Engineering plant facilities 03 mechanics fundamentals

MECHANICS

L | C | LOGISTICS PLANT MANUFACTURING AND BUILDING FACILITIES EQUIPMENT

Engineering-Book

ENGINEERING FUNDAMENTALS AND HOW IT WORKS

September 2014

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PumpA pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action. Pumps can be classified into three major groups according to the method they use to move the fluid: direct lift, displacement, and gravity pumpsPumps operate by some mechanism (typically reciprocating or rotary), and consume energy to perform mechanical work by moving the fluid. Pumps operate via many energy sources, including manual operation, electricity, engines, or wind power, come in many sizes, from microscopic for use in medical applications to large industrial pumps.

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Mechanics Fundamentals

Some positive displacement pumps use an expanding cavity on the suction side and a decreasing cavity on the discharge side. Liquid flows into the pump as the cavity on the suction side expands and the liquid flows out of the discharge as the cavity collapses. The volume is constant through each cycle of operation

Positive displacement pumps, unlike centrifugal or roto-dynamic pumps, theoretically can produce the same flow at a given speed (RPM) no matter what the discharge pressureThus, positive displacement pumps are constant flow machinesHowever, a slight increase in internal leakage as the pressure increases prevents a truly constant flow rate

A positive displacement pump makes a fluid move by trapping a fixed amount and forcing (displacing) that trapped volume into the discharge pipe.


A positive displacement pump must not operate against a closed valve on the discharge side of the pump, because it has no shutoff head like centrifugal pumps. A positive displacement pump operating against a closed discharge valve continues to produce flow and the pressure in the discharge line increases until the line bursts, the pump is severely damaged, or both.

A relief or safety valve on the discharge side of the positive displacement pump is therefore necessary.

The relief valve can be internal or external. The pump manufacturer normally has the option to supply internal relief or safety valves.

The internal valve is usually only used as a safety precaution. An external relief valve in the discharge line, with a return line back to the suction line or supply tank provides increased safety


Energy FactsEnergy FactsEnergy FactsPump Types

Types

Mechanical pumps may be submerged in the fluid they are pumping or be placed external to the fluid.

Pumps can be classified by their method of displacement into positive displacement pumps, impulse pumps, velocity pumps, gravity pumps, steam pumps and valve less pumps.

Positive displacement pump


Screw pump Rotary positive displacement pumps

Reciprocating pump

Gear pump

Roots-type supercharger


Flexible impeller Velocity pumps Centrifugal pump

Hence the power, P, required by the pump:

where ΔP is the change in total pressure between the inlet and outlet (in Pa), and Q, the fluid flow rate is given in m³/s;

η is the pump efficiency; which depends upon the pump's configuration and operating conditions (such as rotational speed, fluid density and viscosity etc.)


A Booster pump is a machine which will increase the pressure of a fluid.

It is similar to a gas compressor, but generally a simpler mechanism which often has only a single stage of compression, and is used to increase pressure of an already pressurized fluid.

A submersible pump (or sub pump, electric submersible pump (ESP)) is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped

The main advantage of this type of pump is that it prevents pump cavitations, a problem associated with a high elevation difference between pump and the fluid surface. Submersible pumps push fluid to the surface as opposed to jet pumps having to pull fluids.

Submersibles are more efficient than jet pumps.


An axial-flow pump, or AFP, is a common type of pump that essentially consists of a propeller (an axial impeller) in a pipe

The propeller can be driven directly by a sealed motor in the pipe or by electric motor or petrol/diesel engines mounted to the pipe from the outside or by a right-angle drive shaft that pierces the pipe

Fluid particles, in course of their flow through the pump, do not change their radial locations since the change in radius at the entry (called 'suction') and the exit (called 'discharge') of the pump is very small

Hence the name "axial" pump


A fire pump is a part of a fire sprinkler system's water supply and can be powered by electric, diesel or steam.

The pump intake is either connected to the public underground water supply piping, or a static water source (e.g., tank, reservoir, lake).

The pump provides water flow at a higher pressure to the sprinkler system risers and hose standpipes

A jockey pump is a small pump connected to a fire sprinkler system and is intended to maintain pressure in a fire protection piping system to an artificially high level so that the operation of a single fire sprinkler will cause a pressure drop which will be sensed by the fire pump automatic controller, causing the fire pump to start.

The jockey pump is essentially a portion of the fire pump's control system


Misalignment is the deviation of relative shaft position from a collinear axis of rotation measured at the points of power transmission when equipment is running at normal operating conditions

Objective of accurate alignment

•Reduce excessive axial and radial forces.•Eliminate the possibility of shaft failure.•Minimize the amount of wear in the coupling.•Minimize the amount of shaft bending.•Reduce the power consumption.


Parallel misalignment

Angular misalignment

‘real world’ misalignment usually exhibits a combination of both parallel and angular shaft centerline positions.


