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Drilling &Completion

Figure 5: Accumulator can be tailored to the project’s need. Figure 6. This comparison shows the cost vs life of the

Source: Technetics Group. Source: Technetics Group.different accumulator types.

Bladders must be replaced from time to essentially the same components as into deeper waters and more extreme time, which requires taking equipment bladder and piston accumulators, but environments. As a result, the equip- offine. use edge-welded bellows to sepa- ment used in these applications is rate the gas charge from the system being pushed to its design limits. fuid, Figure 4. As the fuid pressure

Determining the type of accumula-

Piston accumulators

Piston accumulators similarly increases, the extending bellows com- tor best suited for an application consist of a pre-charged pressure ves- presses the gas. depends on a number of factors, notably the importance of reliability, sel, fuid port, and gas-charge service Provided the bellows have a low cost of maintenance and the envi- port or valve, Figure 3. But instead spring rate, pressure is essentially bal- of a bladder, they use a metal piston anced on both sides. Since no perme- ronment in which it will operate. with an elastomeric or non-elasto- ation or dynamic seals are involved,

Figures 5 and 6 provide a summary meric seal. As system fuid pressure bellows accumulators can be designed comparison of the different types of increases, the gas is compressed by to provide extremely long life. accumulators in terms of these and the sliding piston and dynamic seal. Among the major advantages offered other factors.

Provided the friction is low, the pres- by bellows accumulators is the fact Bladder or piston accumulators can sure on both sides of the piston is that they are maintenance-free and be used for applications where they essentially balanced. do not require recharging. The bel- can be easily accessed for mainte-

These accumulators are relatively lows form a hermetic seal between nance, loss of the gas charge is not inexpensive, can outlast bladder the gas charge and the fuid, allowing critical, and the environment is not accumulators, and can ft in a smaller them to remain in service for the life extreme. However, where mainte- envelope than bellows accumulators. of a system without loss of the gas nance shutdowns are prohibitively

With fewer components they also offer pre-charge. In addition, a service-gas costly, the risk of failure is great, and high reliability with proper mainte- port can be used to produce a variable the operating environment is extreme, nance. However, as with elastomeric pre-charge to accommodate different bellows accumulators are the best bladders, typical piston seal materials service depths. Moreover, bellows can solution. are not compatible with some fuid be made of corrosion-resistant materi- environments and have a more limited als to withstand extreme environments Aaron Glafenhein has a BS degree in temperature range, when compared to that bladder and piston accumulators Astronautical (Aerospace) Engineering metal bellows. In addition, the piston cannot. from Embry-Riddle Aeronautical seals must be replaced periodically, However, bellows accumulators are University, Daytona Beach. He has which can be quite costly when oper- more labor-intensive to produce than worked on HPMFAs for NASA SLS and ating on the ocean foor, as it requires bladder and piston devices, and in gen- Ares rockets, Airbus A350 and the F-35 bringing the accumulator to the eral cannot be repaired if the internal Joint strike fghter. Aaron has been surface. And like bladder accumula- components are damaged. with Technetics Group for almost four tors, piston accumulators tend to lose They also may require a slightly years. their pre-charge, requiring periodic larger envelope than the other types. repressurization. Tim Leister has been with Technetics

Group eight years and has a BS degree

Choosing equipment in Mechanical Engineering from

Bellows accumulators As the demand for oil and gas increases, the industry is moving

Bellows accumulators consist of Purdue University.

OE | August 2013 oedigital.com 42 038_OE0813_DC2_Accumulators.indd 42 7/22/13 12:34 AM