As generally defined, “Accumulators are gas-charged pressure vessels usually installed on hydraulic systems to store energy and dampen pulsation”. Accumulators need to be gas-charged in order to operate. Nitrogen remains one of the most preferred and suitable gases for accumulators which is due to its distinct characteristic features. Oxygen is highly inflammable, and should not be used. An accumulator-mounted hydraulic system can make use of smaller pumps since the accumulator is designed to store excess energy from the pump during occasions of low energy demand in the hydraulic system. When required, the stored energy is made available for prompt usage. It is interesting to note that the energy supplied by an accumulator is by far greater than that of the pump.
Accumulators perform surge or pulsation absorbing functions amongst several other applications. They help in supplying cushioning effects to the hydraulic hammer, thus absorbing the rapid shocks caused by the sudden start up and shut down of the power cylinders in a hydraulic circuit. When properly installed, accumulators are capable of working independent of human interventions.
We would be considering the three main types of accumulators, namely the bladder, diaphragm and piston accumulators. Their functions range from light to very heavy duties. Accumulators are generally governed by both local and international laws.
Let’s now examine what more there is to these highly efficient pressure equipment otherwise known as accumulators. We’d begin by looking more closely at their functions.
First on our list is the accumulator’s pressure maintenance function in the hydraulic system. The fluctuating temperatures of liquid in the hydraulic system usually has a direct implication on the system’s overall pressure. Hydraulic fluid leakage also results in pressure drop. This issue can be rectified by an accumulator. The efficiency of an accumulator-mounted hydraulic system far surpasses that of a non-accumulator-mounted hydraulic system. In the case of the former, the accumulator performs a complementary role on the pressure changes by supplying or absorbing the deficient or excess amount of hydraulic fluid, as the case may be. A non-accumulator-mounted system would suffer unchecked lags during periods of pressure changes.
Next is fluid dispensing. Accumulators do perform fluid-dispensing functions. They could be used to dispense small volumes of fluids like lubricating greases and oils, when required.
One of the most essential functions of the accumulators is their energy storage capacity. Hydropneumatic accumulators essentially require a gas, in addition to hydraulic fluid, to operate. Fluids are fundamentally incompressible therefore cannot store pressure energy. Hydraulic fluids can only be compressed by 1% under a pressure of about 2,500 psi. This relative incompressibility however, makes fluid a perfect vehicle for power transmission in the hydraulic system.
In contrast to fluids, gases can be compressed into smaller volumes under high pressures. The compressibility property of gases is employed in the operational mechanism and construction of accumulators. Potential energy is stored in the compressed gas, and released on demand. When pressure is increased, the gas volume is compressed. On the other hand, when the pressure drops, the compressed gas consequently expands; thus forcing the fluid into the circuit -to locations where they are required.
In the bladder accumulator, the compressed gas exerts pressure against the bladder which separates the gas and the hydraulic fluid. This pressure, in turn, causes the bladder to force the fluid into the hydraulic system. The same applies to the diaphragm and piston accumulators. In each case, the diaphragm or piston, forces the fluid into the system.
Besides the energy storage functions, accumulators are also capable of power storage by supplementing the power supply functions of the hydraulic pump. The hydraulic pump saves up considerable amounts of potential energy in the accumulator during its idle periods of operation. This stored power is made available, by the accumulator, to the system when it requires emergency or peak power. Accumulators act as an auxiliary power source in the event of power failure in the system.
The accessories designed to work alongside the accumulators are; the adapter, pressure gauge, pressure gauge with shut-off valve, unloading valve, safety fuse and protective case.
The pressure gauge is installed on the gas side; it ensures the permanent monitoring of the pre-charge pressure. The function of the Pressure gauge (with shut-off valve) is similar to that of the pressure gauge. The only difference is the inclusion of the shut-off option for safety and control.
The unloading valve is used to safely and efficiently relieve hydraulic pressures from the system when it is de-energized. Its needle valve facilitates the seamless blowdown of the accumulator.
The safety fuse, as the name implies, is used to ensure the safety of the system. It can be installed on either the accumulators or the gas bottles. Its basic function is to help check against possible over-pressurization of the gas as a result of external heat or hydraulic pressures.
Before we go further to learn about some details of individual accumulators, it is vital that we take note of some operational facts;
Different manufacturers are known to have specific recommended pre-charge pressure value for their accumulators. Let’s consider the energy storage function as an example. The bladder accumulator is usually pre-charge to about eighty (80) percent of system’s minimum hydraulic pressure; while the piston is pre-charge to about a hundred (100) psi value below the minimum system pressure. Pre-charge pressure is a determinant for the volume of fluid that would be left in the accumulator at minimum system pressure.
Furthermore, it’s important to note that accurate pre-charge is attained when the accumulator’s gas side is filled with a dry inert gas, e.g. nitrogen, when no hydraulic fluid is present in the fluid side. Charging commences when the hydraulic fluid is administered into the fluid section; this should only take place at an accumulator pressure value higher than the pre-charge pressure value. Carrying out this processes at the appropriate pre-charge pressure is key to prolonging the accumulator’s lifespan; proper care must always be taken to ensure that the pre-charging is accurately carried out. Carefully adhering to operational instructions when operating the accumulator always go a long way in determining its service life. For example, if the operator does moderate the gas/system pressure without performing a corresponding action on the pre-charge pressure, the accumulator could deteriorate quickly, even if it was best in the world.
