HYDRAULIC PUMPS

INTRODUCTION :-
In hydraulic systems the pump is the main driving unit which converts the mechanical energy in to hydraulic energy.Pump pushes the fluid in to the system by creating a positive flow in the load direction.Pump converts the mechanical horsepower in to the hydraulic horse power.

All pumps work on the same principle by generating an increasing volume on the intake side and a decreasing volume at the discharge side.But different pumps vary in the way they are constructed and the way they operate.

TERMS RELATED TO PUMP :-
Displacement :- 
The flow capacity of the pump is normally expressed as the displacement per revolution.Displacement is the volume of fluid displaced per one revolution.Normally displacement is expressed in cc/rev or gallon per revolution.
Pump delivery or flow:-

Pump delivery or flow is defined as the volume of fluid displaced per minute.It is normally expressed in terms of liter per minute(LPM) or gallon per minute(GPM).

*pump delivery is directly proportional to the pump drive shaft speed.

*pump delivery depends on the system operating pressure.At no load conditions it provides maximum flow but under certain operating pressure it provides minimum flow.If the pressure is very high the pump flow is bypassed to the tank as the pump is design is made for certain maximum pressure.
Volumetric efficiency :- 
Theoretically pump delivers a volume that is equal to the displacement volume per revolution but practically the delivery of pump is less than the theoretical value due to internal leakage.Volumetric efficiency is defined as the ratio of actual flow delivered to the theoretical delivery.

Volumetric efficiency%=(actual out put/theoretical output) x 100 

For example if a pump's theoretical output is 10 gpm but in practical testing it delivers 9 gpm then volumetric efficiency will be 90 percent. 

PUMP RATINGS:-


Pump is generally rated by it's maximum operating pressure capability and the maximum output flow or delivery at a given drive speed.

*Pump pressure capability is specified by the pump manufacturer depending on the maximum service life expectancy of the pump.

*If the operating service conditions are heavy or high pressure then the pump service life is reduced and may get severe damage.

PUMP POWER CALCULATION FORMULA  :-

HYDRAULIC HORSE POWER = GPM(flow) X PSI(pressure max)X 0.000583 

HYDRAULIC KILOWATTS = LPM(flow) X BAR(pressure max) X 0.001667

TYPES OF PUMP:-
there are basically two types of pump
1.Non positive displacement type.
In case of non positive displacement type pump the restriction to the flow is only friction in the pipe and the potential head(weight of fluid itself).
Normally these pumps work on the principle of centrifugal force hence called as centrifugal pumps.The fluid coming into the pump inlet is thrown out by rapidly moving impeller.There is no positive seal between the inlet and outlet ports.
It is possible to completely block the outlet port while the pump is running.As the resistance to flow increases the out put is reduced .Output depends on the impeller driven speed.
2.Positive displacement type.

Positive displacement type pump is most commonly used in industrial hydraulic systems.It delivers a specific amount of volume per every stroke or cycle or revolution.It is again classified into two types.
a.Fixed displacement type.
b.Variable displacement type.
examples of positive displacement pump are gear pumps,vane pumps and piston pumps. 

CYLINDER BUCKLING CALCULATIONS

hydraulic cylinders selection mostly depend on the critical buckling strength and bending strength of the cylinder apart from the other criteria like bore diameter,rod diameter and stroke length.

Buckling is a kind of failure where the cylinder fails under axial compressive load. In science, buckling is a mathematical instability, leading to a failure mode. Theoretically, buckling is caused by a bifurcation in the solution to the equations of static equilibrium. At a certain stage under an increasing load, further load is able to be sustained in one of two states of equilibrium: an undeformed state or a laterally-deformed state.

 In practice, buckling is characterized by a sudden failure of a structural member subjected to high compressive stress, where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding.

while calculating buckling we normally use the most conventional formula of mathematician Leonhard Euler.He derived a formula that gives the maximum axial load that a long, slender, ideal column can carry without buckling. 

An ideal column is one that is perfectly straight, homogeneous, and free from initial stress. The maximum load, sometimes called the critical load, causes the column to be in a state of unstable equilibrium; that is, the introduction of the slightest lateral force will cause the column to fail by buckling. The formula derived by Euler for columns with no consideration for lateral forces is given below. However, if lateral forces are taken into consideration the value of critical load remains approximately the same.

