11312_09_Progressing Cavity Pumps, Downhole Pumps and Mudmotors

8/14/2019 11312_09_Progressing Cavity Pumps, Downhole Pumps and Mudmotors

1/14


2/14


3/14

content, pum p life is inversely proportional to a value betweenthe square and the cube of the pum p speed. Low er speeds willcontribute to longer life bu t, for the same flow rate , the slowerpump will be larger and almost certainly more expensive on afirst-cost basis.

T h e Am erican Petroleum Institute (API) publishes a stan-dard, AP1-676, for ro tary positive displacement pum ps. It is agood source for pump specifications but, perhaps more impor-tant, it contains a data sheet to be completed by the user or buyerfor inquiry purposes and the seller or manufacturer for quota-tion purposes. It collects the most relevant data needed to spec-ify user requirements, the nature of the liquid to be pum ped, andthe conditions of pump operation. It is a good overview tha t cov-ers all usual pum ping needs, but it does no t collect informationabou t solids co ntent Therefo re, add at least:

Weight % solids Maximum particle size Maximum fiber length Description of solids, especially as it relates to abrasionIn classic pump selection, the system net positive inletpressure available (NPIPa) must at least equal, and preferably

exceed, the pum p net positive inlet pressure required (N PIP r).If th e reader is familiar w ith centrifugal pum ps, the com para-ble terms are NPSHa (net positive suction head available) andNPSHr (net positive suction head required). For PC pumpstha t frequently deal with a non hom ogeneous fluid (i.e., havingsolids and/or gas or froth) delivered to the pump in clumps or


4/14

batches rather than continuously, the NPIP concept becomesslightly less clear. Empirical data gathered by pump manufac-turers provides guidance on maximum pump speed relative tokeeping the in let pum p cavity reasonably filled before it closesoff to inlet flow. Many PC pumps can be provided with anauger screw welded to the connecting rod (coupling rod) thatforce-feeds inlet fluid into the pumping elements. Indepen-dently driven auger screws can also be provided on hopper-style pumps that will force thick, high-viscosity solids materialinto the pum ping screw.

Rotor-to-stator rubbing speed is the maximum surfacespeed of the ro to r outside diam eter as it rotates and oscillateswithin the stator. Since the rotor and stator normally have aninterference fit at operating temperature, rubbing speedshould be limited to about 16 ft/sec on clean, lower-viscosityliquids (no solids). T his means that larger displacement p um pswill need to be operated at lower speeds than small displace-ment pumps, a fact that applies to most all-rotary positive dis-placement pumps.Example 1: Pum p SizingFluid: Secondary waste water sludgeFlow Rate: 100 gpmDifferential Pressure: 50 psidMaximum Particle Size: 0.25 in.

First, categorize the degree of abrasiveness from Table 1.In our example, it is "medium." If in doubt, select the next-worst classification.


5/14

TABLE IMax.RubbingSpeed Max. *

Abrasives Viscosity (ft/sec) rpm Fluid ExamplesHeavy Very


6/14

TABLE 2

Sze

Gap1rpm*

Dispame(ma*rpm)

Dispame(ma*gm)

RunS

(ft/sec)p

1rpm

MmmPaceSze

(n MmmFibLnh

(n

ABCDEFGHJKLMNPRST

0.0530.0990.1980.3960.7931.6513.3036.605

13.21025.09936.32849.53866.05095.773

178.336330,251627.477

3,0003,0003,0003,0002,5002,0001,6001,2001,000

800700650575500425330275

1.63.05.9

11.919.833.052.879.3

132.1200.8254.3322.0379.8478.9757.9

1089.81725.6

0.220.300.420.490.640.790.981.331.571.952.212.462.663.153.844.595.71

0.030.040.040.060.080.100.120.150.200.270.270.370.370.550.790.981.18

1.01.21.41.41.41.71.71.92.43,13.13.93.95.18.39.89.8

* Mechanical speed limit.** Theoretical (neglecting slip).


7/14

TABLE 3Size RPM per 100 gpm Rubbing Speed (ft/sec)K 398 Tj6L 275 6.08M 202 4.97N 151 4.02P 104 3.28

TABLE 4Maximum PSID/Stage

Unequal Wall Equal WallAbrasives 1:2 Geometry 2:3 Geometry 1:2 GeometryHeavy 15 22 30Medium 35 52 70Light 60 90 120None 87 130 175

A conventional stator is manu factured using com m oncylindrical pipe as the outer structure. It is strong and rela-tively inexpensive. However, when the elastomeric liner is in-jected between the pipe inside diameter and the removablecore in the molding process, the wall thickness of the elas-tom er varies along the length of the stator from the minimumdesign thickness to perhaps twice that thickness. This type ofstator is know n as an uneq ual wall stator, the m ost com m ontype in service today (Table 4).T h e equal wall stator has evolved and now dem onstratesimproved pump capability. It is manufactured using a cast


8/14

outer metallic shell containing the same lead thread as themolded inside diameter will have. Thus, the thickness of theelastomer lining is uniform thro ug ho ut the length of the sta-tor. While the cast stator is more expensive than pipe, itrequires less volume of elastomer. If the elastomer is costly(e.g., fluorocarbon), the total cost of the sta tor m ay be lower,reducing both initial pu m p cost and replacem ent stator costs.

