Hydraulic Bladder Accumulator ( working principle)

Introduction

The main task of the hydraulic accumulator is to accumulate fluid under pressure and return it when necessary. Since the accumulator contains fluid under pressure, it is compared to a pressure tank and must be sized for the maximum operating pressure according to the country regulations where it is installed.

The fluid is preloaded by weight, a spring or a compressed gas to obtain the energy accumulation and volume compensation.

Figure 1 below shows all details of the Bladder Cylinder

Figure 1

The bladder Accumulator is divided into two types:

Figure 2 - Blader Cylinders

The working principle remains exactly the same

Working Principle

The old technology accumulators were high-pressure containers with a sight glass to show fluid level. They were filled approximately half with oil and half with nitrogen (gas), with no separation barrier. This type of accumulator is not used on the new plants, but in old plants still in service (they are not recommended)

The old technology (without Bladder) was removed from new projects for safety reasons. This accumulator type is without positive isolation between precharge pressure (nitrogen) and hydraulic oil. Therefore, if for some reason, such as human error or hydraulic pumps themselves trip, the entire tank remains without hydraulic oil. As soon as the oil runs out, it starts to pass the gas (nitrogen) in the hydraulic circuit. The result could be devastating. The risk is very high; it can vibrate pipes, or even the gas can circulate in the storage tank (this is a huge problem). Since the tank work at atmospheric pressure or better with a few mbar, in this case, it can swell like a children's balloon when everything is fine; when it is wrong, the tank can open like a tuna box. Even if the breath (vacuum) and overpressure valve are installed, if the gas flow is very high, the PSV valve is insufficient or intervenes with delay. With 150 or 200 bars of pressure inside the cylinder with a capacity of 120 litres is a significant flow. There are many incidents like this around the world. For safety reasons, the new technology is mandatory (accumulator with Bladder). Sometimes it can happen even with new technology when the bladder gets broken, but in this case, exists a poppet valve at the bottom of the hydraulic cylinder, which closes to prevent that gas from will go online in the hydraulic circuit.

Having said this, let's move on to the operation of the bladder accumulator.

The new technology is very reliable for these points below:

• Bladder Accumulator has positive isolation between gas and hydraulic oil.
• To protect the hydraulic circuit, the Bladder Accumulator have a poppet valve to close the communication between the cylinder and the hydraulic circuit. If the cylinder remains without oil inside, the rubber of the bladder itself pushes the poppet and closes the valve (see Figure 1 above).

Even if the Bladder Accumulators are more reliable, also they have a problem, especially when the accumulator is new or the Bladder has been replaced with a new one. Below are concerns:

When the Bladder is deflated, or improper internal installation or even not original for that cylinder (maybe too long) or during transportation, something happens, the inner bladder can rest between the poppet and spring (see Figure 3 below)

Figure 3 - Poppet Valve

When the refilling starts, the Bladder will begin to inflate, and at the same time, the Bladder itself will try to push the poppet to close the poppet valve (more pressure, more power). If parts of the Bladder are seated between the spring and poppet will damage immediately. The poppet and spring will destroy the Bladder entirely from the bottom, as in figure 4 below.

Figure 4 - Rubber of the Blader

Figure 4 above shows how the Bladder can be broken.

In order to protect the personnel and Bladder itself should be paying attention to the bladder installation of the cylinder. The material must be original as per manufacture. Our suggestion is: Any charge of the Bladder must be inspected before, even when the cylinder is new. The internal Bladder should be installed directly on location (in the field).

Consider If all fluid is evacuated at high rates, bladders can get caught in the poppet valves, and pistons can be deformed when metal hits metal or can also damage completely.

Inspection and procedure for a pre-charging kit of Bladder Cylinders

First of all, before starting, there are some special tools that should be known (see Figure 5,6 below)

Charging & Gauging Units tools

Figure 5 - Standard kit pre-charge Accumulators

The tools included are:

• Gauging/Charging Assembly Valve
  
• Flexible hose
• Adaptors connection
• Pressure gauges with various ranges

The most important part is the Gauging/Charging Assembly valve (see Figure 6  below)

Figure 6 - Gauging/Charging Assembly

Figure 7 below shows the nozzle of the cylinder.

