High-Pressure Internal Gear Pump: The Power Heart of Industrial Fluid Transfer

In hydraulic systems, chemical processing and food industries, the high-pressure internal gear pump has become a top choice for engineers thanks to its high efficiency, low noise and long service life. This article explains the working principle, key specifications and selection criteria when sourcing a 32 MPa gear pump for demanding industrial duty cycles.

1. Working Principle: How Internal Meshing Achieves High-Pressure Delivery

The core structure of an internal gear pump consists of a driving internal gear, a driven external gear, a crescent plate (or support block) and the housing. The pitch circles of the two gears are close together on one side, while the other side is separated by the crescent plate, forming a suction chamber and a delivery chamber.

When the driving internal gear rotates the external gear in the same direction, the teeth in the suction chamber gradually separate, increasing volume and creating negative pressure that draws fluid in. In the delivery chamber, teeth continuously mesh and reduce volume, forcing fluid out. This continuous process delivers stable fluid output.

The key difference from low-pressure gear pumps lies in the clearance compensation mechanism. High-pressure internal gear pumps use axial and radial pressure compensation: pressure oil from the delivery zone is introduced behind a floating side plate, pushing it inward to maintain axial clearance between 0.03 and 0.04 mm, controlling over 70–80% of leakage. Radially, the support block pushes the internal gear ring into contact with the external gear tooth tips under back pressure, forming a radial seal in the high-pressure zone. This dual compensation allows the internal gear pump to maintain high volumetric efficiency even at low speeds and low viscosity.

2. Key Specifications for 32 MPa High-Pressure Duty

Pressure Rating and Volumetric Efficiency

32 MPa is a typical rated pressure point for high-pressure internal gear pumps. In straight-line conjugate internal gear pump designs, maximum working pressure can reach 32 MPa with volumetric efficiency of 98%. Models using axial and radial pressurisation can achieve a maximum pressure of 35 MPa at a rated pressure of 32 MPa. For high-pressure duty, system maximum working pressure should be held at 80–90% of the pump's rated pressure to reserve margin for pressure surges.

Displacement and Speed Range

High-pressure internal gear pumps cover a wide displacement range, with common models from 5.4 mL/r to 249.9 mL/r. In the 32 MPa high-pressure class, displacements from 10 to 125 mL/r are typically well covered, though some models may have a slightly lower pressure rating (25 MPa) at certain displacements such as 20 mL/r. Each specification must be checked individually during selection.

Speed ranges for high-pressure internal gear pumps extend from a minimum of 600 r/min to a maximum of 3,000 r/min. Speed selection must match medium viscosity: if viscosity is high and speed is also high, the internal rotor may slip around the medium, causing failure to draw fluid or insufficient flow.

Noise and Flow Pulsation

The trapped-oil volume change rate of straight-line conjugate tooth profiles is only one-tenth that of involute gears, significantly reducing noise and pressure pulsation. Models using modified involute short-tooth designs can keep noise at low levels. Low-pulsation output directly benefits part consistency in injection moulding holding phases and precision machining applications.

3. Selection Criteria for a 32 MPa Gear Pump

Step 1: Confirm Medium Viscosity and Temperature

Internal gear pumps tolerate a wide viscosity range, from 0.2 cP to 1,000,000 cP. However, higher viscosity requires correspondingly lower pump speed to ensure full filling of inter-tooth volumes. Medium temperature must fall within the pump model's allowable range — standard models typically handle -20 to +80°C, with high-temperature versions reaching 120°C. For food or chemical media, confirm that pump body material (such as 316L stainless steel) and seal materials are compatible.

Step 2: Calculate Flow and Determine Displacement

Positive-displacement pump flow is approximately linear with speed. The theoretical flow formula is:

Q = Displacement (mL/r) × Speed (rpm) ÷ 60000 (in L/min)

During selection, first calculate required displacement from the system's rated flow requirement, then confirm available motor speed. The operating point should fall within the pump's high-efficiency zone (typically 60–90% of rated flow). Prolonged operation outside this range causes overheating or accelerated wear.

Step 3: Verify Pressure Rating and Surge Margin

A 32 MPa pump does not necessarily achieve that pressure rating at every displacement. Some models drop to 25 MPa or lower at larger displacements. During selection, the rated and maximum pressure for the target displacement must be verified individually. System relief valve settings are generally 10–15% above working pressure.

Step 4: Assess Suction Conditions

Internal gear pumps have better self-priming capability than external gear designs, but basic conditions still apply: inlet absolute pressure is recommended at ≥0.8 bar, suction height ≤0.5 m, and flooded suction is preferable. Suction pipe diameter should not be smaller than the pump inlet, and piping should be short and straight with minimal bends. Suction piping must be strictly sealed to prevent air ingress, which causes cavitation and noise.

Step 5: Confirm Shaft Seal and Mounting Interface

Standard mechanical seals (carbon/silicon carbide or tungsten carbide against tungsten carbide) suit most hydraulic oils. Rotation direction must match motor direction. The case drain port must return to the tank separately, with line back pressure generally held within 0.5–1 bar to avoid damaging the shaft seal.

4. Applicable Duty and Selection Decision

High-pressure internal gear pumps suit hydraulic systems requiring high pressure, low pulsation and compact structure. Typical applications include injection moulding machine hydraulic power units, die casting machinery, forklift hydraulic systems and shearing and bending machines.

In the following situations, a 32 MPa internal gear pump offers clear technical advantages: the system requires holding pressure with high pressure stability; installation space is limited but high power density is needed; medium viscosity varies widely and stable output is required.

The core decision logic in selection is: confirm medium properties and system pressure requirements first, then match displacement and speed to flow demand, and finally verify suction conditions, seal form and mounting interface one by one. Neglecting any step may prevent the pump from achieving expected performance in actual duty. Understanding the physical meaning behind the parameters is more useful for making the right choice than simply memorising the figures.