Chapter VISE

7/30/2019 Chapter VISE

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Chapter VI

Slurry Piping Systems

Slurry piping system, like nay other system, consists of slurry pumps, pipelines and

valves. Special considerations relevant to this systems are introduced by the factthat slurry in not homogeneous phase (unlike gas or liquid)

Slurry can be described as liquid with solids suspended therein. Typical industrialinstances of slurry handling are as follows:

Coal practices suspended in water ( in coal washeries), crystals suspended insolvent in crystallization ( wax manufacturing process) feed to any filtration

equipment. Pulp suspension encountered in papermaking, sludge encountered ineffluent treatment etc. One or more of various considerations such as solidconcentration determines behaviour of slurry. (Usually expressed in wt. % of solid),practical size of solids nature of solids (soft, hard, abrasive), properties of liquid(density, viscosity, chemical nature) etc.

The design of the system (including selection of proper materials) is governed byfollowing general considerations:

a) No solid deposition in system.b) No change in slurry composition from inlet to outlet of the system.c) Minimum wear and tar or erosion.

The design and engineering of the system components is discussed in the followingorder :

i) Line sizing and pressure drop.ii) Special consideration

iii) Pumps for slurry.iv) Instrumentation.

1. Line Sizing And Pressure Drop. :-

The basic steps in design are as follows :

1.1 Identify slurry characteristic .1.2 Select slurry concentration1.3 Select trial pipe size1.4 Calculate critical velocity1.5 Compare design velocity with critical velocity


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1.6 Calculate design friction loss

1.7 Calculate system pressure gradient and pump discharge pressure .

Explanations and / or clarifications relevant to each of above steps are discussed

below .

Step 1.1 Slurry Characteristics.

Slurries can be classified into broad categories as homogenous or heterogeneous.The type of slurry (Whether homogeneous or heterogeneous) dictates rheologicalproperties and there fore characterization is important .

Homogeneous Slurry .

In this type of slurry , solid particles are uniformly distributed in liquid medium . Suchslurries are characterized by high concentration of solids having small (fine) particlesize .Typical examples are sewage sludge , clay slurry .

Cement kiln feed slurry .

Such slurries usually exhibit non-Newtonian flow behavior (effective viscositychanging with shear rate) . Majority of them show behavior like Bingham plastic (noshear rate up to yield stress and Newtonian behavior for stress beyond yield stress ).

Heterogeneous Slurry

The solids are not uniformly distributed in liquid in such Slurries .In a horizontal pipethe concentration of solids is higher at lower levels and lower at upper levels .Such a

Slurry is characterized by low concentration of large size particles . Phosphate rockSlurry is typical example of this type .

Mixed behaviour of Slurry .

Many Slurries encountered in industry may show behavior in between homogeneousand heterogeneous .This would be more so when particles of different sizes

constitute the Slurry . In such a situation dominant characteristics have to beidentified and design procedure adopted to arrive at safer design . A typical exampleof this type is Slurry of coal particles in water .

Step 1.2 Slurry concentration (Solid content of Slurry)


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Solid concentration of Slurry becomes an important consideration for followingreasons :

Solids concentration governs the Slurry specific gravity (and hence pumping cost)and sometimes the rheoloy of the Slurry.

A typical graph for Slurry sp. Gr. Is shown on Figure 1.

(Note : Chart based on solid and liquid sp. gr. of 2.7 and 1.0 respectively)

At certain solid concentration Slurry may be difficult to transport or unstable. In suchsituations solids concentration would have to be selected for proper transportation.Solids concentration in static settled Slurry would be a useful guidance in thisrespect. Generally solid concentration 10-15 % below static settled Slurryconcentration would prove stable and convenient for handling.

Solid concentrations in settled Slurries could depending on nature of solid, vary overa wide range (10-50%) Generally solids with particle size of 0.4-0.5 micron mayform a stable Slurry for solid concentration up to 40% .

Step 1.3 Trial pipe size selection.

Since the throughout of Slurry (volume /time) is known selection of pipe size(particularly pipe Inside dia) will determine the velocity of Slurry through the pipe

(vol. flow rate = velocity area of cross section).

The velocity calculated is referred to as design velocity.


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Exercise 1. Flow rate of Slurry 3000 lit/minTrial pipe size 200 mm ID

Design velocity 300010 460 (0.2)2

= 1.60 m/s

The design velocity is significant with reference to the critical velocity .Critical

velocity is an important parameter for a Slurry. When Slurry flows at velocity belowthe critical velocity solids in slurry may start separating out and setting in a horizontal

pipe . The critical velocity is analogous to transition velocity in flow of homogeneous

fluids , the velocity at which laminar flow ceases to exist (Reynolds No. NRC = 2100).