Figure 7 - Charge Nozzle

The final configuration will be like in Figure 8 below.

Figure 8 - Charging Tool

Note: This type of connection is universal, but some companies use a different connection. The working principle remains the same (carefully read the instruction of the cylinder and clarify with the manufacturer which type of connection is correct).

Procedure to Inspect the bladder cylinders

1. Drain the hydraulic oil fully from the drain point (DBB valve from the bottom side of the cylinder). Very careful when the level oil of the cylinder reaches the end. The drain of the DBB valve (hydraulic side) must be opened very slowly. Usually, when the oil finishes, the rubber of the blader will press the poppet valve (see Figure 3 above), which closes the communication between the cylinder and drain. This should be the normal condition of the cylinder, but if the tubing (drain line) begins to vibrate or become cold, close the valve and stop the job immediately. This means the Bladder is damaged, and the poppet valve didn't close with the consequence that nitrogen (or other gas) will exit from the drain of the cylinder. If the drain line is connected to the oil return tank will be a huge problem (as mentioned above). To protect the oil return tank and personnel, continue with step 2. If there is no issue with the cylinder, jump to step 3.
   
2. IIsolate the DBB valve fully from the circuit (already isolated), then remove the tubing from the drain point between DBB and the return oil tank. Install a container and absorbent, then open very slow the drain valve of DBB and wait until the hydraulic oil and nitrogen are entirely removed (depressurized) from the cylinder (safety glasses are mandatory at all times).
   
3. Remove the sealing cap, as shown in Figure 7. Carefully when it unscrews the sealing cap, maybe the internal check valve is leaking, and nitrogen exits as soon as unscrew the sealing cap. In this case, must be depressurized the entire cylinder and replace the check valve ( see Figure 10 below)
4. Installation of gauging/charging assembly. Before installing, the gauging/charging assembly on the bladder cylinder must be unscrewed the knob completely to move back the gas chuck (See comment on Figure 6 above).
   
5. Install the hose between the nitrogen and the hydraulic cylinder, including the gauging/charging assembly on the hydraulic cylinder and the pressure regulator on the nitrogen bottle. If more pressure is required and the nitrogen cylinder is not enough, the booster pump is required (pneumatic). For more details, see Figure 9 below.
6.  Ensure the bleed valve is closed on the gauging/charging assembly tool.
   
7. Once steps are verified perfectly, especially #1, 3 and #4, turn the knob of the gauging/charging assembly tool to push the gas chuck until the knob becomes hard. This indication means that inside the Bladder, there is pressure. Continue to turn slowly until the pressure gauge starts to indicate.
   
8. If the system requires more pressure, it can be refilled through the nitrogen bottle. Before opening the main valve of the nitrogen cylinder, the pressure reducer should be unscrewed to release the pressure fully, then open the main valve, and then adjust the pressure slowly with the pressure reducer.
   
9. When the pressure reaches the established value, shut off the main valve on the nitrogen bottle. Check any visible leak or pressure drop from the pressure gauge (Wait 10/15 minutes). If the pressure is too high, you can always discharge it from the vent above the gauging/charging assembly tool.
   
10. Once the desired pressure is reached, retract the gas chuck by the knob of the gauging/charging assembly tool.
    
11. Open the bleed valve to release any trapped pressure between the main valve of the nitrogen cylinder and the gauging/charging assembly.
12. Remove the gauging/charging assembly from the bladder cylinder and check with soap if there is any leak from the internal nozzle of the cylinder (check valve maybe not close properly). Generally, if the leak is big or small, should be replaced the internal nozzle (see figure 10). Anyway, as soon the plug with the seal is installed, the leak will disappear (Internal O-Ring permitting. See Figure 7) However, we always recommend that in case of a leak, the internal check valve should replace since the cost of the fitting, including the check valve, is not expensive.
    

