HYDRAULICS

12 Jun

Many machines and processes use a fluid for developing a force to move or hold an object, or to control an action. The term hydraulic refers to a liquid. A number of fluids can be used for developing the force.  In a hydraulic system, oil, water, or other liquids can be used.  Oil is the most common.

E O 1.5

G iven th e ap p rop riate in form ation , CA L C U L A T E th e pressure or force ach ieved  in a h yd rau lic p iston .

E O 1.6

D E S C R I B E th e b asic op eration  of a h yd rau lic s ystem .

Introduction

Although any liquid can be used in a hydraulic system, some liquids have advantages over others.  Oil is a liquid often preferred as the working fluid.  Oil helps to lubricate the various sliding parts, prevents rust, and is readily available.  For practical purposes, oil does not change its volume in the hydraulic system when the pressure is changed.

P ressure and F orce

The foundation of modern hydraulic powered systems was established when a scientist named Blaise Pascal discovered that pressure in a fluid acts equally in all directions.  This concept is known as Pascal’s Law. The application of Pascal’s Law requires the understanding of the relationship between force and pressure.

Force may be defined as a push or  pull exerted against the total area of a surface.    It is expressed in pounds.   Pressure is the amount of force on a unit area of the surface.   That is, pressure is the force acting upon one square inch of a surface.

The relationship between pressure and force is expressed mathematically.

F  =  P x A

where:

F  =  force in lbf

P  =  pressure in lbf/in.2, (psi) A =  area in in.2

Example 1:

In a hydraulic system, the oil pressure at the inlet to the cylinder is 1500 psi, and the area of the piston over which the oil pressure acts is two square inches. Calculate the force exerted on the piston.

Solution:

Since F = P x A, the force of the oil on the piston is calculated as follows.

F  =  1500 lbf/in.2  x 2 in.2
=  3000 lbf

Example 2:

A hydraulic valve requires a force of 1848 lbf to be opened.  The piston area is 3 square inches.  How much pressure does the hydraulic fluid have to exert for the valve to move?

Solution:

H ydraulic O peration

The operation of a typical hydraulic system is illustrated in Figure 8.    Oil from a tank or reservoir flows through a pipe into a pump.  Often a filter is provided on the pump suction to remove impurities from the oil.  The pump, usually a gear-type, positive displacement pump, can be driven by an electric motor, air motor, gas or steam turbine, or an internal combustion engine. The pump increases the pressure of the oil.   The actual pressure developed depends upon the design of the system.

Most hydraulic systems have some method of preventing overpressure.  As seen in Figure 8, one method of pressure control involves returning hydraulic oil to the oil reservoir.   The pressure control box shown on Figure 8 is usually a relief valve that provides a means of returning oil to the reservoir upon overpressurization.

The high pressure oil flows through a control valve (directional control).   The control valve changes the direction of oil flow, depending upon the desired direction of the load.  In Figure

8 the load can be moved to the left or to the right by changing the side of the piston to which the oil pressure is applied.  The oil that enters the cylinder applies pressure over the area of the piston, developing a force on the piston rod.  The force on the piston rod enables the movement of a load or device.  The oil from the other side of the piston returns to a reservoir or tank.

H azards

The hazards and precautions listed in the previous chapter on air compressors are applicable to hydraulic systems as well, because most of the hazards are associated with high pressure conditions. Any use of a pressurized medium can be dangerous.  Hydraulic systems carry all the hazards of pressurized systems and special hazards that are related directly to the composition of the fluid used.

When using oil as a fluid in a high pressure hydraulic system, the possibility of fire or an explosion exists. A severe fire hazard is generated when a break in the high pressure piping occurs and the oil is vaporized into the atmosphere. Extra precautions against fire should be practiced in these areas.
If oil is pressurized by compressed air, an explosive hazard exists if the high pressure air comes into contact with the oil, because it may create a diesel effect and subsequent explosion. A carefully followed preventive maintenance plan is the best precaution against explosion.
Summary
The important information in this chapter is summarized below.

