handbook de pulpas.pdf

C v Alfthan 10.6.2003 1 Hanbooktext 7.doc

OUTOKUMPU TECHNOLOGY

Sampling of slurries

1. Introduction ........................................................................................................................ 3 1.1 Characteristics of slurry ............................................................................................ 3 1.2 Goals of sampling system design ............................................................................. 4

2. Sampling for on-line analyzers ........................................................................................... 5 2.1 Principles for representative sampling with fixed cutters or nozzles. ........................ 5

2.1.1 Well mixed slurry ........................................................................................... 5 2.1.2 Isokinetic sampling ........................................................................................ 6 2.1.3 Dimensioning of cutters and nozzles............................................................. 6

2.2 Data needed to specify sampler ............................................................................... 7 2.3 Pressure flow samplers............................................................................................. 7

2.3.1 PSA pressure pipe sampler........................................................................... 7 2.3.2 Sector sampler .............................................................................................. 9 2.3.3 SPA Suction pipe sampler........................................................................... 10

2.4 Gravity flow samplers.............................................................................................. 11 2.4.1 LSA launder cutter box samplers ................................................................ 11 2.4.2 SKA stationary knife sampler ...................................................................... 12 2.4.3 Suitable sampling points for gravity flow samplers ...................................... 12

2.5 Two stage gravity samplers .................................................................................... 13 2.5.1 CPS Codelco primary sampler (patented)................................................... 13 2.5.2 Two stage cutter sampler (patented)........................................................... 15

3. Sample transport .............................................................................................................. 16 3.1 Principles of dimensioning sample piping ............................................................... 16

3.1.1 Pumping ...................................................................................................... 16 3.1.2 Gravity pipes ............................................................................................... 16 3.1.3 Pressure pipes ............................................................................................ 16

3.2 Route of pressure pipes.......................................................................................... 17 3.2.1 Principles of selecting the slopes and route of pressure pipes.................... 17 3.2.2 Examples of pressure pipe routes............................................................... 19 3.2.3 Pressure pipe pressure requirements and route ......................................... 22

3.3 The route of gravity lines......................................................................................... 22 3.3.1 Airlocks........................................................................................................ 22 3.3.2 Reasons for airlocks.................................................................................... 23 3.3.3 Effects of airlock .......................................................................................... 24

3.4 Installation of pipe................................................................................................... 25 3.4.1 Recommended sample pipe........................................................................ 25 3.4.2 Pipe curvature ............................................................................................. 26 3.4.3 Contractions in pipe diameter...................................................................... 26 3.4.4 Installing the pipe ........................................................................................ 26

3.5 Avoiding pipe blockages. ........................................................................................ 27 3.5.1 Volume objects............................................................................................ 28 3.5.2 Long stiff objects like sticks ......................................................................... 28



4. Composite sampling......................................................................................................... 30 4.1 Cross-stream samplers........................................................................................... 30

4.1.2 Flexible hose pulp samplers........................................................................ 31 4.1.3 Circular path cross-stream samplers........................................................... 32 4.1.4 Straight path cross-stream samplers........................................................... 32

4.2 Multiple fixed cutters ............................................................................................... 33 4.3 Automatic filtering units........................................................................................... 34 4.4 Combination of multiple fixed cutters and cross-stream samplers for large flows... 34



Sampling of slurries

1. Introduction

Many instruments necessary for process control cannot measure the full process flow and/or the instrument is utilized for measuring many streams to share costs. Such on-line instruments are typically elemental and particle size analyzers. They will usually need a continuous or semi continuous sample stream.

For metallurgical balance calculations and product quality assurance usually laboratory analysis of composite samples are used to get the best possible accuracy. Due to the delays laboratory assays are not so good for process control even if they are more accurate than the process analyzer values.

One frequently forgotten part of the sampling is the flow of a primary sample to the analyzer or to a central secondary sampling location and the return of the sample to the process. This is also a critical part of the system.

Very important for overall accuracy of assays is how the sample is presented to the measurement head of the analyzer or resampled within the analyzer system, but these issues are not treated here.

