TRANSPORTASI FLUIDA Dr. Ir. Ahmad Rifandi, M.Sc. Cert. IV Teknik Kimia - POLBAN
Dec 29, 2015
TRANSPORTASI FLUIDA
Dr. Ir. Ahmad Rifandi, M.Sc. Cert. IV
Teknik Kimia - POLBAN
PENGERTIAN-PENGERTIAN DASAR
Properties of Fluids
A fluid is any substance which flows because its particles are not rigidly
attached to one another. This includes liquids, gases and even some
materials which are normally considered solids, such as glass.
Essentially, fluids are materials which have no repeating crystalline
structure.
Buoyancy is defined as the tendency of a body to float or rise when
submerged in a fluid. We all have had numerous opportunities of observing the
buoyant effects of a liquid. When we go swimming, our bodies are held up
almost entirely by the water.
Compressibility is the measure of the change in volume a substance undergoes
when a pressure is exerted on the substance. Liquids are generally considered to
be incompressible. For instance, a pressure of 16,400 psig will cause a given
volume of water to decrease by only 5% from its volume at atmospheric pressure.
Gases on the other hand, are very compressible. The volume of a gas can be
readily changed by exerting an external pressure on the gas
PENGERTIAN-PENGERTIAN DASAR
Relationship Between Depth and Pressure
Anyone who dives under the surface of the water notices that the pressure on his
eardrums at a depth of even a few feet is noticeably greater than atmospheric
pressure. Careful measurements show that the pressure of a liquid is directly
proportional to the depth, and for a given depth the liquid exerts the same
pressure in all directions
As shown in Figure 1 the pressure at
different levels in the tank varies and
this causes the fluid to leave the tank
at varying velocities. Pressure was
defined to be force per unit area. In
the case of this tank, the force is due
to the weight of the water above the
point where the pressure is being
determined
PENGERTIAN-PENGERTIAN DASAR
PENGERTIAN-PENGERTIAN DASAR
CONTINUITY EQUATION
PENGERTIAN-PENGERTIAN DASAR
Volumetric Flow Rate
The volumetric flow rate ( V ) of a system is a measure of the volume of luid
passing a point in the system per unit time.
The volumetric flow rate can be calculated as the product of the rcosssectional
area (A) for flow and the average flow velocity (v).
Example: A pipe with an inner diameter of 4 inches contains water that
flows at an average velocity of 14 feet per second. Calculate
the volumetric flow rate of water in the pipe.
Solution: Use Equation 3-1 and substitute for the area.
PENGERTIAN-PENGERTIAN DASAR
Mass Flow Rate
Example:
Solution:
The mass flow rate (m˙ ) of a system is a measure of the mass of fluid passing
a point in the system per unit time. The mass flow rate is related to the
volumetric flow rate as shown in Equation 3-2 where r is the density of the fluid.
The water in the pipe of the previous example had a density
of 62.44 lbm/ft3. Calculate the mass flow rate
Conservation of Mass
In thermodynamics, energy can neither be created nor destroyed, only
changed in form.
The same is true for mass. Conservation of mass is a principle of
engineering that states that all mass flow rates into a control volume are
equal to all mass flow rates out of the control volume plus the rate of
change of mass within the control volume
CONTINUITY EQUATION
The inlet diameter of the reactor
coolant pump shown in Figure 3 is 28
in. while the outlet flow through the
pump is 9200 lbm/sec. The density of
the water is 49 lbm/ft3.
What is the velocity at the pump inlet?
Example:
Solution:
Example:
A piping system has a "Y" configuration
for separating the flow as shown in Figure
4. The diameter of the inlet leg is 12 in.,
and the diameters of the outlet legs are 8
and 10 in. The velocity in the 10 in. leg is
10 ft/sec. The flow through the main
portion is 500 lbm/sec. The density of
water is 62.4 lbm/ft3. What is the velocity
out of the 8 in. pipe section?
Flow Regimes
All fluid flow is classified into one of two broad categories or regimes.
These two flow regimes are laminar flow and turbulent flow.
Flow Velocity Profiles
Not all fluid particles
travel at the same
velocity within a pipe.
The shape of the
velocity curve (the
velocity profile across
any given section of the
pipe) depends upon
whether the flow is
laminar or turbulent.
