ROLE
OF ROTODYNAMIC PUMPS IN OIL TANKER OPERATIONS
By
Engr.
Obed F. White
&
Engr.
Vince O. Ajala
ABSTRACT
Rotodynamic pumps play a key role in ensuring the reliability,
survivability and safety of oil tankers. Study of their operating reliability
is therefore of both theoretical and practical interest. This study is intended
to be expository in nature in that it will tend to reveal to the reader an
adept knowledge of rotodynamic pumps while at the same time not neglecting the
vast roles that rotodynamic pumps can play in oil tanker operations. As a
look-up into the roles of rotodynamic pumps, the reliability of turbo- and
electrically-driven pumps incorporated into cargo, condensate-feed, and cooling
systems on-board oil tankers as well as the types and functioning capabilities
of pumps of which can function in like manner will be discussed.
Centrifugal pumps as well as axial-flow pumps and mix-flow pumps (combination
of centrifugal pump and axial-flow pump) are all incorporated in oil tankers
for carrying out various activities most especially for oil suction and
discharge purposes. Encompassing a profound knowledge of these pumps as well as
their principle of operation would serve a great deal in helping the field
engineer in choosing which pump is best suited for a particular pumping
activity as well as in carrying out oil tanker operation. However it is
noteworthy to know that knowing the right pump for a given job would not only
relieve on-job cumbersomeness but would also encompass on-job safe practices.
1
INTRODUCTION
A rotodynamic pump is a kinetic
machine in which energy is continuously imparted to the pumped fluid by means
of a rotating impeller, propeller, or rotor, in contrast to a positive
displacement pump in which a fluid is moved by trapping a fixed amount of fluid
and forcing the trapped volume into the pump’s discharge. Examples of rotodynamic pumps include adding
kinetic energy to a fluid such as by using a centrifugal pump to increase fluid
velocity or pressure.
The rotodynamic pumps or dynamic
pressure pumps include the centrifugal pumps, the axial-flow pumps and the
mixed-flow pumps.
Generally, the operating principle
of the rotodynamic pumps is such that the fluid to be pumped receives a
tangential acceleration imparted as a result of the revolving speed of the
impeller in contact with the fluid. The liquid flow through the pump is induced
by centrifugal force imparted to the liquid by the rotation of the impeller.
Based on the nature of work to be
performed, rotodynamic pumps are built to various specifications in accordance
with their capabilities. Centrifugal pumps can be used for sea water
circulation duties, pumping of bilge water, and ballast water. The axial-flow
pump is ideal for condenser circulating duties especially where scoop injection
can occur due to ship rolling through the pump and condenser. Also, due to its
reversibility and high through put, the pump is suited for heeling and trimming
duties.
Worthy of note are innovations such
as the inculcation of both the centrifugal and axial-flow pump into one system
known as the mixed-flow pump. The mixed-flow type of rotodynamic pump with its mounted inducer at the pump suction
is ideal in cargo operation as it reduces the npsh requirements and thus
eliminates the need for a stripping pump during cargo oil pumping in tanker
ships.
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OIL TANKERS AND PUMPING OPERATIONS
A
tanker is a specialized ship intended for the carriage of bulk liquid cargo. An
Oil tanker again is further divided into 2 basic types, namely Crude Oil Tanker
and Product Oil Tanker.
For
both of the above the cargo of oil is carried within the tanks similar to the
holds of other ships, the difference being that the bulkheads are extra
strengthened to take in the load, and the hatch or rather the tank openings are
very small, the sole purpose of having them is for Man Entry and for small
repair work in the dry docks.
The
cargo of oil is loaded on to the ships tanks by pipelines, which are fixed on
the ship (permanent structure), the shore pipelines are connected to the ships
pipelines at the manifold on either side of the ship. Note that some special
ships also have manifolds at the bow and at the stern.
The
shore pipelines may be connected using flexible steel rimmed rubber hoses
(small ports/ Ship to ship transfers/ SBM) – the flexible come in small lengths are connected to each
other to make them long pieces.
The
shore pipelines may also be connected with rigid loading arms – also called ‘chiksons’, which are remotely controlled and
take in the roll of the ship to a certain extent but the fore and aft movement
of the ship has to be kept to a minimum.
