Marine Auxiliary Machinery[1]

Marine Auxiliary

MachineryPUMPS

(part II)

Prepared by:4cl. Balilla

4cl. Maraggun4cl. Nolasco4cl. Salvador

Learning objectives

After successfully completing this lesson, you will be familiar with:

1. The purpose of relief valve on displacement pump2. The need for priming on centrifugal pump3. Cavitation on centrifugal pump4. Name components of a centrifugal pump and materials

they are commonly made5. Differentiate between gland packing and mechanical seal.6. Distinguish different types of displacement pump and

how they are categorized

Displacement pump-Relief valve

Introduction

Positive Displacement pumps are knows as "flow generators", while Centrifugal pumps can be thought as pressure (or head) generators.

Displacement pump-Relief valveDID YOU KNOW?

A PD pump "does not know and does not care" what happens on the system side.

As a result, a PD pump will try to move the same amount of fluid, regardless of the position of a discharge valve resulting in pump discharge pressure building up very rapidly if valve begins to close .

It will cause

Displacement pump-Relief valveWHAT IS THE SOLUTION?

To prevent them, a relief valve should always be employed with a PD pump.

What is a relief valve?The relief valve (RV) is a type of valve used to control or

limit the pressure in a system or vessel which can build up by a process upset, instrument or equipment failure, or fire. A valve in a container in which pressure can build up it opens automatically when the pressure reaches a dangerous level

Displacement pump-Relief valve

Centrifugal pump duties are usually for the movement of large volumes of liquid at low pressures, although higher pressures can be achieved with multi-staging.

A centrifugal pump can be further defined as a machine which uses several energy transformations in order to increase the pressure of a liquid. The energy input into the pump is typically the fuel source energy used to power the driver.

Centrifugal pumps

Inline pump Endsuction pump

Doublesuction pumpSubmersible pump

Centrifugal pumps - Priming

Centrifugal pumps although suitable for most general marine duties, suffer in one very important respect; they are not self priming and require some means of removing air from the suction pipeline and filling it with the liquid.


Where the liquid to be pumped is at a higher level than the pump, opening an air release cock near the pump suction will enable the air to be forced out as the pipeline fills up under the action of gravity. This is often referred to as "flooding the pump".

Alternatively, an air-pumping unit can be provided to individual pumps or as a central priming system connected to several pumps.

The water ring or liquid ring primer can be arranged as an individual unit mounted on the pump and driven by it, or as a motor driven unit mounted separately and serving several pumps, known as a central priming system.


Priming Devices Essential part of centrifugal pumps • Centrifugal pumps cannot pump air • Provides water to the impeller ��

Result of air pressure differential • Reduces atmospheric pressure within • pump cavity

Uses• Testing• Drafting




Pump above water

Cavitation is a problem condition which may develop while a centrifugal pump is operating. This occurs when a liquid boils inside the pump due to insufficient suction head pressure. Low suction head causes a pressure below that of vaporisation of the liquid, at the eye of the impeller.

The resultant gas which forms causes the formation and collapse of 'bubbles' within the liquid. This, because gases cannot be pumped together with the liquid, causes violent fluctuations of pressure within the pump casing and is seen on the discharge gauge. These sudden changes in pressure cause vibrations which can result in serious damage to the pump and, of course, cause pumping inefficiency.

To overcome cavitation:

1. Increase suction pressure if possible.2. Decrease liquid temperature if possible.3. Throttle back on the discharge valve to decrease

flow-rate.4. Vent gases off the pump casing.

When the pressure falls below the vapour pressure of the liquid at a given temperature, boiling occurs and small bubbles of vapour are formed. These bubbles will grow in the low-pressure area and implode when theyare transported to an area of pressure above vapour pressure. The term given to this local vaporisation of the fluid is Cavitation

The collapsing of the bubbles is the area of Cavitation we are concerned with, as extremely high pressures are produced, which causes noise and erosion of the metal surface.

The area of pipeline

This cavitation effect

To reduce cavitation

• The area of pipeline where Cavitation mainly occurs is the pump suction, where the liquid is subjected to a rapid rise in velocity, and hence a fall in static pressure.

• This Cavitation effect on the pump can cause damage on the casing and impeller.

