A mud/gas separator (poor boy degasser) sizing worksheet will assist drilling personnel with the sizing calculations. The worksheet provides a quick and easy evaluation of most mud/gas separators for a specific well application. A brief discussion of other mud/gas separator considerations is provided, including separator components, testing, materials, and oil-based-mud considerations. This paper reviews and analyzes existing mud/gas separator technology and recommends separator configuration, components, design considerations, and a sizing procedure. A simple method of evaluating mud/gas separation within the separator vessel has been developed as a basis for the sizing procedure.


The mud/gas separator (poor boy degasser) is designed to provide effective separation of the mud and gas circulated from the well by venting the gas and returning the mud to the mud pits. Small amounts of entrained gas can then be handled by a vacuum-type degasser located in the mud pits. The mud/gas separator controls gas cutting during kick situations, during drilling with significant drilled gas in the mud returns, or when trip gas is circulated up.

This paper discusses design considerations for mud/gas separators (poor boy degasser). The purpose of this paper is to allow drilling rig supervisors to evaluate mud/gas separators properly and to upgrade (if required) the separator economically to meet the design criteria outlined in this paper, and to provide office drilling personnel with guidelines for designing mud/gas separators before delivery at the drill-site.

Principle of Operation

The operating principle of a mud/gas separator is relatively simple. The device is essentially a vertical steel cylindrical body with openings on the top, bottom, and side, as shown in Fig 1. The mud and gas mixture is fed into the separator inlet and directed at a flat steel plate perpendicular to the flow. This impingement plate minimizes the erosion wear on the separator’s internal walls and assists with mud/gas separation. Separation is further assisted as the mud/gas mixture falls over a series of baffles designed to increase the turbulence within the upper section of the vessel. The free gas is then vented through the gas vent line, and mud is returned to the mud tanks.

Operating pressure within the separator is equal to the friction pressure of the free gas venting through the vent line. Fluid is maintained at a specific level (mud leg) within the separator at all times. If the friction pressure of the gas venting through the vent line exceeds the mud-leg hydrostatic pressure within the separator, a blow through condition will result sending a mud/gas mixture to the mud tanks. As one can readily see, the critical point for separator blow through exists when peak gas flow rates are experienced in the separator. Peak gas flow rates should theoretically be experienced when gas initially reaches the separator.

Types of Mud/Gas Separators

Three types of mud/gas separators commonly are used today: closed bottom, open bottom, and float type. The principle of mud/gas separation within each type of vessel is identical. Differences can be found in the method of maintaining the mud leg, as discussed below.

The closed-bottom separator, as the name implies, is closed at the vessel bottom with the mud return line directed back to the mud tanks, as shown in Fig. 1. Mud leg is maintained in the separator by installation of an inverted V-shaped bend in the mud return line. Fluid level can be adjusted by increasing/decreasing the length of the V-shaped bend.

Fig. 1- Closed-boHom mud/gas separator

Commonly called the poor boy,2,3 the open-bottom mud/gas separator is typically mounted on a mud tank or trip tank with the bottom of the separator body submerged in the mud, as shown in Fig. 2. The fluid level (mud leg) in the separator is controlled by adjusting the fluid level in the mud tank or by moving the separator up or down within the tank. Mud-tank height can restrict the maximum mud leg obtainable for open-bottom mud/gas separators.

Fig. 2-open-bottom mud/gas separator

Fluid level (mud leg) is maintained in a float-type mud/gas separator4 by a float/valve configuration, as shown in Fig. 3. The float opens and closes a valve on the mud return line to maintain the mud-leg level. Valves can be operated by a manual linkage system connected from the float to the valve, or the valve can be air-operated with rig air. Mud-leg height can be controlled by adjusting the float assembly.

Fig. 3-Float-type mud/gas separator

There are some inherent problems in the use of float-type mud/gas separators. The manual linkage separator has experienced problems with linkage failure resulting in improper opening or closing of the mud-return-line valve. Air-operated valves fail to function if rig air is lost, resulting in no control of fluid level within the separator. Mud-return-line valves are prone to plug with solids, preventing mud flow-back to the mud pits.

Because of these problems, float-type mud/gas separators are not recommended and a closed-bottom separator is preferred. Open bottom separators are acceptable; however, one should be aware that they are restricted to a maximum mud leg, somewhat lower than the mud-tank height. Although float-type mud/gas separators are strongly discouraged, these separators can be modified easily for disconnection of the float, removal of the valve, and installation of a mud leg in the mud return line.

