What Is a Vacuum Degasser? Gas Removal Principles, Maintenance & Selection

Among the various components of a drilling solids control system, the Vacuum Degasser is often misunderstood. Some rig crews treat it as an optional device; others confuse it with a mud gas separator. This article provides a standalone, comprehensive explanation of what a vacuum degasser is, covering the physics of gas entrainment, how Vacuum Degassers differ from other gas removal methods, practical maintenance procedures, and selection criteria. The goal is to give you a complete reference that does not rely on previous articles. Wherever appropriate, AIPU Solid Control vacuum degassers are used as the reference example.

1. The Problem: Why Gas Entrainment Is Dangerous

Before understanding the solution, you must understand the problem. Drilling fluid is designed to be a nearly incompressible fluid. When formation gases (methane, H₂S, CO₂) enter the mud, they form small bubbles that are dispersed throughout the liquid. These bubbles do not rise quickly because the mud’s viscosity and the bubble’s small size keep them trapped.

Consequences of entrained gas:

• Reduced hydrostatic pressure – A small fraction of gas by volume can reduce mud density by 0.1–0.3 SG, significantly lowering the safety margin against formation pressure.

• Pump cavitation damage – When gas bubbles enter a mud pump, they collapse violently during compression, causing pitting on cylinder liners and valve seats.

• Shear thinning and rheology changes – Gas‑cut mud exhibits erratic viscosity, making hole cleaning unpredictable.

• Flammable or toxic gas release – Methane can accumulate under rig floors; H₂S is lethal even at low concentrations.

A vacuum degasser is the most effective tool to eliminate these hazards.

2. Defining the Vacuum Degasser

A vacuum degasser is a closed vessel that applies negative pressure (below atmospheric) to drilling fluid to strip out entrained and dissolved gases. It is not a separator for large gas kicks – that is the role of a mud gas separator. Instead, it continuously polishes the mud during normal circulation, removing the small bubbles that the mud gas separator cannot handle.

Key characteristics:

• Operating pressure: -0.02 to -0.04 MPa (gauge)

• Gas removal efficiency: ≥95% for properly sized units

• Typical location: After shale shakers, before desanders/desilters

3. How a Vacuum Degasser Works – Physics and Mechanics

The operation relies on two physical principles: Henry’s Law and bubble expansion.

3.1 Henry’s Law

Henry’s Law states that the amount of dissolved gas in a liquid is proportional to the partial pressure of that gas above the liquid. When you lower the pressure above the mud, dissolved gas comes out of solution – just like opening a soda bottle releases carbon dioxide.

3.2 Bubble Expansion

A small bubble at atmospheric pressure (say 1 mm diameter) will expand dramatically when the surrounding pressure drops. In a vacuum of -0.03 MPa (absolute pressure about 0.07 MPa), the bubble diameter can increase 3–4 times. The larger bubble has much higher buoyancy and rises quickly to the surface, where the vacuum pump can remove it.

3.3 Mechanical Process

1. Mud entry: Gas‑cut mud enters the degasser through a large inlet (e.g., 20 inches). It strikes a distributor or rotating impeller that spreads it into a thin film or small droplets, maximizing surface area.

2. Vacuum creation: A vacuum pump (usually 1.1–5.5 kW) continuously evacuates the vessel, maintaining low pressure.

3. Gas breakout: Under vacuum, bubbles expand and burst. Released gas is drawn out through a vent line (1‑1/2 inches).

4. Degassed mud discharge: Clean mud collects at the bottom and exits through a 6‑inch outlet, returning to the mud tank.

This process is continuous and does not interrupt mud circulation.

4. Types of Vacuum Degassers – A Detailed Comparison

Not all vacuum degassers are built the same. Based on internal design, there are three common types:

Each type suits different rig layouts. The centrifugal type is best for tight spaces; tank‑mounted units are ideal for high‑flow operations; float‑controlled units are preferred for low‑power or automatic applications.

5. Vacuum Degasser vs. Atmospheric Degasser vs. Mud Gas Separator

To avoid confusion, it is essential to distinguish these three devices.

In practice: Most modern rigs have a mud gas separator at the flowline for safety during kicks, and a vacuum degasser for routine gas removal. Atmospheric degassers are rarely used on new rigs because of their lower efficiency.

6. Key Specifications of AIPU Vacuum Degassers

AIPU Solid Control manufactures vacuum degassers that meet API standards. Below are representative specifications for standalone reference.

AIPU APLCQ300 Vertical Vacuum Degasser

AIPU APZCQ Series (Tank‑Mounted)

All AIPU degassers are available with:

• Custom electrical systems (380V/50Hz, 460V/60Hz, etc.)

