LESSON 4 LECTURE POSITIVE DISPLACEMENT PUMPS PA RTS LIST & FUNCTION SUB OBJECTIVE At the end of this lesson the t!inee "ill #e !#le to$ %De 'onst !t e !n ( ndest !ndi n) of Posi ti *e D is+l !,e'ent P('+s- P!ts List !nd F(n,tion%. DISPLACEMENT PUMP As defined in the lesson "l", that a displacemen t pump in which energy is periodically added by application of force. To one or more movable boundaries, of any desired number of enclosed fluid containing volumes resulting in a direct increase in pressure up to the value required to more the fluid through valves or parts into the discharge line. See Fig. 5!for classification of displacement pumps. /. RECIPROCA TIN0 PUMP A reciprocati ng is a positive displaceme nt pump which at constant speed, deliveressentially the same capacity at any pressure within the capability of the driver and the strength of the pump. Thus reciprocating pump is most useful in the field of high pres sure and low capacity . #n some applic ation the cons tant delivery at vary ing pressure is a definite advantage and act as a metering device. /% CLASSIFICATION A re ciprocating p ump may be defin ed as a pump that op erates using a bac$ and forth straightline motion and frequently they driven by an air or steam reciprocation piston. %otary driven reciprocating pumps are usually powered by electric motor and or by internal combustion engines. A&'A ()*& )+%S * -*)A(#)A / -A#( T*(A()* )+%S * 0-0 -A#(T*(A()* /*SS+( 1 0A2*
At the end of this lesson the t!inee "ill #e !#le to$
% De'onst!te !n (ndest!ndin) of Positi*e Dis+l!,e'ent P('+s- P!ts List !ndF(n,tion
%. DISPLACEMENT PUMP
As defined in the lesson "l", that a displacement pump in which energy is periodicallyadded by application of force. To one or more movable boundaries, of any desirednumber of enclosed fluid containing volumes resulting in a direct increase in pressureup to the value required to more the fluid through valves or parts into the dischargeline. See Fig. 5! for classification of displacement pumps.
/. RECIPROCATIN0 PUMP
A reciprocating is a positive displacement pump which at constant speed, deliver essentially the same capacity at any pressure within the capability of the driver andthe strength of the pump. Thus reciprocating pump is most useful in the field of highpressure and low capacity. #n some application the constant delivery at varyingpressure is a definite advantage and act as a metering device.
/% CLASSIFICATION
A reciprocating pump may be defined as a pump that operates using a bac$ and forthstraightline motion and frequently they driven by an air or steam reciprocation piston.
%otary driven reciprocating pumps are usually powered by electric motor and or byinternal combustion engines.
-ost of the pump bodies and plungers are made of stainless steel or other corrosionresistant alloys. sually, the clearance between the plunger and the pumpbody is very small. #n many cases the plunger and the pump body are /A00*&4ground or polished as a set or pair to form a closetolerance fit. This close fiteliminates lea$age within the pump and helps to meter the fluid discharge moreaccurately.
The plunger in this pumps is prevented from lea$ing fluid by several sets of )*'%+( 4' shaped crosssection pac$ing. The pac$ing materials may be rubber,neoprene, nylon Teflon, or other materials depending on the fluid being pumped.(ormally ad6usting the length of the stro$e, through a calibrated connecting rodad6ustment controls the amount of fluid discharge.
3. DIAPRA0M PUMP
The reciprocating pumps used in the power and desal plants, are diaphragm pumps.They are use for chemicals dosing in the systems.
STRUCTURE FUNCTION AND MATERIALS OF DIAPRA0M PUMP
Fig. 11 is a simplified section drawing of diaphragm pump, one of the chemicaldosing pumps.
. An electric motor
3. A pair of couplings
!. 7orm which is connected to couplings
1. 7orm wheel assembly. This is shown in main cross section drawing
0ower is transmitted by an electric motor 4, which connects worm 4! to pair of couplings 43.
7hen an electric motor is used to drive the pump, the worm transmits the power tothe worm wheel 41 and the ( cran$ shaft, via a slotted $ey way and $ey. The(cran$ is attached eccentrically to the center of the shaft. )am 45 provide eccentricmotion to the connecting rod 4: plunger connected with connecting rod, providedreciprocating motion and which pushed the oil lowered the diaphragm 4;. +n theother side of the diaphragm fluid pushed out through the discharge valve.
Fi) 414 5!6 Se,tion D!"in) of Di!+h!)'5#6 Vie" A1A
The principle of stepless stro$e ad6ustment 4from => to ==>, with the pumpoperating or idle, depends upon the upward and downward movement of the )ran$.
This movement changes the total value of the eccentricity of the cam and the )ran$from => to ==>.
