Comparing PD Pump Designs for Transferring Dewatered Sludge Cake

8/17/2019 Comparing PD Pump Designs for Transferring Dewatered Sludge Cake

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Comparing PD Pump Designs for Transferring Dewatered

Sludge Cake

Written by Michael L. Dillon seepe! "nc.

Pumps & Systems, September #$$%

The concept of using a positi&e displacement pump to transfer dewatered biosolidscommonly called sludge cake is not new. 'owe&er the designs of the pumps used ha&e

e&ol&ed to increase their utility and reliability and ha&e fallen in and out of fa&or for a&ariety of reasons.

Justification for Pumping

(sing pumps to con&ey sludge cake offers numerous ad&antages o&er using con&eyors for

the same purpose. )ecause pumps can mo&e cake through &ertical sections of pipe they can

consume much less &olume in a building than a con&eyor that is limited in its degree ofincline. *nclosing cake in a pipe is also much cleaner and eliminates all of the bearings and

other wear points associated with a con&eyor.

The tradeoff is that a typical pump is more technically complicated to apply and maintain

than a con&eyor. Pump speeds must also be matched to the output of the dewatering de&icewhich can either re+uire some e!tra labor increased in&estment of an automated system

and,or increased maintenance skills.

Due to the &iscosity fibrous nature and high pressures associated with pumping sludge cakeonly positi&e displacement pumps can be considered. -ll positi&e displacement pumps

capture li+uid in a defined ca&ity. Due to the high apparent &iscosity of sludge cake two

maor considerations are important in any pump design/

• The design must incorporate a mechanism to feed the material into the pump ca&ity

because atmospheric pressure will not be sufficient to mo&e the cake through the

restrictions that typically block the entrance to the ca&ity.

• The internal mechanisms in the pump must be relati&ely free of restrictions that

increase backpressure when mo&ing such a &iscous product.

The most common reciprocating design is a type of piston pump adapted from designstypically used for pumping concrete. These pumps re+uire some sort of screw feed de&ice

and a non0restricti&e check &al&e system to accommodate the two re+uirements listed abo&e.

1f the rotary PD pumps the most commonly used is the progressi&e ca&ity 2PC3 design.Since PC pumps ha&e ca&ities that are sealed by a compression fit between the rotor and

stator no &al&es e!ist to restrict the discharge. 4arious feed mechanisms are used on the

inlets of these pumps.

Feed Mechanisms

The feed screws used to feed both of these designs really ha&e two functions/ to push the

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the PC pump manufacturer. 8or a piston pump 1*M a separate rotating auger mechanism

must be added to feed the reciprocating pumping mechanism.

The tradeoff here is that the piston pump is almost always more e!pensi&e and has more

mo&ing parts but the cake being discharged from this pump is usually of a higher &iscosity.- typical complaint with a PC pump is that its cake looks runny and acts more like a li+uid

than a cake. 5ote though that the solids percentage has not been reduced. 5o water has

been added. "n a few hours the sludge will re&ert to a more cake0like condition.

)ut without precautions the cake from a PC pump can cause more of a housekeeping problem. To some users this is an ad&antage/ when filling any container including a dump

truck a more li+uid0type product has fewer air pockets and flows easier into corners than a

true cake. 'owe&er some landfill operators may want a more cake0like product.

Continuous vs. Pu"sating F"ow

-nother big difference between PC pumps and piston pumps is e&idenced in system

pressures. PC pumps produce flow on a continuous basis. "n theory they should not pulsate.

"n practice air pockets are often captured in cake as it falls from a centrifuge or belt press

into the pump. The air gets compressed and sometimes collects before it can be e!pelledmaking it often seem as though the pump is pulsating. The pump does not really pulsate: it

continually pumps.

;eciprocating pumps pulsate. *ach cylinder must be e&acuated. The piston must reco&er its

entire length then the cylinder must be filled. Most reciprocating piston pumps arehydraulically actuated. They often remain in an open position to allow filling for an

e!tended period compared to a mechanically operated reciprocating pump where pistons

simultaneously pump and reco&er.

Figure &. Simp"e schematic of a

hydrau"ica""y actuated two-piston reciprocating cake pump.