Beginning with the bladder accumulators, let us examine each of these pressure equipment.
As earlier mentioned, the Bladder Accumulator utilizes a rubber bladder to hold gas charge. This plain, flexible rubber is encased within a steel shell. The bladder accumulator generally consists of the fluid and the gas sections, as well as the bladder which acts as a gas-proof screen. The fluid attached to the bladder is linked to the hydraulic circuit thereby allowing the bladder accumulator to efficiently suck in the appropriate volume of fluid. When pressure is increased, the gas volume is compressed. Conversely, when the pressure drops, the compressed gas consequently expands thus forcing the fluid into the circuit to locations where they are required.
The bladder accumulator utilizes large cavity openings for rapid discharge. The large size of these openings makes it possible for the accumulator to remain insensitive to debris. Another interesting feature is the free movement of the bladder which facilitates prompt and rapid responses, thus eliminating system lags.
This completely repairable pressure equipment is adaptable for various sizes. The bladder can be easily replaced in the event of failure or damage. The recommended mounting position is in the vertical plane. It could also be mounted in the horizon plane (in situations of low cycle applications). The horizontal mounting may however result in slight reduction of the stored volume. The bladder type accumulator is essentially designed to have a limited 4:1 ratio (of maximum pressure to gas-charged pressure); this is necessary in order to protect the bladder from excessive distortion strains.
Subject to manufacturer’s model and design, gas accumulators are available in three varieties; the standard bladder, low flow bladder and high flow bladder accumulators. They generally have a wide range of specific applications which will be discussed later.
It is imperative that certain basic operational conditions and parameters be considered during the selection and mounting of accumulators in general. Some of the conditions/parameters include: the nominal volume, operating pressure, effective gas volume, maximum flow rate, permitted operating temperature, etc. Others include pre-charge gas pressure value, recommended/suitable gas and fluids and so on. It is vital that the manufacturer’s operating manual be read to the detail.
Let’s now look at the diaphragm accumulators. The bladder and the diaphragm accumulators are sometimes referred to as bladder accumulators. This is due to the very similar basic principle of operation which they both employ. The difference is that instead of the rubber bladder as in the case of the bladder accumulator, the diaphragm accumulator makes use of a diaphragm as its gas-proof screen.
A diaphragm accumulator can be exposed to higher compression ratios of up to 8:1 since its gas-proof screen is not distorted to the same level as the bladder. They may also be mounted in the vertical or horizontal positions. Where strong vibrations are expected, ensure you firmly secure the accumulator to prevent it from loosening while working.
The diaphragm accumulators are mostly obtainable in smaller sizes that range between 0.07 and 3.5 litres. They are also insensitive to debris, and are known to be very responsive to quick changes in pressures as they are strain-free.
Always ensure that a residual fluid volume of approximately 10% of the effective gas volume remains in the accumulator. This is to ensure the efficiency of the accumulator, as well as protect it from damage.
Unlike the bladder accumulators, the diaphragm accumulators cannot be fixed once damaged.
The third on our list of accumulators is the Piston accumulator. The piston accumulator is fundamentally a hydraulic cylinder without a rod. They can perform heavy duty functions. On the one side of the Piston is the charged gas, while the other end is supplied with hydraulic fluid.
Like other accumulators, a typical Piston Accumulator consists of a fluid section, gas section and the piston acting as the gas-proof screen. The gas section is pre-charge with an inert gas (usually nitrogen) from the gas bottle. Characteristic of the operational mechanism of gas accumulators, the fluid section is connected to the hydraulic circuit so that the piston accumulator pulls in fluid at high pressure and compressed gas volume; at low pressures, the compressed gas is expanded, thus driving the fluid into the circuit.
The piston accumulator can be easily fixed by simply replacing the Piston seals. Cases of distortion rarely occur with the piston accumulator as is the case with bladder. A remarkable feature of the piston accumulator is that it can be operated at very high compression ratios; the limitations are subject only to the design of the vessel. Unlike the bladder and diaphragm accumulators, the piston are sensitive to debris due to the gliding contact nature of the energetic piston seals. These movements the pistons suffer strains. For this reason, they are not mostly recommended in applications as pulsation dampers where quick responses are required.
Generally, the piston speed velocity should be limited and can vary from 1 to values not greater than 4m/s. High flow rates in relation to the kind of piston used should be considered when determining the speed velocity of the piston accumulator. An aluminum-type piston is usually recommended because it can be easily accelerated due to its light weight.
The piston has guide rings that help protect against metallic friction in the accumulator wall. Furthermore, if the accumulator is to be used with corrosive fluids, the parts that come in contact with the fluid can be plated with nickel for protection, or completely built from materials that are resistant to corrosion. Component accumulator materials suited for low temperature applications are available on request from various manufacturers. It is vital that one has the exact detail about the operational conditions the piston accumulator would be subjected to, before ordering one. There are certain important standards to be considered when selecting an appropriate sealing system. These include: the effective pressure differential, design pressure, switching cycles, temperature fluctuation, operating fluid and the micron rating among others. The type of piston used in the manufacturing of an accumulator is what makes it vary one from another. Sealing systems vary according to the type of piston used. This is because each piston has its own seal-arrangement which is a major speed and efficiency determinant for the accumulator.