EULER'S FORMULA :- 

where

= maximum or critical force (vertical load on column),

= modulus of elasticity,

  = area moment of inertia,

= unsupported length of column,

= column effective length factor, whose value depends on the conditions of end support of the column, as follows.

For both ends pinned (hinged, free to rotate), = 1.0.

For both ends fixed, = 0.50.

For one end fixed and the other end pinned, = 0.699....

For one end fixed and the other end free to move laterally, = 2.0.

is the effective length of the column.

 Examination of this formula reveals the following interesting facts with regard to the load-bearing ability of slender columns.

1. Elasticity and not compressive strength of the materials of the column determines the critical load.
2. The critical load is directly proportional to the second moment of area of the cross section.
3. The boundary conditions have a considerable effect on the critical load of slender columns. The boundary conditions determine the mode of bending and the distance between inflection points on the deflected column. The closer together the inflection points are, the higher the resulting capacity of the column.

 The strength of a column may therefore be increased by distributing the material so as to increase the moment of inertia. This can be done without increasing the weight of the column by distributing the material as far from the principal axis of the cross section as possible, while keeping the material thick enough to prevent local buckling. This bears out the well-known fact that a tubular section is much more efficient than a solid section for column service.
Another bit of information that may be gleaned from this equation is the effect of length on critical load. For a given size column, doubling the unsupported length quarters the allowable load. The restraint offered by the end connections of a column also affects the critical load. If the connections are perfectly rigid, the critical load will be four times that for a similar column where there is no resistance to rotation (hinged at the ends).
Since the moment of inertia of a surface is its area multiplied by the square of a length called the radius of gyration, the above formula may be rearranged as follows. Using the Euler formula for hinged ends, and substituting A·r² for I, the following formula results.

where is the allowable stress of the column, and is the slenderness ratio.

Since structural columns are commonly of intermediate length, and it is impossible to obtain an ideal column, the Euler formula on its own has little practical application for ordinary design. Issues that cause deviation from the pure Euler strut behaviour include imperfections in geometry in combination with plasticity/non-linear stress strain behaviour of the column's material. Consequently, a number of empirical column formulae have been developed to agree with test data, all of which embody the slenderness ratio. For design, appropriate safety factors are introduced into these formulae. One such formula is the Perry Robertson formula which estimates of the critical buckling load based on an initial (small) curvature. The Rankine Gordon formula is also based on experimental results and suggests that a strut will buckle at a load Fmax given by:

where Fe is the Euler maximum load and Fc is the maximum compressive load. This formula typically produces a conservative estimate of Fmax.

Normally for factor of safety for linear buckling is taken more than 3.5.


hope this helps the needful..
If any query about this topic please comment..

HYDRAULIC CYLINDER BASICS

The most important linear actuator in a hydraulic system is a Hydraulic cylinder.It converts the hydraulic energy in to mechanical energy.Hydraulic cylinder can be of any size depending on the purpose for which it is built.The type of work to be done and power requirements determine the size of the hydraulic cylinder.

Types of hydraulic cylinders:-

1. Single acting cylinder.

Single acting cylinders are pressurized at one end only and the other end is vented to the reservoir.

2. Double acting cylinder.

Double acting cylinders can be pressurized from both ends resulting in two way motion of the actuator i.e it can extend and retract.

Basic double acting cylinder.

spring return single acting cylinder

single acting cylinder(ram cylinder)

telescopic cylinder

Operation:-

Hydraulic cylinders get their power from pressurized hydraulic fluid, which is typically oil. The hydraulic cylinder consists of a cylinder barrel, in which a piston connected to a piston rod moves back and forth. The barrel is closed on each end by the cylinder bottom (also called the cap end) and by the cylinder head where the piston rod comes out of the cylinder. The piston has sliding rings and seals. The piston divides the inside of the cylinder in two chambers, the bottom chamber (cap end) and the piston rod side chamber (rod end). The hydraulic pressure acts on the piston to do linear work and motion.
Flanges, trunnions, and/or clevises are mounted to the cylinder body. The piston rod also has mounting attachments to connect the cylinder to the object or machine component that it is pushing.
A hydraulic cylinder is the actuator or "motor" side of this system. The "generator" side of the hydraulic system is the hydraulic pump which brings in a fixed or regulated flow of oil to the bottom side of the hydraulic cylinder, to move the piston rod upwards. The piston pushes the oil in the other chamber back to the reservoir. If we assume that the oil pressure in the piston rod chamber is approximately zero, the force F on the piston rod equals the pressure P in the cylinder times the piston area A:

