Pump­ing Solids

Chemical Industry Digest - - What’s In? - S. L. Ab­hyankar

– S L Ab­hyankar, De­signer, Trainer, Con­sul­tant for Pumps, Valves, Pump­ing Sys­tems The author says that this is one of the most dif­fi­cult ex­er­cises in pump­ing; the se­lec­tion of the pumps and sys­tems needs a good un­der­stand­ing of the solids and slurry to be pumped. Ar­ti­cle pro­vides im­por­tant cri­te­ria to be con­sid­ered.

Synop­sis To pump solids ef­fec­tively and ef­fi­ciently, the pump se­lec­tion de­pends on a good un­der­stand­ing of the char­ac­ter­is­tics of the solids and/or the slurry to be pumped. Con­cep­tual as­pects as well as some of the im­por­tant cri­te­ria to be con­sid­ered are cov­ered along­with the kind of pumps used for such op­er­a­tions.

Pump­ing solids is no easy job. Not only is it dif­fi­cult, it is also com­plex. It is com­plex, be­cause solids to be pumped are of var­i­ous, shapes, sizes and char­ac­ter­is­tics. Men­tion of some typ­i­cal ex­am­ples will give a bird’s eye view of the com­plex­ity of pump­ing solids.

● One good ex­am­ple of com­plex solids is sewage. Sewage is com­plex and is to­tally het­eroge­nous.

● An­other ex­am­ple is pa­per pulp.

● Yet an­other is ce­ment grout.

● Yet an­other is pump­ing car­ba­mate, which has a melt­ing point of about 150oC. So, it will tend to so­lid­ify at am­bi­ent tem­per­a­ture. That means to start a pump to pump car­ba­mate, what­ever car­ba­mate would have set­tled in the pump from pre­vi­ous run­ning needs to be first melted or liq­ue­fied by heat­ing. The pump should have a heat­ing jacket (See Fig. 8). Sim­i­lar is the case with pump­ing tar or bi­tu­men. Pump-man­u­fac­tur­ers man­u­fac­tur­ing pumps for pump­ing solids would be test­ing the pumps pri­mar­ily with water. So, for se­lec­tion of pumps for pump­ing solids the pump­ing duty i.e. rate of flow and head or pres­sure, has to be first trans­lated into equiv­a­lent duty with water.

Dif­fer­ent types of solids have dif­fer­ent meth­ods for trans­la­tion of pump­ing duty to duty with water. These meth­ods are based on some typ­i­cal char­ac­ter­is­tics of the ap­pli­ca­tion viz.

1. Per­cent­age con­tent of solids by vol­ume or by mass. It be­comes use­ful to know the density of both: of the solids and of liq­uid with solids. Per­cent­age con­tent of solids is also called as con­sis­tency in ap­pli­ca­tions like pa­per-pulp.

2. Max­i­mum size of solid par­ti­cles - This con­sid­er­a­tion very much in­flu­ences the width of hy­draulic pas­sages in the de­sign of the pump a. If the solids are like flakes, a grinder or pul­veriser be­fore the pump would make the pump­ing eas­ier. An ex­am­ple is of re­cy­cling the flakes-laden cool­ing water in steel plants.

Pump-man­u­fac­tur­ers man­u­fac­tur­ing pumps for pump­ing solids would be test­ing the pumps pri­mar­ily with water. So, for se­lec­tion of pumps for pump­ing solids the pump­ing duty i.e. rate of flow and head or pres­sure, has to be first trans­lated into equiv­a­lent duty with water.

b. If the solids are of the type of sticks or long fi­bres, a chop­per be­fore the pump would make pump­ing as much eas­ier. See Fig. 3

3. Whether the solids are settling type, e.g. sand-laden water as han­dled by dredg­ing pumps a. Settling types of solids at low con­cen­tra­tions may cause ero­sive wear. Re­silient lin­ing such as rub­ber-lin­ing would have bet­ter life against ero­sive wear. See Fig. 4 b. Settling types of solids at high con­cen­tra­tions may cause abra­sive wear. Sur­face hard­ness would pro­vide bet­ter life against abra­sive wear. c. Also oper­at­ing speed of the pump should be

low. d. Smooth sur­faces as ob­tain­able in coat­ings can help make the par­ti­cles to ‘glide’ in­stead of they caus­ing abra­sion. See Fig. 7 e. The coat­ings can also help pro­vid­ing cer­tain amount of cor­ro­sion-re­sis­tance.

