1. MARINE TRAINING INSTITUTE OF ACCRE
Fore: C/E Y. YOHANA.
GENERAL ENGINEERING
QUESTIONS and ANSWERES
STEERING
STERN TUBE
SHAFT BEARING
PUMPS
Name of student: NUDLER EDUARD
Date: 24.05.2004.
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2. CENTRIFUGAL PUMPS
Q. Give a number of examples for reducing of axial pressure/force in the
centrifugal pumps.
A.
There are three types of axial pressure (force balancing):
a) The sealing device 2and1, equalizing hole 3 –connects suction and discharge
space.
b) Balanced impeller (used in multistage pumps).
c) Simple balancing disk 1 and chamber 5.
d)
Q. What is the purpose of the shafts “PROTECTING SLEEVE”?
A. Pump shafts are usually protected from
erosion, corrosion, and wear at the seal chambers, leakage joints,
internal bearings, and in the waterways by renewable sleeves. Unless
otherwise specified, a shaft sleeve of wear, corrosion, and erosion-resistant
material shall be provided to protect the shaft. The sleeve
shall be sealed at one end. The shaft sleeve assembly shall extend
beyond the outer face of the seal gland plate. (Leakage between the
shaft and the sleeve should not be confused with leakage through the
mechanical seal).
Q. What is the function of a “WEAR RING”?
A. The function of the wear ring is: Wear ring provides an easily and
economically renewable
leakage joint between the impeller and the casing.
For efficient operation it is important to ensure that leakage from the high
to low-pressure side is kept to a minimum. This is achieved by the use of
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3. wearing rings. Traditionally these are fitted to the casing, to increase the
longevity of the impeller wear ring tires may be fitted.
If the clearance becomes too large the pump efficiency will be lowered causing heat
and
vibration problems. Most manufacturers require that you disassemble the pump to
check the wear ring clearance and replace the rings when this clearance doubles.
Q. What is the function of a “DIFUSER”?
A. The function of a “ DIFUSER” is included in the casing of pump.
Circular casing have stationary diffusion vanes surrounding the impeller
periphery that convert velocity energy to pressure energy. Conventionally, the
diffusers are applied to multi-stage pumps.
The casings can be designed either as solid casings or split casings.
Solid casing implies a design in which the entire casing including the
discharge nozzle is all contained in one casting or fabricated piece.
A split casing implies two or more parts are fastened together. When the
casing parts are divided by horizontal plane, the casing is described as
horizontally split or axially split casing. When the split is in a vertical
plane perpendicular to the rotation axis, the casing is described as
Vertically split or radically split casing. Casing Wear rings act as the seal
between the casing and the impeller.
Q. What are the parameters that are responsible for the capacity and pressure in a
centrifugal pump?
A. Capacity
Capacity means the flow rate with which liquid is moved or pushed by the pump
to the desired point in the process. It is commonly measured in either gallons per
minute (GPM) or cubic meters per hour (m 3 /hr). The capacity usually changes with
the changes in operation of the process. For example, a boiler feed pump is an
application that needs a constant pressure with varying capacities to meet a
changing steam demand.
The capacity depends on a number of factors like:
Process liquid characteristics i.e. density, viscosity
Size of the pump and its inlet and outlet sections
Impeller size
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4. Impeller rotational speed RPM
Size and shape of cavities between the vanes
Pump suction and discharge temperature and pressure conditions
For a pump with a particular impeller running at a certain speed in a liquid, the
only items on the list above that can change the amount flowing through the pump
are the pressures at the pump inlet and outlet. The effect on the flow through a
pump by changing the outlet pressures is graphed on a pump curve.
As liquids are essentially incompressible, the capacity is directly related with the
velocity of flow in the suction pipe.
D
B RPM
Q. Why are centrifugal pumps not provided with safety valve?
A. A centrifugal pump does note create pressure, it only provides flow.
Pressure is just an indication of the amount of resistance to flow.
Q. How can we regulate the PRESSURE and CAPASITY of a
Centrifugal pump?
