What is in this article?:
- Fluid Power eBook â Fluid Power Circuits Explained
- Chapter 1: Hydraulic Accumulators
- Chapter 2: Air Logic Circuits
- Chapter 3: Air-Oil Circuits
- Chapter 4: Slip-In Cartridges
- Chapter 5: Counterbalance Valve Circuits
- Chapter 6: Fluid Power Cylinders
- Chapter 8: Directional Control Valves
- Chapter 9: Filtration
- Chapter 10: Flow Control Circuits
- Chapter 11: Flow Divider Circuits
- Chapter 12: Fluid Motor Circuits
- Chapter 13: Pressure Intensifier Circuits
- Chapter 14: Proportional Control Valve Circuits
- Chapter 15: Pumps
- Chapter 16: Reducing Valves
- Chapter 17: Regeneration Circuits
- Chapter 18: Pressure-relief Valves
- Chapter 19: Rotary Actuator
- Chapter 20: Sequence Valve Circuits
- Chapter 21: Servovalve Circuits
- Chapter 22: Synchronizing Circuits
- Chapter 23: Sample Actual Circuits
Chapter 1: Hydraulic Accumulators
Hydraulic accumulators make it possible to store useable volumes of non-compressible fluid under pressure. A 5-gal container completely full of oil at 2000 psi will only discharge a few cubic inches of fluid before pressure drops to 0 psi. The same container filled with half oil and half nitrogen gas would discharge over 11/2 gal of fluid before pressure dropped to 1000 psi.
Figures 1-1 through 1-4 show symbols used for different types of accumulators. Figures 1-5 through 1-8 are simplified cutaways showing construction of different types of accumulators.
All accumulators except Fig ACC4 will have a pressure decrease as fluid discharges. A weight-loaded accumulator maintains pressure until all oil is used.
When using an accumulator, it is necessary to install a manual or automatic function to de-pressurize all fluid before working on the circuit. Several manufacturers make automatic discharge valves that work well. These automatic discharge valves are explained at the end of this section.
Most hydraulic accumulators are used in one of four applications:
1. Supplement pump flow in circuits with medium to long delays between cycles.
2. Hold pressure in a cylinder while the pump is unloading or stopped.
3. Have a ready supply of pressurized fluid in case of power failure.
4. Reduce shock in high velocity flow lines or at the outlet of pulsating piston pumps.
The following circuit images show some circuits using accumulators for the operations mentioned in 1 to 4 above. Other accumulator circuits and information follow.
Using accumulators to supplement pump flow
Some hydraulic circuits require a large volume of oil for a short time; for example to move a large cylinder rapidly to clamp a part. After clamping, the circuit needs little or no additional fluid for period of time while curing takes place. When a circuit has extended dwell time, an accumulator can be used to downsize the pump, motor, tank, and relief valve. The cost of accumulators usually offsets savings on these smaller components, but downsizing saves on operating costs.
The conventional pump, directional valve, and cylinder pictured in Figure 1-9 show horsepower and flow requirements needed for a 12.5 sec cycle time. The advance cycle requires full power, while returning the cylinder needs minimal force. Reduction of the pump and motor size is not possible if the cylinder cycles rapidly. However, if there was a 45 sec wait between cycles, the pump and motor could be almost 70% smaller with an accumulator circuit.
This reduced flow and horsepower are possible when using accumulators and the circuit shown in Figure 1-10. The extra expense of the accumulators offsets the reduced price for the power unit, but operating cost is less for the life of the machine. The directional valve and piping from the accumulators to the cylinder still has to handle the 125 gpm flow.
Using a gas charged accumulator in a pump supplementing circuit will increase maximum system pressure. The extend portion of the cycle needs at least 2000 psi working pressure, which requires filling the accumulators with fluid above 2000 psi so they can discharge oil and not drop below minimum pressure. The maximum system pressure should be as high as can be tolerated. The higher the maximum allowable system pressure, the smaller the accumulators. The drawback of high pressure is that the circuit is at this pressure when the cycle starts. If this higher pressure can cause damage or other problems, it should be lowered to a safe level.
Accumulator circuits normally have flow controls because there is a volume of oil at elevated pressure that can discharge almost instantaneously. Placing a flow control at the accumulator outlet allows free flow from pump to accumulator and adjustable flow to system.
The circuit in Figure 1-10 has a minimum pressure of 2000 psi and a maximum pressure of 3000 psi. This pressure is the limit of most hydraulic components. A 22-gpm pump driven by a 40-hp motor now meets the force and cycle time specified. All pump flow continuously goes to the circuit instead of being unloaded most of the time as in conventional circuits.
