as a desalter feed water and 3 ppm NH3 injection to VDU Top tower.We have two VDU trains, so, one is working ok and one is having LVGO bed fouling problem,where one thing we suspect that lowering of LVGO draw temp. from 80-100 to 60-80 C has caused the problem of NH4Cl sublimation,but there is really a waxy mist entrainment thoughtprocess is going on because vapor velocity in LVGO bed seems high as high as 0.55 ft/sec as per predicted ASPEN steady state model.
We are not measing the chlorides in the desalted crude.