Mud Gas Separator Poor Boy Degasser

The height and diameter of an atmospheric separator are critical dimensions which affect the volume of gas and fluid the separator can efficiently handle. As the mud and gas mixture enters the separator, the operating pressure is atmospheric plus pressure due to friction in the gas vent line. The vertical distance for the inlet to the static fluid level allows time for additional gas break-out and provides an allowance for the fluid to rise somewhat during the operation to overcome friction loss in the mud outlet lines. As shown in Figure 39, the gas-fluid inlet should be located approximately at the midpoint of the vertical height. This provides the top half for a gas chamber and the bottom half for gas separation and fluid retention. The 30 in. diameter and 16 ft minimum vessel height requirements have proven adequate to handle the majority of gas kicks. The separator inlet should have at least the same ID as the largest line from the choke manifold which is usually 4 in. Some separators use tangential inlet, which creates a small centrifugal effect on the gas-fluid mixture which causes faster gas break-out.

The baffle system causes the mud to flow in thin sheets which assists the separation process. There are numerous arrangements and shapes of baffles used. It is important that each plate be securely welded to the body of the separator with angle braces.

A 8 in. minimum ID gas outlet is usually recommended to allow a large volume of low pressure gas to be released from the separator with minimum restriction. Care should be taken to ensure minimum back pressure in the vent line,. On most offshore rigs, the vent line is extended straight up and supported to a derrick leg. The ideal line would be restricted to 30 ft in length and top of the line should be bent outward about 30 degrees to direct gas flow away from the rig floor. If it is intended that the gas should be flared, flame arresters should be installed at the discharge end of the vent line.

As stated previously, when the gas pressure in the separator exceeds the hydrostatic head of the mud in the U-tube, the fluid seal in the bottom is lost and gas starts flowing into the mud system. The mud outlet downstream of the U-tube should be designed to maintain a minimum vessel fluid level of approximately 3 1/2 ft in a 16 ft high separator. Assuming a 9.8 ppg mud and total U-tube height of 10 ft the fluid seal would have a hydrostatic pressure equal to 5.096 psi. This points out the importance for providing a large diameter gas vent line with the fewest possible turns to minimise line frictional losses.

The mud outlet line must be designed to handle viscous, contaminated mud returns. As shown in Figure 39 an 8 in. line is recommended to minimise frictional losses. This line is recommended to minimise frictional losses. The line usually discharges into the mud ditch in order that good mud can be directed over the shakers and untreatable mud routed to the waste pit.

During well control operations, the main purpose of a mud gas separator is to vent the gas and save the drilling fluid. This is important not only economic reasons, but also to minimise the risk of circulating out a gas kick without having to shut down to mix additional mud volume. In some situations the amount of mud lost can be critical when surface volume is marginal and on-site mud supplies are limited. When a gas kick is properly shut in and circulated out, the mud gas separator should be capable of saving most of the mud.

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There are a number of design features which affect the volume of gas and fluid that the separator can safely handle. For production operations, gas oil separators can be sized and internally designed to efficiently separate gas from the fluid. This is possible because the fluid and gas characteristics are known and design flow rates can be readily established. It is apparent that 'gas busters' for drilling rigs cannot be designed on the same basis since the properties of circulated fluids from gas are unpredictable and a wide range of mixing conditions occur downhole. In addition, mud rheological properties vary widely and have a strong effect on gas environment. For both practical and cost reasons, rig mud gas separators are not designed for maximum possible gas release rates which might be needed; however, they should not handle most kicks when recommended shut-in procedures and well control practises are followed. When gas low rates exceed the separator capacity, the flow must be bypassed around the separator directly to the flare line. This will prevent the hazardous situation of blowing the liquid from the bottom of the separator and discharging gas into the mud system.

Figure 39 illustrates the basic design features for atmospheric mud gas separators. Since most drilling rigs have their own separator designs, the Drilling Supervisor must analyse and compare the contractor's equipment with the recommended design to ensure the essential requirements are met.

The atmospheric type separator operates on the gravity or hydrostatic pressure principle. The essential design features are:

  • Height and diameter of separator
  • Internal baffle arrangement to assist in additional gas break-out
  • Diameter and length of gas outlet
  • A target plate to minimise erosion where inlet mud gas mixture contacts the internal wall of the separator, which provides a method of inspecting plate wear
  • A U-tube arrangement properly sized to maintain fluid head in the separator.
Poor Boy Degasser
Figure 39 - Mud Gas Separator

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Responses

  • Tracy Artripe
    Might want to run spell check and also correct the following statement... "they should not handle most kicks when recommended shut-in procedures and well control practises are followed." Mud gas separators today will handle most licks.
    9 years ago
  • ANTONIA MACDONALD
    What is purpose of having a mud seal in the mud gas separator?
    9 years ago
  • Roxy
    How to caculate the pressure of mud seal on poor boy degasser?
    9 years ago
  • kerry
    How to calculate flow through mud gas separator?
    8 years ago
  • angelico
    What would be cause of a pressure buildup in the poorboy degasser?
    8 years ago
  • ofelia
    How to test maximum flowrate of a poor boy degasser?
    7 years ago
  • Lauryn
    How calculate poor boy degasser capacity?
    7 years ago
  • Toby
    What are the scientific principles for mud and gas separator?
    7 years ago
  • Timoteo
    How to survey for Mud Gas Separator?
    7 years ago
  • ave
    How to calculate mud seal pressure on degasser?
    7 years ago
  • jasmine gamgee
    How to calculate mud gas seperator volume?
    7 years ago
  • Tim
    How to calculate mud leg level in a seperator?
    6 years ago
  • rhoda took-took
    Why is mud gas separator called poor boy?
    6 years ago
  • Wegahta Robel
    What is the minimum barrel a mud gas separator can handle?
    6 years ago
  • SAMLAD
    Why is a mud gas separator design called a poor boy?
    5 years ago
  • Hamilcar
    What is a mud seal referred to in a poor boy mud gas separator?
    5 years ago
  • janne
    How to maintain mud leg height?
    5 years ago
  • FETHAWIT
    What is the poor boy degasser bypass line called?
    5 years ago
  • asphodel
    How to calculate back pressure on Mud Gas Seperator vent line?
    4 years ago
  • mhret
    Why call liquid gas seperator poorboy?
    4 years ago
  • sarah
    How to measure gas separator mud leg height?
    4 years ago
  • ralf koertig
    How much volume is in poor boy degaser?
    3 years ago
  • mike
    How to calculate MGS blow through?
    3 years ago
  • Miles
    How to measure mud seal in a poor boy gas separator?
    3 years ago
  • Ty Reid
    Which condition use mud gas separator?
    2 years ago
  • Jaden Sinclair
    What pressure should a poorboy degasser be bypassed?
    11 months ago
  • bethany munro
    How to size a poor boy gas seperator?
    11 months ago
  • tarquinia
    What happens if you fill your poorboy degasser with too much mud?
    9 months ago
  • donnamira
    What API COVERS MUD GAS SEPERATORS?
    8 months ago
  • Benjamin
    How does a poor boy degasser work?
    7 months ago
  • Annika
    How to calcul mgs mud seal?
    7 months ago
  • benjamin
    How to sfop boys in mud?
    6 months ago
  • FREDRIK
    How a poorbuoy degasser works?
    30 days ago
  • fiori
    Is bending accepted in mud gas separator lines?
    16 days ago
  • gabriella
    Why we need to keep poor boy full with mud?
    13 days ago

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