For practical purposes, if the Reynolds number is less than 2000, the flow
is laminar. If it is greater than 3500, the flow is turbulent. Flows with
Reynolds numbers between 2000 and 3500 are sometimes referred to as
transitional flows
An ideal fluid is one that is incompressible and has no viscosity. Ideal fluids
do not actually exist, but sometimes it is useful to consider what would
happen to an ideal fluid in a particular fluid flow problem in order to simplify
the problem.
Ideal Fluid
The flow regime (either laminar or turbulent) is determined by evaluating the
Reynolds number of the flow (refer to figure 5). The Reynolds number,
based on studies of Osborn Reynolds, is a dimensionless number
comprised of the physical characteristics of the flow. Equation 3-7 is used to
calculate the Reynolds number (NR) for fluid flow.
Reynolds Number
The conservation of energy principle states that energy can be neither created
nor destroyed. This is equivalent to the First Law of Thermodynamics, which
was used to develop the general energy equation in the module on
thermodynamics. Equation 3-8 is a statement of the general energy equation
for an open system.
General Energy Equation
Bernoulli’s equation results from the application of the general energy
equation and the first law of thermodynamics to a steady flow system in
which no work is done on or by the fluid, no heat is transferred to or from
the fluid, and no change occurs in the internal energy (i.e., no temperature
change) of the fluid. Under these conditions, the general energy equation
is simplified to Equation 3-9.
Persamaan Bernoulli Sederhana
Energi dalam sistem tertutup terdiri atas:
“pressure”, “motion”, dan “position”
Persamaan Bernoulli Sederhana ……lanjutan
pressuremotion
position
Apabila “motion” diubah tetapi “position” tidak berubah maka “pressure”
akan berubah tetapi jumlah energi dalam sistem tetap tidak berubah.
Contoh: apabila kita menekan ujung selang plastik ketika kita menyiram
kebun maka tekanan air akan berubah dan air akan memancar lebih kuat
dari ujung selang karet.
Multiplying all terms in Equation 3-10 by the factor gc/mg results in the
form of Bernoulli’s equation shown by Equation 3-11
The Bernoulli equation can be modified to take into account gains and
losses of head. The resulting equation, referred to as the Extended Bernoulli
equation, is very useful in solving most fluid flow problems
Contoh Soal
Water is pumped from a large reservoir to a point 65 feet higher than the
reservoir. How many feet of head must be added by the pump if 8000
lbm/hr flows through a 6-inch pipe and the frictional head loss is 2 feet? The
density of the fluid is 62.4 lbm/ft3, and the cross-sectional area of a 6-inch
pipe is 0.2006 ft2.
Penyelesaian
To use the modified form of Bernoulli’s equation, reference points are
chosen at the surface of the reservoir (point 1) and at the outlet of the pipe
(point 2). The pressure at the surface of the reservoir is the same as the
pressure at the exit of the pipe, i.e., atmospheric pressure. The velocity at
point 1 will be essentially zero.
Using the equation for the mass flow rate to determine the velocity at point 2:
Now we can use the Extended Bernoulli equation to determine the
required pump head.
Head Losses
The head loss that occurs in pipes is dependent on the flow velocity,
pipe length and diameter, and a friction factor based on the roughness
of the pipe and the Reynolds number of the flow. The head loss that
occurs in the components of a flow path can be correlated to a piping
length that would cause an equivalent head loss.
Head loss is a measure of the reduction in the total head (sum of elevation
head, velocity head and pressure head) of the fluid as it moves through a
fluid system
Frictional loss is that part of the total head loss that occurs as the fluid flows
through straight pipes. The head loss for fluid flow is directly proportional to
the length of pipe, the square of the fluid velocity, and a term accounting for
fluid friction called the friction factor. The head loss is inversely proportional to
the diameter of the pipe.
Friction Factor
The friction factor has been determined to depend on the Reynolds number
for the flow and the degree of roughness of the pipe’s inner surface
The quantity used to measure the roughness of the pipe is called the relative
roughness, which equals the average height of surface irregularities (e)
divided by the pipe diameter (D).
The value of the friction factor is usually obtained from the Moody Chart (Figure
B-1 of Appendix B). The Moody Chart can be used to determine the friction
factor based on the Reynolds number and the relative roughness.
Determine the friction factor (f) for fluid flow in a pipe that has a Reynolds
number of 40,000 and a relative roughness of 0.01
Using the Moody Chart, a Reynolds number of 40,000 intersects the curve
corresponding to a relative roughness of 0.01 at a friction factor of 0.04.
