Mechanism of Accumulator (Koomey Unit)

This topic describes how an accumulator (Koomey Unit) works. First of all, I will start with accumulator bottles. The accumulator bottles are used to store hydraulic pressure for closing/opening all blow out preventers. Each bottle, which has a rubber bladder inside, has a storage volume of 10 gallons. The rubber bladder inside is pre-charged to 1,000 psi with Nitrogen (N2). Each bottle will be pressured up 200 psi over the pressure charge pressure. At this stage, 1.7 gal of hydraulic fluid is used in order to compress N2 to minimum system pressure of 1,200 psi (200 psi over pre charge pressure), called “minimum operating pressure”. Hydraulic oil will be pumped into the bottle until pressure in the bladder reach 3,000 psi, called “Operating Pressure”. Volume of hydraulic fluid used to pressure up from 1200 psi to 3000 ps,which is called “Useable Fluid”, is equal to 5 gallons,


Note: I will show you how to use Bolye’s Law to figure out usable fluid later.

Accumulator Pressure Charging System

In an accumulator unit as shown in the figure below as an example, there are one electric pump and two pneumatic pumps, which will be automatically pumped hydraulic fluid, which is stored in a reservoir tank when pressure in bottles is below set pressure in order to maintain bottle pressure at 3,000 psi. The electric pump is mainly used to maintain pressure. Pneumatic pumps will be use in case of 1) the electric pump failure and 2) helping the electric pump to increase bottle pressure quicker.



Note: you may see different set up depending on the rig but the concept is the same.


Hydraulic fluid under pressured is divided into two following ways:

1. The first way is used for controlling an annular preventer. Hydraulic under pressure of 3,000 psi in bottles passes a pressure regulator (in the figure is a regulator “A”) in order to regulate pressure from 3,000 to 500-1500 psi for operating annular preventer. Pressure used to operate the annular preventer must be less than pressure used to operate rams preventer because high pressure will damage a rubber seal inside of the annular preventer.

2. Another way is used to control rams preventers (VBR’s rams, blind/shear rams, pipe rams) and HCR valves. Hydraulic under pressure of 3,000 psi in bottles passes a pressure regulator (in the figure is a regulator “B”) in order to reduce pressure from 3,000 to 1,500 psi for operating rams preventers and HCR valves. The regulated hydraulic fluid under pressure of 1,500 goes to a manifold which is used to divide hydraulic pressure in many ways to operate equipment. This is normally called “Manifold Pressure”.

All pressure activated equipment as annular preventer, VBR’s and HCR valves have two hydraulic fluid ports. One is for opening equipment and another one is for closing equipment. You can operate the BOP equipment by pushing remote switches or handles at the accumulator unit. In case of opening equipment, pressured hydraulic fluid will be transmitted to the open port and hydraulic power will drive an internal mechanism inside equipment in order to open equipment. Fluid in closing side will be pushed out and go back to a reservoir tank. On the contrary, in case of closing equipment, the process is quite as same as opening process but only fluid flow direction is opposite.


Reference book: Well Control Books

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34 Responses to Mechanism of Accumulator (Koomey Unit)

  1. Eddy says:

    why they use nitrogen (N2) not hydraulic fluid?

  2. rob says:

    Thank you for this simple explanation… I took a whole class about this and your few paragraphs were easier to understand than the whole class!

  3. Carl says:

    What Happens to the oil in the bottles when you move the Bypass
    Handle to the “High Bypass” side?
    And dose this work without power to the uint?
    Could you use this “Stored Pressure” as an Emergency back up?

    • Oil in the bottles will not be bleed off so the level is the same when you put in the bypass side. With by pass, you will lock the function of BOP in either open or closed direction. The only time that the by pass position is used is when rig move.

  4. Abdoul Garba says:

    Thanks to explain 1000 words in few words. very good explanation

  5. Mahmoud Nader says:

    . the bypass vavle is used to increase the pressure on the annular preventer when required .. Is that true .. If not ,Kindly explain the function of the bypass vavle in the unit.

  6. Ubong Clement says:

    Nice piece…

  7. Rraz says:

    While rig move bottles are pressurized or not,if pressurized then what are the precaution in rig move to handle pressurized bottles.

  8. ashraf says:

    its beter to keep 1000 psi in the bottle during rig move to safe the rubber or pladder from damage
    is that right?
    thanx for the lesson

  9. ghanem says:

    thanks for the explanation I just couldn’t understand the last part about the two hydraulic fluid ports “open and close” kindly simplify this part ..
    thanks in advance

  10. ghanem says:

    thanks for the explanation I just couldn’t understand the last part about the two hydraulic fluid ports “open and close” kindly simplify this part ..
    thanks in advance

  11. ASN says:

    Could you please do an article on choke manifolds.

    Thank you. This website has been a great help.

  12. Maikel McClint says:

    A colleague told me that an accumulator should have a electrical driven pump and a backup air pump. What is the difference between those two and how dangerous is it not to have a backup pump? I am a Geologist so trying to understand this all…

    Thanks in advance!

    • Maikel,

      You need to have independent source of energy to operate the BOP’s. Primary energy source to close the BOP’s is hydraulic pressure stored in the accumulator bottles. The electric and air pump will kick in once the pressure in the accumulator dropping below approximately 90% of the accumulator pressure.

  13. Mohammed says:

    Thanx for these information

  14. Wesley says:

    There are 3 pressure gauges in the Koomey unit, what are these gauges for and their functions.


  15. kareem says:

    Could you please send me a complete schematic diagram for koomey

  16. mostafa awad says:

    For the stored N2 in the bottles under 1000 psi, can this pressure be used in the case that both electric and penumatic pumps are down to close the annular bop

  17. Suvas pandya says:

    Drawing of hydro electric Switch and setting procedure

    in advance

  18. EMAD says:

    How much does it take to the electric/air pumps to recharge the 1200 psi bottle after full operation?

  19. Bahaa Elmasry says:

    Why we are using only N2 not any other gas or air?


  20. amir says:

    good evening dear

    why Pre-charge nitrogen gas 1000 psi on koomy N2 bottle?

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