Hydro Pnuematic Accumulators

The accumulator are pressure storage reservoir in which non compressible fluids are stored under pressure. accumulators are connected to pump discharge lines for compensating for loss of fluid pressure in the hydraulic circuit.

They are used for

1. Emergency operation

2. pulsation Damping

TYPES OF ACCUMULATOR

1. Bladder type accumulator

2. Diaphragm type accumulator

3.Piston type accumulator

Bladder Accumulator 

Fluids are not in compressible and cannot store pressure energy, but gas can be compressed so it is used for storing fluids in accumulator.The accumulator consists of a shell. The shell consist of fluid section and gas section. The gas section has a bladder which is filled with nitrogen gas. Surrounding the bladder is the fluid.The bladder can be expanded and compressed for storing and releasing the fluid.The fluid section is connected to the hydraulic section ie poppet valve for drawing the fluid into the accumulator when the fluid pressure is higher and compresses the bladder with gas.so the fluid is stored inside the accumulator.when the fluid pressure in the line drops the bladder expands and forces the stored fluid in the hydraulic circuit with the pump flow

The accumulator bladder is pre pressurised with nitrogen gas upto 2.3bar. The bladder gas pressure is set 70%  to 80 %of the hydraulic circuit pressure.

Diaphragm type accumulator

These accumulators have a rubber plate or diaphragm as the separating element. This element is welded or screwed together between two spherical shells (or compartments). The compartment above the diaphragm is filled with nitrogen. The compartment below is directly connected to the hydraulic circuit. Diaphragm accumulators are useful if the required fluid storage capacity is low (i.e. 4 litres or less).

Diaphragm accumulators have most of the advantages of bladder-type units, but can handle gas compression ratios of up to 8:1. However, they are limited to smaller volumes, and their performance can sometimes be affected by gas permeating across the diaphragm.

Special Equipment are available to check the pressure of the nitrogen and it should be check once per year. If nitrogen pressure is high it can be reduced. By using mobile nitrogen charging devices the pressure can be added or refilled

PUMP CAVITATION

CAVITATION
   
            When a liquid boils at a suction line of a pump it is said to be cavitation. As the liquid boils it forms cavity of gases in the suction line. this happens when liquid pressure is well below the vapour pressure of the liquid.Boiling starts when pressure of the liquid is reduced to vapor pressure of the liquid at the actual temperature.

Vapour pressure of water: 
The pressure at which the water turns into vapor at the given temperature.
for example: water turns into vapor at 40deg at pump suction pressure below 0.07 bar, so pump suction pressure should not go below 0.07 bar at 40 deg to avoid cavitation.

Causes of cavitation
Cavitation can causes serious damage to the pump in the form of vibration and abnormal sound. cavitation leads to bearing and mechanical seal failure, shaft breakage , pitting on the impeller.
The damage occurs when these bubbles pass to the high pressure region of the pump.The bubbles explode at the vanes of the pump.

Problems due to cavitation
Loud noise
reduced flow
pitting formation in the impeller

Net positive suction head
The Minimum pressure required at the suction port of the pump to keep the pump from cavitating

Net positive suction head available
The pressure available at the suction side of the pump and it should be calculated and it should be more than NPSH R

Net positive suction head required
It is the minimum pressure required at  the pump suction to keep the pump away from cavitation and to be provided by the manufacturer.

Net positive suction lift
If the pump is positioned above the tank it has some net positive suction head. if the pump is raised above the tank the net positive suction head reduces and at some point the head becomes zero and the fluid starts to evaporate.