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As shown in Figs. 2 and 3, a P C pum p basically consists of ahydraulic section (a ro tor inside a stator) and a drive frame trans-m itting the shaft rotation to the hydraulic section by means ofa connecting rod. The connection is accomplished by variousmeans: a pin joint, a universal joint (such as Cardan), or aflexible joint arrangem ent.
A driver (motor, diesel, or oth er prim e m over) is coupledto an input shaft, transm itting its energy into the mechanicalenergy of shaft rotation. The shaft is supported by bearings inthe pump drive frame, and drive shaft rota tion is translated intoeccentric motion of a roto r in the hydraulic section. T h e rotorforms a cavity betw een itself and a stator, as will be explainedin detail. T h e eccentric m otion of the ro tor displaces the fluidwithin the cavity, which "moves" (i.e., "progresses"hence,
Chapter 2TH REE MA IN TY PES O FPROGRESSING CAVITY MACHINES:PCPs9 D H P s5 AND D H M s
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FIGURE 3Progressing cavity cross section (courtesy Monoflo Pum p).
the term) along the axis from suction (inlet) to discharge (exit)(i.e., pumping the fluid against the discharge pressure). Sincethe fluid is actually mechanically displaced, PC pumps belongto a subclass of rotary pum ps.
If such a pump is connected to a generator instead of anelectric m otor or any other load (e.g., a drill bit), and the ro to r
FIGURE 2Typical progressing cavity pump (courtesy M onoflo Pum p).
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is allowed to rotate driven by the differential pressure, thepump essentially becomes a hydraulic motor, per terminologyof PD machines. (N ote the similarity with centrifugal pum psoperating as hydraulic recovery turbines when operated inreverse.) It utilizes the fluid's energy to convert it to themechanical energy of the rota ting shaft from the eccentricrotor, via the connecting rod, to the drive frame shaft, and tothe load. Thus, when mechanical energy is converted tohydraulic energy, the unit operates as a pump; conversely,when the hydraulic energy is converted to mechanical rota-tion, the un it operates as a hydraulic motor. In oil explorationdrilling, such reverse operating units are called "mudmotors,"which u tilize "m ud" as a working fluid to drive the roto r insidethe stator, as well as to lubricate the drill bit and flush out thedebris. In that case, the hydraulic section (rotor and stator) iscalled a "power section," denoting its utilization to transmitpower to the drill bit. A complete drilling unit (a mudmotor)is shown in Fig. 4, and con tains the power section itself, bear-ings, seals, drill bit, stabilizers, and o ther auxiliary equipm ent.Such a un it (referred to as a "strin g" by oil drillers) consists ofa mudmotor, piping, and a drilling mechanism, all of whichare inserted into and operate in a hole (i.e., future po tential oilwell). At the surface, the m ud is pum ped into the hole, usuallyby large, high-p ressure triplex reciprocating pum ps. A goodoverview of this can be found in Ref. 2.
T h e commercial logistics of the supplied equ ipm ent dif-fer for mudmotors and PC pumps. Surface-mounted PCpumps are typically installed in facilities such as process and
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FIGURE 4Complete units: A downbole mudm otor (left, DH M) (courtesy ofDresser Security Com pany), and a downhole pump (right, DHP)(courtesy of PCM Company).
Saver Sub
Power Section
Lobe Coupling Section
Bearing A ssembly Section
Drive System
Coupling! Drive Head
Wellhead
Sucker Rod
kSucker RodTubing
Rotor
Centrafizer Stator
Stop Bushing
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petrochem ical plan ts, paper m ills, utilities, wastewater trea t-m ent plants, etc. T h ey requ ire m uch less space than the com -plete "string" of downhole mudmotor applications. A surfacepump manufacturer supplies a complete pump unit, oftenmounted on a base plate and coupled to an electric motor oroth er driver. T h e custom er o r a contractor's millwright crewconnects the piping and electric power, and the pum p is readyto operate. In the case of mudmotors (DHMs), however, thepow er section is typically supplied by a subvendor (often thesame one who manufactures complete PC pumps and has thefacilities and know-how to produce power sectionscuttingthe rotors and injecting the elastomers into stator tubes). Thesubvendor often then supplies power sections to a mudm otor(complete unit) manufacturer, who cuts the required threadson the stator tube to connect to the rest of the mudmotor.T he se m udm otors are then sold (or, m ore often, rented) to thedrillers who ow n the rigs and perform drilling con tract jobsfor the oil com panies. Dow nhole pum ps (see Fig. 4, D H P ) arelogistically handled according to the specifics of the industryin which they are applied, similar to DHMs. They are essen-tially ro tor/sta tor sections, driven via a long drive shaft (in sec-tions), or by a submersible m otor.
Power sections (which are actually pumping sections fordownhole pumps) are supplied unthreaded by the subvendors,for subsequent completion by the "string" manufacturer. Thiscom mercial practice is pa rt of a trad ition, history, and culturein the oil industry da ting back m any years.
PC pum ps have a low "lobe ra tio" (to be discussed in detailin Chapter 4), such as 1:2 (a one-lobed rotor inside of a two-
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lobed stator), although 2:3 lobe ratio designs have recentlybeen introduced. M ud m oto rs usually have multilobe configu-rations to as high as 9:10. Geom etry and space constraints in -side the drilled hole require that power sections must be asshort as possible (i.e., having fewer stages), with m ore pressuredrop per stage (i.e., m ore pow er and to rqu e concentration perunit length). Multilobe design satisfies such requirements,albeit with limitations and drawbacks that will be discussed inChapter 4.
In the rem ainder of this book, we will concentrate on thedetails of the hydraulic section (rotor/stator) of pumps: surface-m ounted, downhole (D H Ps), and motors (DH M s), includingtheir geometry, performance calculation and evaluation, andelastomer behavior. References are provided for more infor-mation on drilling applications, pumping cond itions, and aux-iliaries (seals, bearings, piping, etc.).
