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:
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