As the ro tor turn s inside an unequa l wall stator, the w orkdone on the elastomer (flexing of the elastomer) varies be-tween w here it is thick versus thin . Th is requires significantlymore starting and running torque than an equal wall statorpum p. T h e nonuniform flexing of the unequal wall stator linergenerates nonuniform frictional heat within the elastomerwith a nonuniform heat dissipation capability that limits itspressure per stage rating. The uniform flexing of the elas-tom er in an equal wall stator allows a pressure ra ting per stagein the order of twice as high as an unequal wall stator designof the same number of stages. At elevated pressure rating,abrasive solids content should be kept minimal for reasonablyequal wall stator life. See Fig. 119 .

A 1:2 geometry in a two-stage pump or a 2:3 geometrysingle-stage pump will meet the specified pumping require-m ents. M ore stages reduce the pressure rise pe r stage and willsignificantly extend pump life, especially in abrasive services.


9/14

FE G U R E 119Standard (left) and "equa l wall" (right) stator designs and comparisonof profiles.

Example 2:Pump SizingFluid: PolymerFlow Rate: 5gpmDifferential Pressure: 150 psidMaximum Particle Size: None; liquid is clean, viscosity low

(100 cP)


10/14

First, categorize the degree of abrasiveness ("none" fromTable 1). F rom Table 2, the smallest pu m p tha t can p rovidethe specified flow is "C ." T h e required speed, neg lecting slip,is 5 gpm / 0.198 gal / 100 rpm x 100, or 25.25 rpm. The re-sultant rubbing speed is 25.25 X 0.42, or 10.6 ft/sec. T h is ru b -bing speed is midrange of tha t recom m ended. A 1:2 geom etrysingle-stage p um p will me et the req uirem ents, as will a two -stage 1:2 geometry unequal wall pump.

Unless a variable speed drive is used, the actual pumpspeed will depend upon the loaded speed of the driver and theactual gear ratio (available from a speed reducer supplier). Inthe first example, the actual available ratios might be limitedto 380 rpm or 405 rpm , in wh ich case flow rates and rub bin gspeeds need to be recalculated at the actual expected speed todeterm ine p um p performance. Obviously, if the flow rate r e -quired is critical, the higher-speed reducer should be chosen.If the required flow rate is not critical, the slightly lower ratiowill produce slightly less flow and draw slightly less power.Note that in the PC pump industry, the standard direction ofrotation is counterclockwise, facing the pump shaft. Most PGpumps are driven through speed reduction mechanisms thatreverse the input drive direction of rotation .

Due to the interference fit of the rotor to the stator, thenecessary pum p starting torqu e can exceed the pu m p run nin gtorque requirement (Fig. 120). Both must be calculated, andthe higher of the tw o is used to size the driver. T h e calculationmethodology is often proprietary to the PC pump manufac-turer. Allowance for speed reduction device inefficiency mustalso be factored into the final power ra ting of the drive needed.


11/14

T

FIGURE I 20 Starting torque relationship (standard design and the equal-wa

T o r q u e f o r ET o r q u e f o r UT o r q u e fo r ET o r q u e fo r UT o r q u eS p e e dT o r q u e is a

T N ET N CT S ET S CTNT f [ n ]

TORQUE REQUIREMENTS

U N - E Q U A L S T A T O RE Q U A L W A L L S T A T D R


12/14

Due to thermal growth of both the rotor and stator, forliquid pumping temperatures outside the temperature rangeof ~ 50-115F, rotor outside diam eters are left oversized (coldservice) or undersized (hot service) to m aintain the requiredamount of interference fit between them. If a rotor is under-sized for high-temperature operation, standard factory testingwill not achieve the same flow rate as will be pum ped when inservice at high temperature. The ambient test will produce alower flow rate due to little or no interference between thenew ro tor and its new stator.

Some PC pumps are available with adjustable stators.These stators are axially slotted on the OD and equipped withclamping bands along their le ng th. As the clamps are tig ht-ened, the degree of rotor-to-stator interference can be con-trolled un der varying pum ping tem perature conditions. T h eycan also compensate for w ear by restor ing a w orn stator to alike-new interference fit.Metallic StatorsM etallic sta tors, also known as rigid stato rs, can be suppliedwhen requirements, operating conditions, and economies arealigned. Metallic stators can handle much higher pressure perstage than elastomeric stators, up to 500 psi per stage. Theyare used with pumpage over ~ 5,000 cp and allow muchshorter pum ps for high-pressure service than would otherwisebe possible for PC pumps. The rotor-to-stator fit is a clear-ance fit. Consequently, removal, cleaning, and reinstallationof the rotor is much easier and quicker. This is especially


13/14

advantageous w here cleaning every shift is a req uirem en t suchas in food plants. Products pumped most frequently in thefood industry include meat emulsions, cookie fillings, cake andcookie icings, glucose , glues, pastes, ho t grease, and molasses,as well as paint, hot resins, varnishes, and similar high-viscositymaterials.

Metallic stators are available in various stainless steels aswell as tool steels. Since the re is essentially no ro tor -to -sta torcontact, product contamination from elastomer wear particlesis eliminated. Metallic stator PC pumps can handle higher tem -peratures as well, to 5000F with drive-end modifications. T hey

FIGURE 121Hollow rotor design.


14/14

Documents

11312_09_Progressing Cavity Pumps, Downhole Pumps and Mudmotors