For the purpose of this paper, a closed-bottom mud/gas separator will be considered for all separator designs.

Fig. 4-Mud/gas separator sizing.

Sizing the Mud/Gas Separator

Table 1 shows a mud/gas separator worksheet to assist with the sizing calculation. The mud/gas separator illustrated in Fig. 4 will be evaluated for sufficient sizing in this paper.


Peak Gas Flow Rate. As discussed previously, the critical time for separator blow-through exists when peak gas flow rates are experienced. Mud/gas separator blow through is defined as inefficient separator operation resulting in a mud/gas mixture returning to the mud tanks through the mud return line.

Two situations can cause separator blow-through.

  1. Friction pressure of the gas venting through the vent line exceeds the mud-leg hydrostatic pressure, resulting in evacuation of fluid from the separator. Friction pressure of the mud through the mud return line is considered negligible because of its short length.
  2. Vessel ID is too small, causing insufficient retention time for the gas to separate efficiently from the mud. This situation is commonly called insufficient’ separator cut.

To estimate a peak gas flow rate properly, we must consider a “typical” kick. The typical kick will depend on the well location, depth, type size, and component ratios of influx. Kick data should be based on previous offset well data and should be a realistic worst case gas kick. The well and kick data in Fig. 5.

Fig. 5-Well configuration

Vent-Line Friction Pressure. The formula used by this paper to calculate friction pressure of gas through a vent line is derived from the Atkinson-modified Darcy-Weisbach equation.

Note that effective vent-line lengths will be significantly affected by the installation of flame arresters or some auto-igniters.  The effect of this additional backpressure should be included in the calculation of vent-line friction pressure.

Mud Leg. As previously discussed, mud-leg hydrostatic pressure must exceed vent-line friction pressure to prevent a separator blow through condition. Minimum mud-leg hydrostatic pressure would occur if an oil/gas kick was taken and the mud leg was filled with 0.26 psi/ft oil. 8 This minimum condition mayor may not occur, depending on the well location. Offset well data should be evaluated to establish a minimum mud-leg fluid gradient. For example, the 0.26-psi/ft mud-leg gradient would be considered extremely conservative if dry gas were expected for the sample problem. A more realistic estimate would approach the gradient of whole mud for the dry-gas case. A realistic mud-leg gradient for a gas/water kick would be the gradient of native salt water.

Separator ID. A blow-through condition may exist because a small vessel ID results in insufficient separator cut. Several complicated models exist to describe gas movement within a liquid.9 A simplified approach, taken in this paper, states that the gas migration rate upward within the separator must exceed the liquid velocity downward within the separator to give 100% separator cut and to prevent a separator blow-through condition. Gas migration rate is estimated at 500 ft/hr, or 8.4 ft/min,9 within the separator. This estimation is conservative and more realistic values would be higher; however, the slow gas migration rate serves as a worst-case scenario. Liquid flow rate through the separator can be estimated as 2xqk; for this paper 2×3=6 bbl/min. This factor of two was determined from gas volume at depth calculations (Boyle’s law) using Drilpro ™ for various depths and kick sizes. Correlation of the data shows that the mud flow from the well approaches twice the mud flow into the well (kill rate) for various kick sizes, kill rate, and wellbore geometries. A more accurate determination of mud flow from the well can be incorporated into the design procedure.


We find that the gas migration rate is greater than the liquid velocity in the separator, 8.4>4.8 ft/min. Therefore, a blow-through condition caused by insufficient separator cut does not exist.

Note that a separator cut < 100% frequently exists with mud/gas separators, and under some conditions, is not a major concern. As stated earlier, the mud/gas separator is designed to provide effective separation of mud and gas with small amounts of entrained gas handled by a vacuum degasser located in the mud pits. Therefore, large active pit volumes may tolerate < 100% separator cut.

Sizing Conclusion. Having evaluated sizing criteria for the mud/gas separator (Fig. 4), we may conclude that the separator is sized sufficiently to handle our worst-case kick properly.

Oil Based Mud Considerations

The effects of oil-based mud on the operation of the mud/gas separation can significantly affect sizing and design requirements. L These concerns are currently being evaluated. However, some conclusions can be made at this stage.

  1. Gas kicks in oil-based mud can approach “possibly soluble” conditions while the kick is circulated from the well.
  2. Gas kicks in oil-based mud that pass through the gas bubble point while being circulated from the well can experience higher Pcmax and Vcmax values than were calculated for a kick of the same initial pit gain in a water-based mud. This results in higher peak gas flow rates through the separator and thus the requirement for a more stringent separator design.
  3. Gas kicks in oil-based mud that do not pass through the gas bubble point until the gas is downstream of the choke will severely affect mud/gas separator sizing and design. Peak gas flow rates will be extremely high relative to those calculated for water-based mud as outlined in this paper. Additional evaluation of the separator sizing should be completed if these well conditions exist.

