Super duplex stainless (2506 UNS 32750 accumulators, dampeners, surge, water hammer, and shock alleviators in extreme, ultra, and super pressure corrosive applications.
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Hydropneumatic Accumulators / Hydraulic Accumulator
The name Hydropneumatic Accumulator covers those hydraulic products that are designed specifically for the storage (accumulation) of liquid under pressure. However, because of the improvements in design of equipment, which has grown from the storage devices, other functions have become loosely associated with the name Accumulator. These other types of units should more properly be called by their function, e.g. – Pulsation Dampers, Liquid Borne Noise Silencers, Line Surge Alleviators, Pump Controllers, Transfer Barriers, etc.
There is yet another area in which the original accumulator was used but where specialist manufacturers now find it necessary to supply purpose built products. These products are shock absorbers for liquids and they fall into three basic categories:
These devices allow large amounts of kinetic energy contained by a pipe with a large mass of fluid flowing at a substantial velocity, to be decelerated over a substantial period of time (at least 0.25 secs) by the increasing resistance caused by the compression of a precharge gas. These units are generally designed to have very large entry orifices that will in many cases allow the volume accepted by the unit to flow back into the system.
These are installed in a system so that their capacitance prevents a shock from occurring by allowing a deceleration time for the fluid, but in nearly every case these units have to have entries which are substantially as great as the diameter of the vessel itself.
These units are capable of dissipating a shock, i.e. absorbing a standing wave which has already been created and is travelling down a pipe. They do so by opening instantaneously upon sensing the beginning of a shock, allowing the shock in against their membrane and closing behind it.
Yet another application for which accumulators were used was to control pumps by allowing a considerable volume displacement to be realized in the system between two different pressure levels, so that pressure switches could be used to operate relays that in turn operated solenoid valves. This was a rather messy arrangement and as a result several manufacturers have developed various pump control accumulators. In all of these Pump Controllers signals and indications are given dependent upon the volume being stored or being discharged by the device.
The first real accumulators were weight-loaded types and these are still applicable where there is considerable headroom available with overhead gantries or lifting mechanisms plus factory space at a reasonable cost. They are nearly always used in a ring main system that supplies a department or complete factory with its hydraulic power.
Weight-loaded accumulators continue to be used to meet heavy industrial requirements and large units usually employ water as the fluid. The large weight-loaded accumulator offers the advantage of extremely high capacity at relatively low cost per unit volume. Construction is rugged and durable, and the units are capable of accommodating shock loads. Only simple control gear is necessary.
The disadvantages of a weight-loaded accumulator are:
The basic design of a weight-loaded accumulator is shown in Fig. 1. A heavy walled cylinder is mounted vertically on a substantial base and carries a ram. A crosshead is attached to the top of the ram, from which is slung a weight box. This is filled with any high density waste, such as ballast, iron scrap, concrete, etc. Alternatively, in the case of smaller units, special made weights may be slung from the ends of the crosshead.
There are two main types, depending on the method of constraining the weights or weight box. On a self-guided design the weight case is provided with internal guides. On externally-guided designs the weight case is constrained against radial movement by external guides or channels, usually mounted on a steel structure. The latter type is normally preferred to large high pressure accumulators to minimize bending stresses.
The ram is raised by pumping fluid (water) into the cylinder. Once raised, the fluid in the cylinder is pressurized by the combination of the weights and ram acting on the cross sectional area of the fluid column. The theoretical pressure available is thus given by:
Pressure (Bar) = 0.03Wt
Where Wt = total weight in kilograms
D = ram diameter in centimeters
Pressure (psi) = 1.274Wt
Where Wt = total weight in pounds
D = ram diameter in inches
In practice the nominal pressure available will be a little less, due to seal friction opposing downward motion. Pressure variations are also likely to occur with differences or variations in falling speed. Thus a pressure variation of 5% is likely to be experienced with a maximum falling speed of 0.3 meters per second (1 foot per second), but may be higher with higher falling speeds. Momentary peak pressures may also be higher or lower than the nominal pressure by an appreciable amount, depending on the rate of deceleration or acceleration of the ram, respectively.
Falling speed can be controlled by the stroke/bore ratio of the ram. A stroke/bore ratio of between 10 and 15 is commonly adopted for accumulators working up to 105 Bar (1500psi), although higher ratios are generally to be preferred for higher pressures. This, however, increases the problem of obtaining mechanical rigidity and also increases the overall height of the accumulator. This could make it unsuitable for indoor installation. As a rough guide, the overall height of a weight-loaded accumulator is at least twice the stroke.
Cast iron cylinders are commonly employed for accumulators working up to 105 Bar (1500psi). Cast steel or forged steel cylinders are used for higher pressures. Honed bores are required, although satisfactory performance may be obtained with rougher bores using leather seals. Rams may be made from cast iron (the original choice and still widely employed), but preferably chrome plated. Stainless steel or alloy steel rams are more usual on smaller sizes of modern weight-loaded accumulators.