ˉ During Cavitation, a liquid/vapour mixture of varying density is produced.

ˉ This results in fluctuations in pressure (caused by the liquid column being drawn in), and causes fluctuations in the discharge pressure, pump power absorbed (shown on the ammeter), and hence pump revolutions.

• To reduce Cavitation we must reduce the 'losses' on the suction side, hence reduce the pipeline friction and NPSH. This means reducing the pump flow rates. To reduce 'losses' on starting, the pump should be started against a closed discharge valveThe volume of the cavities depends on the design of the impeller and the pump housing, and they affect the flow around the impeller and the pump’s ability to handle sand and air.Example picture open centrifugal p/p.pdf page 19

Centifugal Pumps - Components


The two main components of a centrifugal pump are the impeller and the volute. The impeller produces liquid velocity and the volute forces the liquid to discharge from the pump converting velocity to pressure. This is accomplished by offsetting the impeller in the volute and by maintaining a close clearance between the impeller and the volute at the cut-water. Please note the impeller rotation. A centrifugal pump impeller slings the liquid out of the volute. It does not cup the liquid.


Parts of a Centrifugal Pump:

1. Impeller 2. Casing3. Suction pipe with a foot valve and a strainer4. Delivery pipe.


1. Impeller: - The rotating part of a centrifugal pump is called ‘impeller’. It consists of a series of backward curved vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor.

Details of impeller


2. Casing: - The casing of a centrifugal pump is similar to the casing of a reaction turbine. It is an air tight passage surrounding the impeller and is designed in such a way that the kinetic energy of the water discharged at the outlet of the impeller and is converted into pressure energy before the water leaves the discharged at the outlet of the impeller is converted into pressure energy before the water leaves the casing and enters the delivery pipe. The following three types of the casing are commonly adopted:

Centifugal Pumps - Components(a) Volute Casing,(b) Vortex Casing,(c) Casing with guide blades.


3. Suction pipe with a foot-valve and a strainer: - A pipe whose one end is connected to the inlet of the pump and other end dips into water in a sump is known as suction pipe. A foot valve which is a non-return valve or one-way type of valve is fitted at the lower end of the suction pipe. The foot valve opens only in the upward direction. A strainer is also fitted at the lower end of the suction pipe.


Suction pipe with a foot-valve and a strainer


4. Delivery Pipe: - A pipe whose one end is connected to the outlet of the pump and other end delivers the water at a required height is known as delivery pipe.

Centifugal Pumps - ComponentsConstructionThe construction of pump consists of a single rotating element and a simple casing, which can be constructed using a wide assortment of materials. If the material that is being pumped is very corrosive, the pump parts, which will come in contact with the liquid, can be constructed out of lead or other materials which are not likely to corrode. If the material that is being pumped is very abrasive , such as grit or ash, the internal parts can be made of abrasion resistant material or coated with a protective coating.

Centifugal Pumps - ComponentsMaterials commonly used to construct centrifugal pump:

Cast Iron, Stainless Steel, CD4MCu, Zinc Free Bronze, 316 Stainless Steel, Titanium, Bronze, 28% Hard Chrome.

This various type of material will be used will be depend on the pump application.

• To connect the motor to the impeller, the shaft has to pass through an aperture in the casing.

• To allow the shaft to rotate freely in the casing aperture there needs to be a gap, but this gap needs to be closed off to stop air from being drawn in from atmosphere or liquid from leaking out during operation.

• There are two common methods. Packing Mechanical seal• The role of the pump, its speed and the type of liquid

being pumped all play a part in deciding which application works best.

Packing

Packing

1. A stuffing box with a soft packing material is the traditional seal for pumps. Normally made from soft impregnated cotton, which takes the form of a length of square cross-section wound spirally onto a tube. This enables the correct length, to suit the external diameter of the shaft, to be manually cut to the correct size.

Packing

2. The stuffing box is then repeatedly filled with sections until almost full, the gland can then be tightened down to provide the axial compressive force. This in turn provides the necessary radial compressive force required to seal the gap due to the sloping bottom face of the aperture.

3. If the force is insufficient the stuffing box will leak, if the force is too great, the additional friction, and consequently heat generated by the rotating shaft can damage the soft packing and/or shaft.

