Electrostatic Desalter/dehydrator/coalescer isolation failure

[GL431]

We used teflon isolators in our electrostatic crude oil desalter. We never had a failure before.

However, due to good reported experience with cast resin isolators at other facilities, we changed from teflon to cast resin also in our facility. The cast resin isolators failed right after start-up on one out of three electrode grids.

The normal voltage level is 15 kV. It went down to 10 kV after the failure. The current increased to 70 A.

I would like to obtain some more specific information on the application of cast resin in this service. Does anybody know:

1) Are there quality control problems with cast resin which do not occur for teflon?
2) Is cast resin fully compatible with the service in mind? (salt water, crude oil, turbulence)
3) Short circuiting is a frequent occurance in this apparatus (when there is much water in the oil). Is cast resin more susceptible to degradation upon short-circuiting than teflon?

Thank you for your help in advance.

 

[EdStainless]

There are some high dielectric resins that might be suitable for your application. In a prior life I gave up on various fluorinated compounds as electrical insulators because over time they develop conductive 'hole' thorough them.
I have seen unfilled PEEK (ans all of its cousins) used for insulator service.
I presume that you biggest concerns relate to moisture absorption and arc tracking.
I would use moisture absorption as the first criteria in a search.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[GL431]

Thank you for the advice, EdStainless.

Do I understand correctly that Teflon (PTFE), which has fluorine in it, develop conductive 'holes' over time?

 

[EdStainless]

I didn't want to use the tradename Teflon.
As far as I remember all of the various types of fluoroplymers, PTFE, PFA, PFE and so on, will develop conductive paths after long term high voltage exposure. I seam to recall that this is worse in wet environments, but it will enven happen in dry ones.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[Three3]

Start up is a rough time for desalters, especially insolator. The process of steaming the crude unit or just the desalter prior introducing oil can expose the vessel to temperatures above the recommended level (typically 350 F/175 C). Exceeding this temperature will crack the insulator, causing a ground fault for that insulator.

Infrequently, a "shot" of hard water through the desalter will leave water soluble salt deposits across the insulator cause a grund.

Just cycling the vessel from normal operation to abient temperature at shut down can crack the insulator because of the differential rates of contraction/expansion. Going slow on the cool down seems to work most of the time. Even so, most hardware manufacturers recommend replacing the insulators at every shut down to avoid this situation. Insulators are fairly cheap compared to lost production values.

 

[0707]

GL431

Our desalters are equipped with Teflon insulators and we have never had problems with them. Our practice during turnarounds is to remove traces of carbon from the insulators and clean them with a degreaser and smooth clothes to avoid dents.

Regards

Luis Marques

 

[GL431]

Hello Three3

Thank you very much for your reply. Do the problems you mention occur for PTFE insulators also (for which we have observed progressing porosity - see EdStainless's remarks above, but no cracking) or only for cast resin insulators?

We have done more asking around (manufacturers and research labs) on the cast resin insulators and it seems that it was a moisture absorption problem with consequent arc tracking.

The PTFE insulators are back in now.