Hydraulic pumps

POWER GENERATING ELEMENTS IN HYDRAULIC SYSTEMS

HYDRAULIC PUMPS:

Working principle:

Its main function is to push the hydraulic fluid and create flow. When driven by motor it basically performs two functions:

1)      It creates partial vacuum at the pump inlet port, which enables the atmospheric pressure to force fluid from the reservoir into the pump and

2)      The mechanical action of the pump traps this fluid within pumping cavities, transports it through the pump and forces it into the hydraulic system.

The pump converts mechanical energy from the prime mover (engine or electric Motor) into pressure energy in the fluid. The hydraulic energy is then used to operate an actuator, often with very precise control.

Pump is regarded as the source of pressure. Loss of pressure may not be always due to pump itself.

The pump does create pressure in that it applies the push that causes flow. But to create pressure there must be resistance to flow. Pressure developed is to the extent just sufficient to handle the load at the output.

Pressure loss can be through any alternate path that offers less resistance to flow (ex. A leak in a valve is more likely cause of pressure drop)

Pump is there to cause flow. Where the flow goes depends on the other parts of the system.

CONSTRUCTION OF PUMPS:

       Hydraulic pumps essentially consist of:

1)      An inlet port, which is supplied with fluid from a reservoir or other source.

2)      An outlet port connected to the pressure line

3)      Pumping chambers to carry fluid from the inlet to the outlet port.

4)      A mechanical means for activating the pumping chambers.

        In most pumps, design is such that the pumping chambers increase in size at the inlet thereby creating a partial vacuum. The chambers then decrease in size at the outlet to push the fluid into the system.

        Fluid flow from reservoir to the inlet of pump may be due to pressure difference due to vacuum or the inlet may be charged (positive pressure) by a pressurized reservoir (ex. Head of fluid above the inlet.)

PUMP CLASSIFICATIONS: Positive and Non-positive displacement type.

    

 This classification tells whether the pump inlet is sealed from outlet or not.

·             If the inlet and outlet are connected hydraulically so that the fluid can recirculate in the pump when pressure builds up, then the pump is Non- positive displacement pump (NPDP).

·                  If the inlet is sealed from outlet, the pump will deliver fluid any time the inlet is kept supplied and the pump is driven. Such a pump is called Positive delivery pump (PDP) and it requires a relief valve to protect it.

CONTRASTING CHARACTERISTICS IN THE OPERATION OF POSITIVE DISPLACEMENT AND NON-POSITIVE DISPLACEMENT PUMPS ARE:

• A non-positive displacement pump (NPDP) provides a smooth, continuous flow. A Positive displacement pump (PDP) has a pulse with each stroke or each time a pumping chamber opens to the outlet port.

• The delivery of a NPDP is reduced by pressure. A high enough outlet pressure can actually stop any output… the liquid simply re-circulates inside the pump.

•  In a PDP pressure affects the output only to the extent that it increases internal leakages.

•  A NPDP, with its inlet and outlet connected hydraulically, cannot create a vacuum sufficient for a self-priming; it must be started with the inlet line full of liquid and free of air.
• PDPs are often self-priming when started properly.

 Note: NPDP are seldom used in industrial and / or mobile hydraulics due to their characteristics   stated above. They may be used, sometimes as a charging pump or replenishing pump.

     

Fixed displacement Vs Variable displacement Pump.

FIXED DISPLACEMENT PUMPS:

1.          In a fixed displacement pump, the amount of flow, which is displaced by each revolution of pump shaft, cannot be varied. The output can only be varied by changing/varying the drive speed. (In industrial hydraulic systems generally constant speed motors like induction motors are used whose speed cannot be varied as required.)In hydraulic systems, control on speed actuators can be done by using flow control valves, which results in heat development as fluid passes through restricted passage in the flow control valve.

2.          If in Fixed displacement pump circuits, the actuators require varying flow rates during operation the fixed displacement pump must be selected for the highest flow required. When less flow is required, the excess oil from the pump must be dumped over the relief valve at maximum system pressure. This converts unwanted energy directly to heat. Fixed displacement pumps are thus recommended for use in constant speed circuits or in circuits where speed control is for short duration, such as end cushioning or short load deceleration.