A. There are a few methods to change pressure and capacity of
Centrifugal pump:
a) By closing the flap or valve on discharge side.
b) Relieving part of the liquid flowing from discharge space to the
suction space.
c) Changing RPM of the motor (if it possible).
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5. Q. a) From what material is the WEAR RING made from?
b) What is the allowable clearance for the WEAR RING?
c) Sketch at least three wear ring types?
A. A) WEAR RING MATERIAL:
Aluminum, Bronze, Brass, Carbon- fiber, Vein forced thermoplastic
material
Stainless alloys.
b) The clearance between the impeller and the casing
Wearing ring must be a maintained as directed by the
manufacturer. When clearances exceed the specified
amount, the casing-wearing ring must be replaced.
The clearance given for wear rings is often a source of
contention especially when dealing with on-ship made rings. A
clearance of 1/1000 of the diameter of the bore is often quoted
although this may be very difficult to achieve in practice
2-STAGE SELF PRIMING P/P
Symbols Clearance in MM
On the
Drawing Manufac Overhaul Max.
ture Clearanc available
size e. clearanc
new Intermed e.
cond. iate
repair.
1-STAGE NON SELF PRIMING P/P
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6. 1-STAGE NON-SELF PRIMING P/P
Symbol Clearance in MM
Q On the
drawing
m³/h
MANUFACTURE Max. Max.
SIZE availabl availabl
NEW COND. e e
clearanc clearanc
e. e.
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7. Q. Wear ring types:
A. There are various types of wearing-ring designs, and selection of the
most desirable type depends on the liquid being handled, the pressure
differential across the leakage joint, the surface speed, and the particular
pump design. In general, centrifugal-pump designers use the ring construction
they have found suitable for each particular pump service.
The most common ring constructions are the flat-type, the L- type and U- type.
Q. What will happen when a centrifugal pump will turn in opposite direction?
Will the pump build up pressure? And what will the power gauge show?
A. When the centrifugal pump turns in the opposite direction of rotation, it
can
cause the following:
a) Reduction in capacity
b) Pressure droop
c) Increased load – high current
Q. If the clearance of the wear ring increases, will the load increase, or
decreases?
A. If the clearance becomes too large, the pump efficiency will be reduced,
causing heat and vibration problems.
When the clearance between the wear ring increases, it will cause
part of the liquids volume to pass over from the discharge
space to the suction space. It will create an internal resistance.
The pump must to wear down (overcome) the resistance of its own
transferring liquid and also resistance of the leakages. The pump
consumes more energy.
The load will be increased. It will be indicate on the Ampere Meter.
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8. Q. What types of bearings are used on a CENTRIFUGAL PUMP?
A.
;
Journal Thrust Support Support Thrust
bearing: bearing. bearing bearing Bearing
Made of Dbl. Row Single Single row- Dbl. row
bronze or Roller row-ball roller Ball bearing
rubber bearing bearing bearing
Heavy- Deep
duty Groove
angular Ball
contact bearing
ball
bearing
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9. Q. Give a number of examples for centrifugal pumps that are used on the vessel
with a NON-RETURN VALVE on the discharge line.
A. 1) Sea water cooling pump
2) F.W. central cooling pump
3) Fire Fighting and General service pump
4) Boiler feed and circulating pump
5) HT cooling pump
Q. What is the significance of checking “PRESSURE TEST”, on?
A centrifugal pump?
A. Pressure test =Head test it is measuring of KINETIC ENERGY.
Significance of using the “head” term instead of the “pressure” term
The pressure at any point in a liquid can be thought of as being caused by a
Vertical column of the liquid due to its weight. The height of this column is
called the static head and is expressed in terms of feet of liquid.
The same head term is used to measure the kinetic energy created by the
pump.
In other words, head is a measurement of the height of a liquid column that
the pump could create from the kinetic energy imparted to the liquid.
Imagine a pipe shooting a jet of water straight up into the air, the height the
water goes up would be the head.
The head is not equivalent to pressure. Head is a term that has units of a
Length or feet and pressure has units of force per unit area or pound per
Square meter.
The main reason for using head instead of pressure to measure a centrifugal
pump's energy is that the pressure from a pump will change if the specific
gravity (weight) of the liquid changes, but the head will not change.