• H₂S‑resistant steel (316L or 2205 duplex) for sour gas fields

• ATEX / IECEX explosion‑proof certifications

• Any RAL color for surface coating

7. How to Select the Right Vacuum Degasser

Choosing a vacuum degasser requires answering four questions:

7.1 What is your maximum mud circulation rate?

The degasser’s mud capacity must be at least equal to your mud pump’s maximum flow rate. Most land rigs need 240–300 m³/h; larger offshore rigs may need 360 m³/h or more.

7.2 What is your expected gas content?

For routine degassing (1–3% gas by volume), any unit with ≥95% efficiency works. For heavy gas cutting (5% or more), choose a larger model to allow sufficient retention time.

7.3 Do you have H₂S (sour gas)?

If yes, you must select a degasser made from H₂S‑resistant materials (316L or 2205 duplex). Standard carbon steel will fail rapidly due to sulfide stress cracking. AIPU offers H₂S‑resistant construction.

7.4 Where will it be installed?

• Limited deck space: Choose a vertical centrifugal degasser like the AIPU APLCQ300.

• Mud tank with open top: Choose a tank‑mounted unit like the APZCQ series.

• Low power / automatic operation: Choose a float‑controlled APVD series.

8. Installation and Integration Tips

To get the best performance from any vacuum degasser, follow these guidelines:

• Place after shakers, before desander/desilter. This order ensures that large solids do not enter the degasser and that gas is removed before fine separation.

• Maintain adequate feed head. For gravity‑fed units, keep at least 0.5 m of mud above the inlet.

• Keep the vent line short and straight. Long or undersized vent lines create back pressure, reducing vacuum efficiency.

• Install a flame arrestor or flare ignitor on the vent line when dealing with flammable gas (methane). AIPU offers the APFI series flare ignitors.

• Do not bypass the degasser. Some crews bypass it to save power, but this leads to gas accumulation and pump damage.

9. Routine Maintenance Checklist

A vacuum degasser has few moving parts, but neglect will reduce its performance.

Daily Checks

• Vacuum level (should be stable between -0.02 and -0.04 MPa).

• Vacuum pump oil level and color (milky oil indicates water contamination).

• Unusual noises or vibrations.

• Vent line gas flow (should be constant when mud is flowing).

Weekly Checks

• Inspect lid seals for air leaks. A leak will kill the vacuum.

• Clean the inlet distributor or baffles if solids have built up.

• Check motor current against nameplate rating.

Monthly Checks

• Change vacuum pump oil (use the grade recommended by AIPU).

• Inspect the rotor (for centrifugal type) for wear or imbalance.

• Check all gaskets and replace any that are hardened or cracked.

Annual Overhaul

• Remove the degasser from service.

• Open the vessel and inspect internal surfaces for corrosion or scale.

• Replace bearings and mechanical seals.

• Pressure test the vessel if required by local regulations.

10. Troubleshooting Common Problems

11. Frequently Asked Questions

Q: Can I run a vacuum degasser with oil‑based mud?
A: Yes. However, avoid excessive vacuum (more than -0.04 MPa) to prevent vaporizing light hydrocarbon components. AIPU degassers have adjustable vacuum control.

Q: How do I know if my mud needs degassing?
A: Signs include: mud weight lower than expected, pump cavitation, frothy mud surface, or gas detector alarms. Even without visible bubbles, dissolved gas can be present.

Q: What is the difference between a vacuum degasser and a vacuum pump?
A: The degasser is the whole vessel. The vacuum pump is one component that creates the low pressure.

Q: How much power does a vacuum degasser consume?
A: A typical AIPU unit uses 15–37 kW for the main motor plus 1.1–5.5 kW for the vacuum pump. The APVD series is more energy‑efficient.

12. Why AIPU Vacuum Degassers Are a Smart Investment

AIPU Solid Control has over 20 years of experience in drilling fluid solids control and treatment. AIPU products have been delivered to more than 30 countries, with over 300 system‑equivalent shipments. AIPU holds multiple international certifications and independent patents.

Choosing an AIPU vacuum degasser gives you:

• Proven degassing efficiency ≥95%

• Robust construction with three‑layer marine anti‑corrosion coating

• Flexibility of H₂S‑resistant steel and ATEX certifications

• Global support from AIPU’s engineering team

13. Conclusion

A vacuum degasser is not a luxury – it is a core component of any safe, efficient drilling operation. It removes entrained and dissolved gases that would otherwise reduce mud weight, damage pumps, and create safety hazards. By understanding what a vacuum degasser is, how it works, how to select and maintain it, you can protect your rig and optimize drilling performance.

AIPU Solid Control offers a complete lineup of vacuum degassers to match any rig size and gas condition. Whether you need a compact vertical unit, a high‑capacity tank‑mounted model, or an energy‑efficient float‑controlled degasser, AIPU delivers reliability field‑proven in over 30 countries.

Equip your rig with an AIPU vacuum degasser – and get the gas out of your mud.