Basic Hydrulic accumulators

Maintaining Hydraulic Accumulators

The correct gas pre-charge pressure must be maintained for proper functioning and optimum service life,as wel as; Periodic inspection.

Testing and certification can be required by law - accumulators are pressure vessels after all.

Accumulator Types:The three types of gas-charged accumulators;

1. Bladder.
2. Piston.
3. Diaphragm.

Bladder accumulators:

A bladder type accumulator, sometimes known as a hydro-pneumatic accumulator, is a metal tank that contains a rubber bladder filled with compressed gas. There is also a poppet valve in the discharge port and a gas valve used to precharge the bladder.

• 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.
• Good dirt tolerance; they are mostly unaffected by particle contamination in the hydraulic fluid.

Piston accumulators:

Handle much higher gas compression ratios (up to 10:1) and flow rates 215 liters (57 gallons) per second. Unlike bladder accumulators.

Unlike Bladder accumulator as the preferred mounting position is vertical to prevent the possibility of fluid getting trapped between the bladder and the shell,Piston type can be mounted in any position.

Piston accumulators also require a higher level of fluid cleanliness than bladder unit,slower response times (greater than 25 milliseconds) - especially at lower pressures.

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.

Maintenance Considerations:

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.

Accumulators are pressure vessels and as such are manufactured, tested and certified according to statutory standards. for example, the relevant standard is the ASME Boiler & Pressure Vessel Code VIII, Division 1.

References:

• Brendan Casey.
• https://www.machinerylubrication.com/Read/2305/hydraulic-accumulators#:~:text=The%20typical%20design%20life%20for,inspection%20and%20recertification%20is%20required.
• https://www.lunchboxsessions.com/materials/hydraulic-accessories/accumulator-basics-lesson