Hydraulics Summary
The relationship between pressure and force in a hydraulic piston is expressed mathematically as:

Oil from a tank or reservoir flows through a pipe into a pump. The pump can be driven by a motor, turbine, or an engine. The pump increases the pressure of the oil.

The high pressure oil flows in the piping through a control valve. The control valve changes the direction of the oil flow. A relief valve, set at a desired safe operating pressure, protects the system from an overpressure condition. Oil entering the cylinder applies pressure to the piston, developing a force on the piston rod.

The force on the piston rod enables the movement of a load or device. The oil from the other side of the piston returns to a reservoir or tank via a filter, which removes foreign particles.

BOILERS
Boilers are commonly used at large facilities to act as primary or backup steam sources. The source of heat that generates the steam varies, but the basic operation of the boiler is the same. This chapter will summarize the operation of a boiler.

Introduction

The primary function of a boiler is to produce steam at a given pressure and temperature. To accomplish this, the boiler serves as a furnace where air is mixed with fuel in a controlled combustion process to release large quantities of heat. The pressure-tight construction of a boiler provides a means to absorb the heat from the combustion and transfer this heat to raise water to a temperature such that the steam produced is of sufficient temperature and quality (moisture content) for steam loads.

Boilers
Two distinct heat sources used for boilers are electric probes and burned fuel (oil, coal, etc.) This chapter will use fuel boilers to illustrate the typical design of boilers. Refer to Figure 9 during the following discussion.
The boiler has an enclosed space where the fuel combustion takes place, usually referred to as the furnace or combustion chamber. Air is supplied to combine with the fuel, resulting in combustion. The heat of combustion is absorbed by the water in the risers or circulating tubes. The density difference between hot and cold water is the driving force to circulate the water back to the steam drum. Eventually the water will absorb sufficient heat to produce steam.
Steam leaves the steam drum via a baffle, which causes any water droplets being carried by the steam to drop out and drain back to the steam drum. If superheated steam is required, the steam may then travel through a superheater. The hot combustion gasses from the furnace will heat the steam through the superheater’s thin tube walls. The steam then goes to the steam supply system and the various steam loads.

Some boilers have economizers to improve cycle efficiency by preheating inlet feedwater to the boiler. The economizer uses heat from the boiler exhaust gasses to raise the temperature of the inlet feedwater.

Fuel  Boiler  Components
Figure 9 illustrates a typical fuel boiler. Some of the components are explained below.

Steam drum – The steam drum separates the steam from the heated water. The water droplets fall to the bottom of the tank to be cycled again, and the steam leaves the drum and enters the steam system. Feedwater enters at the bottom of the drum to start the heating cycle.

Downcomers – Downcomers are the pipes in which the water from the steam drum travels in order to reach the bottom of the boiler where the water can enter the distribution headers.

Distribution headers – The distribution headers are large pipe headers that carry the water from the downcomers to the risers.

Risers – The piping or tubes that form the combustion chamber enclosure are called risers. Water and steam run through these to be heated. The term risers refers to the fact that the water flow direction is from the bottom to the top of the boiler. From the risers, the water and steam enter the steam drum and the cycle starts again.

Combustion chamber - Located at the bottom of a boiler, the combustion chamber is where the air and fuel mix and burn. It is lined with the risers.

Summary
The important information in this chapter is summarized below.

Boilers Summary
Boilers are vessels that allow water in contained piping to be heated to steam by a heat source internal to the vessel. The water is heated to the boiling point. The resulting steam separates, and the water is heated again. Some boilers use the heat from combustion off-gasses to further heat the steam (superheat) and/or to preheat the feedwater.

The following components were discussed:

The steam drum is where the steam is separated from the heated water.
Downcomers are the pipes in which the water from the steam drum travels to reach the bottom of the boiler.

Distribution headers are large pipe headers that carry the water from the downcomers to the risers.

Risers are the piping or tubes that form the combustion chamber enclosure. Water and steam run through the risers to be heated.
The combustion chamber is located at the bottom of the boiler and is where the air and fuel mix and burn.

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