SSP Pumps in Sugar Processing

Inside View This document has been produced to support pump users at all levels, providing an invaluable reference tool. It includes information on the Sugar processes and provides guidelines as to the correct selection and successful application of SSP Rotary Lobe Pumps. Main sections are as follows: 1. Introduction 2. General Applications Guide 3. Cane Sugar Processing 4. Beet Sugar Processing 5. Refining 6. Pump Specification Options 7. The SSP Advantage 8. Pump Selection and Application Summary

The information provided in this document is given in good faith, but Alfa Laval Ltd, SSP Pumps is not able to accept any responsibility for the accuracy of its content, or any consequences that may arise from the use of the information supplied or materials described.

Contents Section 1.0: Introduction

Page 3

Introduction of SSP Pumps in Sugar Processing

Section 2.0: General Applications Guide

5

Overview of the pump ranges currently available from SSP Pumps and which particular pumps to apply within various application areas

Section 3.0: Cane Sugar Processing

7

Description of how cane sugar is processed and where to find SSP rotary lobe pumps 3.1 Harvesting 3.2 Extraction 3.3 Evaporation 3.4 Crystallisation 3.5 Storage

7 7 8 9 10

Section 4.0: Beet Sugar Processing

13

Description of how beet sugar is processed and where to find SSP rotary lobe pumps 4.1 Harvesting 4.2 Extraction 4.3 Purification 4.4 Evaporation 4.5 Crystallisation

13 13 14 14 15

Section 5.0: Refining

19

Description of the sugar refining process and where to find SSP rotary lobe pumps 5.1 Affiniation 5.2 Carbonatation 5.3 Decolourisation 5.4 Evaporation/Crystallisation 5.5 Separation/Drying

20 21 22 23 24

Section 6.0: Pump Specification Options

27

Description of various pump specification options available on SSP rotary lobe pumps 6.1 Mechanical Seals 6.2 Heating Jackets and Saddles 6.3 Wear Plates 6.4 Bi-lobe Rotors

27 28 29 30

Section 7.0: The SSP Advantage

31

SSP Pumps comparison with other pump technologies 7.1 Other Technologies 7.1.1 Gear Pumps 7.1.2 Sliding Vane Pumps 7.1.3 Progressing Cavity Pumps

33 33 33 35

Section 8.0: Pump Selection and Application Summary

39

SSP Pump selection guidelines summary for the different pumped media found in Sugar Processing

1.0 Introduction Sugar is currently produced in 121 countries and global production now exceeds 120 million tonnes a year. Approximately 70% is produced from sugar cane, a very tall grass with large stems which is mostly grown in tropical countries. The remaining 30% is produced from sugar beet, a root crop resembling a large carrot grown mostly in the temperate zones of the northern hemisphere. Historically, sugar was only produced from sugar cane and then only in relatively small quantities. This resulted in it being considered a great luxury, particularly in Europe where cane could not be grown. Even today, it is difficult to ship food quality sugar across the world so a high proportion of cane sugar is made in two stages. Raw sugar is produced where the sugar cane grows and white sugar is produced from the raw sugar in the country where it is required. Beet sugar is easier to purify and most is grown where it is required so white sugar is produced in only one stage. As a recognised market leader in pumping technology SSP Pumps has been at the forefront of supplying rotary lobe pumps to the sugar industry for over 50 years. SSP rotary lobe pumps are to be found in numerous sugar processes, where their reliable low shear flow characteristics are ideally suited to the transfer of such wide-ranging media as magma, massecuite and thick juice.

3

2.0 General Applications Guide This section gives an overview of the pump ranges currently available from SSP Pumps and which particular pumps to apply within various application areas in the Sugar Industry. Within the various sugar industry processes many opportunities exist for utilising SSP rotary lobe pumps, not only for the final product but other processes such as by-products, sampling and waste.

Walk the Process

By-Products

Raw Material

Sampling

The Process

Waste

Final Product

Services

Opportunities

By-Products

Sampling

Waste

The Process

Raw Material

Final Product

5

Within the sugar industry typical application areas for SSP Pumps are to be found in: • • • • • • • •

Carbonation Crystallisation Evaporation Recovery Separation Storage Tanker Loading Transfer

The table below indicates the typical pumped media found and which pump series can be generally applied: Media Handled Glucose High / Low Green Syrup Liquid Sugar Magma Massecuite Molasses Sugar Syrup Thick Juice Treacle

S ▲ ▲ ▲ ▲ ▲ ▲ ▲

Pump Series D ▲ ▲ ▲ -

G ▲ ▲ -

The table shown below gives a general guide as to the SSP pump series required to suit the application

General Requirements S

Pump Series D

G

1000000

1000000

1000000

Pumped Media Max. Viscosity - cP

200°C (392°F) 200°C (392°F) 200°C (392°F)

Max. Pumping Temperature

-20°C (-4°F)

-20°C (-4°F)

-20°C (-4°F)

