ROLE OF ROTODYNAMIC PUMPS IN OIL TANKER OPERATIONS

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.

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.