Variations in the overall design include the type with a fixed ram and sliding cylinder, and also the differential weight-loaded accumulator. The latter is essentially a lower capacity device, but one capable of providing very high fluid pressures with a relatively low loaded weight. Construction takes the form of a fixed ram with the lower part fitted with a sleeve to increase its diameter. The cylinder slides over the ram and is fitted with weights, the sliding system being either internally or externally guided. The difference in ram diameter and sleeve diameter is relatively small, providing a substantial pressure amplifying effect – Fig. 2.
Control of a weight-loaded accumulator is essentially simple and straightforward. Normal safety devices employed with a continuous running pump include a relief valve which opens automatically to relieve pressure should the ram be raised beyond its normal upper position, and a bypass valve to relieve the system of pressure build-up should all controls fail with the pump still working. If the pump is operated on a "stop-start" basis, pump switching can be controlled mechanically by tappets on the moving assembly operating the pump starter or pump motor at the bottom of the stroke and stopping the pump when the ram reaches the top of its stroke. A typical control circuit is shown in Fig. 3 with automatic starting and stopping of the pump, together with an offloading valve to reduce peak load on starting up the pump.
Spring-loaded accumulators were created to fulfill the same function on individual circuits and had the benefit that as they did not rely upon gravity, they were less detrimentally affected by ‘G’ forces and these were therefore used in early mobile hydraulic equipment. Their only common use today is in systems where there is very considerable temperature change, which would make the use of a gas-loaded accumulator unsuitable.
A spring-loaded accumulator is shown in diagrammatic form in Fig. 4. This comprises a free piston sliding in a cylinder, with a compression spring (or springs) mounted in the blind end of the cylinder. The accumulator is charged by pumping fluid into the cylinder, displacing the piston towards the blind end and compressing the spring. The fluid is thus pressurized by the spring force.
The pressure generated by a spring-loaded accumulator depends on the initial loading and spring rate of the spring, and thus is not constant throughout the stroke of the piston. Pressure will vary from a maximum when the spring is fully compressed, to a minimum when the spring is fully extended, unless constant rate springs are employed.
The particular advantage of a spring-loaded accumulator is that it is compact in design, light in weight and can be used in any attitude. Basically, however, it is only suitable for relatively small capacities and low pressures. It is also not generally suitable for high cycling rates because of the limited cycling life of mechanical springs.
The first gas-loaded accumulators were just large versions of the non-separator pulse bottle. They later incorporated probes through the side of the vessel which sensed when the liquid level was close to the liquid outlet port and at this point signaled a valve to close to prevent the gas loading, normally called precharge gas, from escaping.
Non-separated gas-loaded accumulators take the form of a cylindrical pressure vessel or pressure bottles mounted vertically, with a fluid port at one end and a gas port at the other. Fluid is first introduced into the vessel and then pressurized by a precharge of gas to the required minimum pressure. Further fluid can then be pumped in, compressing the gas (and increasing the pressure). Because of the high compressibility of gases, a large volume of fluid can be accommodated, capable of being delivered from the accumulator above the minimum pressure specified. (Fig. 5).
The particular disadvantage offered by such a simple system is the mixture of fluid and gas at the interface. This limits the amount of fluid that can be drawn off without incurring the danger of drawing off gas into the hydraulic system. Also the cylinder must be mounted vertically. On the other hand such accumulators are simple and comparatively inexpensive and well suited to handling large volumes of fluid, and thus find widespread application. Volume can be readily increased by multiple installations.
The shape of the pressure vessel is invariably tall and narrow so that the contact area between gas and fluid is small. Even so, probably not more than two thirds of the fluid volume can be used without the danger of gas being drawn out into the hydraulic circuit. The pressure vessel may be connected directly to a high pressure air compressor operating against a level regulator, and with pressure regulator valves and controls.
Initially the accumulator is partially filled with fluid and charged with compressed gas at a pressure corresponding to the nominal hydraulic pressure required. If fluid is withdrawn on demand the gas expands, causing a slight reduction in pressure. If further fluid is introduced the precharge of gas is compressed, increasing the fluid pressure. Thus the degree of pressure variation is controlled by the ratio of the gas volume to the fluid volume; if necessary the gas volume can be increased (and pressure variation decreased) by introducing a further supply of gas. Additionally, capacity can be increased by adding further accumulators in parallel. The system is particularly flexible, both as regards control of pressure variation and capacity available.
In normal practice a pressure variation of 10% maximum is generally considered satisfactory. Volume ratios are normally calculated on the basis of isothermal conditions, when a 100 to 90 variation in pressure between high and low fluid levels is equivalent to a gas volume increase from 9 to 10, or a gas:liquid ratio of 9:1. A rather higher ratio (10:1 or 11:1) would normally be adopted, however, to allow for a reserve of fluid, i.e. a sufficient fluid volume in the bottom of the liquid bottle to ensure that the gas-rich boundary layer is maintained at a minimum working height well above the fluid port. An equally important factor is the optimum positioning of the fluid port to avoid any possibility of vortex formation at low fluid levels, which could allow gas to be drawn out through the liquid port.