As the cylinder cycles, the accumulators supply fluid at a rate set by the flow control. Pump flow adds to accumulator flow to set the required cycle time. Cylinder cycling could be made faster than specified by increasing outlet flow from the accumulator.
The fixed-volume pump in Figure 1-10 unloads through a special accumulator relief/unload/dump valve, which sends all pump flow to the accumulators and cylinder until the system reaches set pressure. After reaching set pressure, the valve opens and unloads the pump to tank at approximately 50 psi. The pump will continue to unload until the system pressure drops about 15%. This pressure drop might be from leakage or at the start of a new cycle. Any time pressure drops, the pump will load and stay loaded until pressure tries to go above 3000 psi. With this valve, stored oil in the accumulators automatically discharges to tank when the pump stops, which makes the circuit safe to work on shortly after locking and tagging off the pump.
Figure 1-11 shows a variation of the accumulator circuit in Figure 1-10. Here a 1-gpm fixed-volume pump and a 5-gpm pressure-compensated pump supply oil until the accumulators fill. A pressure switch, set at about 2900 psi, unloads the fixed-volume pump through a solenoid-operated relief valve. After the fixed-volume pump unloads, the pressure-compensated pump finishes filling the accumulators and holds maximum pressure without fluctuations and with minimal heating.
The accumulator dump valve in this circuit will stay closed as long as the pumps are running. When the pumps stop, this valve quickly and automatically discharges the accumulators to tank.
Full-time pressure with fixed-volume pumps
Some circuits need pressure at all times to hold position or maintain force. The circuit in Figure 1-12 holds pressure on the cylinders when they stop, but excessive heat generation makes it a poor choice. Flow controls keep pressure in the circuit while a cylinder is moving.
Some designers use the circuit shown in Figure 1-13 to simultaneously reduce energy loss and maintain holding pressure. This double-pump circuit provides high flow (to move the cylinders rapidly) and low flow (for pressure holding). While the system is at holding pressure, the high-flow pump goes to tank through an unloading valve. Only the low-flow pump goes across the relief valve. Although energy loss is drastically reduced, it is still excessive.
The circuits shown in Figures 1-14 and 1-15 use a small accumulator to hold pressure on the actuators while unloading the pump at minimum pressure. This makes it possible to use a less expensive fixed-volume pump instead of a pressure-compensated pump, with little or no energy loss or heat generation.
The pump in Figure 1-14 unloads through an accumulator relief/unload/dump valve. This valve sends all pump flow to the accumulator and cylinders until the system reaches set pressure. After it reaches set pressure, the valve opens and unloads the pump to tank at approximately 50 psi. The pump will continue to unload until the system pressure drops about 15%. This pressure drop might be from leakage or it could be at the start of a new cycle. The pump loads again and fills the circuit until pressure tries to go above 2000 psi. While the pump unloads, the accumulator makes up for any leakage so pressure at the cylinders only drops about 15% maximum. The length of time the pump unloads depends on the size of the accumulator and the amount of system leakage. With the accumulator relief/unload/dump valve, stored oil in the accumulator discharges to tank when the pump stops. This makes the circuit safe to work on shortly after locking and tagging out the pump.
Notice the variation of the above pressure holding circuit in Figure 1-15. Here the pump unloads through a normally open, solenoid-operated relief valve controlled by a pressure switch. The accumulator and actuators fill from the pump until system pressure reaches 2000 psi. At 2000 psi, the pump unloads through a solenoid operated relief valve at approximately 50 psi. The main advantage of the circuit in Figure 1-15 is that pressure drop is adjustable by more or less than the fixed 15% allowed by the unloading valve in Figure 1-14.
To have a safe accumulator circuit, it is necessary to have a means to discharge stored energy at shutdown. The circuit in Figure 1-15 uses a high-ratio pilot-to-close check valve. The pilot ratio is about 200:1, which means 25 psi in the pilot line can hold as much as 5000 psi in the circuit. Most unloading circuits have at least 25 psi while unloaded, so this valve works well. When the pump shuts off, pressure drops to zero, the pilot-to-close check valve opens, and stored energy dumps to tank.
Another way to automatically discharge the accumulator at shutdown is with a normally open, solenoid-operated, 2-way directional valve. This directional valve connects to the accumulator pressure line and on to tank. Starting the pump motor also energizes the solenoid on the normally open 2-way valve, causing it to close. As long as the pump runs, this valve blocks the flow path to tank. When the pump stops, the solenoid is deenergized, and the valve shifts to port stored energy to tank.
