Accumulator Capacity – Usable Volume per Bottle Calculation (Surface Stack)

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

Reference book: Well Control Books
Formulas and Calculations for Drilling, Production and Workover, Second Edition

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7 Responses to Accumulator Capacity – Usable Volume per Bottle Calculation (Surface Stack)

  1. Franc says:

    is the usable volume per bottle also known as VR..? which is stated in API 16-E

  2. blair seppa says:

    Having spent $700.00 of my own money, to take the ENFORM BOP course,the instructor filled our heads with so many usless confusing eqations. We as a class that paid good money,Alfonso confused the class from the begining. When i pay, i expect to gain some knowledge.I will give my concerns to ENFORM about the quaity of the instruction, mabe take the gas laws off drilling also the accumulator capacity . This four day course started march 12 ending march15 with the test 2012. Be prepared when taking this BOP course in Calgary or Edmonton,and hope to GOD you dont get Alfonso as an insructor.

  3. abu alhuda mohamed says:

    this the best web site i have ever learn and learn more and more every day thanks alot for your sharing experince.
    Abu Alhuda Mohamed
    Sr. Drilling supervisor

  4. Mohammad Mirsadeghi says:

    Hi, I would like take this time appreciating you for sharing such this useful information

  5. Albert Bassett says:

    The best way to understand this formula is to concentrate on how much N2 there is in the bottle.
    If you start to compare N2 and oil ,, you will get mixed up.

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