8 9 9 9 9 9

9 9 9 8 9 8

9 9 9 8 9 8

Max. Capacity - m³/h

106

120

680

Max. Capacity - US gall/min

466

528

2992

Max. Discharge Pressure - bar

20

15

10

Max. Discharge Pressure - psig

290

215

145

Min. Pumping Temperature Ability to pump abrasive products Ability to pump fluids containing air or gases Abilty to pump solids in suspension CIP capability Dry running capability (when fitted with flushed mechanical seals) Self draining capability

Performance

6

3.0 Cane Sugar Processing The processing of sugar from sugar cane can be divided into the following steps: • • • • •

3.1

Harvesting Extraction Evaporation Crystallisation Storage

Harvesting

Cane grows very tall, up to 3 metres, in good growing regions. Harvesting is done either by hand or by machine. Hand cut cane is cut about ground level and assembled in bundles. These bundles are then transferred to a large vehicle and transported to the mill. Most machine-cut cane is chopped into short lengths but otherwise handled in a similar way.

3.2

Extraction

The cane must be processed as soon as possible after delivery to the sugar mill. Typically, cane is processed within 24 hours of cutting. Cane preparation is critical to good sugar extraction. This is achieved with rotating knives and hammer mills called shredders. The extraction is conducted as a counter-current process using fresh hot water pumped through the chain of multiple roller mills or the continuous diffuser. The more water that is used, the more sugar is extracted but the more dilute the mixed juice is. In best milling practice more than 95% of the sugar in the cane goes into the juice. A typical mixed juice from extraction will contain 15% sugar. The residual fibre produced from crushing the cane is known as ‘bagasse’, which also contains the un-extracted sugar and 45-55% water. The bagasse is subsequently sent to the boilers to be used as fuel.

7

3.3

Evaporation

The dark-green mixed juice from the mills is acid and turbid. The clarification process uses heat and lime as clarifying agents. The mixed juice is preheated before milk of lime {calcium hydroxide Ca(OH)2}, to approximately 0.5 kg per tonne of cane, is added to the juice. The lime neutralises the natural acidity and insoluble lime salts, like calcium phosphate precipitate. Heating the limed juice to boiling coagulates the proteins and some of the fats, waxes and gums. The juice goes through a gravitational settling tank, known as a clarifier, where the solids settle out and clear juice exits. The mud from the clarifier still contains valuable raw sugar so it is filtered on rotary drum vacuum filters where the residual juice is extracted and the mud can be washed before discharge, producing a sweet water. The filtered juice and sweet water is returned to the clarified juice; the press cake is discarded or used as fertiliser. The clear juice is heated in a number of juice heaters and concentrated in a multi-stage evaporator to 60-75 Brix and is now called syrup or raw syrup.

8

A direct consumption white sugar can be manufactured from concentrated cane juice if sulfur dioxide (SO2) or carbon dioxide (CO2) is used in conjunction with lime i.e. the sulphitation or carbonation process. The sulphitation process can be carried out either as an acid or alkaline sulphitation depending on whether the sulfur dioxide (SO2) or lime is added first. In the carbonation process, first quick lime and carbon dioxide (CO2) are produced in a limekiln. The quick lime is mixed with water to produce lime milk and added to the juice. The lime is precipitated with carbon dioxide (CO2) in two steps, 1st and 2nd carbonation. Before concentrating the juice it is often decolorized by adding (sulfur dioxide (SO2).

Cane

Bagasse

Purification Screen

2nd Carbonation CO2

Lime Kiln

Milk of Lime

Sulphitation SO2 1st Carbonation Filter

Pre-heating

Clarified Juice

Liming CO2 SSP Pump Application Cake Processing

3.4

Crystallisation

The syrup is boiled in the vacuum pans, crystallised and separated into white sugar and remaining sugar syrup. Physical chemistry assists with sugar purification during the crystallisation process as there is a natural tendency for the sugar crystals to form as pure sucrose, rejecting the non-sugars. Thus, when the sugar crystals are grown in the mother liquor they tend to be pure and the mother liquor becomes more impure. Most remaining non-sugar in the product is contained in the coating of the mother liquor left on the crystals. The mother liquor still contains valuable sugar, so the crystallisation is repeated several times.

9

The crystallisation step i.e. boiling, takes place in a vacuum pan: a large closed kettle with steam heated pipes. The mixture of crystals and mother liquor from a boiling, known as ‘massecuite’, is dropped into a receiving tank called a crystalliser where it is cooled down and the crystals continue to grow. This also releases the pan for a new boiling. From the crystalliser the massecuite is fed to the centrifuges. In a raw sugar factory it is normal to conduct three boilings. The first or ‘A’ boiling produces the best sugar which is sent to storage. The second ‘B’ boiling takes longer and the retention time in the crystalliser is also longer if a reasonable crystal size is to be achieved. Some factories re-melt the ‘B’ sugar to provide part of the ‘A’ boiling feedstock, others use the crystals as seed for the ‘A’ boilings and others mix the ‘B’ sugar with the ‘A’ sugar for sale. The ‘C’ boiling takes proportionally longer than the ‘B’ boiling and considerably longer to crystallise. The sugar is usually used as seed for ‘B’ boilings The diagram on the following page shows where typically SSP rotary lobe pumps can be found in the cane sugar process. Location 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

