Frequently asked questions
AC/DC Electrostatic Fields
What are the limitations of AC/DC electrostatic fields?
AC/DC fields rely on an electrostatic gradient between a postive and negative plate. If you put a conductor between these two plates, an electrical short will occur, eliminating the gradient, which then eliminates the effectiveness of AC/DC fields.
Additionally AC/DC fields rely on the movement of water particles to collide to cause coalesce of the water droplets into larger size molecules. Put a barrier in the way of this movement such as stabilized emulsions and high surface tensions, then movement is restricted.
What are typical conductors that will short an AC/DC field?
- High water contents, typical in stabilized emulsions and heavy oil applications.
- Conductive oil, found in heavy oil applications.
- Chemicals such as polymers used in drilling, and water based chemical injection solutions.
Heavy Oil experiences shorting issues with AC/DC fields. What other issues do AC/DC fields experience with Heavy Oil?
AC/DC fields rely on the movement of water molecules to impact into each other in order to create larger water molecules. Put a barrier in the way of this movement then this limits the effectiveness of an AC/DC field. Typical barriers are stabilized emulsions and high surface tensions, long chain hydrocarbons etc., as experiened with heavy oil.
What does modulating AC/DC fields do?
In theory by modulating a frequency, this changes the acceleration of the water particle in an AC/DC field. Vendors that offer this type of solution, state that it helps breaks emulsions, however shorting issues still occur, surface tensions still need to be overcome, as well as movement through long chain hydrocarbons present in heavy oil. This still limits conductive operations, requires the extensive use of chemicals in heavy oil operations, and barriers to movements still exist. As a result a modulating AC/DC will in essence just increase capital cost and a more complicated operating experience.
Can I add a control system to an AC/DC system to limit issues of shorting experienced with high water contents and conductivity issues?
The short answer is "no"!
By adding an SCR/Thyristor control to the front on the AC/DC system, all you are doing is setting up a way to automatically reset the power every time a short occurs. However, this does not eliminate the short. The short still occurs and the AC/DC gradient is still eliminated.
Is there an AC attration between the AC/DC grids and the interface layer in an AC/DC electrostatic field?
Well first of all a visible interface does not exist in 99% of all electrostatic operations. It is more of an "emulsion pad" that varies in water content as you move up the pad.
Secondly there is no alternating current experience on the grids. In all AC/DC systems the AC current is changed to constant DC on the grids.
So how an AC field is created between the grids and interface is a disussion that is ongoing.
Why does one vendor offer a much smaller AC/DC vessel size that others?
Again a quick answer is "they should all be about the same size".
Electrostatic dehydration and desalting relies on the removal of water molecules from oil under Stoke's Law, which is a Law of Science applied to everyone. Assuming that gravity, operating viscosities and differential densities are the same for all vendors, the only way you can increase separation under Stoke's Law is to increase the water droplet size.
In order to obtain smaller AC/DC vessels than others there must be some way the the droplet size being created is much larger than the droplet sizes of the same AC/DC system being offered by others. Unfortunately there is a limit to the largest droplet size that can be created using electrostatic technology, which is called Gauss' Law, which applies force to the water droplet. Too much force actually rips the water molecule apart making them smaller, not larger. This maximum force is the same for every vendor. So in essence the maximum droplet size for all vendors is the same. Therefore all vendors should have about the same vessel size.
Things to note here.
- Operating viscosities can only be estimated if enough viscosity points are offered to meet ASTM D2270 & D341calulations. Do not provide enough data points and one can assume a much lower operating viscosity, providing a smaller vessel sizing!
- Operating viscosities can be manipulated using chemicals. So one can assume a lower operating viscosity, providing a smaller vessel then "blame it" on chemicals.
- Smaller vessels increase vertical velocities,which require the water particule size required to be much larger to meet the same separation. So as you descrease vessel size, the problems of operation goes up exponentially.
Client should take a look at these points when comparing vessel sizes. Do not always take the vendors word for a guarantee. If "it does not work" the client ends up with the problem. Conservatism is a must with electrostatic separation, especially with heavy oils that almost have the same densities as water!
I have an existing AC/DC system that has emulsion/heavy oil operation/conductive issues. Is there any thing I can do about this?
You can convert the system to AC. This would require a change of internals inside the vessel, and a different transformer power supply.
Alternatively AMR Process, has a patent pending, operational, retrofit solution, that converts the AC/DC system to AC, by just changing the transformer (no internals changes required). It also allows the client to change back to AC/DC system, with a flick of a switch if oil conditions change. AMR Process is the only supplier in world with this technology.
Electrostatic Oil Treater Entrance Bushings/Power Unit Cable Failures
Why do Entrance Bushings fail?
Traditional entrance bushings have a metal cap screwed onto a soft Teflon® NPT thread. This thread will fail due to damage of the thread on screwing the metal cap on and off before it is even put into operation. As a result, it is necessary to put O ring seals in place. Over time this O ring seal will fail too. This leads to a failure of the entrance bushing with oil leaking into the HV connection, shorting the transformer on the HV side, and creating extensive carbon disposits the HV compartment of the transformer.
How to I clean up an entrance bushing failure?
You need to drain all the transformer oil from the HV comparment and hook up.
Replacement or extensive cleaning of the HV tubing/compartment is required (it needs to be spotless).
Brake oil can be used to clean the HV side of the transformer (again it needs to be spotless).
Throw away and replace the expensive, failed entrance bushing.
Throw away and replace the HV wire.
Refill with clean transformer oil.
Can I limit, or eliminate Entrance Bushing failures?
Absolutely! Ask for a Dual-Lock® Entrance Bushing for initial installation and/or replacement of the existing Entrance Bushing. A Dual-Lock® Entrance Bushing, has a total of six seals inside the bushing, and additionally has metal on metal NPT hook up (eliminating the issues of stripping the soft Teflon® before being put into operation). The Dual-Lock® Entrance Bushing can retrofit 99% of all existing installations.
How can I tell when an entrance bushing fails?
You will see constant maximum amperage on the transformer power supply, off spec oil, and discoloration of the HV transformer fluid.
Sometimes, my HV wire shorts without an Entrance Bushing failure. Why is that?
There are a few causes:
- The HV wire relies on insulation around the core wire. If that insulation becomes damaged in some way, then a short can occur. This is seen with HV wire that have soft insulation coatings, that become nicked or damaged before/during installation, or twisting of the HV wire during insulation.
- Water or air bubbles in the HV housing.
- Dirt in the HV housing (common problem).
What is the correct installation procedure for the HV bushing?
1. Remove fluid from HV compartment of transformer so the HV wire can be hooked up. You will need a few extra litres of transformer fluid during refilling (this is always included by AMR Process).
2. Keep removed fluid clean and away from rain etc.
3. Hook up entrance bushing, hook up transformer to vessel HV housing, thread wire from transformer to entrance bushing, secure wire at entance bushing, secure wire at transformer.
4. Add transformer fluid slowly to HV comparment of transformer. Stop every few minutes. Tap on vessel flange and housing to eliminate any air bubbles. Be patient, this will take a few hours.
5. Wait 24 hours before energizing to allow any final air bubbles to escape.
How can I eliminate the damage to the HV wire insulation to avoid shorting issues?
First option is to be really, really, really careful, and make sure the wire is not damaged before installation/during installation and is not twisted during installation.
Alternatively, you can use interlaced, non-arcing, hard insulated wire, with a secondary insulation, that AMR Process supplies as standard.
Additionally the Dual-Lock® Entrance Bushing has simple set screws for securing HV wire and an anti-twisting design during hook up.