Other Mud/Gas Separator Considerations

Fig. 6-Mud gas separator components

Fig. 6 shows other separator components. A minimum 8-in.-ID mud return line is recommended for closed-bottom separators. Smaller lines may encounter problems with solids plugging the line. A larger-ID line would be considered beneficial. The impingement plate should be perpendicular to the separator inlet line and field replaceable.

Baffles within the separator should be located in the upper part of the separator and may continue into the lower part of the vessel. Typically, baffles consist of near-horizontal plates. The plates may be solid or have holes in them. The baffles should not impede the flow of liquid through the separator, which would cause fluid buildup above the baffles. Solids buildup in the baffles can also be a problem if the baffles are too restrictive.

An upper manway should be located on the upper part of the separator to permit visual inspection of the interior of the separator. The manway should be large enough to permit replacement of the impingement plate and equipped with a replaceable rubber seal to prevent leakage.

Closed-bottom mud/gas separators should be designed with a minimum 1-ft sump at the bottom of the vessel. The sump will help prevent solids from settling and plugging the mud-return-line outlet.

A lower manway should be located on the lower part of the separator to permit sump cleanout or unplugging of the mud return line. The manway should be equipped with a replaceable rubber seal to prevent leakage.

The mud/gas separator should be equipped with a valved inlet on the lower section of the vessel to permit mud to be pumped into the separator. Mud can be pumped into the lower section of the separator during operation to decrease the possibility of solids settling in the mud return line. The valved inlet also permits cleaning solids from the lower portion of the separator, especially after separator use.

A siphon breaker or anti-siphon tube may be required to prevent having to siphon mud from the separator into the mud tanks, especially with configurations that require the mud return line to be extended below the separator elevation to allow mud to return to the mud tanks. The siphon breaker is simply an upward-directed open-ended pipe attached to the highest point of the mud return line.

All separators must be built in compliance with the ASME Boiler and Pressure Vessel Code, Sec. VIII, Div. I with all materials meeting requirements of NACE Standard MROJ-75-8412 (1980 Revision). All welding on the vessel must meet ASME requirements.

New mud/gas separators should be hydrostatically tested to 188psi to give a maximum working pressure of 150 psi, as recommended by ASME. Periodic nondestructive testing should include radiographic examination of wall thickness and ultrasound verification of weld continuity. 12 At each initial hookup, every separator should be circulated through with water at the maximum possible flow rate to check for possible leaks in the connections. Frequency of testing should depend on anticipated and historical use of the separator.

Bracing the mud/gas separator has always been a major problem. When gas reaches the surface, separators tend to vibrate and, if not properly supported, can move, resulting in near-catastrophic problems. Thus, it is critical that all mud/gas separators be sufficiently anchored and properly braced to prevent movement of both the separator body and the lines.

Trouble Shooting an Insufficiently Sized Separator

Frequently, the situation arises where a mud/gas separator is picked up with the rig contract, and the drilling rig supervisor and engineer must evaluate the suitability of the separator for the well-location. This evaluation typically should be conducted during the rig bid analysis process. If the separator is insufficient or marginal, it may be more economical to upgrade the existing separator to meet the sizing criteria as an alternative to renting or building a suitable one.

Small Vessel ID. We frequently do our calculations and determine that our vessel ID is too small. Reducing the kill rate will improve this situation; e.g., if the kill rate for the previously sized separator were reduced from 3 to 1.5 bbl/min, then from Eq. 7: vL = [(2 x 1.5)/362]/1,029 =2.4 ft/min.

Thus, reducing the kill rate also reduces the liquid velocity rate in the separator, which increases the mud/gas retention time and improves the efficiency of mud/gas separation.

Also note that a gas migration rate of 500 ft/hr (8.4 ft/min) is a worst-case scenario and values could be higher. Therefore, when vessel ID is considered, a marginal separator probably would be sufficient because of this built-in safety factor. Higher gas migration rates may also be used in the sizing procedure, as previously discussed. Fig. 7 shows the effect of kill rate on the calculation of minimum separator ID for different gas migration rates.

