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Gas-charged accumulators are ubiquitous on modern hydraulic systems. They carry out numerous functions, which include energy storage and reserve, leakage and thermal compensation, shock absorption, and energy recovery.
While accumulators present a number of advantages in hydraulic system operation and can provide many years of trouble-free service, they are a maintenance item.
For example, the correct gas pre-charge pressure must be maintained for proper functioning and optimum service life. Also, periodic inspection, testing and certification can be required by law - accumulators are pressure vessels after all.
The three types of gas-charged accumulators you'll encounter on hydraulic systems are bladder, piston and diaphragm.
The most popular of these is the bladder type. Bladder accumulators feature fast response (less than 25 milliseconds), a maximum gas compression ratio of around 4:1 and a maximum flow rate of 15 liters (4 gallons) per second, although "high-flow" versions up to 38 liters (10 gallons) per second are available. Bladder accumulators also have good dirt tolerance; they are mostly unaffected by particle contamination in the hydraulic fluid.
Piston accumulators, on the other hand, can handle much higher gas compression ratios (up to 10:1) and flow rates as high as 215 liters (57 gallons) per second. Unlike bladder accumulators, whose preferred mounting position is vertical to prevent the possibility of fluid getting trapped between the bladder and the shell, piston accumulators can be mounted in any position.
But, piston accumulators also require a higher level of fluid cleanliness than bladder units, have slower response times (greater than 25 milliseconds) - especially at lower pressures - and exhibit hysteresis. This is explained by the static friction of the piston seal which has to be overcome, and the necessary acceleration and deceleration of the piston mass.
Diaphragm accumulators have most of the advantages of bladder-type units but can handle gas compression ratios up to 8:1. They are limited to smaller volumes, and their performance can sometimes be affected by gas permeation across the diaphragm.
When charging the gas end of a bladder or diaphragm accumulator, the nitrogen gas should always be admitted very slowly. If the high-pressure nitrogen is allowed to expand rapidly as it enters the bladder, it can chill the bladder's polymeric material to the point where immediate brittle failure occurs. Rapid pre-charging can also force the bladder underneath the poppet at the oil-end, causing it to be cut. If pre-charge pressure is too high or minimum system pressure is reduced without a corresponding reduction in pre-charge pressure, the operation of the accumulator will be affected and damage may also result.
Excessive pre-charge of a bladder accumulator can drive the bladder into the poppet assembly during discharge, causing damage to the poppet assembly and/or the bladder. This is a common cause of bladder failure.
Low or no pre-charge also can have drastic consequences for bladder accumulators. It can result in the bladder being crushed into the top of the shell by system pressure. This can cause the bladder to extrude into or be punctured by the gas valve. In this scenario, only one such cycle is required to destroy the bladder.
Similarly, excessively high or low pre-charge of a piston accumulator can cause the piston to bottom out at the end of its stroke, resulting in damage to the piston and its seal. The good news is that, if this happens, an audible warning will result. Even though piston accumulators can be damaged by improper charging, they are much more tolerant of it than bladder accumulators.
Held to Standards
Accumulators are pressure vessels and as such are manufactured, tested and certified according to statutory standards. In the United States, for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.
All pressure vessels manufactured to these standards are considered to have a finite service life depending on the number of pressure cycles experienced during normal operation. The typical design life for a hydraulic accumulator is 12 years.
In many jurisdictions, periodic inspection and recertification is required. This particularly applies to hydraulic accumulators which have relatively large volumes and operate at high working pressures. Inspection may be required at predetermined intervals (i.e. every two, five or 10 years) or when a certain percentage of usable design life is deemed to have been reached.
Depending on the volume and pressure rating of the accumulator, recertification may involve one or more of the following: visual inspection, ultrasonic thickness testing and/or hydrostatic pressure testing.
So if you're responsible for hydraulic equipment which incorporates an accumulator, familiarize yourself with the relevant regulations that apply in your locale.
And along with every other component on your hydraulic machines, it's your responsibility to make sure all accumulators are properly maintained and safe to use.