To avoid the bulky design necessary to accommodate a 10:1 or higher gas:fluid volume, a more suitable arrangement is to employ a separate liquid bottle with a high fluid content connected to additional gas bottles – Fig. 6. This diagram also shows the controls necessary in diagrammatic form. The low level control on the accumulator is made to cut in whilst there is still an adequate volume of fluid remaining, and normally before the design low level is reached. This allows a time safety margin for the switchgear to shut down the accumulator or bring the pump to maximum output before the actual design low level is reached. A stop valve is also needed to prevent complete emptying in the event of exceptionally heavy demand or failure of the fluid pump.
Controls are normally automatic, and could include monitoring of the actual fluid level inside the liquid bottle. This could be by continuous pressure measurement, or by floats coupled indirectly to external indicators, or electronically.
Once precharged, no gas is actually used up during cycling, other than small amounts that may be lost by absorption. Thus no periodic topping up of the precharge is necessary, unless there is a definite discharge of the gas into the fluid circuit through mal-operation or malfunction of the accumulator.
Non-separated accumulators of this type for general industrial applications cover pressure ranges from 35 Bar (500psi) to 420 Bar (6000psi). A typical installation would comprise a single liquid bottle with three auxiliary gas bottle of similar size and the control system already shown in Fig. 6.
Fig. 7 Float Type Accumulator
Non-separated gas-loaded accumulators were later replaced by float type accumulators (Fig. 7). The purpose of the float being to actuate signals and, most important, to decrease the area of the fluid in contact with the area of the gas so as to cut down the rate at which the hydraulic liquid absorbed the gas and particularly to prevent turbulence in the hydraulic liquid from taking away the precharge gas by replacing the saturated hydraulic fluid interface with unsaturated hydraulic fluid. These units are still extensively used in very large installations where the cost of making a mold tool for the manufacture of a large membrane to separate the gas from the liquid is prohibitive and where the cost of honing the bore of a large pressure vessel in order to run a seal piston separator in it is equally prohibitive. They are also still used in installations which are remote from the availability of spare parts, e.g. for blow out preventer standby hydraulic energy on oil rigs floating at sea; and systems in less developed semi-industrial countries where seals and rubber technology are not readily available.
Pre-charge assembly to fit between
back-up bottle pipework
Separator Type Accumulators
Separator type accumulators employ a similar principle of accommodating a pressurized (compressible) gas and (incompressible) hydraulic fluid in a single pressure vessel, but in this case gas and fluid are separated by a physical barrier. Thus no gas saturated boundary layer is involved and the capacity of such an accumulator is virtually its full fluid volume. They are the most common types of accumulator in use today, particularly as they can be rendered in compact form.
Separator type accumulators can be broadly classified as having either rigid or flexible separators. The former use a piston mounted in a cylinder as a separator. Flexible separators include diaphragms, bags, tubes, bladders and bellows constraining the gas charge and are mounted within a suitable pressure vessel.
Piston Accumulators Diaphragm Accumulator
Membrane Type Accumulators
The first membrane type accumulator was the diaphragm type because the diaphragm is a relatively small molding across the center of a sphere and therefore before membrane molding technology was as advanced as it is today the largest possible units could be made with the minimum molding difficulties. These units are still in manufacture because there are a large number of extremely useful elastomers with good temperature and liquid compatibility capabilities, but which are not easily moldable in intricate shapes where the elastomer has to flow a long distance inside a closed mould tool. They are therefore found in exotic liquid systems and in extreme temperature services.
Membrane or diaphragm type accumulators are normally spherical or near spherical in shape and comprise two hemispherical shells clamped or screwed together with a flexible diaphragm between them – Fig. 8. The spherical volume is thus roughly separated into two halves, one forming the gas chamber and the other the fluid chamber.
Diaphragm designs and materials may vary considerably. The favored construction is a convoluted form in synthetic rubber, the convolutions providing minimum creasing with maximum flexibility of movement, and in particular, maintenance of flexibility at lower temperatures where elastomeric materials tend to harden. A rather simpler form of diaphragm is shown in Fig. 9.
The gas volume is charged at a lower pressure than the fluid volume. When the accumulator is initially charged with gas to the required pressure the diaphragm is fully flexed with the gas occupying the full volume of the accumulator. Fluid is then pumped into the high pressure side, compressing the gas and establishing a balance with equal pressure on each side of the diaphragm, and therefore no stress (other than pure compression) is placed on the diaphragm material.
Under normal operation the pump will build up gas pressure in the accumulator to the required balancing point, when an unloading valve is operated automatically to bypass the pump flow (for example to a reservoir). A check valve in the unloading valve prevents the accumulator from discharging to the reservoir. A safety device would also be incorporated to ensure that should the fluid chamber become completely exhausted the diaphragm cannot extrude through the fluid port under gas pressure.
Accumulators of this type are compact and light, but best suited to systems where demand is intermittent and the fluid volumes required from the accumulator flow are not very large, as this type of accumulator is not normally made in large sizes.
Bag Type Accumulators
Bag type accumulators became possible with improved rubber fabrication techniques and have developed to the extent that the gas containing bags are now fabricated by gluing and bonding a number of molded components together. At ambient temperatures and with hydraulic fluids which do not attack the bonding materials (and where the characteristics of the molding materials themselves are good enough), these units remain the most common in use.