3.5

Application Thick Juice Thick Juice Massecuite ‘A’ High Green Syrup High Green Syrup High Green Syrup Massecuite ‘B’ Low Green Syrup Low Green Syrup Low Green Syrup Low Green Syrup Massecuite ‘C’ Molasses Affination Massecuite Affination Massecuite Magma Magma Clear Juice Clear Juice Massecuite Thick Juice

Storage

The final raw sugar forms a sticky brown mountain which is stored. Although the sugar could be used at this stage it invariably becomes dirty in storage and has a distinctive taste. It is therefore refined when it gets to the country where it will be used. Additionally, as all the sugar cannot be extracted out of the juice, a sweet by-product is formed known as ‘molasses’. The molasses is used extensively as a livestock feed or in distilleries as part of the fermentation process.

10

Crystallisation Process Diagram

‘C’ Boiler

‘B’ Boiler

Standard Liquor Concentration

‘A’ Boiler

11

From Evaporation

6

12

7

Centrifuge

2

3

10 8

13

4

9

White Sugar

Clear Juice Molasses

1

5

Affination ‘B’ Melter

19 Clear Juice

Boiler 17

14 18 15 ‘C’ Melter

20

16

Centrifuge Refined Sugar SSP Pump Application

21

4.0 Beet Sugar Processing The processing of sugar from beets is traditionally a seasonal process. After harvesting the campaign lasts normally for 3 months, which is generally in Europe from middle of September until end of December. Larger sugar mills introduce a ‘second’ campaign, which is called thick juice campaign. With this campaign, they do not produce any beets, but refine the stored thick juice to sugar. The thick juice being produced in the beet campaign is stored for the thick juice campaign. The extension of the campaign gives a better use of the equipment in the sugar mill. As the thick juice campaign uses just the later stages of the sugar process, there are no differences in the process. The processing of sugar from sugar beet can be divided into the following stages: • • • • •

4.1

Harvesting Extraction Purification Evaporation Crystallisation

Harvesting

The beets are dug out of the ground and being dirtier than sugar cane have to thoroughly washed and separated from any remaining beet leaves, stones and other waste material before processing.

4.2

Extraction

After the removal of stones and dirt, the beets are conveyed to the slicers for cutting into very thin Vshaped slices, known as cossettes. This increases the surface area of the beet for easier extraction of sugar. The sugar is removed from the beets by means of diffusion. During diffusion the sugar is extracted from the cossettes by hot water, leaving a pulp containing very little sugar but when mixed with molasses and dried it is used as animal feed. A typical raw juice from diffusion will contain perhaps 14% sugar and the residual pulp will contain 1 2% and a total of 8 - 12% solids.

13

4.3

Purification

The raw juice contains proteins, pectins, inorganic salts and colouring substances. These impurities have to be removed before the evaporation. Firstly, milk of lime {calcium hydroxide Ca(OH)2}, is added to the juice vessel and carbon dioxide (CO2) is bubbled through the liquor. This coagulates most of the colloidal matter which is precipitated with the lime as calcium carbonate. The resultant juice is passed through a clarifier in which the solids are separated and scraped to the outlet as mud and pumped away, generally to a press where the ‘sweet water’ is returned to the milk of lime make-up tank. The juice from the clarifier is again treated with carbon dioxide (CO2) in the second carbonation vessel to precipitate more impurities and is the filtered by vacuum, bag or press type system. The filtered juice now passes through a decalcification process (similar to water softening) and using ion exchange resins to remove calcium hardness, followed by sulphitation {sulphur dioxide (SO2) gas bubbled through the juice to remove colour and control acidity}. Any reject matter from the second carbonation filters is returned to the raw juice line. The purified sugar solution has a sugar content of 14 - 16 Brix.

4.4

Evaporation

At this stage the juice is fairly thin and is pumped into a multi-stage evaporator which concentrates the solution to 60 - 75 Brix. It is subsequently pumped and stored as ‘thick juice’.

S6 pumps handling Thick Juice in Denmark

14

4.5

Crystallisation

This final stage involves boiling the thick juice in vacuum pans, crystallising and separating into white sugar and remaining sugar syrup. Three vacuum pans are used, ‘A’, ‘B’ and ‘C’. Pan ‘A’ concentrates the thick juice to a state where crystals start to form. The liquor is pumped to crystallisers in which further crystals form as it cools and here it is known as ‘massecuite’. From the crystallisers the massecuite is pumped to a centrifuge where the crystals are washed thoroughly to remove all adhering syrup. The liquor spun from Pan ‘A’ centrifuge is known as ‘High Green Syrup’ and is pumped to Pan ‘B’ where ‘second product’ crystals are formed. The mixture is again put through crystallisers and the ‘second’ or ‘B’ massecuite is then centrifuged. The resulting liquor now known as ‘Low Green Syrup’ is passed on to Pan ‘C’ and the crystals from ‘B’ centrifuge are conveyed to a dissolver where they are mixed with thick juice from storage. This mixture is known as ‘magma’ and is then thinned by the wash water from ‘A’ centrifuge to lower its viscosity. The syrup is pumped through a filter and returned to the thick juice tank to ultimately be reprocessed through Pan ‘A’. The Low Green Syrup now in Pan ‘C’ continues through the process as in ‘B’ and after centrifuging the crystals they are dissolved in ‘A’ syrup to form the magma, diluted with centrifuge ‘B’ wash water and pumped to Pan ‘B’. The liquor from centrifuge ‘C’ is known as ‘molasses’ and is pumped to storage or the pulp mixer. The white sugar crystals produced from Pan ’A’ are dried, granulated and bagged. G7 pump handling Molasses in Ukraine

