Hydraulic accumulators store energy, enhance the performance of fluid-power systems and provide for emergency backup functions. An accumulator consists of a separation element (bladder, piston or diaphragm) that divides gas and fluid sections. The energy storage capacity is directly affected by the precharge pressure of nitrogen gas inside an accumulator.
In a presentation at IFK 2020, Christian Nisters, Frank Bauer and Marco Brocker of HYDAC Technology GmbH, Sulzbach/Saar, Germany, explained that the performance and availability of hydraulic applications strongly depend on the proper functioning of a hydraulic accumulator, and that the proper precharge pressure is essential. Thus, it is mandatory to check the precharge pressure continuously during the operation; otherwise it leads to an inefficient use of the accumulator.
In this context HYDAC has developed a “smart” product — the P0-Guard — for predictive monitoring of the precharge pressure without the need for measurements on the gas side of the accumulator.
There are two main reasons why a correct precharge pressure is so important. First is to ensure the optimal accumulator performance across the operating pressure range and, thus, maintain high machine output. And second, for system availability, to avoid damage and destruction of the accumulator’s separating element and, in turn, optimize machine service life.
The conventional way to check precharge pressure of a hydraulic accumulator is to measure pressure on the gas side. The check can be carried out by permanent devices like pressure gauges or pressure transducers, or by non-permanent devices like HYDAC’s FPU testing unit.
Making a check on the gas side is the easiest and most direct measurement method; a gauge gives a direct and visual indication of the gas pressure. But on the other hand, there are some disadvantages and risks to be considered. For example, it’s not always easy to access the gas side to carry out measurements. It may even be necessary to use a ladder to reach the gas side of the accumulator. Also, using permanent gauges or pressure sensors increases the risk of potential leak points. To improve visibility, pressure gauges can be installed at eye level, but the additional piping increases costs and further ups the risk of leakage. And the accumulator fluid side has to be non-pressurized and thermal-stabilized during the precharge check in order to get a representative and comparable measurement result.
As a better option, HYDAC has developed the P0-Guard EDS 3400 for predictive monitoring of accumulator precharge pressure without any need to measure pressure on the gas side. It is a compact electronic pressure switch with integrated accumulator monitoring functions. By monitoring and evaluating the oil-side pressure gradients, the P0-Guard notes any significant drop of the precharge pressure and activates an output warning signal in case of a critically low precharge value.
The measurement principle of the unit is based on the physical conditions of the different bulk modulus of a hydraulic fluid and the gas medium inside the accumulator. Due to the fact that at common hydraulic operating pressures, the bulk modulus of the hydraulic system is much higher than the bulk modulus of gas, there is a significant gradient change in the characteristic curve on pressurizing and depressurizing the accumulator. On the fluid side pressure, this gradient change occurs during depressurization when the separation element approaches the bottom of the accumulator.
The P0-Guard searches for the closing pressure during the discharge process of the accumulator and compares it to a selectable minimum pressure. And it compensates for influencing factors such as the flow rate, temperature boundary conditions and the installation position of the accumulator. When this limit value is undershot, the unit transmits a signal to the controller.
The function, reliability and stability of the P0-Guard algorithm were validated in various tests at HYDAC’s in-house test department. The monitoring algorithm provides stable data even under the influence of disturbances like ripples or shocks on the hydraulic pressure signal. And due to the fact that the P0-Guard is based on the hardware of a HYDAC pressure switch, users benefit from the advantages of a long-term proven product as well as a high sampling rate (which is essential for accurate functioning of the implemented algorithm).
The monitoring function can be used in a standard switching mode or IO-Link mode. In the switching mode, the output for precharge pressure monitoring switches off when the detected closing pressure drops below the defined minimum value. In the IO-Link mode, the P0-Guard communicates bidirectionally with the connected IO-Link master. The master is able to change setting parameters or to read the relevant values. Beside its main monitoring function, an additional output can either be used as a switching output for an accumulator charging function or as an analog output for the actual hydraulic pressure signal.
As a manufacturer of hydraulic accumulators, HYDAC takes advantage of the P0-Guard on most accumulators operating in their production plants. It monitors the actual precharge conditions of all hydraulic accumulators, ensures optimum performance, and generates historical trend data from which predictive data and maintenance forecasts are derived, said the authors.
The P0-Guard provides a smart precharge-pressure monitoring tool, which can be used as standalone device as well as embedded in an industrial network system. The algorithm works independent of the accumulator type and, therefore, can be used for diaphragm, metal bellows, bladder and piston accumulators, and for backup gas bottles. It works on hydraulic circuits with multiple accumulators in parallel and detects the lowest precharge pressure. And it permits predictive planning of maintenance intervals to support and optimize a preventive maintenance philosophy.