CAUTION! ALWAYS CHECK AN ACCUMULATOR CIRCUIT FOR PRESSURE BEFORE WORKING ON IT. NEVER ASSUME THE AUTOMATIC UNLOADING SYSTEM WORKED!
Accumulators used for fast response and over-pressure control of pressure-compensated pumps
Because most pressure-compensated pump circuits have closed-center or two-position directional valves (such as the one shown in Figure 1-16), they stay at full-pressure, no-flow until a valve shifts. After any directional valve shifts to start an actuator’s movement, pressure in the circuit starts to drop. When the pump sees a pressure drop, its internal mechanism starts shifting as fast as possible to start fluid flowing. Pump shifting times vary, but no matter how fast they shift, the actuator’s initial response will be slowed down.
With an accumulator installed, as shown in Figure 1-17, the pump is still at no-flow when the circuit is at rest. However, there is a ready supply of oil at pressure available. As a cylinder starts to cycle, as seen in Figure 1-18, fluid flows directly to the actuator from the accumulator and pressure starts to drop. This pressure drop causes the pump to go on stroke, but now pressure drop is minimal. The cylinder takes off quickly and smoothly, and the pump has time to respond to the flow need.
On the other end of the cycle, if the pump is at full flow and all valves center or all the actuators hit the end of stroke, the flow requirement suddenly drops to zero. The pressure-compensated pump is still flowing at the maximum rate and pressure starts to climb. The pump will continue at full flow until pressure reaches 80-98% of the compensator setting. There has been zero flow needed for some time, but the pump does not know this until pressure is near maximum. When pressure reaches compensator setting, the pump starts to shift to no flow. All pump flow during shifting time has no place to go, so this excess flow generates a pressure spike of five to ten times the compensator setting. This pressure spike can cause premature failure of the pump, plumbing, and actuators.
A common fix for this pressure spike is to add a relief valve near the pump outlet, set 150 to 200 psi higher than the pump compensator (as shown in Figure 1-16). This relief valve should reduce the pressure spike, but it does not lower it as much as it appears. A relief valve remains closed until pressure reaches 90 to 98% of its setting. Once the relief reaches maximum pressure, it starts to open, but by the time it actually relieves, the pressure may be 11/2 to 3 times its set pressure. This reduced spike is better, but still is not as good as what an accumulator could provide.
Other problems can occur with relief valves. For example, if the relief valve setting is at or near the pump-compensator setting, the pump can start oscillating on-off flow. As the pump nears its pressure-compensator setting and starts to compensate, the relief valve starts to relieve. A flow path is created when the relief valve begins to open, so downstream pressure drops, causing the pump to go back on stroke. The drop in pressure allows the relief valve to close, so downstream pressure builds up again. This oscillation cycle repeats rapidly, causing damage to the pump and possible line failure due to shock. In another example, if the relief valve setting is lower than the pump compensator, all pump flow goes to tank at relief pressure, generating excess heat. To avoid these problems, use the correct procedure when setting pressures on a relief valve used to reduce pressure spikes.
An accumulator absorbs excess pump flow with minimal pressure override or shock. While fluid from the pump compensates from full flow to no flow, as seen in Figure 1-19, it has a direct path to the accumulator. Because the accumulator has a compressible gas in it, it takes in the small amount of excess flow produced while the compensator is reacting. Pressure increase from this additional fluid is imperceptible.
To size an accumulator for fast response of the circuit, plan to have somewhere between 1 and 5 sec of actuator flow before pressure drops below the minimum it takes to move it. A rule of thumb is to have 1 gal of accumulator for every 10 gpm of pump flow.
Using an accumulator as an emergency power supply
A conventional hydraulic system will not operate unless the pump is running. Some machines must be able to cycle to a safe condition after a power or pump failure. Use an accumulator to store enough energy to move the actuators to a safe condition after the pump quits. The operator or setup person can manually cycle the machine into a safe condition by using the stored energy.
The hopper gate cylinder shown in Figure 1-20 must close in case of a power failure. If the gate stays open, the entire hopper could overflow the truck under it, then dump on the ground. This circuit uses a pressure-compensated pump that maintains pressure with minimal heating during normal operation. An accumulator F stores the first pump flow, check valve D stops accumulator back flow, and normally open directional valves C isolate the accumulator from the cylinder and tank during normal operation.