The
combined pipeline system of the shore and the ship deliver the oil to the cargo
oil tanks directly via the drop lines. These are as the name suggests
pipelines, which drop to the bottom of the tanks vertically from the pipeline
on deck – thus bypassing the pump room. There
are various cross- over valves, which are opened in order to load a group of
tanks. The shore system starts to pump/ delivers by gravity (some Persian Gulf
ports) at a slow rate, so that any leakages can be detected and to check
whether the right tank is receiving the oil or not, once the shore and the
shipside are satisfied the pumping – loading of the cargo is increased. In case of any
subsequent leakages that are detected the ship valves should not be shut
abruptly, rather the shore has to be informed first and then only the ship
valves are to shut, this to prevent pressure surge from bursting the pipelines.
To prevent this surge from
affecting the pipelines the cargo valves have set times at which they close – this depends on the size of the
valves – typically a 550mm valve would shut
at about 24 seconds, whereas a 250mm valve would shut at 6-8 seconds.
After the ship completes her
loading the stage is set for the unloading or discharging operation.
While loading the cargo had by
passed the pump room, now however the cargo from the tanks is allowed to flow
to the pump room through the bottom pipelines. Just within the pumproom and at
the pumproom bulkhead are situated isolation valves known as ‘Bulkhead Master valves’, by opening the valves the oil is
led to the pump suction valve and on opening that the oil flows to the
centrifugal pumps. Turbines, which are situated in the Engine Room, commonly
drive these pumps; the shaft penetrates the ER bulkhead and drives the pump
situated at the bottom of the pumproom.
The pump accelerates the flow of
the oil into the discharge pipeline and this oil is thus led on the deck
pipelines and to the manifold from where it flow through the flexible pipeline
or the hard loading arm to the shore pipeline system.
However, having had a brief
knowledge of rotodynamic pumps and oil tankers, it is pertinent for elaborations
to be made on the types and functioning capabilities of rotodynamic pumps as it
would notwithstanding reveal the roles of these pumps in oil tanker operations.
ROTODYNAMIC [CENTRIFUGAL] PUMPS
TYPES AND NOMENCLATURE
Rotodynamic pumps may be classified
by such methods as impeller or casing configuration. End application of the
pump, specific speed, or mechanical configuration is based primarily on
mechanical configuration.
SCOPE
This standard covers rotodynamic
pumps with centrifugal (radial), mixed flow, and axial flow impellers, as well
as regenerative turbine and Pinot tube type pumps, of all industrial/commercial
types except vertically suspended diffuser turbine pumps. It contains
description of types, nomenclature, and definitions.
DEFINITION OF
ROTODYNAMIC (CENTRIFUGAL) PUMPS
Rotodynamic pumps are kinetic
machines in which energy is continuously imparted to the pumped fluid by means
of a rotating impeller, propeller, or rotor. The most common types of
rotodynamic pumps are centrifugal (radial), mixed flow, and axial flow pumps.
Centrifugal pumps use bladed
impellers with essentially radial outlet to transfer rotational mechanical
energy to the fluid primarily by increasing the fluid kinetic energy (angular
momentum) and also increasing potential energy (static pressure). Kinetic
energy is then converted into usable pressure energy in the discharge
collector.
TYPES OF ROTODYNAMIC PUMPS
Rotodynamic pumps are most commonly
typed by their general mechanical configuration. The broadest characteristics,
which include virtually all centrifugal pumps, are discussed in the following
paragraphs:
Overhung impeller type
In this group, the impeller(s) is
mounted on the end of a shaft that is cantilevered or “overhung” from its bearing supports.
These pumps are either close
coupled, where the impeller is mounted directly on the driver shaft; or
separately coupled, where the impeller is mounted on a separate pump shaft
supported by its own bearings. One variation of this design is the submersible
type, where a close-coupled pump/electric motor unit is designed to operate
while submerged in the liquid it is pumping or another liquid.
Close coupled
Close-coupled pumps are commonly
characterized by the following attributes:
The pump and driver share one
common shaft; the driver bearings absorb all pump thrust loads (axial and
radial).
The driver is aligned and assembled
directly to the pump unit with machined fits.
Short coupled
Pumps described as short coupled
have a coupling arrangement in which the motor is supplied with a flange
adaptor that mounts directly to the casing or body of the pump, thereby
permitting the use of a single or solidly coupled shaft. A variation of this
design is a magnetically coupled seamless pump, which uses a series of magnets
mounted directly on the motor shaft.
Short coupled pumps are commonly
characterized by the following attributes:
The pump and driver have separate
shafts; the pump has an integral bearing housing to absorb all pump thrust
loads (axial and radial). The driver is aligned and assembled directly to the
pump unit with machined fits.
Rigidly coupled
Pumps described as rigidly coupled
have their shaft rigidly coupled to the driver shaft.
