MMET 303

Hydraulic Control Valves: Directional Control Valves - Four Way Valves (Two Positional)

- It contains four ports. There are two designs based of the spool number of positions: two positions and three positions. Spool Position 1: Flow can go from P to A and B to T Spool Position 2: Flow can go from A to T and P to B Usually used to control flow directions to double acting cylinders

When cavitation occurs

- Performance of the pump is severely damaged as volume flow rate delivered drops - Pump vibrates and becomes noisy - Would destroy pump in a short time

1 W =?

1 (N*M)/S or 1 J/S

Components of a Pneumatic System

1. An air tank 2. A compressor 3. Electric Motor 4. Valves 5. Actuators 6. Piping

Two Principles of Pressure Described by Pascal

1. Pressure acts uniformly in all directions on a small volume of a fluid 2. In a fluid confined by a solid boundaries, pressure acts perpendicular to the boundary

Components of Hydraulic System

1. Tank (reservoir) 2. A pump to force oil through 3. A motor 4. Valves to control oil Direction 5. An Actuator 6. Piping

Two Types of Non-Newtonian Fluids

1. Time independent (does not change with time) 2. Time Dependent

Newtonian

A fluid that behaves in accordance with Newton's Law of Viscosity (Ex: Water, oil, gas, alcohol)

Non-Newtonian Fluid

A fluid that does not behave in accordance with Newton's Law of Viscosity.

Dynamic Viscosity

A measure of the resistance to flow of a fluid under an applied force.

Sudden Enlargement

AS fluid flows from a smaller pipe into a larger pipe through a sudden enlargement, its velocity abruptly decreases, causing turbulence, which generates an energy loss

Kinetic Pumps

Add energy to the fluid by accelerating it through the action of a rotating impeller

Pneumatic Systems

Air as the medium because air is very abundant and can be readily exhausted into the atmosphere afterwards.

As Relative Roughness D/E increases, the value of the Reynolds Number at which the zone of complete turbulence begins ...

Also increases

Viscosity index (VI)

An arbitrary measure for the change with variations in temperature

Positive Displacement Pumps: Piston Pumps For Fluid Power

Axial piston pump, which uses a rotating swash plate that acts like a cam to reciprocate the piston.

Saybolt viscometer

Based on principle that the ease of fluid through an orifice is an indication of viscosity

Velocity Profile of Laminar Profiles

Can be thought of a series of concentric layers of the fluid sliding along each other. This results in a parabolic shape for the velocity profile

Most Commonly used kinetic pump

Centrifugal Pumps

Velocity Profile of Turbulent Profiles

Chaotic with significant amounts of intermixing of the particles of fluid and a consequent transfer of momentum among the particles. This results in a nearly uniform velocity across much of the cross section

How to ensure cavitation does not occur?

Computing the Net Positive Suction Head (NPSH)

Hydraulic Control Valves: Directional Control Valves - Three-Way Valves

Contains 3 ports and are typically of the spool design Spool Position 1 - Flow can go from pump port P to outlet port A. Tank port T is blocked Spool Position 2 - Flow can go from port A to Port T. Port P is blocked by the spool. Typically used to control flow directions to single acting cylinders.

Hydraulic Control Valves: Flow Control Valves

Control the flow rate which is used to control the actuator speed.

Relative Roughness

D/E

Friction loss in Laminar Flow Alternative

Darcy's Equation or Hagen-Poiseuille

Hydraulic Radius for Non-circular Cross sections

Defined as the ratio of the net cross-sectional area of a flow stream to the wetted perimeter of the section

Absolute Temperature

Defined so zero point corresponds to the conditions where molecular motion stops.

Types of Pumps: Positive Displacement Pumps

Deliver a specific volume of fluid for each revolution of the pump shaft or each cycle of motion of the active pumping active pumping elements Produces very high pressures at moderate volume flow rates

Fluid Power Systems

Designed specifically to do work. Work is accomplished by pressurized fluid

Fluid Motors, Turbines, Rotary Actuators, and Linear Actuators Energy

Devices that take energy from a fluid and deliver it in the form of work.

Positive Displacement Pumps: Screw Pumps

Do not have the same pulsating flow problem as gear, piston, and vane pumps.

Valves and Fittings

Elements that the control the direction or flow rate of a fluid in a system Energy Losses occur whenever there is a restriction, change in flow velocity, or a change in the direction of the flow Minor Losses

Ha

Energy added to a fluid by device such as a pump

Fluid Friction

Energy in system is converted into thermal energy in the form of energy losses Major Losses

HL

Energy losses from the system due to friction in pipes or minor losses due to valves and fittings

Hr

Energy removed from a the fluid by a mechanical device

Laminar Flow

Flows at a very low velocity, the flow is smooth and steady. Uniform diameter, little to no evidence of mixing various parts of the stream.