The gate cylinder needs at least 1500 psi, so the pump compensator is set for 2000 psi. This ensures that the accumulator has enough fluid to extend the cylinder when necessary. Because the solenoids on valves C are energized by the pump start command, the accumulator is completely isolated from the cylinder and tank as long as the pump runs. When solenoid B of the 4-way directional valve shifts (as seen in Figure 1-21), the gate opens as fast as the pump moves it.
When solenoid A shifts the 4-way directional valve, as seen in Figure 1-22, the gate closes as fast as the pump moves it. When the power is on, the cylinder extends or retracts partially or all the way at the operator’s command.
If the gate cylinder is partially or completely open and power fails, the circuit automatically goes to the condition shown in Figure 1-23. In this condition the pump stops, the 4-way directional valve centers, and the normally open 2-way shutoff valves C open.
When power fails, the accumulator has a direct path to the cap end of the cylinder while rod-end oil flows to tank. The cylinder will extend and close the gate using the stored energy in the accumulator. Place warning signs at the gate indicating this equipment can operate at any time without operator intervention.
When using an accumulator for emergency power supply it is difficult to automatically drain it during normal operation. Automatically draining the accumulator would defeat its purpose as an emergency power supply. Add a manual drain valve E, with warning signs to tell maintenance persons to manually drain the accumulator before working on the gate circuit.
Size emergency-power accumulators to hold enough oil to move all actuators to the home position before pressure drops to dangerous levels. Most manufacturers provide formulas in their catalogs and offer several offer excellent computer programs to size accumulators for emergency-power supplies.
Using accumulators for leakage makeup
Some hydraulic circuits, such as in laminating presses, need to hold at pressure for long periods. A pressure-compensated pump could maintain pressure, but energy loss from pump leakage generates heat. Another way to hold pressure for long periods is with a fixed-volume pump and an accumulator. Figure 1-24 shows a press cylinder that must stay extended under pressure for several minutes.
Tee small accumulator D into the cylinder cap-end line through flow control C. Flow control C allows the accumulator to fill quickly but discharge slowly when directional valve A centers or shifts to retract the cylinder. Flow control C should pass enough flow to let the accumulator discharge quickly without system shock when directional valve A shifts to retract the cylinder. Any oil left in the accumulator when the directional valve centers will make the cylinder extend a small amount. Tee dump valve B into the cylinder cap-end line to automatically discharge the accumulator when the pump stops. Tee pressure switch E into the cap-end cylinder line to set pump load and unload pressure. Pressure switch E sets high and low pressures to control maximum and minimum tonnage.
When the pump starts, Figure 1-25, backpressure check valve F gives 75 psi pressure, closing accumulator dump valve B and supplying pilot oil for solenoid pilot-operated directional valve A. When directional valve A shifts, the cylinder starts to extend, Figure 1-26, at whatever pressure it takes to overcome the counterbalance valve. The signal to the extend coil of directional valveA goes through the normally closed contacts on pressure switch E. Because gas pre-charge pressure in the accumulator is approximately 85% of working pressure, no fluid will enter it yet.
When the cylinder contacts the work, Figure 1-27, pressure increases and oil fills the accumulator. Upon reaching the maximum working pressure set by pressure switch E, the normally closed contacts open, de-energizing the solenoid on directional valve A. Directional valve A spring centers, the pump unloads, and oil stored in the accumulator maintains pressure while making up for cylinder and valve leakage.
Bypass at the cylinder seals and/or valve causes pressure to drop slowly to the low-pressure setting of pressure switchE. This low-pressure setting is normally adjustable but must be high enough to keep the parts firmly together. Upon reaching the low-pressure setting, pressure switch E shifts, allowing the normally closed contacts to shift directional valve A to refill the accumulator. Upon reaching maximum working pressure, directional valve A again spring centers to unload the pump, while the accumulator holds its pressing force and makes up for leaks.
A pilot-operated check valve in the cap-end cylinder line between the directional valve and the pressure switch would have less leakage than the blocked port of the spool valve. With a pilot-operated check valve and resilient seals in the cylinder, it is possible to maintain pressure for 2 to 5 min or more. Use an all-ports-open directional valve with the pilot-operated check valve. This accumulator circuit maintains pressure in the cylinder while unloading the pump. It also conserves energy while using an inexpensive fixed-volume pump.
Using accumulators as shock absorbers
Accumulators can reduce damage from shock in some circuits if correctly applied. In other applications, an accumulator may add shock by releasing stored energy too quickly.
