Why so much pressure across my Spiral Heat Exchanger?

[valene]

We have a Spiral Heat Exchanger(SHE) downstream of our FWKO(free water knock-out) & Treater vessel where we want to cool the produced water(Alberta SAGD heavy oil) from 110C to 92C using glycol entering at 60C. The SHE is not performing to spec in terms of pressure drop & temp. Maximum duration is about 1 week before temp of produce water(PW) crepes up from 92 to 98C and/or pressure drop increases from 150 to 300 kPa. Our SHE sizing shows pressure drop suppose to be only 73 kPa. Our initial suspicion was fouling due to oil carryover and when we opened up 1 unit we saw particulates i.e. organic asphaltene like substance on first couple of the inner rings of the Spiral and powdery like inorganic substance(brown in color) on the last 3 outer rings of the SHE before the PW leaves the exchanger. After we chemicaly cleaned the unit and thermally flushed the hot side with 135C boiler blowdown we flow the PW again with the same results after 1 week. This time we're suspecting not only fouling but gas entrainment because we open the high point vent on the PW return leg and it vented anywhere from 1-3 hrs. Is this entrained gas that somehow got carried with the PW especially from the FWKO or is this CO2 dissolved gas that escapes when pressure drops across the SHE? We have a Group Separator upstream of the FWKO that supposedly has remove majority of the gas. We've taken a sample of the gas for analysis. Now we've even notice air in the cold glycol side so looking into the source of the air. I'm guessing the main impact on the exchanger performance is the gas on the hot side making this into a 2 phase flow exchanger and air in the glycol line. This SHE was not design for 2 phase flow. I'm not ruling out fouling due to oil contamination but maybe not a huge impact. If anyone can shed some opinion I'm ready to listen.
Thks

 

[Compositepro]

If fluid velocity is not great enough to keep gas flushed out of the heat exchanger then there must be a vent for the gas. Otherwise the heat exchanger will not work. This also applies to sedimentation problems.