Using twin wire

Off Set Angular

RPM mm mm / 100 mm

0000 - 1000 0.13 0.10

1000 - 2000 0.10 0.08

2000 - 3000 0.07 0.07

3000 - 4000 0.05 0.06

4000 - 5000 0.03 0.05

Alignment Tolerance


Laser Alignment

Advantages:1. Easy to use.2. Use Reverse Indicator Method.3. Machine does the calculations.4. 0 -20m max. working distance.5. Selectable high resolution 0.1, 0.01, 0.001mm.6. No indicator sag.7. Soft foot measurement program.8. Horizontal shaft alignment with mim 600 rotation.9. Vertical shaft alignment program.10.Thermal or offset compensation.


An electric motor is an electric machine that converts electrical energy into mechanical energy.

In normal motoring mode, most electric motors operate through the interaction between an electric motor's magnetic field and winding currents to generate force within the motor

In an electric motor the moving part is the rotor which turns the shaft to deliver the mechanical power

The rotor usually has conductors laid into it which carry currents that interact with the magnetic field of the stator to generate the forces that turn the shaft


The stationary part is the stator, usually has either windings or permanent magnets

In between the rotor and stator is the air gap. The air gap has important effects, and is generally as small as possible, as a large gap has a strong negative effect on the performance of an electric motor.

Windings are wires that are laid in coils, usually wrapped around a laminated soft iron magnetic core so as to form magnetic poles when energized with current

In magnetic motors, magnetic fields are formed in both the rotor and the stator. The product between these two fields gives rise to a force, and thus a torque on the motor shaft


AC motor has two parts:

A stationary stator having coils supplied with AC to produce a rotating magnetic field,

A rotor attached to the output shaft that is given a torque by the rotating field.

An induction motor is an asynchronous AC motor where power is transferred to the rotor by electromagnetic induction, much like transformer action

An induction motor resembles a rotating transformer, because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side

Poly phase induction motors are widely used in industry.


Currents induced into this winding provide the rotor magnetic field.

The shape of the rotor bars determines the speed-torque characteristics.

At low speeds, the current induced in the squirrel cage is nearly at line frequency and tends to be in the outer parts of the rotor cage.

As the motor accelerates, the slip frequency becomes lower, and more current is in the interior of the winding.

By shaping the bars to change the resistance of the winding portions in the interior and outer parts of the cage, effectively a variable resistance is inserted in the rotor circuit.

However, the majority of such motors have uniform bars.


The motor's armature current is supplied through the stationary brushes in contact with the revolving commutator, which causes required current reversal and applies power to the machine in an optimal manner as the rotor rotates from pole to pole.

A commutator is a mechanism used to switch the input of certain AC and DC machines consisting of slip ring segments insulated from each other and from the electric motor's shaft.

In absence of such current reversal, the motor would brake to a stop.

In light of significant advances in the past few decades due to improved technologies in electronic controller, sensor less control, induction motor, and permanent magnet motor fields, electromechanically commutated motors are increasingly being displaced by externally commutated induction and permanent magnet motors.


Motor

Power (horsepower), Where rpm is shaft speed and T is torque, a motor's mechanical power output Pem is given in British units with T expressed in foot-

pounds,

in SI units with shaft speed expressed in radians per second, and T expressed in Newton-meters, (watts).

For a linear motor, with force F and velocity v expressed in Newton's and meters per second,

(watts).


Electromotive forceSince the armature windings of a direct-current motor are moving through a magnetic field, they have a voltage induced in them. This voltage tends to oppose the motor supply voltage and so is called "back electromotive force (emf)".

The voltage is proportional to the running speed of the motor. The back emf of the motor, plus the voltage drop across the winding internal resistance and brushes, must equal the voltage at the brushes. This provides the fundamental mechanism of speed regulation in a DC motor.

If the mechanical load increases, the motor slows down; a lower back emf results, and more current is drawn from the supply. This increased current provides the additional torque to balance the new load


An engine-generator is the combination of an electrical generator and an engine (prime mover) mounted together to form a single piece of self-contained equipment.

A diesel generator is the combination of a diesel engine with an electric generator (often an alternator) to generate electrical energy

Sizing of diesel generators is critical to avoid low-load or a shortage of power

The primary advantage of engine-generators is the ability to independently supply electricity, allowing the units to serve as backup power solutions

It is the size of the maximum load that has to be connected and the acceptable maximum voltage drop which determines the set size


Diesel engines should be run at least 60% to 75% of their maximum rated load. Short periods of low load running are permissible providing the set is brought up to full load, or close to full load on a regular basis.

Internal glazing and carbon build-up is due to prolonged periods of running at low speeds or low loads

Running an engine under low loads causes low cylinder pressures and consequent poor piston ring sealing since this relies on the gas pressure to force them against the oil film on the bores to form the seal.