1.1 Characteristics of slurry

To keep slurry in suspension the flows are always turbulent. The turbulence keeps the slurries more or less well mixed in contrast to for instance dry solids on a conveyer belt. The finer the solid the more complete is the mixing. 100 micron could be set as the particle size above which segregation in the vertical direction in the gravitation field starts to play a significant role. In the horizontal direction mixing is still very effective.

This mixing makes it possible to use simple robust samplers and still in most cases get a representative sample.

Trash is a significant part of the slurry, as a minimum threads and plastics from the explosives and some stones. The worst case is a lot of woodchips from old mine props. Occasionally there are of course all kinds of other trash like nails or screws or pieces of metal wire. The sampling system must be well engineered to be able to handle also this trash



The size of the sample cutter opening should be preferably 20 mm or more not to get clogged. More narrow cutters down to 8 mm can be used, but small fixed cutters constantly in the process flow should have an automatic cleaning device.

1.2 Goals of sampling system design

• Reliability is the most important characteristic expected from the system. It should operate without blockages at all process conditions regardless of trash and changes in the process flow rates.

• Maintenance should be minimal

• Investment cost should be low.

• The representativity of the sample should be good.

• Sample flow rate should match the requirements of the analyzer system and the sample transport.

Not only the normal operation of the plant must be considered but also

• Power failures

• Loss of instrument air

• Startup of sample line or analyzer system

• Shut down of sample line or analyzer system

Fail safe and automatic startup should be the goals.

Sampling for on-line Instruments should be first and foremost reliable. Control is based very much on trends that require frequent and reliable assays. The assays are not quite as accurate as laboratory assays so some concessions can be done to the representativity of the sample.

Thus almost exclusively fixed cutter or nozzle samplers are used. For samples fine enough practically no error is introduced using a round central nozzle in a vertical stream or a vertical cutter in a horizontal stream of rectangular cross-section. For coarser samples one must take care. A long enough section with turbulent mixing should precede the sampler.

Sampling for product quality or metallurgical balance composite samples requires more. Each increment to the composite sample should not only be representative of the concentrations but also of the momentary solids flow. A sample fulfilling these requirements can in



practice only be taken from a gravity flow. Because the composite sample of only some liters shall represent the plant flow of a shift or a day sampling must be done in several stages. The samplers are typically crosscut samplers of one type or another or for large streams multiple vertical cutters. These sampling systems are often much more expensive and maintenance intensive than the simple sampling systems for on-line analyzers.

2. Sampling for on-line analyzers

2.1 Principles for representative sampling with fixed cutters or nozzles.

2.1.1 Well mixed slurry

The fixed sampler takes sample from a fixed part of the cross-section of the process stream. This means that the process flow in that part must be representative of the whole stream.

In vertical pressure pipes a round nozzle can be used. (In the center) The flow is well mixed after a pump or after a suitable length of vertical pipe. (Longer than 10*diameter of pipe) There is no force causing segregation in the horizontal direction.

In gravity pipes or launders with reasonable slope vertical cutters are used that take an equal slice of all horizontal layers, which might have different contents due to segregation. Usually the cutter has a constant opening for a rectangular process cross-section.

Mostly the slurry is reasonable well mixed in the horizontal direction but there are situations when this is not the case.

1. Two streams are joined before the sampler. A Y-joint is the most dangerous. There is very little mixing. A T-joint is better because there is more mixing between the two streams

2. There is a long horizontal run of the pressure process tube before a vertical part from which a sample is taken with a round nozzle.

3. There is a horizontal bend of a gravity flow just in front of the sample cutter. The process flow is forced by the centrifugal force to the outer wall and the process flow is not any more nicely layered in the vertical direction.

There are also situations, which generally are believed to cause severe demixing while actually it is not so.



A bend in a full pipe is believed to cause demixing. “The proof” is the severe wear of the outer part of the bend. Actually the centrifugal force in the bend is only about the order of magnitude of gravity and thus the horizontal segregation effect is similar to the vertical segregation in a short horizontal section. In the middle of the stream the coarse particles move towards the outer edge of the bend, but in practice almost as many particles move out of the center as moves into the center from the inner curve. Only at the inner curve there will be a thin watery layer and at the outer curve there will be a layer with preponderance of coarse solids. A flow with the speed of 2 m/s at a radius of 0.4 m will give a centrifugal force of gravity. The centrifugal force is proportional to the square of the velocity and inversely proportional to the radius.