From now on, we shall be looking more closely at the bladder and the piston accumulators. This is because your decision for a suitable type of accumulator requires more than assumptions.
Accumulators are generally used to enhance pump flow. They make power available, dampen shock, minimize pump rhythms, and help override possible consequences of internal leakages. They are also efficient in energy saving, minimizing cost of equipment, help check operational expenses, as well as extend equipment life.
Having decided based on several benefits obtainable from accumulators in an hydraulic system, the next thought that readily comes to mind is wondering which of the two designs would serve you better. Interestingly, for a vast array of applications, these factors may not matter. However, the bladder accumulator is generally preferred for some applications, and the piston in others.
Bearing in mind that we are using the collective name, ‘Bladder Accumulator’, for both the bladder and diaphragm accumulators, let us do a proper comparison between the bladder and piston accumulators:
With a volume capacity of not more than 15 gallons, the maximum flow rate obtainable from a bladder accumulator is generally 220 gm-1. A higher flow rate spells ‘damage’ for the bladder.
The pistons, having a similar size to that of the bladder, can tolerate higher rates to the tune of 800 gm-1.The maximum velocity obtainable for the pistons is 120 in. sec-1. The piston accumulator is the best bet in any situation where the hydraulic system is in demand for a flow rate greater than 220 gm-1.
As earlier mentioned, the bladders are rubber materials. They have an operational working-temperature limit of 2500 F; the lower limit being “minus 400 F”. Depending on the seal type, the pistons generally an operational working-temperature range of “minus 45 to 3200 F”. The invariable votes in the pistons for ant application whose temperature range falls beyond that of the bladders.
Furthermore, considering the fact that bladders do not give prior warning or indication before damage, and that on damage, their stored gas is consequently released into the system. It is therefore strongly advised that you take into serious considerations the potential effect or hazard that could occur if the pre-charge gas finds its way into the hydraulic system.
The small-sized seal of the piston accumulators in an advent of failure, do so gradually. Consequently, gas migration into the fluid section is slow. Piston accumulators are recommended in hydraulic operations where sudden gas infusions into system wouldn’t result in harm, or hazard.
The size of the accumulator is another important factor to consider. The bladder accumulator is restricted in size. Besides that, it isn’t available in flexible capacities. For example, it is required that you purchase a 5-gallon bladder accumulator unit even if your hydraulic system requires a 2.5.
Conversely, piston accumulators have varying sizes for a very wide range of applications. They also have the capability to work in conditions of where very pressures are involved
Also, the piston accumulator can be fitted with an automated position sensor to determine if the piston is accurately located in the accumulator during operation. This would help in determining if its pre-charge value is correct. The bladder’s pre-charge value can only be manually determined before the system is energized.
The piston accumulators does not exclusively hold sway over the bladders. The bladder accumulators do possess some fantastic features that make them ideal for a variety of applications.
One of such is that the bladders are a perfect choice of accumulators when the operation involves an agitated system. The bladders are more adapted for short ‘to and fro’ movements; whereas the seal of the pistons negatively react to such. Besides the fact that the bladder accumulators are tolerant to system agitations, they are also lightweight
Although pistons’ failures are gradual in comparison to the bladder that fails suddenly, there are situations where the ‘gradual failing’ of the piston could actually cause harm to the application. In this case, an ‘immediate failure and replacement’ as obtainable with the bladder, is preferred.
The piston accumulators are known to share similar characteristics with the cylinder. If left idle for a long period of time, the piston would require more pressure than the usually for startup. In such a situation, the bladder accumulator is preferred especially if the application does not require high pressure for its operation.
In the aspect of servicing and maintenance, the bladder is quite “a piece of cake” when compared to piston accumulators. The bladder can be easily installed, cleaned from dirt and replaced when damage. Using a piston does not present you as much privilege.
When dealing in applications where weight is significant, Having all other operational conditions in place, the bladder accumulator is recommended. Bladder accumulators with similar ratings as the piston generally have a weight value which is about half that of the piston.
The Gas bottles play a significant role in applications that involve massive accumulators. The use of gas bottles alongside the accumulators significantly helps in reducing overall cost of equipment. The use of these gas bottles with typical bladder accumulators are not recommended. The framework of the bladder accumulator does not give room for such. In applications where a gas bottle must be used with the bladder accumulator, a “transfer barrier” should be employed. The bladder could get immediately damaged if this barrier is not used.
Besides their overall cost effectiveness when incorporated, gas bottles rarely require any kind of maintenance. They also help to maximize space in that they do not require any specific mounting position.
A note of caution: Repairs on accumulators should only be executed when the pressure and the fluid have been completely expelled. Under no account should soldering, wielding or mechanical work be carried on the accumulator’s shell; once connected, the hydraulic line should be entirely vented.