Accumulator (Koomey) is a unit used to hydraulically operate Rams BOP, Annular BOP, HCR and some hydraulic equipment. There are several of high pressure cylinders that store gas (in bladders) and hydraulic fluid or water under pressure for hydraulic activated systems. The primary purpose of this unit is to supply hydraulic power to the BOP stack in order to close/open BOP stack for both normal operational and emergency situation. Stored hydraulic in the system can provide hydraulic power to close BOP’s in well control operation, therefore, kick volume will be minimize. Accumulators should have sufficient volume to close/open all preventers and accumulator pressure must be maintained all time.
This post you will learn how to calculate usable volume per bottle by applying Boyle’s gas law:
Use following information as guideline for calculation:
Volume per bottle = 10 gal
Pre-charge pressure = 1000 psi
Operating pressure = 3000 psi
Minimum system pressure = 1200 psi
Pressure gradient of hydraulic fluid = 0.445 psi/ft
For surface application
Step 1 Determine hydraulic fluid required to increase pressure from pre-charge pressure to minimum:
Boyle’s Law for ideal gase: P1 V1 = P2 V2
P1 V1 = P2 V2
1000 psi x 10 gal = 1200 psi x V2
10,000 ÷ 1200 = V2
V2 = 8.3 gal
It means that N2 will be compressed from 10 gal to 8.3 gal in order to reach minimum operating pressure. Therefore, 1.7 gal (10.0 – 8.3 = 1.7 gal) of hydraulic fluid is used for compressing to minimum system pressure.
Step 2 Determine hydraulic required increasing pressure from pre-charge to operating pressure:
P1 V1 = P2 V2
1000 psi x 10 gals = 3000 psi x V2
10,000 ÷3000 = V2
V2= 3.3 gal
It means that N2 will be compressed from 10 gal to 3.3 gal. Therefore, 6.7 gal (10.0 – 3.3 = 6.7 gal) of hydraulic fluid is used for compressing to operating pressure.
Step 3 Determine usable fluid volume per bottle:
Usable volume per bottle = Hydraulic used to compress fluid to operating pressure – hydraulic volume used to compress fluid to minimum pressure
Usable volume per bottle = 6.7 – 1.7
Usable volume per bottle = 5.0 gallons
Well Control Book Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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