Fouling of cooling water exchanger

[dieselpet07]

Hi,

In petroleum refinery particulary in Coker plant most of the cooling water heat exchangers especially debutanizer overhead cooling water condenser , wet gas compressor interstage cooler are getting fouled severly including leakage also found with in 4-5 months of operation eventhough cooling water quality like pH,TDS & other necessary parameters are being maintained but still fouling/leakage observed.The detail analysis of foulant/depositions (inside the tubes)at the time of shut down / maintenance of exchanger has also been carried out and it was found mostly Fe2O3 -60% & others are PO4-3 & almost ~97 % of Fe2o3 in the fouling depostis analysis which was deposited on outside surface of the tubes.The exchangers MOC is C.S.The one of the mystry that could not able to identified properly - the frequent fouling/leakage problem of most of C.W.HEs are started after comissioning & taking in line of new Cooling tower.(Pl. note that the operating & quality parameters of C.W.is well with in specs in comparision to other C.T.s in the refinery).

Now the question is what could be the probable & root cause of these severe fouling/leakage of HEs ?

Thanks in advance to all of you..

 

[EmmanuelTop]

Hi dieselpet07,

You are telling us that your cooling tower water is in perfect condition and you get severe fouling on the CW side of heat exchangers?

As a first step, you could do the following:

1. Check what is your average (including maximum peak value) of CW outlet temperature. Regardless of water quality, it should not exceed around 40C.

2. Take a sample of CW at the inlet and outlet of process coolers/condensers while the plant is running and check for usual contaminants concentration.

I have seen similar things happen while I was working in refinery, but the cause of fouling was mostly "mechanical" - pluggage of tubes with sand, wood branches, shell and snails (believe it or not - I even expected to catch a fish sometimes). If CW outlet temperature is maintained below 40C, you should not have these "chemical" causes of fouling.

Best of luck,

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[SeanB]

How are you controlling the outlet temperature of your exchangers? Are you bypassing flow on the process side or the cooling water side to control temperature?

 

[dieselpet07]

Hi emmanueltop & seanB,

Thanks for your reply.

The cooling water exchanger which gets maximum failure/leakage is debutanizer overhead condenser in the coker complex and in the c.w. outlet also there is no any Temperature guage/TT or not even c.w. flow transmitter.There is no any bypassing of the flow..but further investigation shows that C.T. circulation cooling water shows little bit higher Fe ppm compare to another cooling tower water that is more than specified limit of 2 ppm i.e. up to 3-5 ppm eventhough corrosion coupon installed near cooling tower in cooling water retrun header shows 0.8 – 1mpy.Furthur,at the time of leakage of debutanizer overhead condenser , in c.w. sulfide was analyzed and it was found 0.3 ppm in c.w. i/l and 0.8 ppm in c.w. o/l while in running normal case it was almost same i.e. 0.08 – i/l and 0.07 – o/l of c.w. I could probably guessing that first failure may be happens at debutanizer overhead condenser and since it contains H2S, the penetration of sulfide/h2s in c.w. circulation flow in coker complex may be leading to other exchanger failure….The deposits which we could see during cleaning/repairing of the same exchanger shows almost 60-70 % Fe2O3 & rest PO4-3 –mainly two contaminants.You can also have a look at the photographs of exchanger opened in s/d.

http://files.engineering.com/getfil...e6&file=C.W._exchanger_Inlet_50%_blockage.JPG

http://files.engineering.com/getfil...120e9e4c4&file=c.w._exchanger_Inlet_tubes.JPG

http://files.engineering.com/getfil...file=c.w._exchanger_Iron_PO4_-_Ca_deposit.JPG

Thanks & Regards,

 

[EmmanuelTop]

I think you definitely have to establish the way for monitoring CW temperature profile (especially the outlet from heat exchangers). If TT are not installed, I suppose you need to have at thermowells at least.

And I bet something really strange happens with your CW quality... How frewuent do you monitor COD & BOD of water?

http://antwrp.gsfc.nasa.gov/apod/astropix.html

 

[dieselpet07]

Dear Emmnueltop,

BOD is not being monitored; COD is being monitored once in week- that remains below limit i.e. below 250 ppm around 228 ppm.

Parameters Design Actual
LPG flow (TPH) 128099 132500
LPG inlet Temperature (Deg C) 61 68
LPG Outlet Temperature (Deg C) 38 46
CW Flow rate (m3/h) 811.5 1130
CW inlet Temperature (Deg C) 32 32
CW Outlet Temperature (Deg C) 45.8 42
Overall heat transfer coefficient
(Kcal/hm2deg C) 202 77.67
CW side Fouling Factor
(m2 h Deg C/Kcal) 0.0004 0.0022

The monitoring of C.W.o/l temp. oftenly being done as and when flow survey has been done.

Recent flow survey shows around 2- 2.7 m/sec velocity in c.w.side.

 

[rgrokkam]

Hello
The pictures remind me of the same damage in one of our effluent lines. It was traced to Microbiologically induced corrosion. Any scope for that?

 

[unclesyd]

As stated in rgrokkam's post there is very strong possibility that you have MIC. Along with MIC you appear to have a very bad bacterial slime problem. I would look over this web site and then contact them for specific recommendation on test kits. I would definitely look a the slime bacterium kit, this also tests for the big 3 of MIC.

This site is an excellent source of information on all aspects of bacterial problems in water systems.

http://www.mic-test.com/home

http://www.mic-test.com/slime