The top half of Figure 1-28 illustrates one way shock is produced. Flow velocity in a hydraulic circuit may be 25 to 30 fps and not cause any problems. However, if oil flow stops abruptly, as seen in Figure 1-28’s middle example, damaging shock can rip out tubing, blow seals, and split pump housings with ease. A column of moving fluid has a lot of energy that can get out of control.
The third example in Figure 1-28 shows a small accumulator teed into the line at the shut off that stops flow suddenly. An accumulator spreads the shock energy over a short period of time and eliminates the potential for damage.
To absorb flow shock, the accumulator is usually pre-charged at about 70 to 80% of system pressure. At this pre-charge pressure, only a small amount of fluid enters the accumulator subsequent to a shock situation. There is also little fluid transfer to take away from or add to the normal pump flow.
When it is necessary to stop a heavy load, such as shown in Figure 1-29, try using an accumulator and a hydraulic cylinder. The accumulator’s pre-charge pressure holds the cylinder extended, thus making it ready for the advancing mass. When the load contacts the cylinder, it mechanically forces it to retract. As the cylinder retracts, fluid flows into the accumulator and gas pressure increases. As pressure increases, the higher resistance slows the mass more. After the load decelerates, the cylinder might try to push the part away. Add valves between the accumulator and the cylinder to control the shock absorber after it finishes decelerating the load.
Some large, slow-turning piston pumps send a shock wave into the circuit every time a piston discharges oil. On the left of Figure 1-30, the piston pump does not have an accumulator at the discharge port. Pressure at the gauge will fluctuate from less than system pressure to well above it without an accumulator.
On the right side of Figure 1-30, adding a small accumulator reduces discharge flow and shock damage. A portion of the sudden discharge flow from an advancing piston goes into the accumulator and discharges smoothly while waiting for the next stroke. The pre-charge pressure for this type of accumulator circuit is about 60 to 75% of maximum system pressure.
Accumulator manufacturers have formulas in their brochures to calculate any situation mentioned here. Some suppliers have computer programs that do all the math after asking for circuit parameters.
Pump supplementing circuit with full pressure when work is contacted
In some cases, a pump-supplementing accumulator circuit can speed up cylinder extension and/or retraction without having to go above working pressure. Normally in a pump-supplementing circuit, the relief valve is set as high as possible above the working pressure to store ample fluid. As the cycle progresses, oil from the accumulator and pump move the actuator quickly, but circuit pressure drops steadily. If pressure drops below the actuator’s need, the pump must refill the accumulator before the cycle finishes. To overcome this problem, a larger pump and/or more accumulators are necessary.
The next circuit shows an accumulator arrangement that provides high volume to move the cylinder rapidly with the relief valve set at working pressure. The accumulator and pump supply volume to fill the large bore cylinder as it extends. The cylinder then moves to working pressure while a check valve isolates the accumulator.
Like all accumulator circuits, there must be time for refilling between cycles, as shown in Figure 1-31. Pre-charge the accumulator to a pressure slightly higher than it takes to retract the cylinder. The cylinder will then retract when directional valve A and normally open, solenoid-operated relief valve H shift. (Also see Figure 1-34.) The large piston rod reduces the return volume, although retract pressure will be higher. When the cylinder fully retracts, pressure climbs and the accumulator starts to fill through check valve E and the bypass check valve around flow control C. Piston-type accumulators are best for this circuit because they can have a low pre-charge pressure and a high final pressure without internal damage. The accumulator can discharge a large volume of oil because the pressure in it is not important when the cylinder needs full tonnage.
When pressure in the circuit reaches 2000 psi, pressure switch G de-energizes the solenoid on normally open, solenoid-operated relief valve H, unloading the pump to tank.
When directional valve A and normally open, solenoid-operated relief valve H shift, Figure 1-32, pump flow and accumulator flow provide a large volume of oil to quickly stroke the cylinder to the work. Because accumulators can discharge at a very high rate, use flow control C to set the desired advance speed. Pressure in the circuit will fall as the cylinder extends and will be well below working pressure before the cylinder meets the work.
When the cylinder contacts the work, Figure 1-33, check valve F keeps pump flow from going to the accumulator. The pump will continue filling the cylinder and pressure will build to whatever it takes to do the work. Check valve F blocks flow to the accumulator to isolate it during the high-pressure work stroke.
When directional valve A shifts to the retract position, Figure 1-34, pump flow goes to the cylinder rod end. The accumulator pre-charge is high enough to force all pump flow to the cylinder, causing it to quickly retract.