Contoh Soal
Penyelesaian
Moody Chart
Moody Chart
The frictional head loss can be calculated using a mathematical relationship
that is known as Darcy’s equation for head loss. The equation takes two
distinct forms. The first form of Darcy’s equation determines the losses in the
system associated with the length of the pipe.
Persamaan Darcy
A pipe 100 feet long and 20 inches in diameter contains water at 200°F
flowing at a mass flow rate of 700 lbm/sec. The water has a density of 60
lbm/ft3 and a viscosity of 1.978 x 10-7 lbf-sec/ft2. The relative roughness
of the pipe is 0.00008. Calculate the head loss for the pipe.
The sequence of steps necessary to solve this problem is first to determine
the flow velocity. Second, using the flow velocity and the fluid properties
given, calculate the Reynolds number. Third, determine the friction factor
from the Reynolds number and the relative roughness. Finally, use Darcy’s
equation to determine the head loss.
Contoh Soal
Penyelesaian
Use the Moody Chart for a
Reynolds number of 8.4 x 107
and a relative roughness of
0.00008.
Minor Losses
The losses that occur in pipelines due to bends, elbows, joints, valves, etc.
are sometimes called minor losses.
Equivalent Length
Minor losses may be expressed in terms of the equivalent length (Leq) of
pipe that would have the same head loss for the same discharge flow rate.
This relationship can be found by setting the two forms of Darcy’s equation
equal to each other
This yields two relationships that are useful
A fully-open gate valve is in a
pipe with a diameter of 10
inches. What equivalent length
of pipe would cause the same
head loss as the gate valve
From Table 1, we find that the
value of Leq/D for a fully-open
gate valve is 10.
Leq = (L/D) D
= 10 (10 inches)
= 100 inches
Contoh Soal
Penyelesaian
The main elements of a pumping system are:
PUMPING SYSTEM
Supply side (suction or inlet side)
Pump (with a driver)
Delivery side (discharge or process)
Energy input = Energy useful + Losses
Efficiency = Energy useful /Energy input
Losses = Mechanical + Volumetric + Hydraulic ⇓
bearings leakage (slip) friction
coupling entrance/exit
Rubbing vortices
separation
disc friction
⇓ ⇓
PUMPING SYSTEM
Work equals force multipled by distance
For a steady motion, the force is balanced by the pressure “p,” acting on area, “A”:
W = (p × A) × L = p × (A × L) = p × V
INPUT POWER, LOSSES, AND EFFICIENCY
Work per unit of time equals power. So, dividing both sides of the equation by “t,”
we get:“Q” is the volume per unit of time, which in
pump language is called “flow,” “capacity,”
or “delivery.”
INPUT POWER, LOSSES, AND EFFICIENCY
Ideal Power = Fluid Horsepower = FHP = p × Q × constant
since all power goes to “fluid horsepower,” in the ideal world
Pressure is measured in psi , and flow in gpm, (US units)
INPUT POWER, LOSSES, AND EFFICIENCY
Untuk pompa sentrifugal pressure biasanya diekspresikan dengan feet of head
PUMP SELECTION
Contoh Soal
LC
Hitung ukuran pipa dan spesifikasi pompa yang diperlukan untuk memompa cairan ortho-
dichlorobenzene (ODBC) dengan kecepatan aliran 10.000 kg/j, suhu 20oC dan jenis pipa
carbon steel (density ODBC = 1306 kg/m3 dan viscosity = 0,9 cp
C 201
1 bar
C 203
2 bar
HE 205
2,5
m
7,5
m
1,0
m
3,5
m
3,0
m0,5
m
20 m
2 m
1 m
4 m
Two storage tanks, A and B, containing a petroleum product, discharge rough
pipes each 0.3 m in diameter and 1.5 km long to a junction at D, as shown in
Figure 3.9. From D the liquid is passed through a 0.5 m diameter pipe to a
third storage tank C, 0.75 km away. The surface of the liquid in A is nitially 10
m above that in C and the liquid level in B is 6 rn higher than that in A.
Calculate the initial rate of discharge of liquid into tank C assuming the ipes
are of mild steel. The density and viscosity of the liquid are 870 kg/m3 and 0.7
mN s/m2 respectively.
Because the pipes are long, the kinetic energy of the fluid and minor losses at the
entry to the pipes may be neglected.