Low cylinder pressures causes poor combustion and resultant low combustion pressures and temperatures

This poor combustion leads to soot formation and un burnt fuel residues which clogs and gums piston rings, causing a further drop in sealing efficiency and exacerbates the initial low pressure


An air compressor is a device that converts power (usually from an electric motor, a diesel engine or a gasoline engine) into kinetic energy by compressing and pressurizing air, which, on command, can be released in quick bursts

There are numerous methods of air compression, divided into either positive-displacement or negative-displacement types

Positive-displacement air compressors work by forcing air into a chamber whose volume is decreased to compress the air.

Piston-type air compressors use this principle by pumping air into an air chamber through the use of the constant motion of pistons.

They use one-way valves to guide air into a chamber, where the air is compressed.


Rotary screw compressors also use positive-displacement compression by matching two helical screws that, when turned, guide air into a chamber, whose volume is decreased as the screws turn

Vane compressors use a slotted rotor with varied blade placement to guide air into a chamber and compress the volume

A type of compressor that delivers a fixed volume of air at high pressures.

Common types of positive displacement compressors include piston compressors and rotary screw compressors

Negative-displacement air compressors include centrifugal compressors.

These use centrifugal force generated by a spinning impeller to accelerate and then decelerate captured air, which pressurizes it


A flow control valve regulates the flow or pressure of a fluid.

Control valves normally respond to signals generated by independent devices such as flow meters or temperature gauges.

Control valves are normally fitted with actuators and positioners

Pneumatically-actuated globe valves and Diaphragm Valves are widely used for control purposes in many industries, although quarter-turn types such as (modified) ball, gate and butterfly valves are also used

The control valve manipulates a flowing fluid, such as gas, steam, water, or chemical compounds, to compensate for the load disturbance and keep the regulated process variable as close as possible to the desired set point


A commonly used control device called a programmable logic controller, or a PLC, is used to read a set of digital and analog inputs, apply a set of logic statements, and generate a set of analog and digital outputs.

For example, if an adjustable valve were used to hold level in a tank the logical statements would compare the equivalent pressure at depth set point to the pressure reading of a sensor below the normal low liquid level and determine whether more or less valve opening was necessary to keep the level constant.

A PLC output would then calculate an incremental amount of change in the valve position. Larger more complex systems can be controlled by a Distributed Control System (DCS) or SCADA system


A solenoid valve has two main parts: the solenoid and the valve. The solenoid converts electrical energy into mechanical energy which, in turn, opens or closes the valve mechanically

A direct acting valve has only a small flow circuit, shown within section E of this diagram

In this example, a diaphragm piloted valve multiplies this small pilot flow, by using it to control the flow through a much larger orifice.

Solenoid valves may use metal seals or rubber seals, and may also have electrical interfaces to allow for easy control.

A spring may be used to hold the valve opened (normally open) or closed (normally closed) while the valveis not activated


net positive suction head (NPSH)

In a Pump hydraulic circuit, it refers to one of two quantities in the analysis of cavitation

Cavitation will occur at the point i when the Available NPSH is less than the NPSH required to prevent cavitation (NPSHR)

For simple impeller systems, NPSHR can be derived theoretically, but very often it is determined empirically.

Note NPSHA and NPSHR are in absolute units and usually expressed in "ft abs" not "psia".

Applying Bernoulli's principle from the suction free surface 0 to the pump inlet i, under the assumption that the kinetic energy at 0 is negligible, that the fluid is non viscose, and that the fluid density is constant


piping is a system of pipes used to convey fluids (liquids and gases) from one location to another

Stress analysisProcess piping and power piping are typically checked by pipe stress engineers to verify that the routing, nozzle loads, hangers, and supports are properly placed and selected such that allowable pipe stress is not exceeded under different situation such as sustain, operating, pressure testing etc.,

It is necessary to evaluate the mechanical behavior of the piping under regular loads (internal pressure and thermal stresses) as well under occasional and intermittent loading cases such as earthquake, high wind or special vibration, and water hammer


A gasket (correct terminology is a "joint" made from "jointing material") is a mechanical seal which fills the space between two or more mating surfaces, generally to prevent leakage from or into the joined objects while under compression.

Gaskets allow "less-than-perfect" mating surfaces on machine parts where they can fill irregularities. Gaskets are commonly produced by cutting from sheet materials

Failure reasons1.Uneven distributed pressing force2.Stress relaxation and torque loss 3.Surface not smooth4.Temperature

Flange fittings generally involve pressing two surfaces to be joined tightly together, by means of threaded bolts, wedges, clamps, or other means of applying high compressive forces


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L | C | LOGISTICS PLANT MANUFACTURING AND BUILDING FACILITIES EQUIPMENT

Engineering-Book

ENGINEERING FUNDAMENTALS AND HOW IT WORKS

MECHANICS

Engineering plant facilities 03 mechanics fundamentals

Engineering

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