The piston moves instead downwards if oil is pumped into the piston rod side chamber and the oil from the piston area flows back to the reservoir without pressure. The fluid pressure in the piston rod area chamber is (Pull Force) / (piston area - piston rod area):

where P is the fluid pressure, F_p is the pulling force, A_p is the piston face area and A_r is the rod cross-section area.

TYPICAL CYLINDER CONSTRUCTION

Parts of a hydraulic cylinder:-

A hydraulic cylinder consists of the following parts

Cylinder barrel

The cylinder barrel is mostly a seamless thick walled forged pipe that must be machined internally. The cylinder barrel is ground and/or honed internally

Cylinder base or cap

In most hydraulic cylinders, the barrel and the bottom portion are welded together. This can damage the inside of the barrel if done poorly. Therefore, some cylinder designs have a screwed or flanged connection from the cylinder end cap to the barrel. (See "Tie rod cylinder", below) In this type the barrel can be disassembled and repaired.

Cylinder head

The cylinder head is sometimes connected to the barrel with a sort of a simple lock (for simple cylinders). In general, however, the connection is screwed or flanged. Flange connections are the best, but also the most expensive. A flange has to be welded to the pipe before machining. The advantage is that the connection is bolted and always simple to remove. For larger cylinder sizes, the disconnection of a screw with a diameter of 300 to 600 mm is a huge problem as well as the alignment during mounting.

Piston

The piston is a short, cylindrical metal component that separates the two parts of the cylinder barrel internally. The piston is usually machined with grooves to fit elastomeric or metal seals. These seals are often O-rings, U-cups or cast iron rings. They prevent the pressurized hydraulic oil from passing by the piston to the chamber on the opposite side. This difference in pressure between the two sides of the piston causes the cylinder to extend and retract. Piston seals vary in design and material according to the pressure and temperature requirements that the cylinder will see in service. Generally speaking, elastomeric seals made from nitrile rubber or other materials are best in lower temperature environments, while seals made of Viton are better for higher temperatures. The best seals for high temperature are cast iron piston rings.

Piston rod

The piston rod is typically a hard chrome-plated piece of cold-rolled steel which attaches to the piston and extends from the cylinder through the rod-end head. In double rod-end cylinders, the actuator has a rod extending from both sides of the piston and out both ends of the barrel. The piston rod connects the hydraulic actuator to the machine component doing the work. This connection can be in the form of a machine thread or a mounting attachment, such as a rod-clevis or rod-eye. These mounting attachments can be threaded or welded to the piston rod or, in some cases, they are a machined part of the rod-end.

Rod gland

The cylinder head is fitted with seals to prevent the pressurized oil from leaking past the interface between the rod and the head. This area is called the rod gland. It often has another seal called a rod wiper which prevents contaminants from entering the cylinder when the extended rod retracts back into the cylinder. The rod gland also has a rod wear ring. This wear ring acts as a linear bearing to support the weight of the piston rod and guides it as it passes back and forth through the rod gland. In some cases, especially in small hydraulic cylinders, the rod gland and the rod wear ring are made from a single integral machined part.

Other parts

• Cylinder base connection
• Seals
• Cushions 

CYLINDER RATINGS:- 


 The rating of a cylinder is designated by the size and pressure capability of the cylinder. 
 The principal size features are
1.piston diameter.(bore diameter of cylinder)
2.piston rod diameter.
3.stroke length. 
The pressure capability is mentioned on the cylinder nameplate by the manufacturer .

to be continued......

HYDRAULIC RESERVOIR DESIGN

figure-1(a typical reservoir cut away view)

Introduction:- 

The main function of hydraulic reservoir in hydraulic system is to store and supply the hydraulic fluid to be used in the system.Hydraulic systems need a finite amount of liquid fluid that must be stored and reused continually as the circuit works, therefore, part of any hydraulic circuit is a storage reservoir or tank.

Reservoir design and implementation is very important, the efficiency of a well-designed hydraulic circuit can be greatly reduced by poor tank design.

