The above subject is divided in two parts as follows:
To determine the optimum value of the pre-charge pressure at which the maximum fluid delivery will occur.
Normally the pre- charge pressures of Nitrogen bottles in a well control unit vary between
750 psi to 1000 psi, depending on the drilling contractor choice.
The accumulator manufacturers such as KOOMEY and PAYNE recommend that 3000 psi
Accumulator should be pre- charged with nitrogen at 1000 psi. they reached the conclusion
That the recommended system will provide more fluid per unit volume at the higher minimum
Pressures used for blow out preventer operation than would a system that is pre-charged to a
Lesser pressure.
Their recommendation can be easily checked by application of BOYLE LAW (P1V1=P2V2).
As an example:
Following are the calculations comparing fluid volumes available for use when applying two
Different nitrogen pre charges and maintaining the final accumulator pressure not less than
1200 psi,. for the calculations purposes , it is also assumed that the accumulator capacity is
80- Gallons.
- With 750 psi nitrogen pre- charge:
80 gals x 750 psi = V2 X 3000 psi
V2=20 gals of gas.
20 gals x 3000 = V2x 1200
V2=50 gals of gas
Therefore,50 gals .gas minus 20 gals .gas= 30 gals of fluid delivered from 3000 psi down to
1200 psi from an 80 gallon accumulator.
(B) With 1000 psi nitrogen pre charge:
80 gals x 1000 psi =V2 x3000 psi.
V2=26.7 gals of gas
26.7 x 3000 psi = V2 X 1200PSI
V2= 66.7 gals of gas.
Therefore, 66.7 gals. Gas minus 26.7gals of fluid delivered from 3000 psi down to 1200 psi
From an 80 gals accumulator.
The above comparison shows that 10 more gallons of fluid will be delivered if the
Pre- charge is increased from 750 psi to 1000 psi, confirming the manufacturer recommendation.
2. To calculate the pre- charge pressure value of a well control unit by application of gas laws.
Assume:
- Total accumulator volume is =180 gals
- Accumulator pressure = 2800 psi
Remove 20 gals hydraulic oil from accumulator and pressure drops to 1200 psi, what is the
The value of nitrogen pre- charge pressure?
if it is assumed that X = volume stored in the accumulator at 2800 psi, therefore ,180-X
= volume of nitrogen in the accumulator at 2800 psi. Now using BOYLE law, P1V1=P2V2:
2800 PSI (180- X) =1200 psi [(180-X) +20]
Or 504000-2800X=216000-1200X+24000.
THEREFORE: X=264000/1600=165gals fluid.
Then the gas volume is 180-165=15 gals.
Finally,(2800psi)(15gals Nitrogen)=(N-pre-charge psi)(180 gals).
N2 PRE-CHARGE PRESSURE=2800×15/180 = 233psi.
The nitrogen pre-charge pressure may be determined in the above manner by simply
Closing and opening a set of BOP rams and noting the accumulator pressure drop.
Conclusion: the given problems demonstrated the effective and useful applications of
BOYLE law in solving well control unit problems. it can also be used for designing the
Required capacity of accumulators per company policies as well as calculating the percentage
Of fluid left in the accumulator after closing and opening HYDRILL a number of BOP rams, and
Hydraulic valves.