2.1.2 Isokinetic sampling

For good representativeness the flow velocity into cutter or nozzle has to be about the same as the velocity of the bulk flow around the cutter or nozzle. The matching is not very critical differences of ± 1m/s are allowed for slurries of normal particle size.

Sample flow q

Process flow Q

Sample cutter

Sampler body

If sample flow is too high (suction in sample tube) fines preferable go into the sample.

If sample flow is too low (cutter or nozzle is plowing the process stream) coarse particles accumulate in the sample.

2.1.3 Dimensioning of cutters and nozzles

From the isokinetic principle follows that the ratio of sampler cutter or nozzle area a to process stream area A is the same as the ratio between required sample flow q to process flow Q.

a/A = q/Q



For a round nozzle of diameter d in a round pressure pipe sampler body we get:

d2/D2 = q/Q

For a cutter of width w in a gravity flow launder of width W

w/W = q/Q

2.2 Data needed to specify sampler

• Process pipe or launder size

• Pressure or head in process pipe and rough filling level for gravity pipes

• Range of flow rate

• Flanges

• Useful to know

o % solids

o Froth factor

o Particle size

Note that correct functioning of the sampler requires:

• Correctly selected location

• Correctly designed and installed sample pipe

• Sampler should be accessible from service platform

2.3 Pressure flow samplers

2.3.1 PSA pressure pipe sampler

TYPICAL APPLICATIONS

• Pumped final tailing lines

• Pumped process lines



The pressure pipe sampler is always installed in a vertical orientation for good representativity, generally immediately above a process pump. A good alternative is after a long (>10 x diameter of pipe), straight vertical stretch of the pipe.

The sample is taken with a nozzle from the middle of the turbulent flow region in the sampler.

Sample nozzle

Body

Flush water

Primary sample

Process flow

Process flow



2.3.2 Sector sampler


• Small pumped process lines

In the case of a very small process flow a major part of the flow must be taken as a primary sample. In this case a suitable nozzle would be too large to be practical. Instead an internal wall cuts a sector of the flow in the vertical sampler to give a sample. One fourth or more of the original process flow can be taken in this way.

Section A-A

Process flow

Process flow

Primary sample flow

Flushing water

A A



2.3.3 SPA Suction pipe sampler


• Feed boxes with several inlets

• Flotation cell discharge or intermediate box

The suction pipe sampler is used when other standard samplers cannot be used. A pipe is inserted into a well-mixed vessel and a sample extracted with the pipe. If there is a direction of flow in the vessel, isokinetic sampling is approximated by directing the opening of the pipe so that the direction of the flow in the vessel is into the pipe opening. The pipe or nozzle is preferable directed downwards so that the nozzle is not filled with solids if the sample flow is cut off.

Tank slurry level aboveSample tube

Flush water

Suction pipe sampler ascontinuous sampler

SlurryTank

Flush water

Suction pipe sampler ason-demand sampler Automatic valves

Sampler outlet

If the sampler is used to take a continuous sample, the sampler and the sample pipe should over its whole length be under the slurry level of the vessel from which the sample is taken. Otherwise the sample flow will not start and if it is manually started, by filling the sample pipe with water, it will normally slowly stop.

With automatic regular flushing suction can be used and the sample taken over the edge of the tank. The tank needs no sampler outlet. The sampler can be taken out and serviced at any time.



2.4 Gravity flow samplers

2.4.1 LSA launder cutter box samplers


• Cyclone overflow pipes

• Distributor pipes

• Concentrate pipes

SPECIAL OPTION

• Remotely controlled periodic mechanical cleaning of cutter

The LSA is suitable for sampling from near horizontal, non-pressurized pipes. A cutter installed in the middle of the box shaped body of the sampler cuts a sample from the process flow through the box.

Processflow

Flush water

Flush waterPrimary sample

Inspection lid Cutter Body

1 meterfree space

Reserve space for valves and pipe curve about 1 meter below sampler

Best sampling results are achieved in pipes with a turbulent flow and a reasonably constant flow rate. The vertical cutter extracts a repre-sentative sample of the slurry flow, even if the slurry is segregated in the vertical direction.