Functions of Hydraulic reservoir:- 

1. Storage of hydraulic fluid.

2. Acts as a heat exchanger.

3. Acts as a De-aeration system.

4. Can be used to support the hydraulic components to save  floor space.

5. Acts As a fluid conditioner.

Components of Hydraulic reservoir:- 

A typical industrial reservoir is made up of steel sheet metal fabricated to a definite shape according to requirement.In case the reservoir is used in any mobile equipment like mobile cranes or material handling equipments the the reservoir sizing and weight is very critical.

A reservoir has different components and accessories(see above picture).

1. Reservoir(sheet metal fabricated structure to suit particular requirement)

2. Strainer

3. Drain plug

4. Baffle plates 

5. Suction pipes (towards the pump)

6. Return line pipe.(from all the return line manifolds)

7. Return line filter

8. Breather

9. Riser.

10.Man hole or clean out plates(for cleaning the reservoir) 

11.Filler cap.

12.Oil level gauge.

13.Temperature gauge.

14.Hydraulic oil cooler (it is not an integral part of reservoir)

15.Oil heater (in case of cold countries)

All the above components are very essential for a good hydraulic tank design as every component has its specific function.

Strainer:-

It is fitted to the suction line inlet.It provides the filtered fluid to the pump.It normally contains some filtering screens (for particular size particulates) which filters the contaminants before it goes to the pump.It is replaced time to time to ensure blockage and reduced flow.

Drain plug:-
It is normally fitted at the bottom of the tank to drain away the fluid while overhauling or

cleaning.The drain plug size should not be so large or so small.one or more drain plugs can be provided depending on requirement.Normally drain plugs are fitted to the welded flanges.Most important factor is to choose proper fittings for the drain plug to avoid leakage issues.

Baffle plates:-

baffle plates are normally the steel plates which are incorporated in the reservoir to divide the fluid in to different chambers so that the fluid has to travel through other ways to get to the suction chamber.Baffle plates divide the reservoir into to chambers return flow chamber and suction chamber.From the suction chamber pump suction lines start. The return line flows are  put into the return chamber so that the return flow has to flow certain distance before it reaches the suction line and in the way get cooled and contaminants get to the bottom of reservoir.No through holes are to be in the baffle, we want the oil flow to go over the top of the baffle.  A small ¼” gap at each end of the baffle allows oil levels to remain equal on each side of the baffle, (majorly of flow goes over the top).

Suction pipe:-

Suction pipes generally have larger diameter as compared to return lines.Suction pipes have smaller length so as to facilitate lesser loss and cavitation.

Return pipe:-

Return pipe comes directly from the return line manifold or the return line filter if provided.The return line end should not be too high from the bottom of the reservoir as too high outlet will cause turbulence to the output flow.Normally the return line ends are taper cut to facilitate the flow direction towards the wall side.This will help the fluid to travel larger distance so that it gets more time to cool,purify and de-aeration.

Breather:-

 The breather cap of the correct size should include a filter media to block contaminants as the fluid level lowers and rises during a cycle. It is important to understand that too small a breather cap will cause a vacuum in the oil tank and cause cavitations in the pumps.  You should never use a filler-breather type cap, as any opening where someone can pour contaminated fluid into the tank should be avoided.  Always provide a quick disconnect just before the return filter used for adding or filling the reservoir, that way all fluid going into the tank is filtered.

 Filler cap:-

 The filler opening is often a part of breather assembly.The opening has removable screen that keeps  contaminants out of the tank while fluid is being added to the reservoir.A cap that would provide tight seal should be chained to the reservoir.
 Oil level gauge:-
 To check the fluid level of the reservoir without opening it and preventing it from contaminants oil level gauge is used.
 

These notes are just basic simple suggestions and are not intended to be detailed design notes in designing reservoirs.  There are many other factors and design notes required to make a good reservoir, and are too many to outline is the forum.  A good reservoir design is the least expensive to build, but the most critical component of any hydraulic system.