Rigidly coupled pumps are commonly
characterized by the following attributes:
The pump and driver have separate
shafts connected by a rigid coupling; the pump has an internal
product-lubricated radial bearing. The driver is aligned and assembled directly
to the pump unit with machined fits. The driver bearings absorb all pump axial
thrust loads and residual radial loads.
Flexibly coupled
Pumps described as flexibly coupled
have the pump shaft flexibly coupled to the driver shaft via a flexible element
drive coupling. Usually of the spacer type.
Flexibly coupled pumps are commonly
characterized by the following attributes:
Pump and driver have separate
shafts; the pump has an integral bearing housing to absorb all pump thrust
loads (axial and radial). With this arrangement the motor may be mounted on a
support that is independent of the pump and not structurally connected to the
pump frame.
High-speed integral gear-driven pumps
High-speed integral gear-driven
single-stage overhung pumps have a speed increasing gearbox integral with the
pump. The impeller is mounted directly to the gearbox output shaft. There is no
coupling between the gearbox and pump; however, the gearbox is flexibly coupled
to its driver. These pumps may be oriented vertically or horizontally.
Integral gear-driven single-stage
overhung pumps are commonly characterized by the following attributes:
Pump, gearbox, and driver have
separate shafts; the pump and gearbox have internal bearings to absorb all
thrust loads (axial and radial). The gearbox shaft is flexibly coupled to the
driver shaft and the motor mounts on a frame supported by the pump and gear
unit.
PUMPS OF OTHER CONFIGURATION
These pumps operate using the same
basic kinetic principles but are configured differently than the conventional
rotodynamic designs. The following examples fall within this description.
Regenerative turbine type
Regenerative turbine pumps are
characterized by a low rate of flow and high head. This design uses peripheral
or side channel vanes or buckets that are typically manufactured integral with
a rotating impeller to impart energy to the pumped liquid. The liquid travels
in a helical pattern through the impeller vanes and corresponding flow passages,
with the liquid pressure increasing uniformly through the passages from inlet
to the discharge.
Pitot tube type
The pitot tube pump is a variation
of a rotodynamic design and uses a pitot tube, in lieu of a volute or diffuser,
to capture flow and convert velocity energy to pressure. The primary feature of
a pitot tube pump that differentiates it from a conventional rotodynamic pump
is that it uses a rotating casing instead of an impeller to impart velocity to
the pumped liquid. The pitot tube design follows conventional pump affinity
rules; however, it is capable of generating higher head than a comparable
rotodynamic design at an equivalent tip speed.
Hydraulic power recovery turbine
A hydraulic power recovery turbine
is a rotodynamic pump that operates by accepting flow in the reverse direction
as normal, by virtue of the fact that a differential pressure is applied across
its connections. Liquid enters the (normal) discharge connection of the pump
and exits the (normal) suction connection. As such, the pump operates as a
turbine, producing useable shaft power as a function of speed and differential
pressure.
Vortex (recessed impeller) type
A vortex or recessed impeller pump
is a rotodynamic pump designed with large, uniform clearances between the open
impeller vanes and the casing shroud. Radial and cup type impellers are used.
The impeller is recessed from the liquid flow path, which induces a vortex
action to the liquid. Correspondingly,
trash and solids can pass through the pump without impinging on the impeller
IMPELLER DESIGNS
Impeller designs are classified as
radial, mixed, or axial flow, depending on their geometry. These designs are
differentiated by specific speed and impeller types as described in the
following paragraphs.
Specific speed (nS) and suction specific speed (S)
The user is cautioned to check
carefully the basis of calculation of specific speed and suction specific speed
before making comparisons because there are subtle but significant differences
in methods used throughout industry and in related textbooks and literature
When calculating the value for
specific speed and suction specific speed, the unit of measurement used for
rate of flow is defined in US gallons per minute (gpm).
Metric units
When calculating the value for
specific speed and suction specific speed, the unit of measurement used within
this standard for rate of flow is cubic meters per second (m3/s).
(An alternative method of
calculating this value is to use m3/h as the unit of measurement for rate of
flow, which then results in a value that is (3600)0.5, i.e., 60 times greater.
Specific speed is an index of pump
performance (developed total head). It is determined at the pump's best
efficiency point (BEP) rate of flow, with the maximum diameter impeller, and at
a given rotative speed. Specific speed is expressed by the following equation:
Where:
ns = specific speed
n = rotative speed, in revolutions
per minute
Q = total pump flow rate, in cubic
meters per second (US gallons per minute)
H = head per stage, in meters
(feet)
NOTE: When calculating specific
speed using units of cubic meters per second for flow rate and meters for head
per stage, 51.6 is the conversion factor for specific speed in US gallons per
minute and feet (i.e., metric 51.6 = US customary units.)