Sample flows out of the sampler by gravity.



2.4.2 SKA stationary knife sampler


• Launders

• Flumes

Flush water

Flush waterPrimary sample

Processflow

1 meterfree space

Cutter

Reserve space for valves and pipe curve about 1 meter below SKA

2.4.3 Suitable sampling points for gravity flow samplers

There are 4 basic requirements for the sampling point:

1) There is no pressure in the process pipe at the selected point. The sampler has a certain flow resistance. Thus ideally process pipe should only be about half full without sampler.

2) The flow velocity in the pipe or launder is not so high that it causes excessive wear. A long slope before the sampler must be small usually less than about 2 %.

3) The process pipe or launder is nearly horizontal at the sampling spot, slope to be less than 20 %.

4) If two different flows are joined a straight length of pipe or launder of 10 times the diameter of the pipe or width of the launder is recommended.



2.5 Two stage gravity samplers

For large flows typically the slit to take a good isokinetic sample becomes too narrow to be practical. Traditionally then the sample has been taken in two stages using a gravity sampler at each stage. However the primary sample of necessity is brought down about a meter or so and frequently cannot rejoin after secondary sampling the main stream without pumping. For this reason a couple of “two stages in one” samplers have been developed.

Both samplers take first a primary sample from gravity flow and next take a secondary pressure sample.

2.5.1 CPS Codelco primary sampler (patented)

A “boot shaped “ primary sampler made of a round tube takes with its open “toe” a sample of the gravity flow process stream. The sample flow is turned into the vertical direction in the boot and returned back into the process flow through an opening in the back. The secondary sample is taken from the upper part of the vertical flow by a round nozzle.

TYPICAL APPLICATIONS:

• Cyclone overflow




Size and level of the boot

The toe of the boot should be well submerged in the incoming primary process flow to give a representative sample. This is a critical issue. This limits for small flows the size of the boot, which is close to the bottom, leaving a gap for stones between boot and bottom of launder.

The boot and the frame holding it, if they are big, can dam the whole flow creating a layer with disorganized flow on top of the fast primary process flow. The boot should not sample this layer.

Nozzle

The nozzle should be in a good vertical flow after some mixing. Thus it should be in the upper part of the vertical flow section

The nozzle size should be selected according to the isokinetic principle depending on primary and secondary sample flow.

The secondary sample flow determines the sample pipeline required according to standard sample pipe dimensioning for pressure samplers.



2.5.2 Two stage cutter sampler (patented)

The sampler is generally suitable for sampling near horizontal non-pressurized pipes or launders with flows higher than 420 m3/h. The primary adjustable cutter in the center of the stream cuts a representative sample from the process flow. The sample is reshaped in the cutter to a broad low cross-section. This makes it possible to use a second cutter to reduce the sample flow to the required size. Best sampling result are obtained for turbulent flows with a reasonably constant flow rate. See pictures below.

Side view of sampler.

Top view of sampler

TYPICAL APPLICATIONS:

• Cyclone overflow


The outflow from the sampler is full pipe flow and the sample pipe should be calculated and designed as a pressure pipe

Reserve space for valves and pipe curve about 1 meter below TSC



3. Sample transport

3.1 Principles of dimensioning sample piping

(Dimensioning formulas and tables in connection with process tube and pump dimensioning)

As a rule of thumb sample pipes requires as a minimum 10% of the tube length as head. This is much more than for normal process pipes. Pipe dimensions are typically 32-50 mm inner diameter.

3.1.1 Pumping

Vertical pumps with integrated pump sumps are recommended. They can run dry without creating problems.

Pumps after pressure flow samplers can also be horizontal pumps. The sample pipe from the sampler should be directly connected to the inlet. The pump speed is adjusted to give the right sample flow

3.1.2 Gravity pipes

Gravity pipes should generally be used after gravity samplers. �

• They are not full

• Slurry flows only downhill

• They have no pressure

• Flow speed and thickness of slurry depends on slope of pipe

3.1.3 Pressure pipes

Pressure pipes should be used after pressure samplers and two stage gravity samplers.

