Selection of hydraulic pumps

 Pumps are selected by considering number of factors into account. Main factors among these considerations are presented in Table 4.11.

SELECTION OF HYDRAULIC PUMPS

(FACTORS INFLUENCING THE SELECTION OF PUMP TYPE)

Pumps are selected by considering number of factors into account. Main factors among these considerations are presented in Table 4.11.

Table 4.11. Factors affecting the selection of hydraulic pump

1. Safe and maximum operating pressures.

2. Maximum flow rate/delivery requirements.

3. Pump drive speed.

4. Type of control.

5. Fluid compatibility.

6. Fluid contamination.

7. Operating environment.

8. Tolerable pump noise level.

9. Compactness and weight-to-power ratio.

10. Efficiency.

11. Cost and economic factors.

12. Availability and interchangeability.

13. Ease of maintenance and spares.

1. Safe and maximum operating pressures: 

• The hydraulic pump is selected based on the safe and maximum operating pressure of the hydraulic system for the particular application.

• In general, when the operating pressure is higher, the flow rate for a given system power will be lesser. This results in smaller pumps, smaller pipe sizes and smaller components. But, at high pressures, the compressibility of the fluid adversely affects the load control.

• Table 4.12 presents the operating pressure ranges for different types of pumps. 

Table 4.12. Comparison of various performance factors for pumps

* Moderately higher than other pumps.

† General industrial requirement is 25-30 W.

2. Maximum flow rate/delivery requirements:

• The second important consideration in selecting a pump is its flow rate capacity or maximum delivery.

• The pump should be capable of delivering the maximum flow rate required by the system.

• If the hydraulic system requires constant delivery, a fixed displacement pump is selected.

• If the system requires varying delivery at fixed levels, a multi-pump system is chosen.

• If the system requires varying delivery, a variable displacement pump can be chosen.

• Table 4.12 presents the flow rate ranges for different types of pumps.

• Usually a pump with a capacity about 10% more than that required is selected (in order to compensate for leakage losses that are caused due to increased operating pressure).

3. Pump drive speeds: 

• The pumps are usually driven by electric motor or internal combustion engine. Since the delivery rate depends on the drivig speed of the pump, which in turn depends on the pump drive speeds.

• Table 4.12 presents the speed ranges for different hydraulic pumps.

• It may be noted that higher the pump speed, the shorter will be its life."

4. Type of control: 

• The various types of pump controls are manual, servo control, pressure compensated control, constant power control and constant flow control.

• A suitable pump control is chosen based on the application requirements such as complexity, accuracy of control, cost, type of machining operation, etc.

5. Type of fluid (fluid compatibility): 

• The pump should be selected based on the type of fluid used in the hydraulic circuit.

• The compatibility between fluid type, pump type, and pump seals are to be taken into consideration.

6. Fluid contamination:

• Depending on the nature of fluid contamination and particulate size of contamination of the hydraulic circuit, the pump type should be selected. 

• For example, if a contaminated (with more dirt) fluid has to be pumped, then non- precision gear pumps such as lobe pumps and gerotor pumps can be selected.

• Irrespective of pump type, pump suction line must have a filter fitted.

7. Operating environment: 

• The effect of operating environment should also be considered while selecting a pump.

• This involves effects of ambient temperature, altitude, humidity and extremes of operating temperatures.

8. Tolerable pump noise level: 

• The operating noise level for a given type of pump varies with the workmanship and make.

• Usually the pump noise level increases with higher operating speed and pressure.

9. Compactness and weight-to-power ratio: 

• It is the ratio of pump weight to the hydraulic power delivered by the pump, expressed as kg/kW.

• This weight-to-power ratio factor is of critical importance in aerospace and military engineering applications.

• The weight-to-power ratio should be as low as possible.

• Table 4.13 presents the weight-to-power ratio ranges for different pumps.

Table 4.13. Comparison of weight-to-power ratio factor for pumps

10. Efficiency: 

• Though the actual pump efficiency depends on design, operating pressure, speed and fluid viscosity, ideally the pump with higher overall efficiency should be chosen for the system requirement.

• Table 4.12 presents the range of volumetric and overall efficiencies for different types of pumps.

• As could be seen from Table 4.12, the reciprocating pumps usually have higher efficiencies than rotary pumps.

11. Cost and economic factors: 

• The pump cost is also a very important factor in selection of a pump.

• The operating and maintenance costs of the pumps are also to be taken into consideration, in addition to its capital cost.

• Usually gear pumps are cheaper; vane and piston pumps are more expensive.

The factors such as availability and

12. Availability and interchangeability: 

• interchangeability are also considered while selecting a pump.

• Now-a-days almost all pump manufacturers adapt international standards, which facilitates direct interchangeability between pumps of different manufacturers.

13. Ease of maintenance and spares: 

• Since pump components worn out after a time and they need to be replaced. Thus the ease of pump maintenance and availability of spare components are also should be considered while selecting a pump.

• Usually gear pumps when worn-out are replaced, whereas in vane pumps, all wearing parts are replaced as a set.

Note 

1. In general, gear pumps are the least expensive but also provide the lowest level of performance. Since the gear pumps are simple in design and compact in size, therefore they are most commonly used in fluid power systems.

2. The efficiency and the cost of vane pump lies between those of gear and piston pumps. However, clean oil with good lubricity is necessary for its continued satisfactory performance.

3. Among all pumps, piston pumps are the most expensive but provide the highest level of overall performance. They produce a non-pulsating flow and can operate at the highest pressure levels. However, because of their complex design, piston pumps cannot be repaired in the field.