Working pressure 

A hydraulic system can be provided either by a high-pressure system using a small flow of fluid, or a low pressure one with a large flowrate. Either could be arranged to give the prescribed force or torque at the hydraulic actuators. 

The choice of working pressure is a major decision for a hydraulic system. Some engineers encourage high pressures as this can reduce sizes and lead to overall compact sets and devices. Pressure loss is not directly dependent on the working pressure. Therefore, the higher the pressure, the smaller the loss will be in proportion, and the higher the efficiency. On the other hand, there have always been some limits on the hydraulic pressure. The working pressure cannot be limitlessly high. If the use of a very high pressure is involving stronger and presumably more costly equipment, it is not obvious whether there will be a net gain. Only a detailed optimization can indicate the optimum pressure.  

Many engineers and experts believe there is no inherent advantage in the use of very high working pressures. To better explain, working pressure should be high and suitable for each hydraulic application, but not very high. Some applications and services need a high working pressure and compactness of high-pressure equipment is often an advantage. However, there are nearly always some limits. An optimization should be done to find the best and optimum working pressure for each application. The selection of working pressure tends to be governed by the maximum working pressure of available cost-effective and reliable equipment, components and parts. Nowadays working pressures above 100 Bar have been commonly used in hydraulic systems. Pressure as high as 250 Barg, 350 Barg, 400 Barg, or more are used in different compact hydraulic units. Once the working pressure has been determined, this usually fixes the rate of flow. It is then necessary to decide on the sizes for the piping, valves and others. 

 Sizing and selection 

As a rough indication, the flow velocity between 3.5 m/s and 5.5 m/s have been used in hydraulic circuits. Although flow velocity outside of this range has also been successfully used. This rough rule can be employed to check the pipe diameter, valve sizes and sizes of other parts and components. Although useful for checking or rough estimates, relying on such simplistic procedure cannot be suitable in many cases. A key point is the pressure drop is not only the function of sizes. Different parts, inlines, valves, devices and components present different performance, functionality and pressure drop patterns, which cannot be determined using simplistic rules as above. 

For the sizing of piping, valves and other hydraulic items, it is recommended to use an estimate of the allowable pressure loss. In other words, in the sizing and selection, firstly the overall allowable pressure loss in the hydraulic circuit should be decided based on operational and functional requirements. Then it is determined how the pressure loss should be distributed between the items and devices (piping, fittings, valves, etc) to achieve the best performance and lowest overall cost. As another indication, the ratio of the pressure loss at full working speed to the maximum pressure can be used for such estimates and sizing.  

Hydraulic pumps 

Hydraulic pumps are important components in any hydraulic system. Usually a positive displacement pump type is used for a hydraulic system. There are many types and models of hydraulic pumps such as reciprocating pumps, piston pumps and gear pumps. Many of these pumps can be arranged as horizontal pumps or vertical pumps. An axial piston pump is a positive displacement pump that has a number of pistons in a circular array within a cylinder block. This cylinder block is driven to rotate about its axis of symmetry by an integral shaft. This type of hydraulic pump is used in high pressure applications. Gear pumps have been used in so-called medium pressures.  

There are different configurations and arrangements for hydraulic pumps. Some systems use two pumps in operating-standby configuration. Some others may use “n+1” (“n” operating and one standby) pumps. Often, it is easier to provide independent streams of flows rather than splitting the flow generated by a pump. Therefore, using multiple pumps or double pumps (two pump casings driven by a driver) are popular in some hydraulic systems.  For Source info, Click here!

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