Figure 1-31 shows the cylinder reaching the top of the stroke. The accumulator now accepts all pump flow through check valve E until pressure switch G unloads the pump.
Two other pump-supplementing circuits with full pressure when work is contacted
Figures 1-35 and 1-36 depict two more ways to use an accumulator for volume and still have immediate high pressure for doing work. Either circuit works equally well with the two pump types shown.
These circuits would normally require a piston-type accumulator. Notice the pre-charge is less than one-third of maximum pressure. The large pressure difference would squeeze the bladder in a bladder-type accumulator so much that holes caused by chafing would allow the nitrogen gas to leak. The minimum pressure in the circuit could be even lower than shown here. If the actuators can move at 300 psi, then use 150 to 200 psi pre-charge.
The circuit in Figure 1-35 uses a pressure-compensated pump and a normally open, poppet-type, 2-way directional valve. All flow goes directly to the accumulator, filling it to maximum pressure with the pump operating. When the cylinders start to cycle, flow from the pump and accumulator move them rapidly. When the cylinders contact the work, pressure is well below the required amount. To get full force, energize solenoid C1. This stops pump flow to the accumulator and raises the cylinders to full pressure. De-energize solenoid C1 when the cylinders finish their work to allow the accumulator to refill.
Energizing solenoid C1 when the actuators are moving is possible with a correctly designed poppet valve. Notice the blocked position of the valve has a check valve symbol, meaning it only stops flow to the accumulator. This type of poppet valve provides accumulator volume to the actuators when pressure is low. However, maximum pressure is immediately available when the cylinders meet resistance. De-energize solenoid C1 at the end of the cycle to refill the accumulator. Some poppet-type directional valves have a very high pressure drop when flowing through the closed check valve. Use a brand designed for low pressure drop in this circuit.
The circuit in Figure 1-36 has a fixed-volume pump with a normally open,, solenoid-operated relief valve and pressure switch to unload the pump at maximum pressure. Minimum system pressure for this circuit is 1500 psi. Therefore, it is important to set the sequence valve in front of the accumulator to this pressure. Set the pressure switch to unload the pump at 1700 psi. Then set the normally open, solenoid-operated relief valve at approximately 1900 psi. Because no oil can go to the accumulator if the system pressure is below 1500 psi, the actuators will always have maximum force anytime they meet resistance. When the cylinders are moving to and from the work, pump and accumulator flow can combine to give rapid movement at reduced pressure. Flow from the accumulator can always go to the cylinders through the bypass check valve. Fluid only goes to the accumulator when pump flow is greater than the system requires. This circuit fills the accumulator anytime the cylinders stop or anytime required volume is less than pump output.
There will be some heating of the oil while the accumulator is filling until system pressure reaches 1500 psi or above. One advantage is that no control circuitry is necessary, even while the accumulator fills anytime actuator volume is less than pump flow.
Non-invasive way to check accumulator pre-charge
It is important to check accumulator pre-charge pressure at regular intervals. Check a new installation each shift for a few days to see if there is a gas-pressure loss. It the gas charge is holding, check pre-charge pressure weekly for the next month. If all is well at the end of a month, then monthly checks should be more than satisfactory.
The normal way to check pre-charge pressure is: (1). Shut down the system. (2). Attach a gauge and charging kit to the accumulator. (3). Open the gas valve and check the pressure reading.
However, this procedure is time consuming, allows some gas to discharge, and may damage the charging valve, which can result in a continuous leak. Outlined below is a simple, non-invasive way to check accumulator pre-charge pressure to see if gas is leaking.
Figure 1-37 shows a partial accumulator circuit. This figure shows an operating hydraulic system, just as the pump stops. At this point, the accumulator relief/unload/dump valve is open, draining pressurized oil stored in the accumulator. As fluid in the accumulator discharges, pressure at gauge PG1 starts dropping. By controlling the flow with a fixed orifice or a flow control, pressure deteriorates slowly when there is oil in the accumulator.
When all fluid is out of the accumulator, Figure 1-38, pressure at gauge PG1 will suddenly drop to zero. Carefully note gauge pressure when it suddenly drops. The pressure seen at the sudden drop is the present pre-charge pressure of the accumulator. This reading is only as accurate as the gauge and the person reading it. It is not a perfect reading, but will be close enough to see if a full-fledged check is needed.
If there is more than one accumulator on the machine, as in Figures 1-39 and 1-40, this test will show the lowest pre-charge pressure. When a low pre-charge pressure shows up, check each accumulator individually until finding those at a lower pressure than required.