15

D6 pump handling Massecuite in USA

The diagram on the following page shows where typically SSP rotary lobe pumps can be found in the cane sugar process. Location 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Application Thick Juice Thick Juice Massecuite ‘A’ High Green Syrup High Green Syrup High Green Syrup Massecuite ‘B’ Low Green Syrup Low Green Syrup Low Green Syrup Low Green Syrup Massecuite ‘C’ Molasses Affination Massecuite Affination Massecuite Magma Magma Clear Juice Clear Juice Massecuite Thick Juice

16

Crystallisation Process Diagram

‘C’ Boiler

‘B’ Boiler

Standard Liquor Concentration

‘A’ Boiler

11

From Evaporation

6

12

7

Centrifuge

2

3

10 8

13

4

9

White Sugar

Clear Juice Molasses

1

5

Affination ‘B’ Melter

19 Clear Juice

Boiler 17

14 18 15 ‘C’ Melter

20

16

Centrifuge Refined Sugar SSP Pump Application

21

5.0 Refining Raw sugar is produced in tropical countries where sugar cane can be grown profitably. It is then shipped in bulk to a refinery in the country where the sugar is required. It now has to be finally cleaned, purified and made ready consumer use. Raw Sugar

Refined Sugar

Molasses

Mud

The incoming raw sugar consists of 95 - 99% sucrose and the remainder is made up of impurities. The purpose of the refining process is to produce white sugar of approx. 99.95% sucrose with excellent shelf life qualities and of food quality specification. In simple terms, the refining process consists of a series of progressive separations of impurities from the sucrose. Most of the impurities finish up in molasses, together with some sucrose that cannot be economically extracted. Some impurities also finish up in intermediate products such as brown sugars, golden syrup and some liquid sugars where colour, taste and aroma are important. S6 pump handling Glucose in UK

19

5.1

Affination

Affination or Washing is the first step in the sugar refinery. The raw sugar crystals have a thin film of impurities on the surface. This thin film is softened by mixing with hot impure sugar syrup (60°C). The resulting magma is fed to centrifuges, which spin off the syrup including the impurities. In the last step of the centrifugal separation, water is sprayed on the crystals to remove the final traces of impurities. Leaving the affination process the sugar crystals have a purity between 98.5 - 99%. These crystals are melted with sweet water of the same purity. This melted liquor is sent to further purification steps. Raw syrup from the Affination process is sent to the Recovery House where a triple combination of crystallising and centrifuging is used. The recovered sucrose is returned to the Melting process whilst the discarded impurities, still containing some sucrose, form Molasses.

From Raw Sugar Store

Weigh Scales

Masse Feed Pipes

Raw Sugar Minglers

Sweet Liquor

Raw Syrup

Affination Centrifuges Affined Sugar Conveyors

SSP Pump Application

To Recovery House Condensate Steam

Recovered Sugar from Recovery House

Melted Liquor To Carbonatation

Melters

20

5.2

Carbonatation

The melted liquor still includes some insoluble material as bagacillo (very fine bagasse), soil, suspended solids and colloids. There are different processes used for carbonatation e.g. pressure filtration with inert filter aid or chemical treatment and filtration. The chemical treatment consists often of a liming step and a carbonisation or phosphatation step. Recovered carbon dioxide (CO2) gas from the boiler flue gas reacts with the lime and forms chalk. The impurities stick to the chalk and can be removed in the filter presses downstream. The optimum liming and impurity absorption is at a temperature around 80°C. Half of the colour is already removed in this step before the liquor is going to the main decolourisation step. In the phosphatation process, phosphoric acid is used to form calcium phosphate as the carrier for the impurities before filtration.