"Hydrostor" Open Bag Type Accumulator
A typical design of bag type accumulators is shown in Fig. 10. The accumulator comprises an outer pressure vessel, normally cylindrical in shape with hemispherical ends, (but sometimes a plain cylinder), and with drawn or forged construction to eliminate welding or mechanical joints. Inserted into this pressure vessel is a bag of elastomeric material, chosen for compatibility with the hydraulic fluid being used (normally nitrile rubber for general use). The bag is drawn into position during assembly through the wide opening which accommodates the poppet valve. The bag and shell are assembled by means of a high pressure valve molded integral with the bag and clamped in place on the top of the shell with an external nut.
The oil port is assembled in the other end of the shell, the joint commonly being closed with an O-ring and the design adjusted so that the lower mouth of the shell will spread at a pressure below the design pressure of the shell as a safety precaution. The valve opening is large to allow an unrestricted flow of oil. The large opening also allows the bag to be removed for inspection or replacement should this be necessary. The poppet type valve provides high volumetric efficiency and also serves to prevent extrusion of the inflated bag when the fluid side is depressurized, or should all the fluid be drawn off. Other safety factors normally include some safeguard to prevent removal of the fluid discharge plug while there is any pressure remaining inside the bag.
In use, the bag is pressurized (with air or nitrogen) to the specified precharge pressure and fluid is pumped into the main chamber to compress the bag. The gas precharge pressure is invariably much lower than the fluid pressure and compression ratios of up to 5:1 may be achieved, according to the particular requirements of the installation. Nominal maximum working pressure with this type of accumulator is commonly 210Br (3000psi), although the same type can also be designed for higher rated working pressure, i.e. 350Br (5000psi) or 420Br (6000psi).
The flexible bag is normally pear-shaped or similarly tapered, this form giving optimum pressure distribution. Some modification of optimum shape may, however, be dictated by the material used and the method of construction. Bag failure is unusual with modern designs, although this can happen at a dangerously low level. This type of failure can occur where an external gas bottle is used and the total gas volume falls appreciably due to a considerable drop in temperature (as may happen overnight).
Bag type accumulators are particularly suitable for use with oil fluids, but can also accommodate other types of fluid provided that the bag material is compatible. Used with water or water fluids it is generally necessary to pre-treat the steel shell to prevent corrosion. Stove enameled interior finish for the shell is a typical treatment.
In the use of a bag type accumulator it must always be remembered that if the bag is punctured, the gas loss is likely to be sudden and total. For this reason, in order to prevent the bag being damaged by contacting the bag anti-extrusion mechanism, the flow rate capabilities of this design are severely limited. If used with gas bottles to increase capacity, it must be remembered that in order to prevent the bag from being forced into the piping system which runs to the bottles, it is necessary to reduce the ratio between the volume of the accumulator and the total volume of the gas bottles. Additionally, to prevent bag damage caused by violent changes of gas temperature entering the bag through small apertures or sintered plugs, the flow rate of bag type accumulators connected to gas bottles must be kept to the minimum. It may even be necessary to increase the number of bag type accumulators connected to the gas bottles. As a result gas bag type accumulators of the closed bag type are often used very largely to house gas rather than to accumulate liquid.
Bag type accumulators are the most versatile of all types and are equally effective for energy storage, shock absorption and "holding", and "reservoir" duties (fluid leakage make-up and fluid volume compensation due to temperature changes). They also provide effective damping of pump pulsations and are widely used for such other duties as hydro-pneumatic springs, pressurized fluid dispensers and transfer barriers between two fluids.
Such devices are frequently called open mouth units. Apart from being manufacturable in bag materials which are glueable, they also have servicing and inspection benefits as the complete cross section of the tube is laid open by the removal of a header plug. This header plug is also used to seal the open mouth of the bag and because the bag has an open mouth the internal shape mould former is easily removed from the molding, thus allowing the bag to be produced in a single operation. The header plug of the assembly therefore provides a surface through which charging valves, venting valves, gauge installations with isolation valves and gas back-up bottle connections can easily be introduced in conjunction with safety overload vent gas depressurization rupture discs. In general, these units are more versatile both in temperature and liquid in ancillary equipment terms.
Piston Type Accumulators
Solid piston accumulators are used where any sudden gas loss would be catastrophic. They are nearly always used in conjunction with back-up bottles. The response time of such devices is frequently as fast as 3 milliseconds. Any gas loss from these units will be relatively easy to determine and it is therefore usually possible to plan their maintenance and the replacement of seals to coincide with machine maintenance intervals.
A basic design of piston type accumulator is shown in Fig. 11. Normally the piston is "free", but in some cases may be connected to a conventional piston rod. Design and construction is relatively straightforward and the type can be made in a wide range of sizes. Cost is, however, relatively high. It is particularly suited for high pressure systems since cylinder stresses are readily determined and standard hydraulic quality cylinder tubes can be employed for the barrels. Construction can follow that of hydraulic cylinders, with tie rod assembly for higher pressures or heavy duties. However, it is necessary to make provisions to prevent disassembly of the end covers when either the fluid or gas side of the accumulator is under pressure.