It may be assumed, as a first approximation, that R/pu2 is the same in each pipe
and that the velocities in pipes AD, BD, and DC are MI , «2. and uj espectively, if
the pressure at D is taken as PD and point D is id m above the datum for the
calculation of potential energy, the liquid level in C. Then applying the energy
balance equation between D and the liquid level in each of the tanks gives:
Contoh Soal
Hitung ukuran pipa dan spesifikasi pompa yang diperlukan untuk memompa cairan ortho-
dichlorobenzene (ODBC) dengan kecepatan aliran 10.000 kg/j, suhu 20oC dan jenis pipa
carbon steel (density ODBC = 1306 kg/m3 dan viscosity = 0,9 cp
Penyelesaian
Penyelesaian
Penyelesaian
Penyelesaian
P = 65,27 psi, Q = 32,93 gpm;
BHP = 2,09 PK= 1,5 kVA
Penyelesaian
Penyelesaian
Penyelesaian
Penyelesaian
Penyelesaian
SYSTEM CURVE
Essentially, any flow restriction requires a pressure gradient to overcome it.
These restrictions are valves, orifices, turns, and pipe friction.From the fundamentals of hydraulics based on the Bernoulli equation, a
pressure drop (i.e., hydraulic loss) is proportional to velocity head:
For the flow of liquid through a duct (such as pipe), the velocity is equal to:
which means that pressure loss is proportional to the square of flow:
From the discussion above, we have established that flow and pressure are
the two main parameters for a given application. Other parameters, such as
pump speed, fluid viscosity, specific gravity, and so on, will have an effect on
flow and/or pressure, by modifying the hydraulics of a pumping system in
which a given pump operates. A mechanism of such changes can be traced
directly to one of the components of losses, namely the hydraulic losses.
SYSTEM CURVE
Essentially, any flow restriction requires a pressure gradient to overcome it.
These restrictions are valves, orifices, turns, and pipe friction.
Hydraulic losses, as a function of flow.
PUMP CURVE
A pump curve shows a relationship between its two main parameters: flow
and pressure
The shape of this curve (see Figure) depends on the particular pump type.
Pump curves, relating pressure and flow. The slope of the centrifugal pump curve is
“mostly” flat or horizontal; the slope of the PD-pump is almost a vertical line.
PUMP CURVE
Therefore, the pump operating point is an intersection of the pump curve
and a system curve (see Figure 6). In addition to friction, a pump must
also overcome the elevation difference between fluid levels in the
discharge and suction side tanks, a so called static head, that is
independent of flow (see Figure 7). If pressure inside the tanks is not
equal to atmospheric pressure then the static head must be calculated as
equivalent difference between total static pressures (expressed in feet of
head) at the pump discharge and suction, usually referenced to the pump
centerline (see Figure 8).
The above discussion assumes that the suction and discharge piping near
the pump flanges are of the same diameter, resulting in the same
velocities. In reality, suction and discharge pipe diameters are different
(typically, a discharge pipe diameter is smaller). This results in difference
between suction and discharge velocities, and their
energies (velocity heads) must be accounted for. Therefore, a total pump
head is the difference between all three components of the discharge and
suction fluid energy per unit mass: static pressure heads, velocity heads,
and elevations
PUMP CURVE
For example,
Note that the units in Equation 16 are feet of head of water. The conversion
between pressure and head is:
PUMP CURVE
FIGURE 6
Pump operating point — intersection of a pump
and a system curves.
Note : Due to the almost vertical curve slope of
rotary pumps (b), their performance curves
are usually and historically plotted as shown on
(c) (i.e., flow vs. pressure).
PUMP CURVE
PUMP CURVE
From our high school days and basic hydraulics, we remember that
the pressure, exerted by a column of water of height, “h,” is
Where γ is a specific weight of the substance, measured in lbf/ft3
.A specific gravity (SG) is defined as a ratio of the specific weight of the
substance to the specific weight of cold water: γo = 62.4 lbf/ft 3 . (SG is
also equal to the ratio of densities, due to a gravitational constant
between the specific weight and density). So,
(To obtain pressure in more often used units of lbf/in2 (psi), divide by 144).
PUMP CURVE
Clearly, if the system resistance changes, such as an opening or a closing of
the discharge valve, or increased friction due to smaller or longer piping, the
slope of the system curve will change (see Figure 9). The operating point
moves: 1 → 2, as valve becomes “more closed,” or 1 → 3, if it opens more.