This results in a relati&ely sporadic discharge from the end of the pump. The screw takes

some time to feed and fill the piston. The piston must accelerate to a midpoint decelerate toa stop and reco&er in the opposite direction but with the same &elocity profile. This not only

results in sporadic flow but the peak &elocities must also be a minimum of twice the

a&erage of a constant flow profile 0 and they can reach three times that of a constant &elocity pump due to the fill time.


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Figure '. %ypica" ve"ocity and f"ow

profi"e with a reciprocating piston-type cake pump.

Piston pumps must inherently generate two to three times the pressure of a PC pump to produce the same flow rates per minute or per hour as that of a PC pump for the same

application. Piston pumps can no doubt generate more pressure than a PC pump and an

eight0stage or nine0stage PC pump is not typically recommended for any run of pipe longerthan 0psi,ft of pipe

and use a larger pipe diameter than a piston pump manufacturer. The piston pump

manufacturer may recommend #0psi,ft of pipe friction loss and a smaller pipe si9e becausethey can generate more pressure.

Pipe Si(ing

" ha&e a firm stance regarding pipe diameter si9ing. "n a PD pump 2?PM ! psi3,>%>< @ 'P

so any decrease in pressure results in a power sa&ings along with less wear and tear on the pump.

- designer should take any opportunity to reduce wear and tear and power consumption so

selecting a larger pipe diameter always yield a long0term sa&ings. ?oing to larger pipe si9es

certainly diminishes the return but this is an easy e!ercise to undertake and must be done one&ery cake pump application.

5omograms estimate friction losses for sludge cake in pipe but some +uick rules0of0thumb

can also be used for initially estimating friction losses. 8or si9ing PC pumps " recommendan estimated apparent &iscosity of #=$$$$0cPs for #$ percent solids and


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handling centrifuge discharge at o&er A= percent solids. "t has also been successfully used to

mi! calcium o!ide 2Ca13 into the sludge to produce an e!othermic reaction in the sludge or

increase the p' significantly to produce a Class ) or Class - biosolids. Total solids coulde!ceed =# percent.

The most effecti&e of these designs include a separate &ariable speed dri&e for the bridge

breakers 2since their primary role is to generate shear3 while the pump speed can be

regulated to meet the discharge rate from the dewatering de&ice.

(ntil about ten years ago all PC pump manufacturers had similar designs until some ofthem decided to adapt the system used by piston pump manufacturers. The ad&antage of this

approach is that the screw feed mechanism can be con&eniently located directly below the

discharge of the dewatering de&ice and the pump like a piston pump can then be placed ina more suitable position.

The unfortunate aspect of this design is now the user really has two pumps to maintain. Thescrew feeder must now generate a positi&e pressure that not only pushes cake into the

pumping ca&ity but also o&ercomes any friction loss in a transition chamber or pipe between the screw feeder and the pump. This is a difficult ob for a simple screw0feeding

de&ice.

-s a result these twin0screw feeders ha&e become +uite complicated and maintenance0

prone. *ach shaft must be reinforced with bearings dri&en with a motor and gears andsealed to pre&ent leakage. 5aturally bearings are not typically designed to operate in an

en&ironment of pressuri9ed sludge so they become a constant point of maintenance

concern.

Progressi&e ca&ity pumps ha&e many uni+ue characteristics. The ca&ity created between therotor and stator does not change its shape as it mo&es through the pump. "t has no &al&es. "t

does not inherently pulsate. "t has relati&ely low internal &elocities because the rotating parts

ha&e relati&ely small diameters and it is a pump designed with a bearing on each end. 1nthe end with the motor the pump has traditional tapered roller bearings lubricated with oil

or grease like any other rotary pump.

1n the other end the polished and hardened helical rotor is rotating inside a rubber0lined

tube. "n other applications this configuration is easily recogni9ed as a marine bearing that iscommon in marine &essels ranging from #>0ft inboard ski boats to Dallas0class attack

submarines. ;ubber with water as a boundary layer is ama9ingly slippery. -ll of the

progressi&e ca&ity pump manufacturers at least in their early years purchased stators frommanufacturers of marine bearings.