Heat dissipation by the wall surface of reservoir:-

 A baffle separates the return line from the pump inlet line, forcing the fluid to take the longest possible path through the reservoir before returning to the pump inlet. This arrangement also mixes the fluid well and provides more time to drop contaminates and de-aerate. In addition, the fluid spends more time in contact with the outer walls of the reservoir to dissipate heat.
A formula for estimating how much heat a reservoir can dissipate is as follows:

HP = 0.001 x (Tf – Ta) x A
HP = maximum HP tank can dissipate.
Tf = maximum fluid temperature as °F.
Ta = maximum ambient air temperature as °F.
A = tank area as sq. ft. in contact with fluid.

Reservoir sizing :-
The main reason the reservoir exists is to store fluid.The accepted general rule for sizing a tank is; the tank volume should be two to four times the pump flow in gpm. This is only a general rule, and some circuits may require more volume, while less fluid may be adequate for other circuits.

With this general rule, the returned fluid theoretically will have two to three minutes in the tank before it circulates again.  The application really determines the reservoir size, for example, in very high ambient temperatures the reservoir should be oversized if possible even with an oil cooler.  It requires time to dissipate heat, allow contamination and air to escape the fluid, and the longer you can give the fluid time to rest before it circulates again the better.

On mobile equipment where space and weight are a premium, the reservoir still must provide all the basic functions, just at a lower level.  More external cooling and filtration is required in most mobile applications to make up what the oil tank could do if sized larger.

The reservoir level during operation should raise and lower no more than is necessary, about 6”-8”.  The more the fluid level changes, the more air is required to enter and leave the reservoir.  Along with this air exchange, comes moisture which produces condensation (water) in the fluid. There also should be about a 4”-6” air pocket above the oil level at all times and sufficient level above the pump from falling below the inlet and to prevent a vortex from developing above the pump inlet allowing air to enter the system.

Another situation where a tank may need to be larger is if the circuit has accumulators. Accumulators need fluid to fill them at start up and space into which to discharge this fluid at shut down. An undersized reservoir may not have enough fluid to keep the pump inlet covered at all times.

HYDRAULIC FLUIDS

Hydraulic fluid is the medium for power transmission in a hydraulic system.The life and performance of the hydraulic components depends on the kind of hydraulic fluid used in the system.The selection of hydraulic fluid is very important in any hydraulic system design.The main purpose of hydraulic fluid in the hydraulic system is  transmission of power but along with that it serves many purpose like lubrication,sealing and heat transfer medium.

PURPOSE OF HYDRAULIC FLUID 

POWER TRANSMISSION :-  As hydraulic fluids  are slightly compressible,whenever force is applied the response to the force is instantaneous.For easy transmission fluid must flow easily through the passage.Too much resistance to the flow creates the power loss.


LUBRICATION :-  Lubrication to the internal components is provided by the hydraulic fluid.So anti wear additives are added to hydraulic fluids which increases the service life of the components.


SEALING :-  Fluid acts as the sealing agent in the high pressure components like pump cylinder etc. because it has a viscous characteristics which creates a film in the leakage area.But high viscosity of the fluid is not recommended.close mechanical fit and viscosity of fluid determines the leakage rate.


COOLING :- Fluid helps in heat transfer as it flows heat through it .

CLEANING :- Hydraulic fluid helps in cleaning the system by flowing the dust and dirt or the system created particles outside the system.When these particles flow in the system either they settle at the bottom of the reservoir by gravity or they get cleaned through the return line filter.

CHARACTERISTICS OF HYDRAULIC FLUIDS
The demands placed on hydraulic systems change as industry requires greater efficiency and speed at higher operating temperatures and pressures.

Selecting the best Hydraulic fluid requires a basic understanding of each particular fluid's characteristics in comparison with an ideal fluid.

An ideal Hydraulic fluid would have these characteristics.

1.Thermal stability. 

2.Hydrological stability.

3.Low chemical corrosiveness.

4.High anti wear characteristics.

5.Low tendency to cavitation.

6.Long life

7.Total water rejection

8.Constant viscosity regardless temperature. 

9.low cost.

Although no single fluid has all the characteristics but the selection requires the knowledge of the system in which it will be used.

like.