Milk of Lime

Flue Gas Vent To Resin Plant

SSP Pump Application

Pressed Liquor

Carbonatation Tanks

Filter Supply Tank Filters

Press Mud

Melter Liquor

21

SSP Pump Application

5.3

Decolourisation

Sugar in itself is colourless. The non-sugar contaminants are responsible for the colour, which is mostly brown. These non-sugar contaminants are removed by adsorption in the decolourisation tanks. The adsorption can be achieved by different materials and chemicals, with anionic resins and ionexchange columns being commonly used today. In combination with these or as stand-alone bone char columns being used as well. The total colour removal in these systems exceeds 90% and the liquor leaving is known as fine liquor. Pressed Liquor from Carbonatation Plant

SSP Pump Application Acrylic 1 Resin Cells

Acrylic 2 Resin Cells

Styrene Resin Cells

Granular Carbon Cisterns

Fine Liquor To Evaporators To Liquid Sugar Plant

SSP Pump Application 22

5.4

Evaporation / Crystallisation

Fine Liquor leaving the decolourisation stage usually has a dry substance quantity of 60 - 67 Brix. Before crystallisation in the vacuum pans it has to be further concentrated to 70 - 75 Brix in an evaporation system. Evaporated fine liquor with 70 - 75 Brix is fed to vacuum pans. Pressurised steam provides indirect heat by means of coils inside the pans. The liquor itself is processed under reduced pressure so as to minimise colour formation. White sugar crystals are grown to the correct size and are discharged together with the residual liquor into a mixing vessel. The sugar crystal / liquor mixture known as ‘massecuite’ is now centrifuged to separate the market quality white sugar crystals from the liquor. Pure, hot condensate wash water is applied to remove residual traces of liquor. The resultant liquor is then spun off and used again for further crystallisation. The final liquor, containing some 88 - 91% sucrose, is now returned to the Recovery House. Fine Liquor from Decolourisation

SSP Pump Application

Evaporator

Vapour Vacuum Pans

Steam

Vapour Condensate Storage Tanks

Steam

SSP Pump Application

Condensate

To Liquid Sugar Plant

SSP Pump Application

Mixers

23

To Centrifuges

5.5

Separation / Drying

The separated white sugar crystals discharged from the centrifuges still contain about 1% moisture. This is removed by passing the sugar into a rotating, two-stage, fluidised-bed dryer, through which filtered, heated air is passed. The moisture level of the emerging sugar is about 0.04% but further drying occurs during conveying. Dry sugar from the Dryers discharges onto conveyor belt systems which take it to various storage silos and packaging processes. From Mixers

SSP Pump Application Storage Silos

White Sugar Centrifuges

Bulk White Sugar Road Tanker Deliveries

‘Jet’ Liquor Sugar Conveyors Vapour

Retail Packets

Hot Air Dryers

25/50 kg Bags and 1 tonne Containers Sugar Conveyors

Special Processes

Further Crystallisation Processes

To Liquid Sugar Plant

24

Approximately 15% of a refinery’s output is sold as liquid sugar. This can be Dissolved Granulated Sugar, Fine Liquor or intermediate liquors from the various crystallising stages. All these products are microbiologically filtered before storage, prior to loading into road tankers for delivery to customers.

S5 pumps handling Molasses in Denmark

25

6.0 Pump Specification Options Dependent upon application, various pump specification options are available on the ranges of SSP rotary lobe pumps as follows:

6.1

Mechanical Seals

Due to the tendency for media containing sugar particulates to harden when in contact with the atmosphere, the standard mechanical seal specification is a single flushed mechanical seal with hard faced tungsten carbide seal faces and FPM elastomers. The presence of a flush media will act as a barrier between the pumped media and atmosphere (acting as an interface film between the seal faces). This will inhibit drying out and hardening of the media, so avoiding seal face stiction (seal faces gluing together). The flush should be a warm compatible fluid e.g. water at a pressure of 0.5 bar max. Using a flushed mechanical seal will increase overall seal life and enable pumps to run dry without pumphead component damage. R90 Single Flushed Mechanical Seal Seal housing ‘o’ ring

Flush liquid

Seal housing Rotorcase

Lip seal Shaft

Stationary seal face

Rotary seal face

27

Shaft sleeve/spacer

6.2

Heating Jackets and Saddles

SSP Series S pumps have the option of being fitted with jackets to the rotorcase cover and/or saddles to the rotorcase. These are used for heating the pumphead so as to maintain the pumped media viscosity and reduce risk of any crystallisation / solidification. The maximum pressure and temperature of the heating fluid is 3.5 bar and 150°C respectively. Heating jackets and saddles should be in operation approximately 15 minutes prior to pump start-up and remain in operation 15 minutes after pump shut down.

Saddle

Jacket

Connections for steam or hot fluid

28

6.3

Wear Plates

Due to the abrasive nature of massecuite and magma in particular, a degree of component wear will take place over time, principally to the rotorcase and rotorcase cover. Pump rotational speed plays a key role in determining the rate of component wear, where put simply the faster the speed, the greater the rate of wear. This factor must be taken into account at the time of pump selection, by applying a maximum speed limit dependent upon the pumped media. Assuming correct pump selection, rotational speed relating to pumped media, the rate of wear should be extremely low. However, to increase abrasion resistance, SSP Series D and G pumps have the option of being fitted with replaceable wear plates. These are manufactured from hardened steel and can be replaced in situ with minimal pump dismantling.