CENTRIFUGAL PUMP
Stuffing Box - A : :
Packing - B : :
Shaft - C : :
Shaft Sleeve - D : :
Vane - E : :
Casing - F : :
Eye of Impeller - G : :
Impeller - H : :
Casing wear Ring - I : :
Impeller - J : :
Discharge Nozzle - K : :
PUMP IMPELLER
EXTERNAL GEAR PUMP
INTERNAL GEAR PUMP
LOBE PUMP
1. As the lobes come out of mesh, they create expanding volume on the inlet side of the
pump. Liquid flows into the cavity and is trapped by the lobes as they rotate.
2. Liquid travels around the interior of the casing in the pockets between the lobes and
the casing -- it does not pass between the lobes.
3. Finally, the meshing of the lobes forces liquid through the outlet port under pressure.
DOUBLE SCREW PUMP
VANE PUMP
1. body
2. rotor
3. piston valve
4. spring
SINGLE SCREW PUMP
PERISTALTIC PUMP
DIAPHRAGM PUMP
FLEXIBLE IMPELLER PUMP
CHAIN PUMP
ECCENTRIC
CAM PUMP
PUMP CHARACTERISTICS
System Curves
For a specified impeller diameter and speed, a centrifugal pump has a fixed and
predictable performance curve. The point where the pump operates on its curve is
dependent upon the characteristics of the system In which it is operating, commonly
called the System Head Curve. ..or, the relationship between flow and hydraulic losses* in
a system. This representation is in a graphic form and, since friction losses vary as a
square of the flow rate, the system curve is parabolic in shape.
By plotting the system head
curve and pump curve
together, it can be
determined:
Where the pump will
operate on its curve.
What changes will occur if
the system head curve or
the pump performance
curve changes.
NO STATIC HEAD - ALL FRICTION
As the levels in the suction and discharge are the same (Fig. 1), there is no static
head and, therefore, the system curve starts at zero flow and zero head and its
shape is determined solely from pipeline losses. The point of operation is at the
intersection of the system head curve and the pump curve. The flow rate may be
reduced by throttling valve.
POSITIVE STATIC HEAD
The parabolic shape of the
system curve is again
determined by the friction
losses through the system
including all bends and valves.
But in this case there is a
positive static head involved.
This static head does not affect
the shape of the system curve
or its "steepness", but it does
dictate the head of the system
curve at zero flow rate.
The operating point is at the
intersection of the system curve
and pump curve. Again, the
flow rate can be reduced by
throttling the discharge valve.
NEGATIVE (GRAVITY) HEAD
In the illustration below, a
certain flow rate will occur by
gravity head alone. But to
obtain higher flows, a pump Is
required to overcome the pipe
friction losses in excess of "H" -
the head of the suction above
the level of the discharge. In
other words, the system curve
is plotted exactly as for any
other case involving a static
head and friction head, except
the static head is now negative.
The system curve begins at a
negative value and shows the
limited flow rate obtained by
gravity alone. More capacity
requires extra work.
MOSTLY LIFT- LITTLE FRICTION HEAD
The system head curve in the illustration below starts at the static head "H" and
zero flow. Since the friction losses are relatively small (possibly due to the large
diameter pipe), the system curve is "flat". In this case. the pump is required to
overcome the comparatively large static head before it will deliver any flow at all.
*Hydraulic losses in
piping systems are
composed of pipe
friction losses, valves,
elbows and other
fittings, entrance and
exit losse (these to the
entrance and exit to
and from the pipeline
normally at the
beginning and end not
the pump) and losses
from changes in pipe
size by enlargement
or reduction in
diameter.
CHARACTERISTIC CURVE OF PUMP
How to Read Pump Curves
STEP 1: The basic pump curves are no different than reading any other
head - flow curve. For a known head value, follow the head over to the
pump curve then drop down to the capacity axis and this will be the flow
rate. What you are trying to figure out here is what diameter impeller is
needed to get the required head and capacity.
STEP 2: The next thing to figure out is what motor is needed to drive
this impeller without overloading. To do this use the dashed horsepower
lines. To the right of the horsepower line is overloading and to the left is
non-overloading.
STEP 3: The last thing to determine is at what pump efficiency the pump
will operate. Look at the U-shaped lines and interpolate to get the
efficiency.
