The height and diameter of a poorboy degasser 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.
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.