Mechanical seals

Mechanical seals

The provision of rotary shaft seals instead of the usual stuffing box and gland, where conditions are suitable, possesses many advantages. The power absorbed is lower and is constant, whereas a gland excessively tightened causes a considerable increase in power absorbed.

Mechanical sealsIn small pumps this may result in overloading the

motor. In addition maintenance costs are reduced, the rotary seal operating for long periods without wear or attention.

A standard seal consists of a stationary carbon ring insert in the casing, or seal cover where such is provided, and against this a metal ring of easy clearance on the shaft sleeve rotates, contact between the faces being ensured by a lightly loaded coil spring.

Mechanical seals

The rubbing faces of both carbon and metal rings are independently lapped to give a dead flat surface.

A synthetic rubber ring, of circular cross-section, contained between shaft sleeve and metal ring, in a groove in the latter, effectively prevents leakage between them.

The diameter of the groove is such that a squeeze is exerted on the rubber ring, thus a sufficient frictional force is provided to rotate the metal ring, with certain exceptions.

Mechanical sealsThe width of the groove is, however, made considerably

greater so that the metal ring is capable of free axial float with accompanying rolling action of the rubber ring.

Materials used for the various seal parts are as follows:1. Carbon stationary ring.2. Synthetic rubber ring.3. Bronze rotating ring with bronze spring for standard

and all gunmetal pumps.4. Stainless steel rotating ring for all iron pumps.

Mechanical sealsFor non-lubricating liquids, such as ammonia,section, contained between shaft sleeve and metal

ring, in a groove in the latter, effectively prevents leakage between them.

The diameter of the groove is such that a squeeze is exerted on the rubber ring, thus a sufficient frictional force is provided to rotate the metal ring, with certain exceptions.

The width of the groove is, however, made considerably greater so that the metal ring is capable of free axial float with accompanying rolling action of the rubber ring

Pumps

Positive Displacement Pumps

The positive displacement pump operates by alternating of filling a cavity and then displacing a given volume of liquid. The positive displacement pump delivers a constant volume of liquid for each cycle against varying discharge pressure or head.

Positive Displacement Pumps

PD Pumps - Rotary

Lobe pumps can handle solids.

A gentle pumping action minimizes product emulsification.

Can operate dry for brief periods of time.

Can handle viscous products.

PD Pumps - Rotary

WCMRC use hydraulic drive screw pumps.

Typical is the Desmi opposite.

This can generate 10 bar discharge pressure.

Max. capacity 30 m³/h.

80 liters per minute at 210 bar max.

PD Pumps - Rotary External Gear pumps come in

single or double configurations.

Handle viscous and watery-type liquids.

Reduced speeds with high-viscosity liquids results in greater efficiency.

Two gears come into and out of mesh to produce flow.

Uses two identical gears rotating against each other.

PD Pumps - Rotary

A vacuum is created by the rollers compressing the hose and moving along• Liquid is drawn into vacuum

Rollers capture liquid between them and move liquid towards discharge

Peristaltic Pump

PD Pumps - Rotary

Vane Pumps Rotating eccentric cam principle

o Not suitable for high viscosity products

Used to move light productso Fuelo Crudeo Watero dispersant

PD Pumps - Rotary

Hydro roller pumps are the most popular pumps worldwide for agricultural tasks. These low-cost, highly-versatile pumps are used for spraying and transferring a variety of fluids including insecticides, herbicides, fungicides, emulsives, aromatic solvents, liquid fertilizers and many other non-abrasive liquid

PD Pumps - Reciprocating

The plunger pump is a positive displacement pump, designed to pump high solids (18-20% solids) content commonly found in raw influent. This technology is specified to pump effluent (run off) and industrial discharge as well. Together with progressive cavity pumps which have been in existence since the 1930’s, the plunger pump first began pumping municipal sludge in the 1920’s.


A piston pump is a type of positive displacement pump where the high-pressure seal reciprocates with the piston. Piston pumps can be used to move liquids or compress gases.


Diaphragm Pump

Using a diaphragm to push and pull the product through a chamber.

Typical example at OSRL is the spate pump 75C.

Capable of moving up to 30m3/hr.

Documents

Marine Auxiliary Machinery[1]