Two types of accumulator are in common use — bladder type and float type (Figure 47). They each have a total capacity (fluid + nitrogen + bladder/float) of 11 gallon although 15 gallon versions are also available. Cameron float type accumulators are available with up to 35 gallon capacity. The volume occupied by the float or bladder is usually taken to be 1 gallon.
The bladder type accumulator contains a rubber bladder that separates the nitrogen from the stored hydraulic fluid. The gas is injected into the bladder through the precharge valve in the top of the bottle and the hydraulic fluid enters the accumulator at the bottom. A poppet valve at the base of the bottle prevents extrusion and damage to the bladder once all the fluid has been expelled.
In the float type accumulator, the gas is introduced at the top of the bottle and is kept separate from the stored fluid by a buoyant float. Escape of the gas through the fluid port at the base of the bottle is prevented by the weight of the float actuating a shut-off valve once all the fluid has been expelled.
The float type design is more complex than the bladder type and although replacing the rubber bladder can be a difficult operation this type of bottle tends to be the most commonly used of the two. Since large numbers of accumulator bottles are often needed, they are installed in banks which have the necessary isolation valving to allow for the re-charging or servicing of individual bottles.
Figure 47 - Accumulator Types
Figure 47 - Accumulator Types
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Hydraulic fluid volume at 3,000psi equals 6.67 gallons. Hydraulic fluid volume at 1,200psi equals 1.67 gallons. Therefore usable fluid operating under the above pressure; equals 6.67—1.67 = 5gallons
NOTE: If the total volume of hydraulic fluid required to operate the various functions of the BOP were to be 225gallons. The required number of 10 gallon accumulator bottles would be;
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Determining the total accumulator capacity required for a particular BOP stack is an important consideration in the design of the control system. The particular criteria to be applied depends on the relevant regulatory authority or on operating company policy. However, the following can be taken as an example:
• The accumulators must be able to provide the fluid needed to function as required the BOP functions and still retain a pressure of 200 psi above the precharge pressure.
The operating pressure of accumulators is generally 3000 psi. A minimum of about 1200 psi is required to hold some annular preventers closed and so this is taken to be the minimum acceptable pressure that should remain in the accumulator after operating the stack functions. A precharge pressure of 1000 psi will therefore ensure that a small liquid reserve will remain in the bottles when the pressure in the system falls to 1200 psi. Figure 48 illustrates these situations for a bladder type accumulator.
Using these pressures, the amount of usable fluid in a bottle can be calculated and knowing the total volume of fluid needed by the various stack functions will allow the required number of bottles to be determined. The calculations are different however for surface and subsea accumulators.
Sea water hydrostatic pressure will be added to the initial gas pre-charge pressure of 1000 psi (subsea bottles), this will reduce the overall useable fluid volumes.
This sea water hydrostatic increase on the stack mounted bottles is to overcome any hydrostatic compression on the hose bundles prior to entering the pods.
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