• They are full

They are dimensioned to give enough friction to offset the available head at a suitable sample flow. The practice to adjust the flow from the pressure sampler with a valve is not recommended due to high wear of valve and risk of blockages.



3.2 Route of pressure pipes

3.2.1 Principles of selecting the slopes and route of pressure pipes

If the pump stops, maybe the supply of electricity is lost; the pipe should drain as well as possible. The slurry should flow out to empty the pipe. No sagging of the pipe is allowed. Any slurry, left in the pipe, will settle, causing an obvious risk of blockage. As the pipe is seldom ideally supported, the practical minimum slope is about 2 %.

No

Care must be taken to ensure that the slurry, which has settled on the bottom of the pipe, will never shift in a "landslide", because this would result in a blockage. The most dangerous moment is when the uphill flow stops and the friction of the slurry against the solid bed is lost.

Slurry flowkeeps solids bedin place

When slurry flow stopsVibrations put solids

landslide in motion

The pipe must be either nearly vertical (so slurry cannot settle during flow) or nearly horizontal, having a slope of less than 20° (35%) (So solids on the bottom will remain immovable). A downstream pipe may have a steeper slope because the solids will not settle so easily on the



bottom of the pipe. However, the downstream slope should be the same throughout the length of the pipe or it should get steeper towards the end.

Air in the slurry causes a different problem. If a negative pressure condition occurs at any point in the pipe, the air will expand and the effective specific weight of the slurry is reduced. The upper part of a pipe followed by a steep downward slope is often such a point. In such cases, the siphon pressure created by the slope is easily lost. It may prove difficult for the air to move downwards, and it gathers in the upper part of the pipe. In the end there will be gravity flow in the steep downstream slope.

Pressure flow Gravity flow

Steep downstream slopes must be avoided if the siphon effect of the pipe is to be utilized.

No emptying

Risk for landslide

Some risk for landslideRisk for loss ofsiphon effect

Slurry flow

70

20

1.2

-1.2

-20

-70

O

O

O

O

O

O



The gray areas are the ranges of pipe slope to avoid when the slurry flows out of the center of the circle. Slope is in degrees.

3.2.2 Examples of pressure pipe routes

Each of the diagrams on the following pages shows at least two route alternatives, a good one and a less good one. The following symbols are used in the figures:

Direction of slurry flow

Direction of emptying of pipe

Emptying of pipe not guaranteed

Forbidden Routing

Undesirable routing to be avoided

Risk for landslide

Sample flowDraining



Bad draining

Settling in vertical

pipe if sample valve is closed



Loss of siphoneffect

Landsliderisk

a valve opening when the pump shuts downis added. Make the connectionof the valveto the main pipe short to avoid solids plugging it up.

This pipe does not drain unless;



3.2.3 Pressure pipe pressure requirements and route

Head H

Length of sample tube is L

L < 10*H is OK10*H < L < 20*H may be OK

Head is not sufficient to startsample flow

The sample pipe should at no point go too close or higher than the zero pressure point of the process pipe; otherwise the sample flow will not start. Because the sample pipe is smaller, it has a very much higher friction than a process pipe. Thus the head should be at least 10 % of the sample pipe length to assure a reliable flow. A 5 % head can sometimes be enough under favorable circumstances. A sampling engineer can decide if the slope is sufficient.

3.3 The route of gravity lines

A gravity line is by definition "a pipe in which the slurry at all points is at ambient pressure". The pipe is continuously sloping down so that gravity gives the energy to overcome the friction of flow. Changes in flow velocity caused by changes in slope cause a varying thickness of slurry flowing on the bottom of the pipe. This assumes that air can freely move about in the upper part of the pipe to accommodate the varying thickness of slurry.

3.3.1 Airlocks

However, airlocks that stop the flow of air are easily created in small diameter sample pipes. If the pipe is open at both ends to air, one airlock is generally not serious but if there are two such airlocks, the section between the two airlocks can become an unwanted pressure pipe.

A breather pipe can of course be installed between the two airlocks preferable close to the upper one. These breather pipes have however



a tendency to clog up unless they are regularly cleaned and flushed. A clear break of the pipe might be a better solution.