Two Types of Fluid Power Systems

Fluid Transport and Fluid Power

Vapor Pressure

Fluid property that determines the conditions under which bubbles form in a fluid

Friction loss in Turbulent Flow

For this flow we use Darcy's Equation where f dependent on Reynolds Number and Relative Roughness. Where, Relative roughness is is ratio of pipe Diameter (D) to the average roughness (E).

Within the zone of complete turbulence, the Reynolds Number ...

Has no effect on the friction factor

Simplex

Has only one piston. Resulting intermittent delivery is often undesirable.

Reynolds Numbers For Critical Region

If 2000< Nr < 4000

Reynolds Numbers For Laminar Region

If Nr< 2000

Reynolds Numbers For Turbulent Region

If Nr>4000

Darcy's Equation

In the general energy equation, hL is defined as the energy loss from the system. This is expressed mathematically as this equation

Friction Loss in Laminar Flow

In this flow the fluid flows in several layers, one on top of the other. This creates shear stress between the layers. Energy is lost in-between these layers by overcoming the frictional forces produced by the shear stress.

Rotary Pump Performance

Is a plot of capacity, efficiency, and power versus discharge pressure

Basic Definition of Reynolds Number

Is used to predict is the flow is laminar or turbulent

Resistance Coefficient

K - Represents a constant of proportionality between the energy loss and the velocity head - Depends on the geometry of the device that causes the loss and sometimes on the velocity of the flow

Positive Displacement Pumps: Gear Pumps Advantages

Low Pulsating Flow Good Capability for Handling High Viscosity Fluids Can be operated in either direction

Rotary Pump Performance: Overall Efficiency

Measure of the ratio of the power delivered to the fluid to the power input to the pump

Rotary Pump Performance: Volumetric Efficiency

Measure of the ratio of the volume flow rate delivered by the pump to the theoretical delivery.

Energy in Pumps

Mechanical Device that adds energy to a fluid

Types of Pumps: Kinetic Pumps

Operate by transferring kinetic energy from a rotating element, called an impeller, to the fluid as it moves into through the pump. Some of this energy is then converted to pressure energy at the pump outlet.

Positive Displacement Pumps: Gear Pumps Disadvantages

Operates only at moderate pressures Not recommended for handling fluids containing solids

2 Types of Pumps

Positive Displacement Pumps Kinetic Pumps

Piston Pumps Disadvantages

Pressure pulsations of output flow, Being able to handle only low viscosity fluids Potentially High Wear of moving parts

Piston Pumps Advantages

Produce very high pressures

Fluid Transport Systems

Purpose is to transport fluid from one location to another to accomplish a task. Ex: Pumping Station that delivers water to homes

Specific Gravity (SG)

Ratio of Specific Weight of a substance to the specific weight of water at 4 degrees Celcius

Kinematic Viscosity

Ratio of dynamic viscosity to density of a fluid

K

Resistance coefficient

Volute

Rotation of the impeller accelerates the fluid outward along the vanes toward the casing

Hydraulic Control Valves: Directional Control Valves - Check Valve

Simplest of the directional control valves - Permits freeflow in one direction and prevents flow in the opposite direction Also offers the pilot-operated control valve - Permits flow in opposing direction only if pilot pressure is applied at the port of the valve.

Double Acting Duplex

Smooths performance curve (More desirable)

Hydraulic System

Systems that use liquids such as petroleum oils, water, synthetic oils, and even molten metals.

Fluid Power

Technology that deals with generation, control, and transmission of power using fluids

Hydraulic Control Valves: Directional Control Valves

They determine the path through which a fluid traverses within a given circuit

Hydraulic Control Valves: Pressure Control Valves

They protect the system against overpressure , which may occur due to a gradual buildup as fluid demand decreases or due to a sudden surge as valves open or close.

Saybolt universal viscometer (SUS)

Time in seconds required to fill 60 cubic centimeters of the container

Ha in pump manufacturing

Total Dynamic Head (TDH)

Variation of Viscosity in gas With Temperature

Viscosity of gasses increases as temperature increases

Variation of Viscosity in liquids With Temperature

Viscosity of liquids decreases as temperature increases

Turbulent Flow

When the faucet is nearly halfway open the water has: - High velocity - elements if fluid mix chaotically within the stream

Cavitation

When the suction pressure at the pump inlet is too low, vapor bubbles form in the fluid in a manner similar to boiling

Reservoirs

Where sludge, water, and metal chips settle and where entrained air is picked up by the oil is allowed to escape. -Dissipation of heat

Surface Tension

Work/Area (N/M)

Specific weight

amount of weight per unit volume of substance

For a given Nr as the relative roughness increases, the friction factor (f) ...

decreases

For a given relative Roughness the friction factor decreases as Reynolds Number ...

increases until the zone of complete turbulence is reached

Power in fluid mechnics

rate at which energy is being transferred

Density (p)

the amount of mass per unit volume