4. If the solids are of non-settling type, they would make the liq­uid tur­bid at low con­cen­tra­tions and vis­cous at high con­cen­tra­tions. For Con­cepts of Vis­cos­ity see ar­ti­cle at http://www.columbia.edu/itc/ldeo/lack­ner/E4900/Themelis3.pdf

It would be use­ful to un­der­stand the dif­fer­ence be­tween ero­sion and abra­sion.

Re­silient lin­ing as of rub­ber would help to make the im­ping­ing par­ti­cles to bounce away and in turn re­duce the sever­ity of ero­sive wear. Smooth sur­faces also help to make the im­ping­ing par­ti­cles to glide away.

Since meth­ods for trans­la­tion of pump­ing duty to duty with water are dif­fer­ent ac­cord­ing to the char­ac­ter­is­tics as de­tailed above, for vis­cous liq­uids, i.e. liq­uids with non-settling solids the method has been to use the chart (See Fig 9).

Ac­tu­ally the chart in Fig. 9 was based on sim­i­lar chart avail­able in ear­lier edi­tions of stan­dards of Hy­draulic In­sti­tute (HI). How­ever there is the stan­dard ISO-TR-17766:2005, which is more re­cent, gives equa­tions and for­mu­las which can be used al­go­rith­mi­cally.

For liq­uids with settling solids, one can re­al­ize that such solids need to be forced to rise. An in­ter­est­ing ar­range­ment is Jet Lift Dredg­ing. (See Fig 10) It can be noted that in Jet Lift Dredg­ing the solids are raised only into the trans­porta­tion pipe and do not get into the

pump at all. That elim­i­nates wear and tear in pumps, due to solids. But the jet prin­ci­ple is not ef­fi­cient in power-con­sump­tion.

When solids are of settling type, the cor­rec­tion of the to­tal head is done by get­ting the cor­rec­tion chart (See Fig. 11) based on Cave’s equa­tions.

That settling solids need to be forced to rise means that the cor­rec­tion is re­quired only to the head of pump­ing and not to the flow-rate. Higher pump­ing head is also as­so­ci­ated with de­ple­tion of pump-ef­fi­ciency. If the pump­ing head needs to be cor­rected by 1.25 times, the pump-ef­fi­ciency will be less by 1/1.25 = 0.8 times of ef­fi­ciency when pump­ing clear liq­uid.

Gen­eral for­mula for power (P) re­quired for pump­ing is P = r* Q*H/102/ h Where r = spe­cific density of liq­uid (for water r = 1) Q = flow-rate in litres per sec­ond

H = To­tal head in me­ters h = ef­fi­ciency of pump

Since to­tal head is Hstatic + Hf, the fric­tional head Hf is to be cal­cu­lated by us­ing charts. Charts for fric­tional head are most com­monly avail­able for pump­ing water and not for every other liq­uid. Moody’s chart should then be a handy tool (See Fig. 12) Method of us­ing Moody’s chart is well ex­plained in IS:2951 (Part 1). Fric­tion in valves and fit­tings can be fur­ther cal­cu­lated as ex­plained in IS:2951 (Part 2).

Since liq­uids whether with non-settling solids (hav­ing some vis­cos­ity) or with settling solids do not flow eas­ily, pump­ing in­stal­la­tions for pump­ing liq­uids with solids should rather be in­stalled with pos­i­tive head above suc­tion.

Since liq­uids whether with non-settling solids (hav­ing some vis­cos­ity) or with settling solids do not flow eas­ily, pump­ing in­stal­la­tions for pump­ing liq­uids with solids should rather be in­stalled with pos­i­tive head above suc­tion. See Figs. 13, 14, 15, 16 and 17.

Most of the dis­cus­sions above fo­cused on cen­trifu­gal pumps. But liq­uids with high con­cen­tra­tion of sol- ids and of high vis­cos­ity are pumped by Pos­i­tive dis­place­ment pumps. Pump­ing of ce­ment grout em­ploys pos­i­tive dis­place­ment pumps only. An­other em­i­nent ex­am­ple is of mud pumps used in oil wells. What the mud pumps pump is not ac­tu­ally mud. What they pump is drilling fluid, which is called as mud, be­cause it very much looks like mud.