Now let's try an example using ZM1570, Performance Data for Models 6650-6671
(5-15 BHp 4" discharge units). For the example we will size a pump for 400 GPM
at 54 feet of total dynamic head.
STEP 1: Locate the point of 400 GPM at 54 feet on the pump curve. This
point is slightly above the 8.31" impeller but well below the 8.63"
impeller so I would go with an 8.38" impeller to hit the duty point.
STEP 2:
Next, draw a new pump curve that passes through the duty point and is
parallel to the existing pump curves. This will give you a close
representation of the actual performance the pump will deliver. Look to
see where this curve crosses the horsepower line to the right of the
design point.
In this example the pump curve crosses the 10 BHp curve at about 48
feet and crosses the 12.5 BHp curve at about 21 feet. We will not oversize
an impeller on a pump if the overload point on the pump curve is greater
than the static head for the system.
So for this example, if the static head is greater than 48 feet then we can
use the 10 BHp unit. If the static head is between 21 feet and 48 feet, use
the 12.5 BHp motor. If the static head is less than 21 feet then use the 15
BHp motor.
STEP 3:
Now let's figure the pump efficiency we can expect. The design point
is about half way in between the efficiency lines of 60% and 63%. So,
for the design point of 400 GPM at 54 feet, we would expect about
61.5% pump efficiency.
As you can tell from the above example, we would consider oversizing
an impeller on a unit and not overload the unit due to engineering the
right pump for the system. If this were the case we would also able to
provide a more competitively priced unit since pricing is based on
motor size (i.e. smaller motors cost less).
The only exception to this rule is a single-phase unit. ZOELLER
COMPANY DOES NOT SELL SINGLE-PHASE UNITS WITH OVERSIZED
IMPELLERS because we feel that this will compromise the life of a
single-phase unit.
The Total Dynamic Head (TDH) is the sum of the total static head, the total
friction head and the pressure head. The components of the total static head for
a surface water and well wate pumping system are shown
TOTAL DYNAMIC HEAD
Total Static Head
The total static head is the total vertical distance the pump must lift the water. When pumping from a
well, it would be the distance from the pumping water level in the well to the ground surface plus the
vertical distance the water is lifted from the ground surface to the discharge point. When pumping
from an open water surface it would be the total vertical distance from the water surface to the
discharge point.
Pressure Head
Sprinkler and drip irrigation systems require pressure to operate. Center pivot systems require a
certain pressure at the pivot point to distribute the water properly. The pressure head at any point
where a pressure gage is located can be converted from pounds per square inch (PSI) to feet of
head by multiplying by 2.31. For example, 20 PSI is equal to 20 times 2.31 or 46.2 feet of head.
Friction Head
Friction head is the energy loss or pressure decrease due to friction when water flows through
pipe networks. The velocity of the water has a significant effect on friction loss. Loss of head due
to friction occurs when water flows through straight pipe sections, fittings, valves, around
corners, and where pipes increase or decrease in size. Values for these losses can be
calculated or obtained from friction loss tables. The friction head for a piping system is the sum
of all the friction losses
Velocity Head
Velocity head is the energy of the water due to its velocity. This is a very small amount of energy
and is usually negligible when computing losses in an irrigation system
Determining Flow and Head
The pump is installed and running, but how do you know if it is
operating at its design point? There is a simple way to check. Knowing
that a pump will provide a certain flow at a given head, we can
determine the point at which the pump is operating. To determine the
head, a few gage readings will be necessary. Take one reading from
the suction of the pump and one from the discharge after the system is
balanced and with all the control valves wide open. The difference
between the two gage readings will give you the head that the pump is
providing. Remember to convert your gage readings to feet of head.
Knowing the head and the impeller size, you can determine the flow of
the pump.
Now that we have the flow and head of the pump, let’s see how close
we are to the design point. Most often, the head will be less than what
we expected, and the flow will be more. Why does this happen? There
are many reasons, but it does no good to blame anyone. Let’s just fix
the problem.
Solutions
Trimming the impeller is one of best solutions. Before we can trim the impeller,
we need to determine where the pump is operating. In the pump curve above,
let’s call point “D” the design point, and draw the system curve that corresponds
with that design point. Point “A” is where we actually are, which we determined
from our gage readings. Along with that is our actual system curve. Remember
that we are concerned with the actual system curve. This shows us how our
system operates, not how it was designed. Operational and design points are
often completely different.