The main disadvantages of piston type accumulators are associated with the piston seal, the most difficult task being that of maintaining an adequate seal when the fluid system is shut down with the gas end still under pressure. A number of designs include a liquid seal for additional protection.
With a liquid seal the fluid port is closed by a probe attached to the piston entering the end cover at the end of its stroke and trapping a certain amount of fluid in the liquid end of the cylinder. This amount of fluid is pressurized by gas pressure on the piston, but the area on which it acts is less than the piston area. As a consequence the pressure generated on the trapped fluid is greater than the gas pressure, preventing gas leakage into the liquid end. Liquid seals also prevent hydraulic shock in the event of the fluid content of the accumulator being fully discharged.
An alternative method of "cushioning" the end travel of the piston is shown in Fig. 12. This is a dashpot-type piston accumulator, where the fluid side of the piston carries an extension of reduced diameter entering a cushion chamber in the cylinder end. Should the fluid level fall to such an extent that the piston nose enters the cushion chamber, dashpot damping is provided over the remainder of the stroke.
Virtually all piston type accumulators suffer to some extent from gas leakage which may develop in use since no effective seals, can be entirely free from wear. Such conditions are aggravated by contamination of the fluid, or corrosion, which could affect the bore finish or seal material. Periodic topping up of the gas charge is therefore normally necessary in order to maintain a minimum working pressure.
Fluid capacity (and thus size of cylinder) can be determined from the following:
V2 = P1 V1 _ P1 V1
Where V1 = total gas volume
V2 = fluid volume
P1 = initial precharge pressure (gas)
P2 = minimum working pressure
P3 = maximum working pressure
(P3 - P2) = maximum permissible pressure variation
For continuous operation, P1 should be taken as near as possible to P2. The actual pressure difference over the working range (P3 - P2) can be adjusted by varying the compression ratio. Where this would require the use of an excessively large cylinder a separate gas bottle can be used to provide the additional gas volume. In this case the gas end of the cylinder would have to be designed with a large size port in addition to the charging valve for the precharge.
Piston type accumulators are useful for handling special duty fluids, which may attack conventional low-cost elastomers used with flexible separator accumulators. They would normally be vertically mounted, but horizontal or angular mounting is not necessarily excluded. They are not so suitable as other types for shock absorbing duties, due to the inertia of the piston and the friction of the piston seals.
A tandem type piston accumulator is shown in Fig. 14 and is used for special duties. It is also called a self-displacing accumulator and comprises an accumulator combined with a pressurized reservoir; it is thus capable of maintaining a constant volume of active fluid in the hydraulic circuit. The gas precharge displaces the tandem piston to fill the low pressure cylinder with fluid. When the system is pressurized, the high pressure (hydraulic) side of the accumulator is filled with fluid and the gas compressed. The fluid to fill the high pressure side is drawn from the low pressure side. With the system working, any fluid withdrawn from the high pressure side is simultaneously replaced on the low pressure side, thus maintaining a constant volume of fluid both in the system and the accumulator.
A further type of piston accumulator is shown in Fig. 15. This is of concentric configuration and is generally referred to as an annular piston accumulator, the outer (annular) volume providing additional gas volume. This has the advantage of providing a substantially larger gas volume without increasing the length of the cylinder or employing a separate gas bottle. Constructionally it has the advantage that the liner can be relatively thin since the gas pressure on the outside is equal to the fluid pressure on the inside, and they thus offset each other.
Membrane-cum Piston Type Accumulators (Fig. 16)
These units contain a hollow piston that is constructed like a diaphragm type accumulator. They have the advantage over pure membrane type accumulators that a piston is not capable of being so easily damaged by approaching the liquid or precharge gas ports and, therefore, they are typically designed for flow rates four times as great as pure bag type units.
The diaphragm within the piston gives an advantage over pure solid piston type units because the dynamic seals on the piston do not have to move for every minute pulse or fluctuation in the system caused by pump or servo oscillation. This advantage enables its dynamic seals to outlast those of a pure piston type accumulator many times over.
Fig. 16 Membrane-cum Piston Type
The arrangement is such that the ports to the chamber are on a pitch circle diameter and the ports to the diaphragm inside the piston are on the centerline. This makes it impossible for the diaphragm to escape down the ports of the system. As the piston moves at a much lower pressure differential than the pressure required to extrude the diaphragm the piston itself acts like the anti-extrusion valve at either end of stroke. As a result of this arrangement there is no anti-extrusion valve and in consequence the diaphragm’s sensitivity, unhindered by an anti-extrusion valve, is capable of dealing with high frequencies from 10Hz to 1000Hz. This combination diaphragm-cum piston unit is also used where the whole of any additional gas volume is required to be stored in additional gas bottles.
