Pressure peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks in hydraulic systems: A risk to sensors and other equipment

Pressure peaks occur in virtually all gas and liquid-filled pipelines. Those pressures arising in just a few milliseconds can exceed the overload pressure of the pressure transducers employed and also destroy them.

Pressure peaks, or very high pressures existing over a short timeframe, are usually noticed only when the damage has already been done. They are the result of pressure surges and also other physical phenomena (cavitation, micro-diesel effect) that occur wherever liquids or gases are transported through pipes. Pressure peaks, however, are less important among gases due to their high compressibility and thus only rarely represent a danger. In the context of water pipes, the term ‘water hammer’ is often used. With these terms, a dynamic pressure change of the liquid is ultimately implied. When, for example, a valve is quickly closed, water flow will stop instantaneously. This triggers a pressure wave, which flows through the medium against the direction of flow at the speed of sound and is then reflected back again. Within milliseconds, there is a sharp pressure increase which can cause damage to pressure sensors and other equipment (damage to pipe fittings and pipe clamps, as well as to pumps and their footings etc.). In the first line, however, it is the measuring devices that are affected, upon which we will be concentrating in the following. These damages can appear as a tiny “rupture” or a deformation (see Figures 1 and 2).

Figure 1: “Rupture” as a result of pressure spike

Figure 2: Deformations due to pressure peaks

If the pressure acting on the pressure transducer exceeds the overload pressure, then this will sustain permanent damage. There are two possible scenarios here: As paradoxical as it may sound, the complete destruction of the measuring instrument due to pressure peak is the mildest of consequences. Users, after all, do notice the damage immediately here. If the sensor is merely deformed as the result of a pressure peak, however, it will continue to operate, but deliver only inaccurate measurements. The financial consequences here are disproportionally higher than with a totally destroyed sensor.

How to prevent damage caused by pressure peaks

The golden path to preventing damage caused by pressure peaks lies in the integration of pulsation dampers or pressure chokes. Other means, such as the use of valves, would not lead to satisfactory results, because they are too slow to react to pressure peaks which actually arise in mere milliseconds.

The purpose of a choke is to dampen pressure peaks so that they no longer exceed the overload pressure of pressure transducers and then damage them. For this purpose, the choke is placed in the pressure channel in front of the sensor cell. As a result, pressure peaks will no longer reach the membrane directly and unchecked, since they must first pass through the choke itself:

Figure 3: Pressure channel with Pressure choke

Because of their very good protection from pressure peaks, the use of pressure chokes remains the best option. This variant, however, does have its pitfalls. It can lead to a blockage of the pressure channel due to calcification and deposits, especially in media with solid and suspended particles. This results in a slowing down of the measurement signal. If chokes are used in relevant applications, then regular maintenance should be carried out here.

A supplementary protection from pressure peaks can be achieved with a higher overpressure resistance, as opposed to the standard one. Whether this is advisable depends upon the particular application: If high accuracy readings are required, these can no longer be achieved in certain circumstances of very high overpressure resistance relative to the measurement range.

The diesel effect in hydraulic systems: Material damage is the outcome

The diesel effect in hydraulic systems: Material damage is the outcome

As the name suggests, the term diesel effect refers to the combustion process in a diesel engine. But it can also be observed in hydraulic systems. In addition to pressure peaks, oil aging, residues and the destruction of seals are the outcomes.

The diesel effect occurs as a consequence of cavitations. We will therefore first consider the formation conditions for cavitations in hydraulic systems before turning to the diesel effect itself.

Cavitation in hydraulic systems

Depending upon gas, temperature, liquid and pressure, hydraulic oils contain dissolved air. Cavitation is ultimately an air expulsion from the hydraulic oil. This occurs when the oil is subjected to a certain pressure or shearing motion. In practice, this occurs in suction lines, pump interior spaces, cross-sectional narrowings and, in hydraulic systems, where pulsations appear. When the moving oil mass shears, voids are formed, into which the finest of air bubbles are released.

The diesel effect

If the air bubbles resulting from cavitation, which also contain oil particles, are subjected to a high pressure, then a drastic temperature increase occurs in those bubbles. This major temperature rise leads to the diesel effect, namely combustions within the hydraulic system, and this combustion process takes place within milliseconds.

The consequences of cavitation and the diesel effect

Cavitation can have a variety of negative consequences, including material damage to pump housings and pressure relief valves, the sucking away of sealing elements such as O-rings, altered flow characteristics, reduced function of pumps and gears due to filling losses, noise, pressure surges with pressure peaks exceeding the system pressure, and the diesel effect, in the form of oil aging, combustion residues and destroyed seals.

The consequences of cavitation and the diesel effect are not always immediately apparent. They are often only noticed when it is already too late and there is a need to repair the hydraulic system. Pressure peaks as a result of cavitation and the diesel effect can also damage the pressure transmitters installed in the system by overshooting. The sudden pressure increase in the system causes the membrane of the pressure transmitter to be “shot through” (read more about this here).

In view of the serious consequences of cavitation and the diesel effect, appropriate measures must be taken to avoid these phenomena. This includes a sufficient filling in the suction chambers and low flow velocities, as well as avoiding sharp edges, deflections and pulsating pressures.