Tendency of solids in Slurry to settle or separate out will be reduced in

pressure of turbulence .The critical velocity for a given Slurry will be determined bydifferent parameters such as size and specific gravity of solids , solids concentrationviscosity of liquid and degree of turbo lance .

Step 1.4 Calculate critical velocity

Calculations of critical velocity for homogeneous Slurry is now described asbelow :

For heterogeneous Slurries the procedure is little complex . The details of theprocedure for heterogeneous Slurry are available in an article by Aude et. Al (1)

For homogeneous Slurry , the procedure for calculating critical velocity is asfollows :

A) If the Slurry shown Newtonian behavior , then critical Reynoldss No.

NRec is considered as 2100 and critical velocity is calculated .

Ex.2 Slurry flow rate 3000 lit / min

Apparent viscosity 60 cpOf Slurry

(coefficient of rigidity)NRec = 2100 , Pipe ID = 200 mm

Slurry sp. gr. 1.61


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2100 = D (Vc) e = 0.2Vc1610

4 0.06

Ve = 2100 0.06 = 0.39 m/s

1610 0.2

B) If the Slurry is non-Newtonian type and exhibits Bingham plastics typebehaviour, then he procedure adopted for calculating critical velocity is as

follow:

B.1 Calculate design Reynolds No. NRe

Ref. Ex.1, NRe = DVe = 0.21.601610

0.06= 8586

To/B.2 Calculate plasticity no. PL = V/DEx. 3 Consider = 50 dynes /sq.cm.

= 5 N/sq.m.

From Ex.2 V =1.60 m/s D=0.2m .

PL = 5/0.06 = 50.2__ = 10.417

1.6/0.2 1.60.06

B.3 Calculate Hedstom No. NHe

NHe = NRe PL= 8586 10.417 = 89440

B.4 Use chart (shown as fig.2) to calculate NRec


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Enter the chart (Fig.2) with value of NHe , find the value critical Reynolds No. NRec.

In present case NRec = 6400 (say)

B.5 Calculate critical velocity (Vc)Critical velocity can be calculated by using one of the following

relationships

NRec = D (Vc) e

Or Vc = NRecV NRe

Ex.4 Since NRec = 6400 and NRe =8586And V = 1.60 for NRe = 8686

Vc = V NRec = 1.60 6400 = 1.193m/sNRe 8586

Step 1.5 Compare design velocity with critical velocity

Vc = 1.193 m/s (from Ex.4)V = 1.60 m/s (from Ex.1V-Vc = 1.6-1.193 = 0.417 m/s

The trial pipe size selected (step 1.3) should be such that(V-Ve) should be 0.3 m/s or more . In the calculations above(V-Vc) = 0.417 m/s , therefore selected pipe size is satisfactory .


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Step 1.6 calculate design friction loss .

Once the selected trial pipe size is satisfactory , the pressure loss can becalculated by usual equation i.e.

AP = 4f V2 L e

2 D

For this calculation , the numerical value off is to be selected from f Vs NRe charts .

Since f depends on roughness of pipe , as a considered (Hazen William factor =100) .

Secondly , it is a common practice to express the friction loss per say 10m of piping .For this purpose equivalent lengths of fittings etc. Have to be taken into account .

Ex.5 Consider F = 0.008

Friction loss per 10m equivalent pipe length

= 4 0.008 (1.6)2 10_ 16102 0.2

= 3297.3 N/m = 3.297 KN/m2

Since Slurry sp.gr. is 1.61

6.4 m of Slurry column = 1 atm (101.3 KN/m2 )

Friction loss /10 m of equlv. Piping = 3.297 6.4= 0.208 m Slurry column

Step 1.7 Calculate pump dish charge pressure

For transport of such a Slurry consider elevation change = 10mAnd total equivalent length =200m

Total head to be developed by pump=10 + 20 0.208 = 10.416 m (of Slurry)

10Since 1 atm =6.4 m Slurry


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Pump discharge pressure required (for flooded suction condition)

=10.4166.4

=1.63 atm

2. Special considerations

2.1 Slope of pipe lines : Slops of horizontal lines should not exceed angle

of repose for Slurry .2.2 Provisions for flushing and draining of pipe lines and manual cleaning .