3.          Fixed displacement pumps are sized precisely for the speed (flow rate) required.

Thus, a Fixed Displacement Pump can be used if:

• ·          System pressure need not be maintained on a stalled actuator.
• ·          The hydraulic circuit does not operate over a broad speed range.
• ·          The pump can be unloaded by the circuit design during idle periods.
• ·          During large portion of the working cycle the actuator need not be operated at very low speed.

VARIABLE DISPLACEMENT PUMPS:

In variable displacement pumps, there is a provision for changing the size of the pumping chambers. The flow, (liters/minute) delivery can be changed by moving the displacement control ring or changing the drive speed or both.

Variable displacement pumps help in energy saving pursuits by delivering fluid flow when and as required by the system.

Because of this characteristic in creating flow against demand only, no heat is produced (developed) and no energy is wasted.

Variable displacement pumps often eliminate or reduce the need for flow control and pressure reducing valves. This offsets high initial cost of such variable displacement pumps.

Classification of Hydraulic Pumps:

Pumps are primarily classified as

i) Positive displacement pumps and

ii) Non positive displacement pumps    

Centrifugal pumps, axial propeller and mixed flow pumps are examples of non-positive displacement pumps. These are pumps which are used to pumps large volumes of fluids at relatively low pressures. They are less expensive, simple to operate, requires less maintenance, low noise level and has ability to pump nearly all fluids without damage to internal parts. Because the inlet and outlets are hydraulically connected, centrifugal pumps are not self priming and must be positioned below the level of fluid or primed to start the pumping action.

Centrifugal pumps have their greatest application as super charging pumps, as liquid transfer pumps and in low pressure hydraulic applications requiring high fluid flow rates, such as in traverse feed mechanisms.

Centrifugal pumps may be staged in series to increase system pressure.

Propeller pumps are also non-positive displacement pumps. Fluid is swept along by the action of the close fitting propeller blades in the pump housing. The fluid moves axially with respect to the direction of the propeller drive shaft. Propeller pumps are installed in water distribution pipes to raise the line pressure.

Following diagram represents the major classification of pumps used in industrial applications:

RATINGS of Hydraulic pump:

Maximum tolerable operating pressure (kPa or bar), flow output (lpm) at maximum drive speed of the pump drive shaft (rpm) and geometrical displacement per pump shaft rotation (m³) are the three main factors by which pumps are rated.

PRESSURE RATING:- It tells us how much pressure the pump can with stand for a given time without damage to its parts. This in turn decides how much load the system can handle. The manufacturer, based on reasonable service life expectancy under specified operating conditions determines pressure ratings. Operating at higher pressures than rated pressures may result in reduced pump service, or serious damage to the pump and the hydraulic system.

FLOW RATING: of a pump can be expressed either in terms of liters per minute (L/min) delivery or displacement in mL/revolution. The flow out put (displacement) from a pump (also called as geometrical displacement) is that volume of hydraulic fluid, which gets transported through the pump by single rotation of the pump drive shaft. i.e., Displacement is the volume, which is “swept” by a pump in one revolution or cycle. This is expressed in milliliters/rev. for a rotary pump or mL/cycle for a reciprocating pump.

           

For a PDP, the volume of fluid discharged by a pump per revolution or cycle is approximately equal to displacement, since leakage is low.

NOTE: Lower the operating pressure lower is the internal leakage. So volume discharged per revolution approaches the pump displacement at zero outlet pressure.

Most pumps have a fixed displacement, which cannot be changed except by replacing certain pump components.

Delivery in Lpm:

This rating refers to nominal flow rate of the pump, expressed in litres

Per minute (Lpm)

                                 

 

Volumetric efficiency:

Volumetric efficiency is equal to the actual output divided by the theoretical output. It is expressed as percentage. In reality the actual output is reduced because of internal leakage and slippage. As the pressure increases, the leakage from outlet back to inlet or drain increases and volumetric efficiency decreases.

Factors to be considered while selecting a Hydraulic pump:

Note: Pumps should never be selected on an empirical basis. A number of important factors are to be considered before the right pump is chosen. They are:

·     Maximum pressure required to produce sufficient force output with the actuators.

·      Maximum (peak) flow required or average flow required when system is using an accumulator.

·     Pump performance, operating reliability, ease of maintenance, initial purchasing cost and pump noise.

·    Pump flow control during non-action stages of the system, fixed displacement, variable displacement.