Indicating piston type accumulator
Indicating Piston Type Units
These units have been developed from the solid piston type accumulator because such accumulators are frequently used for pump unloading systems. They have a tail rod, which indicates the volume stored in the unit at any time. This is an advantage that can rarely be obtained from a membrane type unit, and even then, not accurately. In addition, by knowledge of the original precharge pressure and the use of a gauge connected to the gas side of the unit, it is possible by correlating the position of the indicator rod to the pressure shown on the precharge gauge to ascertain that the unit has not lost any gas precharge; this may be done at any time (i.e. one does not have to totally discharge the unit of liquid before checking the precharge pressure to ascertain whether any of it has been lost). The tail rod is frequently used to actually cut in or out of circuit one or more pumps and this can be done either non-electrically, by directly tripping the unloading valve, or by tripping switches. The result is that it is not necessary to have more than a few psi pressure (typically 5psi) between one pump cut in and the next or between pump cut in and cut out, even when working at 5000psi when these indicating type units are connected to gas bottles. This amount of control is extremely difficult to obtain even with the use of the most sensitive pressure switches, and it is this degree of controllability that has caused these units to be classified as "pump controllers" rather than accumulators.
Tubular accumulators are intended primarily to act as pump pulsation dampers or shock absorbers, rather than energy storers. They are comprised of inner and outer cylinders with the inner cylinder tube being perforated and covered with a rubber sleeve – Fig. 17. The unit is precharged with gas in the annular space between the two cylinders, fluid flow being through the inner cylinder, but with access to the inner side of the rubber tube through the perforations.
Fig. 18 Tubular Accumulator or Transfer Barrier
with all metal bellows
Bellows Type Accumulators
The bellows type accumulator is another form of tubular accumulator in which the rubber sleeve is replaced by a stainless steel convoluted sleeve – Fig. 18. Again, it is intended to be used as a pulsation damper or shock absorber with fluids incompatible with elastomeric sleeves (e.g. in high temperature hydraulic systems). The operating principle is essentially the same as that of a tubular accumulator.
General formula PVn = a constant
For fully adiabatic condition, n = 1.4. For applications where the accumulator has sufficient time to return to normal temperature before a second discharge is required, n = 1. Where fairly rapid cycling is required practical values of n usually lie between 1.1 and 1.3, depending on the amount of heat produced and the actual time of cycling. For example, 1.1 for low to moderate rates of cycling and some heating effect and 1.3 for rapid cycling where heating effects are very apparent.
Nominal or average pressure = Ph + Pl
Pressure variation (%) = Ph + Pl x 100
Ph = pressure at high fluid level
Pl = pressure at low fluid level
Vh = fluid volume at high fluid level
Vl = fluid volume at low fluid level
Vf = effective fluid capacity of accumulator
Vhg = volume of gas at high fluid level
Vlg = volume of gas at low fluid level
Vf = PlVl ( 1 _ 1 )
This formula gives the fluid volume (Vf) or effective fluid capacity required for a given working range from P2 (minimum) to P3 (maximum).
Pl = Vf P2 P3
Vl (P3 - P2)
For a practical solution, Pl must not exceed P2, Vf must not exceed the capacity of the accumulator, and P3 must not exceed the maximum pressure rating of the accumulator. If necessary the critical volume (Vl) can be increased by adding a gas bottle.
Gas-Loaded Accumulators with Auxiliary Bottle (Fig. 19)
Relevant formulas are:
Vf = V2 – V3
P1 = Vf P2 P3 .
(VlV4)(P3 - P2)
For working between given pressure limits P3 to P2
P2 = P3 (V2 – V3)
(V2 + V4)
Volume Relationship for Non-Separated Accumulators
For any given pressure difference (P2 – P1)
Vf = P2 – P1
where Vg = gas volume
Thus for an x% pressure difference
Vf = x .
Or for a 10% pressure difference
Vf = 10 = 1
V2 90 9
Compression Ratio (see Fig. 20)
Compression ratio = V2 + V4
V3 + V4
With no gas bottle, V4 = 0
Compression ratio = V2
Compression ratios are generally in the range 1.5:1 to 3:1, depending on the application with 2:1 a typical average figure. This may be further modified, and the pressure difference over the working range reduced, by coupling the accumulator to an auxiliary gas bottle.
The choice of size and type of piston accumulator is largely dictated by the particular application. Thus relatively large capacities are required to cope with continuous operation and high demand. A much smaller size could be used where the accumulator has only to supply peak demand or is worked only intermittently, or is mainly employed as a shock absorber. Where the accumulator is employed solely as a source of emergency power the size can be calculated on the demand required.
For continuous operation with piston type assemblies the inflation pressure (P1) should ideally be equal to the lower or cut-in value of the system working pressure (P2), as this will give the greatest swept volume over the working pressure range and thus minimize the number of pressure cycles. For intermittent use, or where the accumulator is used as a source of emergency power, lower inflation pressures and consequently higher compression ratios can be used.
"Minitrol" Shock remover
in section showing working characteristics
Accumulator Selection Chart (Christie Hydraulics Ltd.)
The graph (below) is universal for all gas-loaded accumulators. The only calculation required is the division of the maximum pressure P3 by the minimum pressure P2 to find a pressure ratio. For example, if P3 = 3000psi and P2 = 2500psi then the gas precharge will be 0.9 x 2500 = 2250psi, and the pressure ratio will be 1.2. Using the graph, locate 1.2 on the vertical axis and take a horizontal line across to the curve. Project this line vertically downwards and it will be seen to cut the horizontal axis at 10.5%. Check the manufacturer's literature to find the column headed "Actual gas volume". This is the volume of the accumulator at condition V1. If this column is not given take the nominal volume of an accumulator, e.g. 20 Liters. Multiply this figure by 0.105, to find the actual volume of oil stored, (i.e. 0.105 x 20 = 2.1 Liters of oil stored between the pressures of 3000psi and 2500psi.