Airlock

Airlock

Breather

Airlock

Airlock

Break in pipe

3.3.2 Reasons for airlocks

• Sharp bends

Section with air that will be slowlydepleted until the tube is so filled that it sucks the upper airlock openand the section is again filled with air. NO

Vertical section between two airlocks can lead to surging, producing oscillating flow. The sharp bend forms an airlock because additional energy is required to quickly change the direction of flow.



• Horizontal section

NO

To get the slurry to flow through a pipe sloping upward or a horizontal or nearly horizontal section of the pipe, some head is required. This means that the pipe fills up. A down-slope of 5 % is generally the minimum requirement to avoid this type of airlock.

• Internal pipe joining piece

NO

If pipes are joined by using a short piece of smaller diameter pipe, the cross-section of the pipe is easily halved at that point. A half full or fuller pipe will then be completely filled at that point

• LSA sampler full with slurry

This situation is to be avoided otherwise no air flows into the cutter. This easily leads to the whole sample pipe becoming a pressure pipe. This makes the slurry flow rate too high if a normal gravity flow pipe size has been used. To correct the situation the sample pipe has to be selected smaller in diameter than otherwise. It is engineered as a pressure pipe. An alternative solution is to add a breather pipe close to the cutter.

• Deaccelerator on top of multiplexer

This is a unit used to fill the multiplexer hose to reduce the speed of the slurry hitting the screen in the multiplexer. It acts, then, by definition as an airlock and to maintain proper gravity flow, no other airlocks are allowed.

3.3.3 Effects of airlock

One airlock creates no problem



Two airlocks, of which the upper upper one can be opened, give the following situation

1.The space between the airlocks is filled with slurry

2.Slurry flow increases = surging

3.Upper airlock is momentarily opened and lets air into space between the airlocks

4.The space between airlocks is filled with slurry

And so on

Two airlocks, of which the upper one cannot be opened, give the following situation:

1. The space between airlocks is filled with slurry

2. Slurry speed increases = surging that continues

Typically the upper airlock that cannot be opened is the LSA, SKA cutter fully submerged in the process flow. Sample flow will most often increase much too much. In this case sample pipe must be reduced in size and calculated like pressure pipe. Sample flow is constant and not a fraction of process flow. Isokinetic condition as for pressure samplers

3.4 Installation of pipe

3.4.1 Recommended sample pipe

The sample pipe must be made of a wear-resistant material. It is recommended that a high-density polyethylene (HDPE) pipe be used.

For smaller pipe sizes coiled pipe can be used. It is however difficult to get the pipe completely straight without heating. The number of joints along the line should be minimized, and the pipe must not be bent beyond its flexible range. Bigger pipe sizes need heating to make the bends.

The inside diameters of the pipes vary between 20 and 50 mm for pressure pipes. The most common standard sizes (inside diameter) are 26, 32 and 41 mm. Gravity pipes are generally 50 mm but larger pipes can be used.



3.4.2 Pipe curvature

To minimize wear, the radius of the bends in the pipes should be at least 500 mm, preferably more. THE STANDARD READY MADE BENDS FOR WATER AND SEWAGE PIPES MUST NOT BE USED.

Minimum radius 500 mm NO

3.4.3 Contractions in pipe diameter

Normally, no contractions in the diameter of the transfer pipe are allowed. REDUCING THE SLURRY FLOW BY THROTTLING IS FORBIDDEN. At the throttling point, the wear increases dramatically as does the risk of blockage. Diameters exceeding the normal diameter of the transfer pipe are allowed, but even then the wear increases at the point, where the diameter is reduced to its normal value.

3.4.4 Installing the pipe

The way in which the pipe is installed is also important for a trouble free operation of the sample line.

It is important that the pipe is properly mounted. Because of the centrifugal forces, pipe bends should be well fastened on both sides of the bend. Special attention should be paid to the multiplexer connection. A pipe with a small angle of inclination must be adequately supported to prevent sagging. It may even be necessary to provide the pipe with a continuous angle iron support or with supporting brackets placed at distances of some 60 cm from each other. A plastic pipe has a high coefficient of thermal expansion, so supports must accommodate pipe motion.