Wear Plate to be fitted to back of rotorcase bore

For all Series D pumps wear plates can be fitted to rotorcase only. For all Series G pumps wear plates can be fitted to rotorcase and for G9 pump models wear plates can also be fitted to rotorcase cover. Additionally rotorcase covers are hardened as standard on all Series D and G pumps. If there is any doubt over the pumped media being handled at the time of pump selection, it is recommended that wear plates are supplied on all massecuite, magma and molasses applications.

29

6.4

Bi-lobe Rotors

On applications where the preservation of sugar crystals is highly desired, Series S and D pumps can be fitted with stainless steel bi-lobe rotors in place of the standard tri-lobe rotors to further minimise any potential crystal damage due to crushing. Bi-lobe rotors are typically able to handle solids of a size 1.5 times that of the tri-lobe equivalent. The majority of the small amount of crystal damage which does occur will take place either in the rotor mesh (the clearance between the two rotors) or the radial clearance. With a tri-lobe form, the rotors will come into mesh three times per pump revolution, whereas bi-lobe rotors only come into mesh twice per revolution. The extent and degree of crystal damage in the pump will be dependent upon the actual size and volume of the crystals in suspension. Should the crystal be significantly larger than the clearances, it will not be able to pass between the clearances, thus little or no crush damage will occur to either crystal or pumphead component. Should the crystal be significantly smaller than the clearances, it will pass between the clearances suffering little or no crush damage. Only when the crystal is approximately the same size as the clearances will there be moderate damage to either pumped media or pumphead component (dependent upon hardness of pumped media compared to component material). Fitting pumps with bi-lobe rotors should be considered on applications where the pumped media has a high crystalline solids content i.e. massecuite and magma.

Stainless Steel Bi-lobe Rotor

30

7.0 The SSP Advantage SSP Rotary Lobe Pumps offers significant advantages over other pump technologies as follows: •

Volumetric efficiency SSP rotary lobe pumps have a high volumetric efficiency typically in excess of 95% on applications where the product viscosity within the pump is 250 cP or greater. With most pump technologies as product viscosity increases, the volumetric efficiency decreases. However this is not the case with rotary lobe pumps, as efficiency increases with increasing viscosity, up to around 500 cP, thereafter efficiency remains constant at around 98%. As many sugar applications are related to media with a high viscosity, this means that to counter the loss of efficiency, competitors offering alternative technologies either have to increase their operating speed (if possible), which in turn increases pump wear rate, or more usually select larger pump units. The selection of a larger pump unit increases cost itself and also means more space required, plus an increase in absorbed power requirement, which results in larger more expensive drives and higher energy costs.

•

Solids handling The main factors contributing to rotary lobe pumps being able to handle solids are:

•

•

o

Cavity size within pump head. Rotor rotation produces a series of distinct cavities, which act to carry media from suction to discharge. The SSP rotary lobe pump design and principle of operation means the nature of these cavities are such that their size is constant throughout each rotor revolution and do not compress the product.

o

No rotor to rotor, or rotor to rotorcase / cover contact. See section 6.4 Bi-lobe Rotors.

o

Pump port geometry (size) Design of inlet port optimised to allow direct entry of media into rotorcase without sudden restriction, which would otherwise contribute to adverse pump hydraulic operation, leading to increased noise, possible NPSH/cavitation problems.

No contact between rotating elements Benefits include: o

Reduced crystal damage, thus optimising pumped media quality and therefore value.

o

Reduced media wetted component wear (rotors, rotorcase, rotorcase cover).

o

Reduced power consumption due to low friction/break out torque.

Dry running capability As there is no contact between rotating elements, the possibility of heat build up due to friction, which would otherwise lead to rapid component failure, is avoided. Provided the option of a flushed seal is selected, to ensure seal face/seat lubrication, the pump is able to run dry indefinitely. This in turn gives the advantage of increased process flexibility.

31

•

Slow start-up speed capability Benefits include: o

Ability to overcome pumped media yield stress in long pipe runs, thus providing greater system design flexibility.

o

Ability to handle high viscosity media, where there would otherwise be a significant risk of drive train overload, resulting in damage to pump shafts, bearings, drive/motor gearbox, and/or flexible coupling.

•

Operating pressures up to 20 bar (range dependent) Ability to handle high viscosity media over long pipe runs, thus providing greater system design flexibility and being able to use one pump technology (supplier) over a wide range of different media/applications, meaning a potential reduction in key supplier numbers and spares inventory.

•

Operating flow rates up to 400 m³/h Benefits include:

•

o

Increased process flexibility.

o

Optimised pump/speed selection, when considering pumped media integrity versus pump component wear.

o

Wide range of flow rate options from a single pump technology, meaning a potential reduction in key supplier numbers, leading to increased purchasing strength.

Compact size Benefits include: o

Optimisation of available space, providing potential for reduced costs.

o

Easy maintenance through easy access.

•

Ease of servicing SSP rotary lobe pumps are designed to be maintenance friendly by trained personnel.

•

Low overall cost of ownership The combination of all the above provides a pump solution with low overall cost of ownership, relative to other competing pump technologies.