Hydraulic line shock is the result of compression waves travelling from a source (e.g. a rapidly closed valve) up-stream to the end of the pipe and back again causing an increase in pressure and noise. This cycle is repeated at a regular frequency (depending on the length and the elastic modulus of the pipe) until the waves are dissipated by friction (Fig. 21).
The size of accumulator necessary to alleviate shock waves can be calculated from:
Vc = 0.004R x P2(0.005L – T)
P2 – P1
Where Vc = accumulator capacity required in gallons
R = rate of flow in the pipe in gal/min
T = normal closing time of the valve in sec (if the valve closure
is instantaneous T = 0)
L = length of pipe in feet
P1 = flow pressure at the valve in psi (this is the static pressure
at the valve when the valve is open and the fluid is at its full
P2 = pressure, the upper limit of which the surge should be
valve closure. This valve should be set at 1-1/2 times the
static pressure in the line when the valve is closed and the
fluid is at rest.
As a general rule the accumulator should be placed as close as possible to the source of the shock. Gas precharge should be approximately 10% below the static pressure at the valve at full flow pressure.
An accumulator has the inherent property of eliminating pulsations of a frequency greater than the cut-off frequency of the device. The cut-off frequency of an accumulator can be calculated from the geometry, fluid density and pressure.
cut-off frequency (radians per second) = 2 .
pL x dV
where p = mass density of the fluid
L = length of fluid chamber
A = cross sectional area of fluid chamber
V = volume of fluid chamber
P = pressure
All gas-loaded accumulators will have a low cut-off frequency. Thus, they will pass all moderate to high frequency pulsations but tend to absorb all low frequency pulsations below the cut-off frequency.
This characteristic is put to advantage to eliminate pump pulsations or pump ripple. In general, however, for satisfactory performance in this respect it is necessary to "size" the accumulator to provide a cut-off frequency higher than the known frequency of the pump. If necessary, two accumulators may be used, one on the inlet side and the other on the outlet side of the pump.
Elimination of Pump Pulsations
Pressure surges caused by pump pulsations are a common cause of shock wave generation in hydraulic systems. The severity of the pressure pulsations and their frequency depends on the type of pump and it’s speed. All positive displacement pumps generate pulsating flow. Piston pumps are the worst in this respect, and the fact that they are the type most used for generating higher pressures aggravates the problem. The use of several piston pumps discharging simultaneously into a pipeline may create very severe pressure surges that are liable to cause damage or failure. Pulsations and pressure surges such as these can be minimized or eliminated only by the use of a bag type accumulator whose absence of inertia or friction permits the extremely rapid response essential for effective pulsation damping.
A formula, which can be used to determine a suitable size of accumulator to eliminate the effect of pump pulsation, is:
V = K x Q .
where Q = pump discharge
K = a factor dependent on the type of pump:
- Simplex pump (single cylinder, single acting): K = 5
- Simplex pump (single cylinder, double acting): K = 2.5
- Duplex pump (two cylinders, double acting): K = 1.3
- Triplex pump (three cylinders, single & double acting): K = 0.45
Note: when the pump rev/min exceeds 100, then the denominator in the equation = 100.
Pressure Holding and Leakage Compensation
In a closed system where pressure must be held against the work by a holding ram for long periods while further duties in the operating cycle call for pump capacity, the use of an accumulator to replenish oil lost through leakage is advantageous.
The accumulator in a blocked circuit eliminates the problems of holding pressure variations created by the varying demands of branch circuits on the pump in open center systems. In addition, system leakages, which are normally present or which develop over a period of time are automatically taken care of.
When lengthy holding times are required, two or more hydraulically operated presses can be run economically with the use of accumulators. External or internal leakage through ram packings, valves or seals, result in piston creep and variation of the load on the work. The accumulator compensates for such leakage, maintaining the correct loading for the required period of time. Providing each press and accumulator is isolated during the holding cycle, the system pump is free to meet the volumetric requirements of the other presses.
Accumulators are now making possible many uses of hydraulic mechanisms, which hitherto have not been feasible. This is especially true where the danger of increased pressure due to thermal expansion of the fluid in closed systems would cause rupture of the lines. The installation of an accumulator, precharged to the normal working pressure in the line, readily takes up the expanded volume and, more importantly, feeds it back into the line as thermal contraction takes place.
Hydraulic push-pull control mechanisms have been greatly complicated and limited in use due to thermal expansion problems. The application of accumulators with a high precharge simplifies them and extends their use.
Transfer Barrier Between Different Fluids
In some systems it is necessary to develop pressure in one side of the circuit, and transfer the pressure developed into another fluid without the fluids intermixing. In this type of application the flexible diaphragm in the accumulator acts as a barrier between the fluids allowing some movement without diminishing the pressure.