We would like to be on the unmodified actual system curve, but where on that
curve? If our load has not changed and our heat transfer is the same, we want to
be at our design flow. That is“I,” the ideal point.
Trimming the Impeller
But how do we get there? Although it’s off our impeller curve, we can trim our
impeller down to the right size. In this particular case, our ideal impeller size falls
between 10-1/2” and 11-1/2 (actually about 11”). Fortunately, trimming an
impeller is not too difficult or expensive, and in fact it pays for itself very quickly.
Notice from the figure that when we trim our impeller we lose some pump
efficiency, but we’re more concerned about the cost of operating our pump and
that cost has dropped tremendously. In this case we have dropped from 85Hp to
40 Hp-that’s a lot. Even if your electric rates are low and you don’t operate all
year long, there is still the potential for great energy savings.
PB = Barometric pressure in feet absolute.
VP = Vapor pressure of the liquid at maximum pumping temperature, in feet absolute.
P = Pressure on surface of liquid in closed suction tank, in feet absolute.
Ls = Maximum static suction lift in feet.
LH = Minimum static suction head in feet.
hf = Friction loss in feet in suction pipe at required capacity
NET POSITIVE
SUCTION HEAD
The NPSH available in a
flooded suction system is:
Atmospheric Pressure (- )
Vapor Pressure (+) Liquid
Height (-) Friction in the
Suction Line
The NPSH available in a suction
lift system is:
Atmospheric Pressure (-) Vapor
Pressure (-) Liquid Ht. (-) Friction
in the Suction Line.
Net Positive Suction Head Available (NPSHA)The net positive suction head available is a function of the pump suction system.
The Net Positive Suction Head is the absolute total suction head in feet.
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head -
vapor pressure of your product - loss in the piping, valves and fittings
Given:
Atmospheric pressure = 14.7 psi
Gage pressure =The tank is at sea level and open to atmospheric pressure.
Liquid level above pump centerline = 5 feet
Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
Pumping =100 gpm. 68°F. fresh water with a specific gravity of one (1).
Vapor pressure of 68°F. Water = 0.27 psia from the vapor chart.
Specific gravity = 1
NPSHR (net positive suction head required, from the pump curve) = 9 feet
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head - vapor pressure of your
product - loss in the piping, valves and fittings
Static head = 5 feet
Atmospheric pressure = pressure x 2.31/sg. = 14.7 x 2.31/1 = 34 feet absolute
Gage pressure = 0
Vapor pressure of 68°F. water converted to head = pressure x 2.31/sg = 0.27 x 2.31/1 = 0.62 feet
Looking at the friction charts:
100 gpm flowing through 2 inch pipe shows a loss of 17.4 feet for each 100 feet of pipe or 17.4/10 = 1.74 feet
of head loss in the piping
The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
Adding these numbers together, 1.74 + 0.6 = a total of 2.34 feet friction loss in the pipe and fitting.
NPSHA (net positive suction head available) = 34 + 5 + 0 - 0.62 - 2.34 = 36.04 feet
The pump required 9 feet of head at 100 gpm. And we have 36.04 feet so we have plenty to spare.
Given:
Gage pressure = - 20 inches of vacuum
Atmospheic pressure = 14.7 psi
Liquid level above pump centerline = 5 feet
Piping = a total of 10 feet of 2 inch pipe plus one 90° long radius screwed elbow.
Pumping = 100 gpm. 68°F fresh water with a specific gravity of one (1).
Vapor pressure of 68°F water = 0.27 psia from the vapor chart.
NPSHR (net positive suction head required) = 9 feet
Now for the calculations:
NPSHA = Atmospheric pressure(converted to head) + static head + surface pressure head
- vapor pressure of your product - loss in the piping, valves and fittings
Atmospheric pressure = 14.7 psi x 2.31/sg. =34 feet
Static head = 5 feet
Gage pessure pressure = 20 inches of vacuum converted to head
inches of mercury x 1.133 / specific gravity = feet of liquid
-20 x 1.133 /1 = -22.7 feet of pressure head absolute
Vapor pressure of 68°F water = pressure x 2.31/sg. = 0.27 x 2.31/1 = 0.62 feet
Looking at the friction charts:
100 gpm flowing through 2.5 inch pipe shows a loss of 17.4 feet or each 100 feet of
pipe or 17.4/10 = 1.74 feet loss in the piping
The K factor for one 2 inch elbow is 0.4 x 1.42 = 0.6 feet
Adding these two numbers together: (1.74 + 0.6) = a total of 2.34 feet friction loss in the
pipe and fitting.