The usual symbol for specific speed
in US customary units is Ns.
An alternative definition for
specific speed is sometimes used based on flow rate per impeller eye rather
than total flow rate. When applying this alternative method to a double suction
impeller pump, the resultant value of specific speed is less by a factor of
1/(2)0.5 (i.e., 0.707 times less).
Suction specific speed is an index
of pump suction operating characteristics. It is determined at the BEP rate of
flow with the maximum diameter impeller. (Suction specific speed is an
indicator of the net positive suction head required [NPSH3] for given values of
capacity and also provides an assessment of a pump's susceptibility to internal
recirculation.) Suction specific speed is expressed by the following equation:
Where:
S = suction specific speed
n = rotative speed, in revolutions
per minute
Q = flow rate per impeller eye, in
cubic meters per second (US gallons per minute)
= total flow rate for single
suction impellers
= one half total flow rate for
double suction impellers
NPSH3 = net positive suction head
required in meters (feet) that will cause the total head (or first-stage head
of multistage pumps) to be reduced by 3%
NOTE: When suction specific speed
is derived using cubic meters per second and meters, the conversion factor to
suction specific speed in US gallons per minute and feet is 51.6. The US
customary symbol Nss is sometimes used to designate suction specific speed.
The value S is an assessment of a
pump's inlet design, including both the stationary casing and the rotating
impeller design elements. Higher numerical values of S are associated with
better NPSH capabilities. For pumps of typical suction inlet design, values
range approximately from 120 to 250 (6000 to 13,000). In special designs,
including inducers, S values can be up to 700 (35,000) or higher depending on
the connected inlet piping, the pump's suction casing arrangement, the range of
flow over which the pump must operate, size and power rating of the machine,
and other considerations.
Radial flow
Pumps of this type with single
inlet impellers usually have a specific speed below approximately 90 (4500) and
with double suction impellers, a specific speed below approximately 135 (7000).
In pumps of this type, the liquid enters the impeller at the hub and flows
radially to the periphery, exiting perpendicular to the rotating shaft.
Mixed flow
This type of pump has a single
inlet impeller where the flow enters axially and discharges in a mixed axial
and radial direction. Pumps of this type usually have a specific speed from
approximately 90 upwards.
Axial flow
A pump of this type, sometimes
called a propeller pump, has a single inlet impeller with the flow entering
axially and discharging nearly axially. Pumps of this type usually have a
specific speed above approximately 200 (10,000)
GENERAL INFORMATION
Size of a rotodynamic pump
The standard Hydraulic Institute
nomenclature for pump size is “discharge opening size by maximum rated nominal impeller diameter (each
indicated in millimeters).” For example, a pump with 80-mm suction, 50-mm discharge openings, and
a 160-mm maximum rated nominal impeller diameter, will be referred in SI terms
as a 50-160 pump.
Conversely, US nomenclature refers
to pumps by using the notation: inlet opening size by discharge opening size by
maximum rated nominal impeller diameter, all measured in inches. The pump
measured above in US customary units may be referred to as a “3 2 6” pump, i.e., the smaller of the numbers is the
discharge size. The pump described above in ISO standards would be referred to
as an “80-50-160” pump.
These methods are in compliance
with methods used in other reference industry standards such as ISO 5199 and
ANSI/ASME B 73.1M.
Duplicate performance pump
A duplicate pump is one in which
the performance characteristics are the same as another, within the variations
permitted by ISO 9906 or ANSI/HI 1.6 test standards, and parts are of the same
type. But, by reason of improved design and/or materials, mounting dimensions
and parts are not necessarily interchangeable.
Dimensionally interchangeable pump
An interchangeable pump is one in
which the mounting dimensions are such that the replacement pump can be mounted
on the existing bedplate and match existing piping and driver, with hydraulic
characteristics and materials to be specified. Interchangeability may involve
some variation, not necessarily significant, as a result of manufacturing
tolerances.
Identical pump (performance and dimensional)
An identical pump is a replica of,
and is interchangeable with, a specific pump. Where it is intended that a pump
is to be identical in all respects, including parts, mountings, connecting
flange dimensions, and materials, it should be identified as identical with
pump serial number XXXXXX. An identical pump will replicate the original pump
in performance and dimensions as closely as the manufacturing tolerances allow.
