Sunday, May 27, 2012

Slurry Piping Systems, Its Design And Instrumentation

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Slurry piping system, like nay other system, consists of slurry pumps, pipelines and valves. Special considerations relevant to this systems are introduced by the fact that slurry in not homogeneous phase (unlike gas or liquid).

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

Coal practices suspended in water ( in coal washeries), crystals suspended in solvent in crystallization ( wax ma nufacturing process) feed to any filtration equipment. Pulp suspension encountered in papermaking, sludge encountered in effluent treatment etc. One or more of various considerations such as solid concentration 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 by following 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 following order:

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 concentration

1.3 Select trial pipe size

1.4 Calculate critical velocity

1.5 Compare design velocity with critical velocity

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 rheological properties and there fore characterization is important.

Homogeneous Slurry

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

Cement kiln feed slurry

Such slurries usually exhibit non-Newtonian flow behavior (effective viscosity changing with shear rate) . Majority of them show behavior like Bingham plastic (no shear 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 pipe the 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 rock Slurry is typical example of this type.

Mixed behavior of Slurry

Many Slurries encountered in industry may show behavior in between homogeneous and heterogeneous. This would be more so when particles of different sizes constitute the Slurry . In such a situation dominant characteristics have to be identified and design procedure adopted to arrive at safer design . A typical example of this type is Slurry of coal particles in water.


Step 1.2 Slurry concentration (Solid content of Slurry)

Solid concentration of Slurry becomes an important consideration for following reasons:

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

A typical graph for slurry special gravity

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

Solid concentrations in settled Slurries could depending on nature of solid, vary over a wide range (10-50%) Generally solids with particle size of 0.4-0.5 micron may form 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.


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 below the 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 by different parameters such as size and specific gravity of solids , solids concentration viscosity of liquid and degree of turbo lance.

Step 1.4 Calculate critical velocity

Calculations of critical velocity for homogeneous Slurry is now described as below:

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

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

A) If the Slurry shown Newtonian behavior , then critical Reynolds’s No.

NRec is considered as 2100 and critical velocity is calculated.


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

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

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:


Step 1.5 Compare design velocity with critical velocity

Vc = 1.193 m/s (from Ex.4)

V = 1.60 m/s (from Ex.1

V-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.

Step 1.6 calculate design friction loss

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


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.


Since Slurry sp.gr. is 1.61

Step 1.7 Calculate pump dish charge pressure

For transport of such a Slurry consider elevation change = 10m

And total equivalent length =200m

Total head to be developed by pump


Pump discharge pressure required (for flooded suction condition)

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

wear prone zone

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 velocity as 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

Different types as given below can be used


3.1 Centrifugal

3.1.1 For very dilute Slurries , solids concentration up to 10 L), sump type pumps 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 displacement pump . A chamber filled with clear liquid is interposed between piston and pump valves . Movement of piston creates a pressure sludge in clear liquid chamber which is 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 for moderate flows and pressure, the discharge in obtained at a steady pressure, well suited for thick slurries.

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 is convenient. 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 speed and 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 these should be provided with back flushing arrangements, connected to pipe line with capillary. Moreover the gauges should be separately supported and 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, page
74 - 87.

Heterogeneous Slurries :

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2 comments :

  1. Good Article,Thank you.

    ReplyDelete
  2. We have a problem in our ammonium sulfate crystallization plant. We have purging system 1/2" pipe line which send a 40% ammonium sulfate and 5% solid content. This purging going to dryer spray nozzle and centrifuge. We found this line fail as after we start the pump then after some time line got choked. We are using hose pump for this purging. Line is heated by electric tracing. Can you give us the solution..?

    ReplyDelete

 

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