-s such this bearing pro&ides an e!cellent support for a feed and,or shearing auger that

enables the PC pump to handle sludge cake and other highly &iscous products without

adding additional wearing parts or maintenance points. Though it contains as much as


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installations e!ceeding four years in operation ha&e e&er re+uired replacement.

- &ariant of the cake pump with a concentrically rotating auger has a separate &ariable

speed dri&e dedicated to the auger. The auger can then be not only used to feed material into

the pumping ca&ity but it can be also used as a separate shear inducing or mi!ing de&ice.

This design can be used to add calcium o!ide or calcium hydro!ide for treating biosolids."ndustrially it has been used to handle pigments discharged from plate and frame filter

presses that e!ceed %= percent solids and to mi! milled grains with hot water to prepare

6wort6 in breweries and distilleries. "t is also being used to handle spent grain dischargedfrom centrifuges in ethanol plants.

)oundary *ayer +n,ection

-nother common appurtenance used when pumping dewatered biosolids is a boundary layer

inection system which consists of an inection pump and an inection flange designed toinsert a layer of li+uid between the pipe wall and the slug of sludge mo&ing through the

pipe.

These systems can typically cut the friction loss associated with pumping sludge cake to

one0half or less. "n certain cases especially when a highly diluted solution of polymerflocculent is used as the boundary layer friction loss can be reduced by as much as =

percent. These can be used really on any type of cake pump as it affects the piping system

and not the pump. *!cept as mentioned abo&e it reduces the differential pressure which in

turn reduces the operating power re+uired and wear and tear on the entire system.

Conc"usion

Cake pumps re+uire some type of feed de&ice to push dewatered solids into the pumping

ca&ity of either a piston0type or a PC0type cake pump. The de&ice can help to reduce the

apparent &iscosity of the cake as well as feed it into the pumping ca&ity.

The efficiency and characteristics of this feed de&ice can define the character of the cakethat is discharged into a storage or transport de&ice. 'igh shear feed de&ices will produce a

cake that is temporarily a lower &iscosity which may be aesthetically undesirable. (sing

e+uipment that is designed for higher capacities and will operate at lower speeds and shearrates can reduce this effect.

Piston pumps are designed to handle higher pressures and to pump cake o&er longerdistances through smaller diameter pipes than PC pumps. - comparison solely based on

pressure capabilities of the two designs is not appropriate since PC pumps produce aconstant output with pressures being one0half to one0third of the pressure spikes that occur

as a function of the &arying &elocities and flows associated with reciprocating pumps.

The feeding systems used on all of these pumps &ary a great deal. Some are +uite

complicated and trade some of the problems associated with highly sheared low &iscosity


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cake discharges with complicated maintenance prone feed systems that amount to using

two pumps where one pump can be effecti&ely used.

(sing appropriate shearing and feeding de&ices along with boundary inection systems

pressures can be minimi9ed and cake can be pumped o&er substantially long distances2e!ceeding #$$$0ft3 with relati&ely ine!pensi&e efficient and reliable

e+uipment.

eferences

• )arrie D. Petrik E. 6;eal0World Sa&ings/ *!panding - Dewatering 8acility6

Pumps Systems 21ctober #$$A3

• Dillon M. L. St. Clair E. -. Eline P. '. 6Predicting 8lowrates from Positi&e0

Displacement ;otary Pumps6 Chemical *ngineering 2Fuly ## >G=3 pp.=%0B$

• 'eald C. C. Cameron 'ydraulic Data Se&enteenth *dition Second Printing

"ngersoll0;and Woodcliff Lake 5.F. 2>GG#3

• 'emphill ). Shanley ;. Mikowski T 6"nno&ati&e Material 'andling at

)iosolids Cake Storage 8acility6 Public Works 21ctober #$$#3

• 'eywood 5. 6'ow to ;educe Pipe 8riction in Slurry 8lows6 Proceedings of the

A>H>$H?).pdf downloaded "nternetdocument 28ebruary #$$%3
http://www.putzmeister.com/pdfs/IP_1318_10_GB.pdfhttp://www.putzmeister.com/pdfs/IP_1318_10_GB.pdf

Comparing PD Pump Designs for Transferring Dewatered Sludge Cake

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