DEGASSERS

A degasser is a device used in drilling to remove gasses from drilling fluid which could otherwise form bubbles.-wiki

The purpose of a degasser is to remove entrained gas from the drilling
fluid. By this definition, then, the degasser has a limited capability for
handling large quantities of gas—typically anything more than about
50–100 scfm (20 scfm of gas at surface pressure will gas-cut 400 gpm of
16-ppg drilling fluid to 10 ppg). Large volumes of gas need to be
removed first by a separator or gas buster.

Entrained gas is composed of such small bubbles that the gravity
displacement of the bubbles is near the viscous drag of the fluid. The
bubbles do not rise to the surface, or rise very slowly. Initial gel strength
is also a powerful force in retarding the bubbles’ rise.
The bubbles must rise to the surface and expand enough to overcome
the liquid film enveloping them before they can break and release the gas.
Fine solids and long-chain polymers in the drilling fluid often make
the film around the bubbles stronger. As an example, foaming in drilling
fluid occurs when the liquid film around the bubbles becomes strong
enough to contain the gas at atmospheric pressure.
The importance of a vacuum to help remove entrained gas from the
drilling fluid can be shown by some simple calculations (see Box 9.3).

 Degasser Operations

The effective throughput of a degasser depends on a number of variables:
1. The vacuum level is limited in part by how high the drilling fluid has
to be lifted to enter the vacuum chamber. Lifts of more than 10 feet
(3 m) are probably counterproductive.
2. The denser the drilling fluid, the more the displacement force of
the bubble upward, but this is not generally as important as the
properties of the fluid fraction of the drilling fluid. A higher drag
coefficient reduces the ability of the bubble to rise. This is related to
viscosity effects and initial gel strength and is generally greater in
higher-density drilling fluids.
3. Polymers and fine solids in the drilling fluid tend to build a tougher
film around the bubble.
4. The more fluid is pumped or ejected, the less the residence time in
the vacuum; or contrary-wise, with more gas, there will be less fluid
throughput.
With any particular drilling fluid, volume of entrained gas, and height of
the degasser suction, there is a limit to the ability of the degasser
to remove all the gas from the drilling fluid. Since it is not possible to
predict all the drilling fluid/gas conditions, degasser planning is based on
experience in the area. Some manufacturers have test curves that will
show the real output of the degasser under fixed conditions.

Degasser Types

Vacuum-Tank Degassers

The original degasser, and the most common form of degasser, is the
vacuum tank (Figure 1). The tank may be a horizontal or a vertical
cylinder. The drilling fluid is pulled into the tank by vacuum action.
The primary vacuum force for filling the tank is created by the jet that
is discharging the drilling fluid, or in some cases by the pump that is
discharging the drilling fluid. The fluid level in the tank is controlled by a
float that opens or closes a vacuum breaker valve.

The separation of gas and liquid starts as the drilling fluid is pulled up
the suction. When the liquid enters the tank, it is distributed over a plate
or series of plates where it flows as a thin film. As entrained bubbles
increase in size, come to the surface, and break, the vacuum pump
discharge pumps the released gas to a disposal line. The size of tank
degassers varies widely, but the standard horizontal tank degasser on
a skid is generally about 12 feet long × 4 feet wide and weighs about
3000 pounds. Some units are quoted at about 1000 gpm of fluid and a
maximum vacuum of about 13 inches Hg.

The throughput of the vacuum tank degasser is controlled by the
discharge jet or pump. The higher the tank is above the surface of the
drilling fluid, the more of the energy from the jet or pump is used to lift
the fluid. The throughput volume of the tank decreases with height.
Most problems with the vacuum tank degasser are because the tank lift is
too high or the jet discharge is not strong enough (Box 9.4).

Pump Degassers or Atmospheric Degassers

The size and weight of the tank degasser lead to the development of
smaller and lighter degassing units. There are several configurations,
but the pump degassers are typically about 3½feet in diameter at the
top and 8 feet long. An impeller in the head pulls up the drilling fluid
and discharges it against the inside of the degassing chamber. Degassing
is accomplished by the reduction in pressure as the drilling fluid is
pulled up to the impeller and then by the impact of the spray discharge. (Figure 2)

Magna-VacTM Degasser

The Burgess Magna-Vac [Burgess Manufacturing Ltd.] is the most
sophisticated and complex design among drilling fluid degassers. It
combines the more efficient vacuum removal of gas with the lighter
weight and smaller size of the pump systems (Figure 3).
The drilling fluid is drawn up from the pits through a rotating pipe by
a vacuum provided by the regenerative vacuum blower on the top of the
unit. The drilling fluid enters the vacuum chamber of the unit through
holes in the top of the rotating pipe, and at that point is further accelerated and sprayed outward against the walls of the vacuum chamber.
The gas is pulled to the vacuum pump through a narrow gap at the upper
edge of the vacuum chamber that excludes liquids. Pressurized gas is
then sent to the flare or discharge line.
The drilling fluid flows to the bottom of the vacuum chamber, where
it is picked up by an evacuation (centrifugal) pump and discharged.
The system is controlled by a buoyant scheduling ring in the vacuum
chamber that controls the height of the liquid in the vacuum chamber by
restricting or opening the entrance to the vacuum chamber.
Depending upon the model type, the unit is about 3½ feet in diameter,
about 6 feet long, and weighs 900 to 1500 pounds. Flow volume is
quoted at 1000 gpm and maximum vacuum at 10–15 inches Hg.