Liquid Ring Vacuum Pumps

A liquid ring vacuum pump is a rotating positive-displacement pump. It uses an eccentrically mounted impeller and a seal liquid—usually water or another liquid compatible with the process—to generate vacuum.

1. The Liquid Ring Forms

As the impeller rotates, centrifugal force moves the seal liquid outward against the inner wall of the pump casing, forming a rotating liquid ring.

2. Gas Enters and Is Compressed

Because the impeller is positioned eccentrically inside the casing, spaces between the impeller blades and liquid ring increase and decrease in volume. As the chamber volume increases, process gas is drawn into the pump. As the volume decreases, the gas is compressed.

3. Gas and Seal Liquid Are Discharged

The compressed gas and part of the seal liquid leave through the discharge port. The gas and liquid can then be separated, while the seal liquid is discharged, partially recirculated, or fully recirculated depending on the vacuum-system design.

This operating principle enables liquid ring vacuum pumps to handle moisture, condensed vapors, and liquid carryover while maintaining reliable continuous operation.

Correct pump selection requires more than matching horsepower or nominal suction capacity. Important process factors include:

Required Vacuum Level

Determine the normal operating pressure, maximum required vacuum, and whether the process requires a single-stage, two-stage, or hybrid vacuum system.

Suction Capacity

Establish the required gas flow at the actual inlet pressure and temperature. Pump capacity should be based on process conditions rather than free-air displacement alone.

Gas and Vapor Composition

Identify the gases, condensable vapors, moisture, chemicals, solids, and expected liquid carryover entering the pump.

Seal Liquid

Confirm which seal liquid is compatible with the process. Water is commonly used, but another process-compatible liquid may be required. Seal-liquid temperature, flow, and vapor pressure should be considered during pump sizing.

Materials of Construction

Select cast iron, stainless steel, elastomer-lined, or other construction based on corrosion, erosion, temperature, and process-compatibility requirements.

Seal and Drive Configuration

Mechanical seals may be appropriate for general applications, while magnetic-drive pumps can provide hermetically sealed operation for processes requiring enhanced containment.

Seal-Liquid System Design

The pump may use once-through seal liquid, partial recirculation, or total recirculation. The correct arrangement depends on utility availability, discharge requirements, contamination risk, and operating cost objectives.

Discharge Conditions

Back pressure, separator design, exhaust treatment, and downstream process requirements can affect pump performance and system configuration.

Installation and Operating Environment

Available space, motor requirements, controls, redundancy, ambient conditions, hazardous-area classifications, and maintenance access should be evaluated before final selection.

Because vacuum-pump performance depends on the complete process, published data should be used for preliminary comparison only. Consult a NASH engineer for final pump sizing and system design.