Another way to check pre-charge pressure is to note the gauge’s pressure reading when turning on the pump. With an accumulator in the circuit, the first pressure reading should be pre-charge pressure. It is difficult to obtain an accurate reading this way with glycerin-filled or orifice-dampened gauges in the circuit. The gauge should also be at or close to the accumulator to keep line losses from adding to the reading.
Hydraulic type accumulator dump valves
When using an accumulator, there must be a way to discharge stored oil before safely working on the circuit. Even when using the accumulator for emergency power supply, install a manual drain valve for safe operation.
A manual drain valve with a gauge near it is the best way to ensure a safe operation. Mark the manual drain valve and place warning signs at all hydraulic component locations indicating there is an accumulator in the circuit and to open the manual drain before performing maintenance.
A common way to discharge stored energy is to use a normally open, solenoid-operated, 2-way directional valve teed into the pressure line with its outlet hooked to tank. Wire the solenoid on the 2-way valve to close when the pump is running. Any time the pump stops, the 2-way solenoid valve de-energizes and discharges stored oil to tank.
A solenoid-operated valve works well in most cases but can cause problems. First, if the valve fails to close or only partially closes, oil dumps across it, generating heat and making it operate sluggishly or not at all. Second, if the valve fails to open when the pump stops, the circuit is unsafe. This is a safety hazard for an inexperienced person who might not detect the problem. Third, additional wiring creates additional costs.
If the circuit uses a fixed-volume pump as shown in Figures 1-41 through 1-44, use an accumulator relief/unload/dump valve for most applications. This valve has an integral adjustable 2-way unloading valve A to unload the pump when reaching set pressure. Also, there is a pilot valve to close shut-off B that stays closed while the pump is running and opens any time the pump stops. Isolation check valve (C) keeps accumulator oil from back flowing to the pump when it stops.
In Figure 1-41 the pump has just started, so pressure jumps to accumulator pre-charge pressure and all flow goes to the accumulator through check valve C. Pilot-operated 2-way shut-off B pilots closed when the pump is running. The pilot-operated, adjustable-spring shut-off Astays closed until set pressure is reached.
Pressure continues to climb until the accumulator is full, as seen in Figure 1-42. When pressure reaches that set on 2-way adjustable-spring valve A, it opens, unloading the pump to tank at low pressure. Even while unloading there is enough pressure to keep pilot-operated 2-way shut-off B closed.
When pressure in the circuit drops approximately 15%, Figure 1-43, unload-valve A closes, again forcing oil to the circuit and accumulator. The pump will load and fill the system any time pressure drops about 15%. This pump load pressure is non-adjustable so it will not work for all circuits.
Some manufacturers offer an accumulator relief/unload/dump valve with an adjustable differential setting. Setting these valves’ load-unload pressure by more or less than the 15% differential is possible.
When the pump shuts off, as in Figure 1-44, pilot pressure to 2-way valve B drops, allowing it to open. Now all stored fluid from the accumulator has a path directly to tank. The accumulator will quickly discharge, making it safe to work on the circuit.
CAUTION! ALWAYS CHECK AN ACCUMULATOR CIRCUIT FOR PRESSURE BEFORE WORKING ON IT. NEVER ASSUME THE AUTOMATIC UNLOADING SYSTEM WORKED!
Hydraulic-type accumulator dump valves (continued)
When using an accumulator with a pressure compensated pump, the packaged dump valve shown works well. (See Figures 1-45 through 1-48.)
A pressure-compensated pump maintains pressure while flow changes to meet the needs of the circuit. When the first actuator in the system starts to move, there is no flow for it until pressure drops. As pressure drops, a pressure-compensated pump will go on stroke quickly but there will be a slight pause before flow actually starts. The addition of the small accumulator shown in Figure 1-45 nearly eliminates the startup pause. This enhances system response while reducing cycle time and pressure fluctuations.
On the other end of the cycle, if the pump is at full flow and all the valves center or all the actuators hit end-of-stroke, flow requirement suddenly goes to zero. The pressure compensated pump is still flowing at maximum and pressure starts to climb. The pump will continue at full flow until pressure reaches 80 to 98% of the compensator setting. There has been zero flow needed for some time, but the pump does not know this until pressure is near maximum. When pressure reaches the compensator setting, the pump starts to shift to no-flow. All pump flow during the shifting time has no place to go, so this excess flow makes a pressure spike of five to ten times the compensator setting. This pressure spike can cause premature failure of the pump, plumbing, and actuators. An accumulator as shown will take in this small volume of oil to minimize the spike.