2.3 Selection of wear resistance materials or higher thickness .2.4 Identification of wear prone points .2.4.1 downstream of weld

2.4.2 Joints for easy replacement of worn out positions .2.5 Use of long radius bends .2.6 No dead spaces

2.7 General observation : Wear /erosion is higher for velocities more

than 2.1 m/s whereas critical velocity may dictate minimum velocityas 1.2 m/s .

2.8 Use valves with maximum port size .2.8.1 Use full part ball valves.

2.8.2 Avoid use of globe value (seat may be plugged by solid deposition)2.8.3 Provide flushing connections for valves .

3. Pump for Slurry


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Different types as given below can be used

3.1 Centrifugal

2.1.1 For very dilute Slurries , solids concentration up to 10 L), sump typepumps could be used .

3.1.2 Impeller may have to be replaced frequently due to erosion .Therefore split casing type design is preferred .

3.1.3 Low efficiencies are to be expected .

3.1.4 Rubber lining may prove useful in many situations .3.1.5 Special wear resistant materials should be used .3.1.6 Flushing connection for shaft sealing arrangement is to be provided .

3.2 Positive displacement (Plunger/piston type ) .

3.2.1 Used for high discharge pressure (about 40 bar)3.2.2 Plunger type design is preferred for abrasive Slurries .3.2.3 Flushing arrangement for plunger packing is desirable .3.2.4 Liners of wear resistant materials inside the cylinder can yield

longer trouble free services .

3.3 Lock-hopper system

As shown and explained in fig.5 useful system for Slurry of coarse particles .

3.4 Surge log pump

Lock hopper system concept is used in conjunction withy a positive displacementpump . A chamber filled with clear liquid is interposed between piston and pumpvalves . Movement of piston creates a pressure sludge in clear liquid chamber whichis utilized to discharge the slurry .Such design have been used for abrasive Slurries .

3.5 Diaphragm pumps

This type of pumps are common choice when lower through puts are to be handled

for low delivery pressure (Typically up to 1000 1pm , up to 3 bar)

3.6 Moyno pump

A proprietary type design corpora ting advancing cavity concept .Used formoderate flows and pressure, the discharge in obtained at a steady pressure, wellsuited for thick slurries.


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4. Instrumentation

Pressure of solids and possibilities of erosion put many restrictions on instruments to

be used .Some relevant observation are as below :

4.1 For measuring slurry concentration, use of radiation density meters isconvenient .However , periodic calibration may be necessary .

4.2 If a side stream is drawn and then returned conveniently measured by

magnetic flow meters (which are rather expensive) .4.3 Flow rate of slurry can be conveniently measured by magnetic flow meters

(which are rather expensive) .

4.4 When positive displacement pump is used for slurry transfer pump speedand displacement can be used to calculate slurry flow rate.4.5 Pressure gauges and other instruments mounted on pipe line are

susceptible to damage due to vibrations.4.6 For measurement of pressure, diaphragm type gauges are recommended

hese should be provided with back flushing arrangements, connected topipe line with capillary. Moreover the gauges should be separatelysupportedand not mounted directly on pipe line.

References/Further reading : (1) Slurry piping system : Trends , T.C. Aude, N.T.Cowper , T.L. Thompson , E.J. Wasp, chemical Engineering , June28 , 1971, page74 - 87.

Heterogeneous Slurries :

More than Particle size(largest 5%)

(micron)

Solid sp.Gr.

(thin) 600-2000(thick) 1.0

(thin) 80-300(thick) 3.0(thin) 60-150(thick) 5.0

Deposition Velocity : Vd = (NFrc) gd

NFr = V2 , NFrc found graph .

gD


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Durand relationship

(f - fw) - overpressure correction to be calculated for all particle sizes .(f - fw) total = (f-fw) i Cv iCd - as Safe design take Cd = 0.44

Ex. D = 8 inch , V=12 ft/sec, = 80 lb/ft , w= 62.3 lb/ftCv= 0.166, Cv=0.44, f = 0.0037, Fw=0.00455

f - fw 100 = 23%f

sp.gr. solid 35% by wt ofwhich Determine

Slurry 1.28 20 Mesh (800micron-40%)

Cv (vol fr.)

Solids 2.70 10 Mesh (2000micron -60%)

for both sizes

Slurry gravity 1.28Basis : 1 kg Volume = 1___ = 7.813 10 -4 m3

1280

14 28 mesh 0.35 0.6 = 0.21 kg28 48 mesh 0.35 0.4=0.14 kg.

Solid gr. 2.70018 28 mesh vol 0.21 =7.78 10-5

27007.7810-5

Cv = 78.1310-5

= 0.0995

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