Normally it takes a lot of time and equipment to check a hydraulic accumulators gas precharge pressure. Also using the normal way of opening the gas charge valve and reading Gas pressure at the Accumulators Gas Valve can introduce contamination that will allow gas leakage thereafter.
Two non-invasive way of checking precharge pressure very fast and with little or no added equipment is as follows:
Both methods require a KNOWN ACCURATE PRESSURE GAUGE at or near the Accumulator Outlet. A Glycerin Filled Gauge works but a Non-Filled gauge reads more quickly and gives closer results especially to the inexperienced eye.
These methods are not dead accurate but quickly indicate a trend of gas loss and can indicate when an accurate check is required.
1) Observe the Pressure Gauge closely when the hydraulic pump is first started. Since acircuit with an Accumuator normally has all flow blocked when not operating, pressoil at a higher pressure than the Gasure should be seen shortly after starting the pump. Since the accumulator has a Gas filled space there is a void that can be filled by pump oil but that void can only be filled by forcing the oil to a pressure higher than the Gas pressure. So, very soon after the pump starts the pressure gauge at the Accumulator will jump to the pressure of the Gas in the accumulator. After this pressure will continue to increase to system pressure. THE SUDDEN PRESSURE JUMP IS THE PRESSURE OF THE GAS IN THE ACCUMULATOR. The time it takes to build to system pressure depends on the size of the Pump (GPM Flow) and/or the size of the Accumulator or Accumulators.
The above is not real accurate but can be used to get a geneal idea. Following is the most accurate non-invasive way to check for a good Accumulator operation.
(2) When the circuit is running at pressure and the Pressure Gauge described above is in place, turn OFF the Pump and observe the Pressure Gauge. If the installation has an Automatic Drain, as it should, pressure at the gauge will start slowly to quickly dropping since the automatic drain will open at pump shutdown to drain the Stored Energy from the circuit. If an Automatic Drain valve is not installed then use the Manual Drain valve, if a Manual Drain valve is not installed, INSTALL ONE after opening some Bleed Point or manually cycling some actuators to relieve the Stored Energy.
As pressure continues to drop there will be a point when the Gauge suddenly drops to Zero. This pressure drop point is the Accumulators Pre-Charge Pressure and should be as noted on a tag attached to the Accumulator or on the Hydraulic Crcuit Schematic.
Their are three possible scenarios at pump shutdown:
The pressure gauge will act as above and Pre-Charge will be above, Below or very near Optimum Pressure
The Pressure Gauge will drop suddenly at Pump Shut down, indicating no Gas Charge since it was Pre-Charged to a pressure higher than system presuure and the oil cannot enter the chamber.
Or, it was never filled or it has a leak and has lost all Gas.
This second method gives a closer reading especially when using a Manual Drain since flow can be restricted and the point at which all oil is evacuated will not happen before you are ready.
Re: Non-Invasive way to check Accumulator Pre-Charge
11/29/2007 1:07 AM
If you want a truly non invasive and even automated method, consider attaching strain gauges to the accumulator casing and applying the same logic as you already have to the system pressures. Your suggested regimes are certainly in use and also automated to register a loss of precharge on for instance Off Highway Vehicle brake and steering systems.