Figure 10 - Internal Check Valve

Warning: Do NOT reduce precharge pressure by depressing the nozzle on the bladder cylinder with a foreign object (such as a screwdriver) as this may damage the valve seat, especially if the cylinder is under pressure it could seriously injure personnel (see Figure 11 below)

Figure 11 - Internal check valve

Procedure to charge the cylinder's Bladder when it is empty or is new.

The most crucial phase is refilling the cylinder when it is empty or new. The procedure remains the same as the inspection procedure written above. There are two-point that must be rigorously considered. Let's see which ones.

When you have reached point 3 of the procedure written above, proceed as follows.

• Once steps #1, #3 and 4 are verified perfectly, turn the knob to push the gas chuck until the knob becomes hard, It should be free because there is no pressure inside. Anyway, the pressure gauge is mandatory.
  
• Once you are sure the cylinder is empty, unscrew the knob (fully) again to disengage the gas chuck from the nozzle of the cylinder.
  
• Proceed to steps four and five and come back here to step 6
  
• Step 6: When the cylinder is new, or the Bladder has been replaced; it is essential that:
  

• When the valve from the nitrogen cylinder is opened and starts to pressurize the hydraulic cylinder, the internal Bladder must be checked to see if nitrogen exits from the Bladder (bottom side). This means that the Bladder is damaged. Usually, also the internal poppet valve, when the pressurization starts, should be moving like in figure 12 below:
  

 Figure 12 - Poppet valve movement

• If there are no nitrogen leaks from the bottom side cylinder and the poppet valve moves regularly, pressurize the cylinder at the required pressure and proceed to step 7 until the end of the steps.
  

Typical connection of Bladder Cylinder

As you can see, two PSVs exist for each side of the cylinder (gas and hydraulic side) to protect the cylinder from the thermal expansion of hydraulic oil. Usually, the rapture disk should be installed on the side gas (up). The reason why two PSVs are installed is that: in case the bladder gets damaged, the rubber can close the internal hole where it is connected to the PSV and then, in case of high pressure, will not open. For this reason, we always recommend installing the Rapture disk and the PSV on the gas side (up).

Recommendations

1. An accumulator should be clamped adequately to the mounting fixture, especially when it is new and should be pressurized.
2. Do not use homemade tools to preload cylinders. Always use universal kits.
   
3. The temperature should be considered when using a booster pump (pneumatic pump) instead of a cylinder for transferring nitrogen to the Bladder; the temperature should be considered. When the booster pump works, pressure will increase simultaneously with the temperature. This condition can damage the internal nozzle of the cylinder. For this activity, use a long hose. Remember, every 10 bars of increasing pressure are 3 degrees more temperature (Celsius).
   
4. The personnel carrying out the refills and inspections must be specialized personnel who know the operating principle, including the knowledge of fittings (BSPP or NPT or whatever).
5. When loading a bladder cylinder, the ambient temperature must be considered.
   
6. Before starting any activity on the bladder cylinder, apply positive isolation from the hydraulic source (Mandatory).
7. The nitrogen charge pressure must be 80% of the maximum working pressure of the system (more nitrogen pressure and less oil will be inside the cylinder).
   

Maximum working pressure

The max working pressure of the bladder cylinder depends on the datasheet (manufacturer). Let's say it is more or less around 350 Bar, not more.

Conclusion

Many companies supply cylinders, one of the best is Hydac, but I have not said that the others are bad.

Unfortunately, there are too many incidents worldwide during accumulators recharge. Probably, there are no adequate procedures due to a lack of knowledge cylinder.

Usually, this type of work is given to mechanical specialists, but in our opinion, the instrumental part should also be involved. The instrumental specialist supervises the job; he knows the universal charge kit, the pressure adjustment, the size of fittings, etc.