№3 March 2009Table of contents Issue Archive
№ 3 (March 2009)
Electric submersible pumps are widely used for the production of deep-seated liquid fluids such as oil, water, or mineral fluids. To help with the lubrication of bearings and cooling down of electric motors, dielectric oils are applied.
By Material supplied by REAM-RTI
There is a pressure drop between an external medium and oil, of which the volume changes subject to temperature. To eliminate this pressure drop, special devices should be used to compensate for volume changes. Today, an elastomeric seal bag (or bladder) is the most common device used for this purpose. It is installed in the protector of the submersible motor’s seal section.
The bladder’s function is to equalize the pressure between the media and to separate those with the help of elastic properties. Nevertheless, stringent requirements are set to its design, the materials used and manufacturing quality assurance (i.e. eliminating any hidden defects with the help of applied technology) to ensure the long term durability (at least 1,000 operation days) while working both with hot motor oil (up to 200 degrees Сelsius and higher) and gas containing external media, such as borehole fluid. The borehole fluid contains carbon dioxide (CO2); it can also contain hydrogen sulfide (H2S) (12 percent and over) and other gases.
Rupture of the bladder or loss of elasticity (cornification) caused by thermal and chemical incompatibility results in engine failures and non-justified increase in well operation costs, which are incomparably higher than the cost of the high quality item.
That is why the leading manufacturers of oil production equipment have to set stringent requirements while selecting manufacturers of bladders and to pay much attention to both the selection of materials used to manufacture the items and to the specifics of their design. National, as well as international standards for electric submersible pump units recommend using the following elastomers to make bladders:
based on hydrogenated rubbers (HNBR, HSN) for oils with temperatures up to 170 degrees Сelsius; and
based on fluoroelastomers (of AFLAS® type) for oils with higher temperatures (up to 205 degrees Сelsius).
With support available on the part of Russian service companies and a number of leading manufacturers of oil production equipment in Russia and the USA, specialists of REAM-RTI developed rubber compounds resistant to explosive decompression (compliance with NACE and NORSOK requirements) and mastered production of a wide range of bladders.
REAM-RTI invented a new technology for producing bladders using blow molding. This method was developed because conventional technologies (injection or compression molding) do not ensure strict control over the variation in the wall thickness of the bladder. It also allows eliminating residual strain in the course of production. Based on the results of outgoing quality inspection of fluoroelastomeric items, processing deficiency can be quite significant, thus affecting their cost. (See Fig. 1 with a tubular billet and a fluoroelastomeric seal bag manufactured using the blow molding method.)
This technology is featured by using an elastomeric tubular billet calibrated and strained during extrusion for subsequent product molding.
The tube extrusion eliminates air bubble generation within the tube walls, and ensures that variation in the tube wall thickness does not exceed 0.2 mm. This enables the production of a bladder with a wall thickness starting from 1 mm and a set geometry (for example, as an out-off-round cylinder).
Items made using this technology are applied by one of REAM-RTI’s customers in new equipment.
Some customers in Russia requested an improvement to low-temperature elastic properties of fluoroelastomers applied for making bladders and bellows. That is why REAM-RTI specialists developed and carried out operational testing of bladders made of modified fluoroelastomers based on AFLAS® 100H of RS-66 grade. A brittle temperature of this material is minus 65 degrees Celsius, and it is distinguished by a higher process ability compared to fluoroelastomer of АF-15 grade (based on the same AFLAS® 100H).
REAM-RTI mastered the production of piston modules for seal sections of electric motors that can be operated under conditions of oil temperatures over 215 degrees Celsius (up to 250 degrees Celsius and higher) and a high gas to oil ratio, as well as in other conditions where there is a risk of failure of the elastomeric bladders. These modules are already supplied to some Russian manufacturers and service companies. Information on these technical solutions has been published by Oil&Gas Eurasia (issues #6, June 2006; #3, March 2007; #10, October 2008) and by Eurasia Offshore (issue #2, February, 2007).