Vertical centrifugal pumps
Vertical centrifugal pumps are also
referred to as cantilever pumps. They utilize a unique shaft and bearing
support configuration that allows the volute to hang in the sump while the
bearings are outside of the sump. This style of pump uses no stuffing box to
seal the shaft but instead utilizes a "throttle Bushing". A common
application for this style of pump is in a parts washer.
Froth pumps
In the mineral industry, or in the
extraction of oil sand, froth is generated to separate the rich minerals or
bitumen from the sand and clays. Froth contains air that tends to block
conventional pumps and cause loss of prime. Over history, industry has
developed different ways to deal with this problem. One approach consists of
using vertical pumps with a tank. Another approach is to build special pumps
with an impeller capable of breaking the air bubbles. In the pulp and paper
industry holes are drilled in the impeller. Air escapes to the back of the
impeller and a special expeller discharges the air back to the suction tank.
The impeller may also feature special small vanes between the primary vanes
called split vanes or secondary vanes. Some pumps may feature a large eye, an
inducer or recirculation of pressurized froth from the pump discharge back to
the suction to break the bubbles.[4]
Multistage centrifugal pumps
A centrifugal pump containing two
or more impellers is called a multistage centrifugal pump. The impellers may be
mounted on the same shaft or on different shafts.
For higher pressures at the outlet
impellers can be connected in series. For higher flow output impellers can be
connected in parallel.
A common application of the
multistage centrifugal pump is the boiler feedwater pump. For example, a 350 MW
unit would require two feed pumps in parallel. Each feedpump is a multistage
centrifugal pump producing 150 l/s at 21 MPa.
All energy transferred to the fluid
is derived from the mechanical energy driving the impeller. This can be
measured at isentropic compression, resulting in a slight temperature increase
(in addition to the pressure increase).
Centrifugal pumps for solids control
An oilfield solids control system
needs many centrifugal pumps to sit on or in mud tanks. The types of
centrifugal pumps used are sand pumps, submersible slurry pumps, shear pumps,
and charging pumps. They are defined for their different functions, but their
working principle is the same.
Magnetically coupled pumps
Magnetically coupled pumps, or
Magnetic drive pumps, vary from the traditional pumping style, as the motor is
coupled to the pump by magnetic means rather than by a direct mechanical shaft.
The pump works via a drive magnet, 'driving' the pump rotor, which is
magnetically coupled to the primary shaft driven by the motor.[6] They are
often used where leakage of the fluid pumped poses a great risk (e.g.,
aggressive fluid in the chemical or nuclear industry, or electric shock -
garden fountains). They have no direct connection between the motor shaft and
the impeller, so no gland is needed. There is no risk of leakage, unless the casing
is broken. Since the pump shaft is not supported by bearings outside of the
pump's housing, support inside the pump is provided by bushings. The materials
of construction of these bushings and the required clearances of the parts may
restrict the kinds of fluids for which this kind of pump may be used.
CONCLUSION
Pumping and piping system cannot be
neglected on-board ships. In lieu of this, proper and efficient pumps must be
used to avoid accidents. All vessels have fresh water, ballast and fuel pumping
system ; a dependable pump like the Rotodynamic Pump is used because their parts
are cheap and easy to maintain. There are no drive seals, therefore the risk of leaks is
completely eradicated. This means that hazardous liquids can be pumped without
spillages compared to other type of pumps. I beseech Ship Builders to use
rotodynamic pumps in their subsequent ship construction in order to reduce the
risk of accident and the seafarers on-board should do proper maintenance so as
to obtain high efficiency of the pumps.
REFERENCE
**Shepard, Dennis G. (1956).
Principles of Turbomachinery.
McMillan. ISBN 0-471-85546-4. LCCN 56002849.
**Reti, Ladislao; Di
Giorgio Martini, Francesco (Summer, 1963). "Francesco di Giorgio
(Armani) Martini's Treatise on Engineering and Its Plagiarists". Technology and Culture 4 (3):
287–298 (290).
**Richards, John (1894). |”Centrifugal pumps: an essay on their construction and operation, and
some account of the origin and development in this and other countries. The Industrial Publishing
Company.” pp. 40–41.
**Baha
Abulnaga (2004). Pumping Oils and Froth. 21st International Pump
Users Symposium, Baltimore, Maryland. Published by Texas A&M University,
Texas, USA.
**Larry Bachus, Angle Custodio (2003). Know
and understand centrifugal pumps. Elsevier Ltd. ISBN 1856174093.