Fig. 22 shows two accumulators employed to ensure an emergency gland-sealing oil supply to a compressor or fan. In the event of pump failure, gas pressure forces contaminated oil to expand the separator bladder, thus expelling clean oil into the system to seal the glands and prevent gas escaping into the plant.
In some cases an additional gas bottle is used to back up the accumulator and thus increase the available capacity. This course is frequently recommended, the object being to allow delivery of a given quantity of fluid at a smaller pressure drop than would occur with the accumulator alone. In a correctly designed system, exact pressures are known and the volume of the accumulator bladder can be controlled so that it need never decrease to less than one-quarter of it’s original expanded size.
The installation of an accumulator in a rigid hydraulic system introduces hydro-pneumatic springing which can be used to advantage in many applications.
For instance, steel mill rolls or sugar mill cylinders (Fig. 23) are required to exert a constant pressure as material passes between them. If foreign matter or oversize material is introduced the rolls must move apart to prevent damage and automatically resume their normal positions at the required pressure. This springing action is accomplished by an accumulator, or a series of accumulators if necessary, of sufficient capacity to absorb and release displaced fluid at almost constant pressure.
Another springing application is incorporated in the balancing of machine parts, a recent innovation representing a great advance in machine tool design. To eliminate any "judder" on the cutting tool, the tool head must be held firmly upwards against the lead screw thread. This was formerly achieved by attaching large cumbersome weights to the head through a system of chains and pulleys. The same function is now performed by a hydraulic ram backed up by an accumulator, the whole unit being much smaller and lighter, and capable of concealment within the machine structure.
In all springing applications, the rapid action of bag type accumulators due to lack of inertia, friction and "stiction" is advantageous, particularly where movements are small and, even more important, where pressures are low.
Unequal volumes of fluid will be displaced in the system by double-acting single-rod cylinders and similar devices. An accumulator in the circuit provides a means of accommodating such volume changes while offering other advantages that a simple tank or reservoir cannot. An accumulator, for example, will return excess fluid at substantially constant pressure.
Two (or more) accumulators in a circuit can be used to provide synchronization of movement of hydraulic cylinders, constant velocity or constant pressure operation, as required. This is merely a case of using the "stored energy" and "constant pressure" characteristics of an accumulator, which are maintained independent of demand or pump output, provided the accumulators are properly sized.
An accumulator is a convenient source of pressure to operate a holding device at constant pressure, regardless of demand from other sections of the system, or for maintaining a high working pressure on a workpiece during a long stand-by period. A particular advantage is that this is accomplished without power being absorbed or heat generated, as would be the case were the "hold" maintained by a continuously running pump.
An accumulator is a ready source of high pressure or increased capacity on a dual-pressure circuit and in such cases may be charged by a separate pump. Thus a large-volume low-pressure system demand could be met by a low pressure pump (with its own accumulator, if necessary) and the second high-pressure service met by an accumulator charged by a small, high pressure pump.
An accumulator can be used as an emergency source of power in the event of failure of the primary power source (for example, failure of the pump driver). They are widely employed in this way on aircraft systems for maintaining operation of emergency services such as undercarriage operation. They are also used on large road vehicles in utilizing hydraulic brakes and steering systems and can be used in industrial applications. The main requirement is that the accumulator should be of a suitable size to meet the emergency demand. Reversion to emergency operation can be fully automatic or manually selected, as required.
Liquids and lubricants can be stored in an accumulator to be dispensed at will under controlled pressures. In cases where constant but extremely slow rates of lubrication are sufficient, an accumulator can provide an automatic supply that requires checking only at long intervals.
In other cases where actuation is necessary for a short period in the event of pump failure or incapacitation of the primary hydraulic lines, the accumulator serves as a source of energy to provide emergency lubrication and prevent damage.
Accumulator Safety Design
The Health and Safety at Work Act 1974 lays a duty on manufacturers to ensure safe design and construction of items when properly used (Chapter 37, Paragraphs 6 (1) a, b, and c of the Act).
The following is a provisional Safety Design Code, which would appear to cover the requirements of the Health and Safety at Work Act and the most stringent requirements in the following countries by the following authorities:
Belgium – Associated Industrial Belgique (A.I.B.); France – Bureau de Mines, A.P.A.V.E.; Germany – T.U.V.; Italy – A.N.C.C.; Sweden – "Swedish Steam Users" and "T.R.C."; Switzerland – S.C.B.D.; Canada and Japan – A.S.M.E. VIII.
Taken from Trade & Technical Press Limited, 1979. Hydraulic Handbook 7th Edition.
For information on shock, pipeline water-hammer and pump start-up and shut-down surge, go to Shockguard (USA div. of Shockguard Ltd)...
Piston Type Accumulators
HydroTrole: Hydraulic and Control Engineering since 1963.
The name hydropneumatic accumulator covers those hydraulic products that are designed specifically for the storage (accumulation) of liquid under pressure. However, because of the improvements in design of equipment, which has grown from the storage devices, other functions have become loosely associated with the name Accumulator. These other types of units should more properly be called by their function, e.g. – Pulsation Dampers, Liquid Borne Noise Silencers, Line Shock Alleviators, Pump Controllers, Transfer Barriers, etc. More about accumulators...