Returning a crude oil export cooler to fully operational condition
Even the most reliable heat exchangers occasionally need to be serviced. So when a customer had trouble with a plate pack which hadn’t been serviced for eight years, they turned to Vahterus Service for help.
The customer had been using a 9HH-324/1/1 openable heat ex- changer for cooling crude oil on an oil drilling platform. They used two similar heat exchangers working in parallel and both of these were installed at the platform. They also had backup plate packs on-site to enable smooth servicing.
The customer had noticed issues with one of the plate packs and discovered that metallic particles had got into the seawater side of it. They decided to send the plate pack to Vahterus Service for inspection, cleaning and refurbishment. The plate pack had been in use continuously for eight years without a stop before servicing.
Vahterus Service Work
As soon as they received the plate pack, Vahterus Service engineers got to work. Before attempting any cleaning, they inspected the unit visually with the aid of mirrors
and an endoscope. Just as the customer reported, the seawater side was contami- nated with metal particles. However, it also suffered from seawater-based biological fouling. And on the oil side, they also found heavy contamination with oily material. This appeared to consist of both asphaltines and waxes from the crude oil. During the inspec- tion, the integrity of the plate pack was also tested and no leaks were found.
The next step of the inspection was a pressure drop test on the seawater side. This was performed at the Vahterus R&D laboratory to try and understand the condition of the plate pack. The tests involved varying the water flow through the heat exchanger and measuring the pressure drop with various water flows using a standard pressure drop test. In these kinds of tests, the water flow can be as high as 250 t/h. This helps show the real condition of an old plate pack and helps identify which future actions are needed.
Before servicing, the measured pressure drop on the seawater side was over three times higher than the values for a clean heat exchanger. Such a high pressure drop indicated fouling in the unit and it was obvious that the seawater side needed to be cleaned. First, the loose material was manually cleaned from the inlet port of the unit and then alkaline cleaning was performed overnight. Our engineers then per- formed another pressure drop test, and while the pressure drop had been reduced it was still about 2.5 times higher than the designed specifications. Therefore, they decided to do another round of chemical cleaning. This involved a combination of acid cleaning followed by higher temperature alkaline cleaning. Finally, the unit was backflushed with a large water flow to re- move all the loose material between the plates.
After this second round of cleaning the sea- water side pressure drop was reduced to values close to that of a new plate pack.
The oil side also needed to be cleaned. The first step was to select the correct cleaning method. In the lab at Vahterus R&D, our engi- neers ran various tests to determine how to re- move the oily contamination from the plate pack.
Their target was to understand the solubili- ty of the contaminants, and then to identify the best choice of solvent and how increasing the temperature would affect the cleaning process. They found that the contaminant was soluble in hydrocarbon solvents and that temperatures over 70°C would reduce the wax viscosity to a level close to where it would start to drain.
After these tests, the shell side of the plate pack was cleaned. First it was sprayed with sol- vent to soften the layer of dirt. Then the plate pack was washed with a pressurised water jet.