1. Maximum and minimum operating and ambient temperature.

2. Type of pump used.

3. Operating pressure

4. Operating cycles

5. Loads encountered by various components

6. Types of control and power valves. 

The table below lists the major functions of a hydraulic fluid and the properties of a fluid that affect its ability to perform that function

Function	Property
Medium for power transfer and control	Low compressibility (high bulk modulus) Fast air release Low foaming tendency Low volatility
Medium for heat transfer	Good thermal capacity and conductivity
Sealing Medium	Adequate viscosity and viscosity index. Shear stability
Lubricant	Viscosity for film maintenance Low temperature fluidity Thermal and oxidative stability Hydrolytic stability / water tolerance Cleanliness and filterability Demulsibility Antiwear characteristics Corrosion control
Pump efficiency	Proper viscosity to minimize internal leakage High viscosity index
Special function	Fire resistance Friction modifications Radiation resistance
Environmental impact	Low toxicity when new or decomposed Biodegradability
Functioning life	Material compatibility

 VISCOSITY :-

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms (and for fluids only), viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity. Put simply, the less viscous the fluid is, the greater its ease of movement (fluidity).

VISCOSITY IS A COMPROMISE FOR A HYDRAULIC FLUID AS HIGHER VISCOSITY CAUSES THE RESISTANCE TO FLOW AND LOWER VISCOSITY CAUSES THE WEAR AND TEAR TO THE COMPONENTS.

TOO HIGH VISCOSITY INCREASES FRICTION RESULTING IN:-

1.HIGH RESISTANCE TO FLOW.

2.INCREASED POWER CONSUMPTION DUE TO FRICTIONAL LOSS.

3.HIGHER TEMPERATURE CAUSED BY FRICTION

4.HIGHER PRESSURE DROP DUE TO RESISTANCE.

5.SLOW OPERATION.

6.DIFFICULTY IN SEPARATING AIR FROM OIL IN RESERVOIR.

TOO LOW VISCOSITY RESULTS IN :-

1.INTERNAL LEAKAGE

2.EXCESSIVE WEAR OF INTERNAL COMPONENTS DUE TO BREAKING OF FLUID FILM BETWEEN THE MATING PARTS.

3.PUMP EFFICIENCY MAY DECREASE DUE TO SLOW OPERATION OF ACTUATOR.

4.LEAKAGE LOSSES MAY INCREASE THE TEMPERATURE.

 

PRINCIPLES OF FLOW & PRESSURE IN HYDRAULICS

In the previous post i had explained you about the difference between hydrodynamics and hydrostatics.Hydrostatics is nothing but hydraulics in common language.Pressure and flow are two big terms in hydraulics.In this post i will explain pressure & flow in details.

Pressure explained :

Basically pressure is created in a hydraulic system when there is a resistance to the flow in the system.If there is a load or force which is opposing the free flow of fluid in the system(friction if neglected) then only pressure is created i.e equal to the force divided by the area perpendicular to the opposing force.


pressure = force / perpendicular area in contact with force.


if there is no opposition to the flow then pressure is equal to zero.

pressure head comes from the weight of the fluid.
e.g. A column of air of one square inch as high as atmosphere gives a pressure of one atmosphere at see level i.e 76 mm of mercury or 14.7 psi. 


a pressure gauge is used to measure the pressure but it shows the gauge pressure which is the difference between absolute pressure minus the atmospheric pressure.

gauge pressure =absolute pressure - atmospheric pressure 

psig  = psia - 14.7

* Liquid always seeks a level depending on pressure.

*according to Bernoulli's theorem the sum of pressure energy kinetic energy and the potential energy is constant for a system.

*If the fluid levels are same then the potential energy will be cancelled out hence we have kinetic energy inversely proportional to the pressure energy.

principles of flow :-

Flow is an action in the hydraulic system that gives the actuator its motion.Pressure gives the actuator it's force but flow is essential for the movement.The flow is created by pump.

There are two ways to measure flow rate one is velocity of flow ( feet per second ) and flow rate (liter per minute ).

velocity is the average speed of fluid particles past a particular point in unit time.Flow rate can be defined as the volume of fluid passing a particular point in unit time.

large volumes are measured in GPM (gallons per minute) or LPM (liters per minute) and small flow rates are measured in CC per min.

1 gpm = 231 inch^3 per minute

1 lpm = 1000 cc^3 per minute.

whenever a liquid is flowing then there must be an unbalanced force to cause motion.When a fluid flows in a streamlined path in a straight path then also there is a pressure difference between the start point and downstream.That pressure drop is due to friction and friction causes the pressure drop causing the flow of fluid in the pipe.