2 replies to this topic

[Cwsrt]

Hi, good day to everyone!

I have some inquiries regarding issues encountered by our plant (refinery). I hope you could spare a little time to voice in your opinions. They'll be highly appreciated.

In our refinery, we have a separate water system using boiler feed water as the cooling medium. The system is composed of a few heat exchangers connected in parallel and an air fin cooler to adjust the heated water's temperature. The supply temperature of this system is at 75-80 deg C (which I think is a bit high) to "cool" pipetill overhead stream, vacuum tower bottoms, and petrochemical rundown coolers.

Recently, we encountered a tube leak at one of the heat exchangers and it was suspected by out plant reliability group that the leak originated from the water side.

The water system does not receive any treatment other than that of the boiler feed water (MMF, RO, deaerators). Is there any other parameter that we may have missed that lead to the tube leak incident? Is the BFW normally corrosive to CS pipes used for cooling water? Please help me on this issue. Thank you.

BTW, here are the parameters of our BFW:
Silica: 1 ppm
Hardness: 0.2 ppm
TDS: 20 ppm
pH: 8-9
O2 scavenger: 0.5 ppm
Polymer: 0.5 ppm

[breizh]

You should talk to the supplier in charge of your water treatment system about problems encountered .

Breizh

[kkala]

1. Boiler feed water can be demineralized water. The latter is corrosive to carbon steel pipe, http://cedb.asce.org/cgi/WWWdisplay.cgi?77662. I have vaguely heard that no inner layer of deposits can be formed to protect carbon steel from oxygen, so corrosion depends on O2 concentration on the water. A local refinery demi water plant (resins) produces water tranfered by stainless steel piping, with dissolved O2 = 7 mg/l (7 ppm) as a maximum. Reported O2 concentration of 0.5 mg/l is low for water entering the deaerator, but high for water entering the boiler (locally 0.005 ppm). Probably BFW of the query is preliminary oxygen scavenged upstream coolers (to eliminate corrosion), finally entering deaerator with 0.5 mg/l O2. Can you please confirm / explain? A simple sketch of whole BFW route would be also useful, indicating any atmospheric storage tank too (air ingression source).
2. Remaining chlorides can also result in (pitting?) corrosion, yet reverse osmosis water seems not to have this risk, ( http://www.finishing.com/521/30.shtml - first response). Reported RO is understood to mean reverse osmosis, but what does MMF mean?
3. In mentioned local refinery, condensate is distinguished between oily (main criterion: if oil pressure in an exchanger is higher than that of used condensing steam ) and "clean" condensate. Oily condensate undergoes deoiling (coalescer + activated carbon), then it passes from special resins to reduce total iron content (to 0.05 mg/l) as well as other heavy metal cations. "Clean" condensate is flashed, then passes from same resins. Both condensates end to a common cone roof tank and can be used as feed water to deaerators or elsewhere. Demi water can feed deaerators too.
Apparently mentioned treatment aims at satisfactory operation of the boilers (not at resisting corrosion). It is noted, since BFW of the query has also a long route, passing from exchangers; so something similar might be considered in future. Recommended BFW specs can be seen at http://www.lenntech.com/applications/process/boiler/boiler-feedwater-characteristics.htm (but O2 before scavenger addition is not well understood), as well as http://www.altret.com/upload/article/200902110704471251523487_boilerwatertreatmentguideline.pdf.
4. Chelants can be corrosive, especially not far downstream of their injection to water pipe,
http://www.matcoinc....r_feedwater.pdf . At least here chelants can be confused with "polymers", check corrosive behaviour of used polymer with its supplier (if not clear).
5. I have only dealt with part of BFW circuit and local Process Dept was not responsible for materials (of piping and equipment); yet above may be of some usefulness, though not complete. Communication with water treatment vendor, suggested in post No 2, can also help.
A tube leak in exchangers can be caused by other reasons beside corrosion anyway (http://www.ehow.com/...xchangers_.html '> http://www.ehow.com/...xchangers_.html ).
Furter contribution to this thread by other members would be welcomed.
Dissolved O2 in the water passing from exchangers can be checked from time to time (and probably chlorides once).
Periodic check of oil content and total Fe content does not concern corrosion, but good operation of the boiler (perhaps there is oil detector in BFW circuit to cause alarm in the control room).

Edited by kkala, 13 August 2012 - 02:07 AM.