Pipe joints can be simply made by using a length of flexible hose of a suitable size to accommodate the plastic pipe inside it. Insert the plastic pipes end to end inside the flexible hose and fasten them with hose



clips on both sides of the joint. Anchor the pipes on a support on both sides of the joint. In the event of pipe blockages, this kind of joints can be dismantled and reassembled to allow the pipe to be cleared. Welding is used to make joints of a more permanent nature. Note the risk of blockages caused by this method. (See two pages down)

Hose clamps

Rubber hose

Plastic pipe

Fasteners of pipe

Note downstream pipe beveled

3.5 Avoiding pipe blockages.

Blocking of pipes can be caused by solids settling in the pipes or by foreign objects. Settling can be avoided by using sufficient flow speed and by a good pipe route as explained in previous sections. Foreign objects are another matter.

Foreign objects can be of several kinds:

1. Volume objects. Seldom stones cause blockages in pipes (Cutters are a different matter) more often lighter substances like rubber and plastic can be the culprit.

2. Long stiff objects like wooden sticks or pieces of metal wire. These pass through the pipe well if they are parallel with the flow-lines, but if they are longer than the pipe diameter and turn across the stream they easily get stuck.

3. Soft objects like plastic sheet or threads. These seldom are the primary cause of blocking but often wrap around objects in their path, like long stiff objects and cause the final blockage.



3.5.1 Volume objects

Volume objects are removed by screening. Large objects can get stuck at points where the flow channel is reduced, like going from a PSA nozzle into the sample pipe. The rule is to make that reduction easily accessible. So, in the PSA, the valve should have an opening at least equal to the nozzle size and the reduction in size occurs after the valve. Thus the valve can be closed and the blockage found after the valve, where it is easy to remove.

3.5.2 Long stiff objects like sticks

Long stiff objects generally follow the flow quite well. The end of the object can scrape against the wall but will continue its path unless the scraping end finds a step that stops its movement. The object then turns across the stream and easily gets lodged. These blockages typically occur at pipe joints and constrictions.

Welded butt joints

NO YES

The stick is stuck

Internal pipe joints

NO

The stick is stuck



External pipe joints

The stick is stuck ?

YES ?Beveled downstream edge

YES !

The stick is stuck

NO YES

Pipe diameter contraction



4. Composite sampling

The composite sample shall correctly represent the production, feed or tail of the process during for instance a shift or a day. That means that at regular intervals a small representative sample is added to the composite the amount of which is also a fixed proportion of the process flow. This second requirement is not required for on-line analysis when only the current assay information is needed.

A well-designed pressure sampler will give a representative sample, but the amount of sample is not proportional to process flow. The amount is dependent of process pressure, which often is more or less constant only slightly dependent of process flow. Thus the sample flow from a pressure sampler will be mostly almost constant independent of process flow. If the process flow is constant as often can be argued for feed and tails at least for some time intervals a pressure sampler of course can be selected for its robustness.

All theoretically correct composite samplers are gravity samplers. If all the sample flow that enters the sample cutter unrestricted can continue its way into the sample proportionality is usually assured.

One challenge is that the composite sample should be easy to handle and thus have a volume of about 5 liters. On the other hand increment samples should be added as often as possible to be representative of the time history of the plant. Thus each increment should be about 1dl. This leads in most cases to multiple sampling stages each one fulfilling the requirements of composite sampling.

For a on the short term reasonable steady process stream, a generally accepted solution to get a representative sample increment, is a cutter that moves across the whole stream giving equal opportunity for all parts of the stream to enter the cutter. This principle is essential to use for strongly segregated process streams like solids on a moving belt. Many slurry streams are so well mixed especially in the horizontal direction so also other more robust fixed cutter solutions can be used.

4.1 Cross-stream samplers

The rules for good sampling have been carefully developed to give good sampling for all kind of materials. For slurries that are well mixed the factors influencing representativity can be relaxed. The factors influencing proportionality must be stringently followed.



4.1.1.1 Rules for representativity

To give equal opportunity for each part of the stream to get into the sample the cutter edges should be parallel for a straight path cutter and edges should be radial for a circular path cutter.

Also speed of cutter movement should be uniform. It should be less than 0.4 m/sec.