Vis­cos­ity Cor­rec­tion fac­tors in Fig. 9 are to be ap­plied only when pump­ing by cen­trifu­gal pumps. In case of pos­i­tive dis­place­ment pumps, higher the vis­cos­ity, there would be less ‘slip flow’ i.e. the flow slip­ping back­wards to­wards suc­tion. Less the slip flow, bet­ter is the vol­u­met­ric ef­fi­ciency of the pump. Whereas this sounds charm­ing, most pos­i­tive dis­place­ment pumps have suc­tion and delivery valves. One can re­al­ize that solids get­ting de­posited at valve seats would ren­der pump­ing dif­fi­cult and in­ef­fi­cient. Yet for pump­ing liq­uids with high con­cen­tra­tion of solids and of high vis­cos­ity pos­i­tive dis­place­ment pumps would mostly be the only op­tion.

Rec­om­men­da­tions for fur­ther read­ing

1. For Con­cepts of Vis­cos­ity see ar­ti­cle at http://www.columbia.edu/itc/ldeo/lack­ner/E4900/Themelis3.pdf

2. Slurry Pump Ba­sics - pub­li­ca­tion by M/s. Metso Min­er­als, can be down­loaded at http://www.metso.com/miningand­con­struc­tion/MaTobox7.nsf/Doc­sByID/119A6A698 EE232B0C2257E7A00457903/$File/Slurry%20Pump%20 Ba­sic_13.pdf

3. Slurry Pump­ing Man­ual - pub­li­ca­tion by slurry pumps di­vi­sion of M/s. Weir Group plc, can be down­loaded at https://www.scribd.com/doc­u­ment/338766642/WeirSlur­ryPump­ingHand­book-pdf

4. “The Ef­fects of Slur­ries on Cen­trifu­gal Pump Per­for­mance” - An ar­ti­cle by Mr. Ge­orge Wil­son can be down­loaded at http://tur­bo­lab.tamu.edu/proc/pump­proc/P4/P419-25.pdf

5. ISO-TR-17766:2005 - Spec­i­fi­ca­tion for pumps han­dling vis­cous liq­uids

6. IS:5120 Technical Re­quire­ments of pumps for ro­to­dy­namic spe­cial pur­pose pumps

7. IS:5600 Spec­i­fi­ca­tions for pumps for sewage and drainage

8. IS:6596 Spec­i­fi­ca­tion for pumps for han­dling pa­per stock

9. IS:9201 Pumps for han­dling slurry

10. IS:2951 (Part 1) - Rec­om­men­da­tion for Es­ti­ma­tion of Flow of Liq­uids in Closed Con­duits, Part I: Head Loss in Straight Pipes Due to Fric­tional Re­sis­tance

11. IS:2951 (Part 2) - Rec­om­men­da­tion for es­ti­ma­tion of flow of liq­uids in closed con­duits, Part 2: Head loss in valves and fit­tings.

De­signer, Trainer, Con­sul­tant for Pumps, Valves, Pump­ing Sys­tems.

Con­sid­ered as the ‘Pump Man of In­dia’, he is a lead­ing con­sul­tant with over 50 years of ex­pe­ri­ence in the pumps field. He was for­mer Technical Ad­vi­sor to the In­dian Pump Man­u­fac­tur­ers’ As­so­ci­a­tion.

Fig. 4 Ex­ploded view of pump with rub­ber-lin­ing

Fig. 3 Chop­per be­fore pump

Fig. 2 S-Type Semi-open im­peller

Fig. 1 Open, semi-open and closed im­pellers

Fig. 9 Vis­cos­ity Cor­rec­tion Chart (Ref. IS-5120-1977)

Fig. 5 In abra­sion, the load is per­pen­dic­u­lar to the sur­face and move­ment is par­al­lel to the sur­face i.e. with zero an­gle of in­ci­dence Fig. 6 In ero­sion, par­ti­cles im­pinge onto the sur­face with a dis­tinct an­gle of in­ci­dence

Fig. 7 Pumps coated to have smooth sur­faces would help to make solids to ‘glide’

Fig. 8 Pump with heat­ing jacket

Fig. 11 Cor­rec­tion for settling solids

Fig. 10 Jet Lift Dredg­ing

Fig. 12 Moody Diagram

Fig. 13 Dry pit sewage pump with ver­ti­cal motor

Fig. 14 In­stal­la­tion with hor­i­zon­tal motor

Fig. 16 Wet Pit pump with hor­i­zon­tal sub­mersible motor

Fig. 15 Wet Pit Pump with ver­ti­cal sub­mersible motor

Fig. 17 Wet Pit pump with side dis­charge, ex­ten­sion shaft and motor

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