32

7.1

Other Technologies

There are several pump technologies which could compete with SSP rotary lobe pumps in this industry, notably Gear Pumps, Sliding Vane Pumps and Progressing Cavity Pumps. 7.1.1

7.1.2

Gear Pumps There are two main types of gear pump; internal gear and external gear. A comparison made between SSP rotary lobe pumps and gear pumps is as follows: •

In a gear pump (internal or external), there is no external synchronisation, i.e. one gear drives the other gear. This means both gears are in constant contact with each other. A lubricant is required to ensure there is no excessive heat generation due to the constant contact. The lubricant is always the pumped media, which means gear pumps cannot run dry, thus reducing process flexibility. Likewise pumped media with poor lubrication properties also leads to overheating and wear problems. Both problems will lead to the rapid destruction of gears which will in turn damage the pump head, leading to pump breakdown, with associated pump rebuild/replacement costs. Gear Rotary Lobe ☺

•

Due to gear contact, component clearances within the pumphead are either very small or non-existent. When the pumped media contains abrasive solids, such as magma, massecuite and molasses, depending upon the relative hardness of the pumphead component to media solid, there will be abrasion. This abrasion will apply to both pumphead components, leading to performance loss resulting in increased maintenance/replacement cost, and to the pumped media with damage to the sugar crystals, so reducing product yield and quality. The rates of abrasive wear and product damage, due to the much smaller clearances, will be much higher than in a rotary lobe pump. Gear Rotary Lobe ☺

•

Due to gear contact, the pump may absorb more power during operation, to overcome the resultant frictional force. Any increase in absorbed power will lead to increased energy usage and therefore cost. Gear Rotary Lobe ☺

•

Gear pumps typically have lower initial unit cost. Gear ☺ Rotary Lobe

Sliding Vane Pumps A sliding vane pump consists of a slotted rotor or impeller supported within a cycloidal cam or liner, which in turn is mounted within a housing. Two cover plates then seal the housing from each side. The rotor is located close to the wall of the cam so a crescent-shaped cavity is formed. As the rotor/impeller rotates and fluid enters the pump, vanes located within the slots are pushed sequentially onto the walls of the liner, by a combination of centrifugal force, hydraulic pressure and push rods located in the slot behind the vane. This creates a distinct suction/discharge region within the pump head. As the pump continues to rotate, the housing and cam force fluid via holes in the cam into the pumping chamber, which then occupies the space created by the movement of vanes, rotor, cam relative to liner and cover plates. As the rotor continues around, the vanes sweep the fluid

33

to the opposite side of the crescent where it is squeezed through discharge holes of the cam as the vane approaches the point of the crescent. The fluid then exits the discharge port. A comparison made between SSP rotary lobe pumps and sliding vane pumps is as follows: •

Abrasive media will cause abrasion to the vanes and liner, resulting in higher maintenance requirements and increased life cycle costs. Sliding Vane Rotary Lobe ☺

•

The principle of operation means that vanes are in sequential constant contact with the liner. Dry running over an extended period of time is therefore not possible as the vanes need lubrication. High friction area between vane and liner will result in wear. Frictional force resulting from constant contact will need to be overcome, leading to increase in absorbed power requirement, therefore energy cost. Sliding Vane Rotary Lobe ☺

•

Vanes are self compensating for wear due to the action of the push rod behind the vane. Should vane wear beyond the rotor groove, it will become lodged between the rotor and liner, leading to pump failure. Sliding Vane Rotary Lobe ☺

•

Minimum rotational speed is required to provide enough centrifugal force to extend vanes to liner. This means ‘slow speed start’ operation is not possible, sometimes being required particularly on high viscosity applications (magma, massecuite, molasses), to overcome product yield stress and allow product movement. To overcome this, product would need either agitation/force feeding into the pump, or preheating or a combination of both, thus requiring additional investment in equipment or increased operating costs. Sliding Vane Rotary Lobe ☺

•

Potential of vane to ‘stick’ inside rotor on high viscosity applications. Sliding Vane Rotary Lobe ☺

•

Sliding Vane pumps are uni-directional, thereby loss of process/system flexibility. Sliding Vane Rotary Lobe ☺

•

Sliding Vane pumps typically have lower initial unit cost. Sliding Vane ☺ Rotary Lobe

•

Sliding Vane pumps only have one shaft seal, compared to two on a rotary lobe pump. Sliding Vane ☺ Rotary Lobe

•

Self adjusting vanes keep volumetric efficiency approximately constant, even when wear has taken place. Sliding Vane ☺ Rotary Lobe

•

Limited product ‘slip’ within the sliding vane pumphead, due to operation principle, provides a better solution on clean low viscosity applications. Sliding Vane ☺ Rotary Lobe

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7.1.3

Progressing Cavity Pumps A progressing cavity pump essentially consists of a single helix metal rotor which rotates within a double helix elastomeric stator of twice the pitch length of the rotor. The circular cross section rotor creates a continuously forming cavity as it rotates. The cavity progresses from suction to discharge, advancing in front of a continuously forming sealing line, within which the pumped media is transferred. The metal rotor is connected via a drive shaft assembly to a geared electric motor. A primary seal is located on the drive shaft assembly, at the suction end of the pump, to prevent media leakage. A progressing cavity pump is sometimes mistakenly called a screw or eccentric screw pump. A comparison made between SSP rotary lobe pumps and progressing cavity pumps is as follows: •