During possible resting periods of the cutter, it must be well protected for sprays and dust.

After the cutter should be a volume that without resistance takes the entire sample.

The cutter width should be least 8 mm.

4.1.1.2 Rules for proportionality

The cutter movement speed should be repeatable

The time intervals between cutter movements should be constant

After the cutter should be a volume that without resistance takes the entire sample.

4.1.2 Flexible hose pulp samplers

This sampler is for small streams. Typically they have been used to take composite samples from sample streams for on-line analyzers.



A pneumatic cylinder flips the hose back and forth over a rectangular cutter. The amount of slurry in one back and forth movement is

s = 2wQ/v ,where

w is the width of the cutter, v its speed and Q is the primary flow as volume/time.

4.1.3 Circular path cross-stream samplers

This sampler is also called a Vezin sampler. The sampler has a sector shaped cutter that rotates around an axis collecting sample from normally a falling stream. This sampler is normally used for relatively small streams typically as a secondary sampler after a previous straight path cross-stream sampler

The sample of one cut is

s = 2aQ/� ,where

a is the angular width of the cutter, � its angular speed and Q is the primary flow as volume/time.

When this sampler is used to reduce the size of one cut of a primary usually straight path cross-stream sampler the reduction ratio r is the ratio of the cutter angle to the full circle.

r = a/2�

4.1.4 Straight path cross-stream samplers

This sampler has a parallel edge cutter roughly perpendicular to the process stream normally falling from a lip. The cutter is mounted on a carriage moving the cutter from a position outside the lip to a position



outside the lip on the other side and back. This sampler is often used for medium sized streams. The sample from a cut in one direction is

s = wQ/v

EXAMPLE

We have a stream with a flow of 6 m3/min. To minimize the sample the smallest allowed width of the cutter 8 mm and the fastest speed of the cutter 0.4 m/s are used. The sample from a unidirectional cut is then 2 liters. This volume is collected in 1-3 seconds in a small tank, which is emptied through a small tube past a flexible hose sampler or a circular path cross-stream sampler, which reduces the sample with a factor of 20 to get the required increment of about 1 dl. We then get for the continuously operating circular path cross-stream sampler a cutter with an angle of 18o between the radial edges.

4.2 Multiple fixed cutters

For large flows more sampling stages are required. Very big cross-stream samplers are very expensive and maintenance intensive.

An alternative that can be used for slurries is to have a mixing tank and after the tank a set of several fixed cutters, which take samples from the middle and both sides of the stream in a box shaped launder. This gives a more representative sample than a single cutter in the middle, but is of course not quite as good as a cross-stream cutter. On the other hand a theoretically slightly inferior sampler with little downtime is in practice to be preferred.

1. The mixing tank should be such that the risk of sanding is eliminated.

2. To prevent clogging of the cutters the width of the cutters should be 20 mm or more.

3. To give a good representativity the ratio of the sample flow to the process flow is preferable 1/10 or bigger



4.3 Automatic filtering units

Automatic filtering units are often used to remove the water from the sample before the sample is taken to the laboratory. This makes it possible to have more or bigger sample increments improving the representativity of the sample. The % solids information is of course lost.

In the picture is an example of a commersially available filtering unit for up to 12 streams

4.4 Combination of multiple fixed cutters and cross-stream samplers for large flows

Large flows require several sampling stages to reduce the increment sufficiently while retaining the representativity and the proportionality to process flow. This means in practice that a series of gravity samplers all fulfilling the representativity and proportionality criteria are used.

Multiple fixed cutters in the first stage(s) give continuous sample flow. Cross-stream samplers on the other hand give discontinuous flow and the sample from the beginning of the cut might be different from the end of the cut. Thus secondary cross-stream samplers must give equal



opportunity for the beginning and end of the cut to go into the sample. This means that many secondary cuts have to be made from the one primary cut. A mixing tank of some sort between the stages is also recommended.

Several gravity sampling stages means that the process stream falls down to a lower level, to which all the primary samples except the final composite sample increments is to be returned. The loss of head generally causes costs in the plant design and should be minimized.

Maintenance and optical inspection of cutters and other parts of the sampling system should be simple.

handbook de pulpas.pdf

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