Fig. 7-Effect of circulating kill rate on minimum separator ID

Vent-Line Friction Pressure Exceeds Mud-Leg Hydrostatic Pressure. Another area of concern is vent-line friction pressure exceeding mud-leg hydrostatic pressure, Pj > Pml’ Several options exist to help alleviate this problem.

  1. Reduce the circulating kill rate. As discussed previously, a reduction in the circulating kill rate may improve a separator’s operation when vessel ID is considered and also when excessive vent-line friction pressures are considered. This reduction in kill rate may be the most economical solution to the sizing concern. For example, if the kill rate for the previously sized separator were reduced from 3 to 1.5 bbl/min, the peak gas flow rate would decrease. Combining Eqs. 1 and 3 and converting, we obtain: (=75.9/1.5=50.6 min and qmax=9,036/50.6=1,443,903 ft31D.

This decrease in peak gas flow rate would significantly decrease the excessive vent-line friction pressure and improve the operation of the separator (Eq. 4). Pj(5.0X 10-12 x410x 1,443,903)217.05 =0.25 psi.

Fig. 8-Effect of kill rate on vent·llne friction pressure.

Fig. 8 shows the effect of kill rate on the calculation of vent-line friction pressure for the previously sized separator.

  1. Increase the mud leg. Another solution may be to increase the height of the mud leg. For example, if we increased the previously sized separator from a 7-ft mud leg to alOft mud leg, the mud-leg hydrostatic pressure should increase (Eq. 6). Pml=lOxO.26=2.6 psi. Thus, the mud-leg hydrostatic pressure increased from 1.8 to 2.6 psi, allowing the separator to operate more efficiently.
Fig. 9-Effect of mud-leg height on mud-leg hydrostatic pressure.

Fig. 9 shows the effect of mud-leg height on the calculation of mud-leg hydrostatic pressure for different mud-leg gradients. Note that the mud-leg height cannot exceed the separator height. The mud leg may also be restricted by bell-nipple elevation. If the mud leg is higher than the bell nipple, additional surface equipment may be required to permit the separator to operate when drilling with significant gas in the mud returns.

  1. Adjust vent-line bends. As shown in Table 1, the type and number of bends in the vent line significantly affect the effective vent-line length, which in turn affects the calculation for vent-line friction pressure. If we were to replace the targeted T-bends on the previously sized separator with right-rounded bends, the calculations for the effective length (Eq. 5) and vent-line friction pressure (Eq. 4) would change: Le=200+(3 X 1)=203 ft and PI =(5.0x 10-12 x203x2,887,806)217.05 =0.5 psi.

Hence, a vent-line friction-pressure decrease from 1.0 to 0.5 psi increases the efficiency of the separator for a given mud leg. In addition, the vent-line friction pressure increases proportionally to the effective length (Fig. 10).

Fig. 10-Effect of effective length on vent-line friction pressure.
  1. Increase vent-line ID. Increasing the vent-line ID is generally the most expensive alternative but may be the only adjustment possible to increase separator efficiency. Larger-ID vent lines will decrease the vent-line friction-pressure calculation. For the previously sized separator, if an 8.0-in.-ID vent line were used, the calculation for vent-line friction pressure (Eq. 5) would change to PI =(S.Ox 10-12 X41OX2,887,806)217.05 =O.S psi.

Again, a vent-line friction-pressure decrease from 1.0 to 0.5 psi will increase separator efficiency for a given mud leg. Fig. 11 shows the effect of vent-line ID on the calculation of vent-line friction pressure for the previously sized separator.

Fig. 11-Effect of vent-line ID on vent-line friction pressure


  1. The principle of mud/gas separation within most commonly used mud/gas separators is identical. Differences can be found in the method of maintaining the mud leg.
  2. A closed-bottom mud/gas separator is the preferred configuration. Open-bottom and float-type separators work well but are subject to limitations and prone to failure.
  3. Sizing of a mud/ gas separator should be specific to individual well conditions.
  4. Modeling of gas flow through a mud/gas separator can be approximated by a simple procedure in a limited time.
  5. A complete list of mud/gas separator components and considerations was compiled to assist with the design of mud/ gas separators.
  6. A trouble-shooting guide was developed to address economical upgrading of an existing insufficiently sized separator to meet sizing guidelines as an alternative to building or renting a new separator.

Some pictures and content of this topic comes from:
Author: G.R. MacDougall (Chevron Canada Resources Ltd.)


  1. Hello, I realized the diagrams used in this text are familiar to me. Then it came to my mind they were published in a paper way back in the day. It would be respectful with the actual writer of that paper to put a reference to his paper here.

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