As with any accumulator installation, safety is important. When shutting a circuit down for maintenance, always drain the accumulators. A manual drain valve works, but the automatic drain shown on the facing page is better. When the pump starts -- and as long as it is running -- a pilot valve closes check valve B to block the drain port. Check valve A isolates the pump from accumulator back flow when it stops or fails. There is no electrical wiring needed, so the accumulator dump valve is invisible to the control circuitry.
The pump is just starting in Figure 1-45, so pressure immediately climbs to accumulator pre-charge pressure. Flow continues until the accumulator is full and system pressure is at its maximum. Pilot-to-close check valve B blocks the drain path to tank when the pump starts. The drain path stays closed as long as the pump is running.
Figure 1-46 shows flow while the circuit is working. Accumulator and/or pump flow will go to the actuators to quickly start them and move them through their cycle. During the working part of the cycle, the accumulator smooths out flow fluctuations, while reducing pressure drops and spikes.
With the system at rest as shown in Figure 1-47, pump flow is zero and the accumulator is full and ready for another cycle.
Figure 1-48 shows how the circuit responds when the pump stops. Check valve A closes to stop back flow and pump motoring. Pressure to pilot-to-close check B drops out, allowing it to open. All accumulator volume now has a path to tank through an orifice that keeps flow at a reasonable rate. In a very short time the accumulator’s stored energy dissipates, making it safe to work on the system.
CAUTION! ALWAYS CHECK AN ACCUMULATOR CIRCUIT FOR PRESSURE BEFORE WORKING ON IT. NEVER ASSUME THE AUTOMATIC UNLOADING SYSTEM WORKED!
Linear pressure-type accumulators
The following circuits use accumulator types with little or no pressure drop as they discharge fluid.
Gas- or spring-loaded accumulators lose pressure as fluid discharges and the gas or spring expands. In a typical circuit using this type of accumulator, the maximum system pressure must be higher than working pressure to allow for this pressure drop. Some circuits cannot operate at these elevated pressures or may need high pressure for the entire stroke. Therefore, they can’t use gas or spring loaded accumulators.
The circuit in Figure 1-49 shows a weight-loaded accumulator, a fixed-volume pump, and a normally open, solenoid-operated relief valve that can replace either circuit shown in Figures 1-10 and 1-11. Notice the maximum pressure and working pressure are at 2000 psi. This is possible because the weight-loaded accumulator does not lose pressure as fluid discharges. Until the accumulator piston reaches bottom, system pressure stays constant.
With a weight-loaded accumulator, the amount of weight on a given piston area sets maximum pressure. To raise or lower maximum pressure, weight must be added or taken off. Set the relief valve on this type circuit 100 to 150 psi higher than system pressure so it does not bypass during normal operation.
The main disadvantage of a weight-loaded accumulator is its physical size. An accumulator for the circuit shown in Figure 1-49 would require a 10-in. ram with a 60-in. stroke for the cylinder to have full force for its entire cycle. This size accumulator needs almost 160,000 lb of weight on the ram to get the required volume and pressure stated. A block of concrete approximately 1080 ft3 in size or about 10 X 10 X 11 ft would be necessary to meet this need. Such high mass eliminates the use of this type accumulator for mobile equipment and also rules out many industrial applications. Using a smaller accumulator ram with a longer stroke reduces weight, but you must make sure column strength is adequate when reducing ram diameter.
The air-cylinder-loaded accumulator shown in Figure 1-50 works the same as a weight-loaded accumulator. There is a slight pressure drop as fluid starts to flow due to piston and ram seal friction but this is usually not enough to cause problems.
Physical size can also be a problem with air-cylinder-loaded accumulators, especially when using low air pressure. Most plant systems operate at 100 to 125 psi so the unit required to handle the cylinder in Figure 1-50 might be a 40-in. bore air cylinder driving a 9-in. ram with a 75-in. stroke. Using air pressure at 250 psi could reduce the accumulator to a 30-in. air cylinder driving a 10_-in. ram for a 55-in. stroke. In either case, these accumulators are still too large for mobile equipment and for many industrial applications.
Air-cylinder-loaded accumulators work best and are more economical to operate using a surge tank for the air cylinder. Surge tanks provide fast flow for discharging high oil volumes with minimal pressure drop. They also make it possible to use a small air compressor because it only has to make up for leaks after the system gets up to pressure. Size the surge tank to allow for a 3- to 8-psi drop when the accumulator discharges during a normal cycle.