Pressure to Head Conversion formula
The static head corresponding to any specific pressure is dependent upon the weight of
the liquid according to the following formula:
Head= Pressure (PSI) ×2.31
Specific Gravity
Q. How do we check CAPASITY of the centrifugal pump?
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10. A. There are two methods to carry out a capacity test for a
centrifugal pump.
a) Calculation by means of formula: Q (m³/h)
Q=m×v×A
m- mass of fluid in (kg)
v- velocity of flow in (m/sec)
A-area of the pipe in(m²)
OR: Q (feeet³/h)
Q=449×v×A
v- velocity of flow in (feet/sec.)
A-area of the pipe in(feet²)
b) In order to measure the capacity, we should fill the specific
volume (tank), which is calibrated by sounding table.
Time of filling must be noted and regarded to this volume.
Q. In what way can we cool a mechanical seal?
A. Where mechanical shaft seals are used, the design ensures
that positive liquid pressure is supplied to the seal faces under all
conditions of operation and that there is adequate circulation of
the liquid at the seal faces to minimize the deposit of foreign
matter on the seal parts.
a) Cooling water is supplied from the discharge side of the pump.
b) Cooling liquid is supplied from the external source:(central cooling
water system or M.E. Jacket cooling system).
Those pump usually are designed for the high temperature liquid, like
Exhaust Boiler Circulating Pump .
c) The high pressure and high temperature liquid, while it
passes through the minimum clearance between the
throttle bush and shaft sleeve, increases the pressure
reduction. The 3-gland packing is sufficient
in order to prevent the liquid from leaking out.
Since the tempo the leaking water is 80º-95ºC considerations is given to the extension
of service life by providing a cooling water jacket around the gland.
Q. What is the difference between parallel and series pumping in a
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11. Centrifugal pump? Give a few examples.
A. Series Pumping
.Pumps in series double the head at the same flow condition point
One pump discharge is piped into the suction of the
second pump producing twice the head capability of
each pump separately. The second pump however must
,be capable of operating at the higher suction pressure
which is produced by number one. This mode of operation
?is a very cost effective way of overcoming high discharge
.heads when the flow requirement remains the same
Parallel Pumping
Identical Pumps
Operating two identical pumps in parallel the assumption is made that the
flow will double.
This however is not the case. In order to calculate the additional flow realized by
running two identical pumps in parallel the following calculations must be made.
ONE P/P RUNNIG TWO P/P RUNNING
TDH FLOW(gpm) TDH FLOW(gpm)
130 100 130 200
120 190 120 380
110 280 110 560
100 360 100 720
0perating point A 0perating point B
85 470 105 660
First let's look at the pump curve and system head curve with one pump running.
Second let's look at the pump curve and system head curve with two pumps
running.
The addition of the second identical pumps produces and extra 190 GPM and does
not double the flow. Point C is the flow produced by each pump when both pumps
are running.
There are some systems, which designed to be operated with two ore more parallel
consumers, for example coolers. Entering in parallel operation with an additional
consumer (cooler), which will be accompanied by additional cooling pump.
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12. For example changing over from the harbor system to sea system.
Series operation of the centrifugal pump usually applied to MULTISTAGE
CENTRIFUGAL PUMPS, like boiler feed pump, hydrafores pump and
Some fire fighting high-pressure pumps.
Q. What forces act on the shaft of a CENTRIFUGAL PUMP?
A. R-Radial thrust which is developed in high-pressure centrifugal pump.
A-Axial force. Without careful design an axial
force is created by the action of the impeller.
This is due to the low pressure acting on the
suction eye whilst the rest of the impeller is a
subjected to discharge pressure.
R
Q. How it is possible to change the relative location of the impeller relative to the
casing in a centrifugal pump?
A. There are tree possibilities that depend on the pumps construction:
1) We can move the impeller closer to the casing by adding a distance washer under the
thrust bearing.
2) We can add packing, which will produce an additional distance between the bottoms
casing and the top casing, that holds the mechanical seal and bearing.
3) We can change the relative location of the impeller on the shaft (by adding a distance piece
between the shaft shoulder and sleeve).
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