Hydraulic fluid types

Fluids may be classified as follows:
• Ideal fluid
• Real fluid
• Newtonian fluid
• Non-Newtonian fluid.

Ideal fluid
An ideal fluid is one, which is incompressible and has no viscosity. Such a fluid is only imaginary, as all existing fluids possess some viscosity.

Real fluid
Simply speaking, a fluid which possesses viscosity is known as a real fluid. All fluids, in actual practice are real fluids.

Newtonian fluid
During the course of our discussion on viscosity, we have seen that shear stress is proportional to the velocity gradient i.e. ra dv/dy. A real fluid in which the shear stress is proportional to the velocity gradient is known as a Newtonian fluid.

Non-Newtonian fluid
A real fluid in which the shear stress is not proportional to the velocity gradient is known as a non-Newtonian fluid.

Orifice plate-type hydraulic flowmeter

Another method by which flow rate can be determined involves the use of an orifice plate-type flowmeter in which an orifice is installed in the pipeline as shown in Figure 2.14.


The figure also shows the presence of two pressure gages, one each on either side of the orifice. This arrangement enables us to determine the pressure drop (AP) across the orifice when the fluid flows through the pipe and given by AP = Pi- P2- The higher the flow rate, greater will be the pressure drop.

The actual flow rate can be determined by the following equation:


Q is the flow rate in m3/s
C is the flow coefficient (C = 0.80 for a sharp-edged orifice and 0.60 for a square-edged orifice)
A is the area of the orifice opening in m^
S is the specific gravity of the flowing fluid and
AP = P1-P2 is the pressure drop across the orifice in psi or kPa.

Rotameter hydraulic flow measurement

The Rotameter also known as variable area flowmeter is the most common among all flow measurement devices. Figure 2.12 shows the operation of a Rotameter. It basically consists of a tapered glass tube calibrated with a metering float that can move vertically up and down in the glass tube. Two stoppers one at the top and the other at the bottom of the tube prevent the float from leaving the glass tube. The fluid enters the tube through the inlet provided at the bottom. When no fluid is entering the tube, the float rests at the bottom of the tapered tube with one end of the float making contact with the lower stopper. The diameter of the float is selected in such a way that under conditions where there is no fluid entry into the tube, the float will block the small end of the tube completely.


When the fluid starts entering the tube through the inlet provided at the bottom, it forces the float to move upwards. This upward movement of the float will continue, until an equihbrium position is reached at which point the weight of the float is balanced by the upward force exerted by the fluid on the float. Greater the flow rate, higher is the float rise in the tube. The graduated tube allows direct reading of the flow rate.

Types of fluid flow

Fluid flow can be classified as follows:
• Steady and unsteady flows
• Uniform and non-uniform flows
• Laminar and turbulent flows
• Rotational and non-rotational flows.

Steady flow
Fluid flow is said to be steady if at any point in the flowing fluid, important characteristics such as pressure, density, velocity, temperature, etc. that are used to describe the behavior of a fluid, do not change with time. In other words, the rate of flow through any crosssection of a pipe in a steady flow is constant.

Unsteady flow
Fluid flow is said to be unsteady if at any point in the flowing fluid any one or all the characteristics describing the behavior of a fluid such as pressure, density, velocity and temperature change with time. Unsteady flow is that type of flow, in which the fluid characteristics change with respect to time or in other words, the rate of flow through any cross-section of a pipe is not constant.

Uniform flow
Flow is said to be uniform, when the velocity of flow does not change either in magnitude or in direction at any point in a flowing fluid, for a given time. For example, the flow of liquids under pressure through long pipelines with a constant diameter is called uniform flow.

Non-uniform flow
Flow is said to be non-uniform, when there is a change in velocity of the flow at different points in a flowing fluid, for a given time. For example, the flow of liquids under pressure through long pipelines of varying diameter is referred to as non-uniform flow. All these type of flows can exist independently of each other. So there can be any of the four combinations of flows possible:
1. Steady uniform flow
2. Steady non-uniform flow
3. Unsteady uniform flow
4. Unsteady non-uniform flow.

Laminar flow
A flow is said to be laminar if the fluid particles move in layers such that one layer of the fluid slides smoothly over an adjacent layer. The viscosity property of the fluid plays a significant role in the development of a laminar flow. The flow pattern exhibited by a highly viscous fluid may in general be treated as laminar flow.

Turbulent flow
If the velocity of flow increases beyond a certain value, the flow becomes turbulent. The movement of fluid particles in a turbulent flow will be random. This mixing action of the colliding fluid particles generates turbulence, thereby resulting in more resistance to fluid flow and hence greater energy losses as compared to laminar flow.

Air-to-hydraulic pressure booster

Air-to-hydraulic pressure booster is a device used to convert workshop air into a higher hydraulic pressure needed for operating cylinders requiring small to medium volumes of high-pressure oil (Figure 2.7(a)).


It consists of an air cylinder with a large diameter driving a small diameter hydraulic cylinder. Any workshop equipped with an airline can easily obtain hydraulic power from an air-to-hydraulic booster hooked into the airline. Figure 2.7(b) shows an application of the air-to-hydraulic booster. Here the booster is seen supplying high-pressure oil to a hydraulic cylinder used to clamp a work piece to a machine tool table.


Since the workshop air pressure normally operates at around 100 psi, a pneumatically operated clamp would require a relatively larger cylinder to hold the work piece while it is being machined.

Let us assume that the air piston has a 10 sq. in. area and subjected to a pressure of 100 psi. This produces a 1000 lb force on the hydraulic cylinder piston. Thus if the area of the hydraulic piston is 1 sq. in., the hydraulic discharge oil pressure will be 1000 psi. As per Pascal’s law this produces a 1000 psi oil pressure at the small hydraulic clamping cylinder mounted on the machine tool table.

The pressure ratio of the pressure booster can be determined as follows:


Turbine-type hydraulic flowmeter

Figure 2.13 is a simple illustration of a turbine-type flowmeter.

This flowmeter has a turbine rotor in the housing, which is connected to the pipeline whose flow rate is to be measured. When the fluid flows, it causes the turbine to rotate. Higher the flow rate, greater is the speed of the turbine. The magnetic end of a sensor, which is positioned near the turbine blades, produces a magnetic field whose magnetic lines of force are interrupted by the rotation of the turbine blades, thereby generating an electrical impulse. An electrical device connected to the sensor converts the pulses to flow rate information.


Hydraulic Jack Principle

This system uses a piston-type hand pump to power a single acting hydraulic cylinder as illustrated in Figure 2.6.

A hand force is applied at point ‘A’ of handle ‘ABC, which pivots about point ‘ C The piston rod of the hand pump is pinned to the input handle at point ‘B’. The hand pump contains a cylinder for aiding the up and down movement. When the handle is pulled, the piston moves up, thereby creating a vacuum in the space below it. As a result of this, the atmospheric pressure forces the oil to leave the oil tank and flow through check valve 1. This is the suction process.

When the handle is pushed down, oil is ejected from the hand pump and flows through the check valve 2. Oil now enters the bottom of the load cylinder. The load cylinder is similar in construction to the pump cylinder. Pressure builds up below the load piston as oil is ejected from the pump. From Pascal’s law, we know that the pressure acting on the load piston is equal to the pressure developed by the pump below its piston. Thus each time the handle is operated up and down, a specific volume of oil is ejected from the pump to lift the load cylinder to a given distance against its load resistance. The bleed valve is a hand-operated valve which when opened, allows the load to be lowered by bleeding oil from the load cylinder back to the oil tank. This cylinder is referred to as single acting because it is hydraulically powered in one direction only.