Hydraulic Orifice

In hydraulics, the orifice is a type of flow control valve. It serves as a local resistance that narrows the cross-section of the line. At an orifice, the volume flow is abruptly stopped and only allowed to pass through very slowly. The short length of the orifice makes it independent of the medium’s viscosity. However, the stronger the pressure becomes, the more resistance they offer to the flow.

Another common term for an orifice is a “throttle plate.” However, this designation is not entirely correct, as orifices and throttles differ technically.

Simple Orifices for Constant Pressure Regulation in Hydraulics

The simplest forms of orifices consist of simple perforated plates. Their cross-section is not variable.

They are used to generate a constant flow behind the opening despite fluctuating pressures. These components are also called “fixed orifices.”

Hydraulic Orifice: Difference from Throttle

Orifices and throttles are similar in their function. Both reduce pressure in a volume flow.

A throttle, especially a non-variable fixed throttle, has a long cross-section that gradually narrows. Throttle valves are also suitable for variable design due to their size. This is difficult to implement with orifices. Furthermore, a throttle cannot generate constant pressure. While the pressure is slowed down by the throttle, it continues to increase behind it with rising system pressure.

However, an orifice generates a clearly defined pressure behind its opening, which remains constant even with increasing system pressure. The reason for this is the turbulence that occurs behind the orifice. This turbulence increases with rising pressure, but it makes no difference to the pressure range behind the orifice. It remains constant above a defined value.

Weaknesses of an Orifice

Orifices are available in various sizes. The smallest orifices, known as needle valves, tend to clog. If a system operates with many orifices, great care must be taken regarding the purity of the medium. Micro and fine particle filters are mandatory when using orifices.

An orifice resists pressure. As long as this resistance is within the technical parameters of the orifice, it performs its task without problems. However, it can break if the limit values are exceeded. This usually leads to a rupture of the entire line in which the orifice is installed.

Furthermore, the abrupt interruption of the volume flow leads to cavitation above a certain pressure. This is the formation of voids in the volume flow. The vacuum thus created quickly collapses, releasing destructive forces. Orifices are therefore much more prone to cavitation than throttles.

Gradual Pressure Reduction through Multi-Stage Orifices

A viable way to avoid cavitation is the installation of multi-stage orifices. Here, several orifices are connected in series, each progressively narrowing in cross-section. They have the advantage over single-stage throttles that they possess a certain variability. By exchanging the individual stages, the volume flow can be somewhat controlled and regulated.

Check Valves with Orifices

Orifices are suitable as check valves when the direction of the volume flow changes. They always offer the same resistance to the volume flow, regardless of the direction from which it comes. If a hydraulic system has different pressures from both sides, the orifice acts as a check valve, creating a pumping effect. The same happens if the pressures are equal but the pressure durations differ.