NPSHA (net positive suction head available) = 34 + 5 - 22.7 - 0.62 - 2.34 = 13.34 feet.
This is enough to stop cavitation also.
Where
N = Pump speed RPM
Q = GPM = Pump flow at best efficiency point at impeller inlet
(for double suction impellers divide total pump flow by two).
hsv = NPSHR = Pump NPSH required at best efficiency point.
Suction specific speed (S or NS) is defined as:
SPECIFIC HEAD
CAVITATION
Discharge Cavitation
Discharge Cavitation occurs when the pump discharge is extremely high. It normally occurs
in a pump that is running at less than 10% of its best efficiency point. The high discharge
pressure causes the majority of the fluid to circulate inside the pump instead of being allowed
to flow out the discharge. As the liquid flows around the impeller it must pass through the
small clearance between the impeller and the pump cutwater at extremely high velocity. This
velocity causes a vacuum to develop at the cutwater similar to what occurs in a venturi and
turns the liquid into a vapor. A pump that has been operating under these conditions shows
premature wear of the impeller vane tips and the pump cutwater. In addition due to the high
pressure condition premature failure of the pump mechanical seal and bearings can be
expected and under extreme conditions will break the impeller shaft.
Suction Cavitation
Suction Cavitation occurs when the pump suction is under a low pressure/high vacuum
condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is
carried over to the discharge side of the pump where it no longer sees vacuum and is
compressed back into a liquid by the discharge pressure. This imploding action occurs
violently and attacks the face of the impeller. An impeller that has been operating under a
suction cavitation condition has large chunks of material removed from its face causing
premature failure of the pump.
BHP = Flow(GPM) X TDH(FT) x SG /3960xEFFICIENCY(%)
Example: BHP = (100 GPM) x (95 Ft) x (1.0) / 3960 x .6
BHP = 4.0
BRAKE HORSE POWER
Horsepower at the output shaft of
an engine, turbine, or motor is
termed brake horsepower or shaft
horsepower, depending on what
kind of instrument is used to
measure it. Horsepower of
reciprocating engines, particularly
in the larger sizes, is often
expressed as indicated
horsepower, which is determined
from the pressure in the cylinders.
Brake or shaft horsepower is less
than indicated...
The affinity laws express the mathematical relationship between the several
variables involved in pump performance. They apply to all types of centrifugal
and axial flow pumps.
With impeller diameter D held constant:
With speed N held constant:
Where:
Q = Capacity, GPM
H = Total Head, Feet
BHP = Brake Horsepower
N = Pump Speed, RPM
THE AFFINITY LAWS
When the performance (Q1, H1, &
BHP1) is known at some particular
speed (N1) or diameter (D1), the
formulas can be used to estimate the
performance (Q2, H2, & BHP2) at some
other speed (N2) or diameter (D2). The
efficiency remains nearly constant for
speed changes and for small changes
in impeller diameter
Example:
To illustrate the use of these laws, refer to Fig. 8 below. It shows the performance of a particular
pump at 1750 RPM with various impeller diameters. This performance data has been determined by
actual tests by the manufacturer. Now assume that you have a 13" maximum diameter impeller, but
you want to belt drive the pump at 2000 RPM.
The affinity laws listed under 1 above will be used to determine the new performance, with N1 1750
RPM and N2 = 2000 RPM. The first step is to read the capacity, head, and horsepower at several
points on the 13" dia. curve in Fig. 9 below. For example, one point may be near the best efficiency
point where the capacity is 300 GPM, the head is 160 ft, and the BHP is approx. 20 hp.
This will then be the best efficiency point on the new 2000 RPM curve. By performing the same
calculations for several other points on the 1750 RPM curve, a new curve can be drawn which will
approximate the pump's performance at 2000 RPM, Fig. 9.
Trial and error would be required to solve this problem in reverse. In other words, assume you want
to determine the speed required to make a rating of 343 GPM at a head of 209 ft. You would begin by
selecting a trial speed and applying the affinity laws to convert the desired rating to the corresponding
rating at 1750 RPM. When you arrive at the correct speed, 2000 RPM in this case, the corresponding
1750 RPM rating will fall on the 13" diameter curve.
PUMP PERFORMANCE CURVE