Figure 15 4a 7hen the stro$e is =>, the ()ran$ is at the bottom.
4b 7hen the stro$e is ==>, the ()ran$ is at the top.
Fi) 418 Pin,i+le of the N1C!n9 !nd sto9e !d:(st'ent
A replenishing chamber consists of an oil relief valve 43= and an Fig. 19 oilcompensating valve 4;.
Fi) 41; Oil Flo" Di!)!' in the Di!+h!)' +('+
The oil relief valve actuates when the pressure in the displacement chamber is higher
than the specified value. 7hen the oil volume in the displacement chamber is greater, thediaphragm touches the plate 41 prior to the completion of discharge stro$e, resulting inan e8cessive pressure rise in the chamber.
An oil compensating valve actuates to provide the oil to the displacement chamber. 7henthe plunger 4! is on a suction stro$e, if the oll volume in the displacement chamber islower, the diaphragm touches the plate 41 prior to the completion of the suction stro$e,producing a lower discharge flow rate and decreasing the pressure in the displacementchamber. The negative pressure induces the entry of the oil in the oil compensating valveto the displacement chamber, by the spring force created by the negative pressure.
-ost rotary pumps have drive shaft which accept drive torque from a power source.The ma6ority of rotary pumps are mechanically coupled to the driving power sourcewith couplings of various type.
>. ROTAR= E?TERNAL 0EAR PUMP
eavyduty geartype pumps are able to withstand rugged operating conditions, aresimple in construction, and are economical in cost and maintenance 4Fig. 1;. ighvolumetric efficiency of gear type pumps depends on maintaining complete sealing of all gear tooth contact surfaces. All gear surfaces are precision finished, and each pair is matched carefully.
2eartype pumps are made with fewer wor$ing parts than many other types of pumps. )astings are made of a special alloy iron, are precision machined, and arecapable of resisting bursting under severe shoc$ loading.
*nd covers for fluid power pumps are usually specified and coded. These details areimportant in specifying and ordering parts. The shaftend cover may be furnished ineither flange or pad mounting, and the portend cover may be provided either with noporting or with end porting arrangements.
The drive shafts are also specified and ordered by code number. They may be either splined or straight$eyed shafts.
A bearing carrier is used on tandem pumps and motors. #t is positioned betweenad6acent pumps or motors. The bearing carrier is also available with tapered thread,SA* thread and straightthread fittings, and either with no porting or with left and?or right hand side porting.
Some rotary geartype pumps for generalpurpose applications use a pac$ed bo8 for the shaft seal. The pac$ing gland should be ad6usted to permit slight seepage for best performance. A mechanical uses less power than the pac$ed bo8, has longer service life under proper conditions, and does not require ad6ustment. Specialmechanical seals, such as 'iton 41===F, and Teflon 45==+F and corrosionresistant, can be supplied for special conditions.
#n the rotary pump, a steam chest located between the casing and the outboardbearing effectively transfers heat to both the pump and the pac$ing. #t can be usedwith hot water, steam, and heat transfer oil@ or it can be used as a cooling chamber.The steam chest is ideal for transferring thic$, viscous liquids, such as asphalt mi8es,creosote, refined sugars, corn starch, etc.
An ad6ustable relief valve 1< in the pump faceplate eliminates outside piping andprotects the pump from e8cessive outlet line pressure@ it also permits the operator toclose the discharge line without stopping the pump, under standard operatingconditions. 'arious spring sies are available to provide ad6ustments over the fulloperating range of the pump from != to == psi.
As aforementioned, the rotary geartype pumps are more simple in design, and theyhave fewer wor$ing parts than the rotary vanetype and pistontype pumps. Thenames of the various parts of a typical geartype pump can be learned from thediagram and parts list in Fig. 1=.
Fig. 1<. An ad6ustable relief valve protects the rotary geartype pump from e8cessiveoutlet line pressure
Screw pumps are a special type of rotary positive displacement pump in which the flowthrough the pumping elements is truly a8ial. The liquid is carried between screw threads onone or more rotors and is displaced a8ially as the screws rotate and mesh. #n all other rotarypumps the liquid is forced to travel circumferentially, thus giving the screw pump with itsunique a8ial flow pattern and low internal velocities a number of advantages in manyapplications where liquid agitation or churning is ob6ectionable.
Basic types as indicated in the introduction, there are three ma6or types of screwpumpsC
. Singlerotor
3. -ultiplerotor timed
!. -ultiplerotor untimed
The second and third types are available in two basic arrangements singleend anddoubleend. The doubleend construction is probably the best$nown version as itwas by far the most widely used f or many years because of its relative simplicity andcompactness of design.