Due to the rotor being in constant rubbing contact with the stator, component clearances within the pumphead are non-existent. When the pumped media contains abrasive solids, such as magma, massecuite and molasses, depending upon the relative hardness of the pumphead component to media solid, there will be abrasion and excessive wear due to both axial and rotational movement. This will result in performance loss and increased maintenance/replacement cost. The rate of abrasive wear, due to zero clearances, will be much higher than in a rotary lobe pump. Progressing Cavity Rotary Lobe ☺

•

Progressing Cavity pumps have no dry running capability. If a progressing cavity pump is allowed to run dry there will be severe pump component damage and probable contamination of pumped media. A rotary lobe pump fitted with flushed mechanical seals has continuous dry running capability without damage to pumphead components. Progressing Cavity Rotary Lobe ☺

Rotary Lobe Pump with flushed mechanical seals enables continuous dry running capability without damage to pumphead components.

Progressing Cavity pump not suited for dry running.

No dry running capability- severe pump component damage and probable contamination of valuable pumped media

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•

For increased pressures, a large increase in physical size is necessary for progressing cavity pumps due to additional stages required. This will also lead to large increase in power required to overcome the increased friction from additional stage(s). Progressing Cavity Rotary Lobe ☺

Rotary Lobe Pump • Up to 20 bar capability from the same pump size. • Power increase only required to overcome pressure. 1 bar

or

20 bar

1 bar

20 bar Progressing Cavity Pump • Large increase in physical size resulting from additional stages. • Large increase in power to overcome increased friction from additional stages required.

•

Progressing Cavity pumps are uni-directional, thereby loss of process/system flexibility. Progressing Cavity Rotary Lobe ☺

•

Progressing Cavity pumps typically have lower initial unit cost. Progressing Cavity ☺ Rotary Lobe

•

Progressing Cavity pumps only have one shaft seal, compared to two on a rotary lobe pump. Progressing Cavity ☺ Rotary Lobe

•

Progressing Cavity pumps with zero clearances have better suction lift capability. Progressing Cavity ☺ Rotary Lobe

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•

Due to its compact design the rotary lobe pump occupies considerably less floor space than progressing cavity pumps, thereby providing potential for reduced costs. Progressing Cavity Rotary Lobe ☺ Planning the space

Saving the space

Major savings in time and manpower to change major pumping components resulting in reduced lifetime costs

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Strength Ability to pump abrasive media Compact size Easy maintenance Easy re-start Low capital investment Low energy consumption Low shear pumping Reduced lifecycle cost (versus others compared) Reduced lifecycle cost (versus Progessing Cavity) Single seal required Growing presence and acceptance High efficiency Large global presence Robust construction Suction capability Traditional concept Wide current acceptance Wide range of displacements

Weakness Ability to pump abrasive media Capital cost Dry running capability High spares cost Large size (versus others compared) Material compatibility Pulsation Pumped media contamination Two seals required Whole life cost Limited presence Limited range of displacements Still gaining acceptance Suction capability Bold typeface shows attributes that are considered relevent in this industry. Grey typeface shows attributes that are considered not relevent in this industry. 38

Gear

Lobe

Pump Technology

Progressing Cavity

A summary comparison of Rotary Lobe, Progressing Cavity and Gear pump technologies strengths and weaknesses is given in the table below:

8.0 Pump Selection and Application Summary This section gives information as to pump selection for different pumped media found in the Sugar Industry. It should be noted that the information given in this section is for guidance purposes only actual pump selection should be verified by our Technical Support after the provision of full pumped media and duty details.

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Sugar Applications Summary Viscosity applicable in pump low = < 50 cP med = 50 - 1000 cP high = > 1000 cP Pumped Media CARBONATATION SLURRY GLUCOSE GOLDEN SYRUP HIGH GREEN SYRUP LOW GREEN SYRUP MAGMA MASSECUITE MOLASSES SUGAR PULP - BEET SUGAR PULP - CANE SUGAR SYRUP THICK JUICE TREACLE

Pump Speed low = < 100 rpm med = 100 - 350 rpm high = > 350 - max rpm pump speed (system conditions permitting i.e. NPSHa etc) Viscous Behaviour Type Newtonian Newtonian Newtonian Newtonian Newtonian Pseudoplastic Pseudoplastic Newtonian Pseudoplastic Pseudoplastic Newtonian Newtonian Newtonian

Viscosity

Speed

Pump Series

Sealing NBR

med high high high med high high high high high med med high

med med med med med low low med low low med med med

D, G S S S, D, G S, D, G D, G D, G D, G D, G D, G S S, D, G S

Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush Single Flush

Elastomer Compatibility EPDM FPM X X X X X X X X X X X X X X X X X X X X X X X X X X

PTFE X X X X X X X X X X X X X