Fig.1 is shown in addition, which displays the information exactly and correctly.
Despite this simple formula, the problem continues to occur during pump operation in parallel that the pumps or the pumping system
do not achieve the expected flow rate.How such problems can avoided will be explained under the following five head-ings:Real versus ideal
conditions
The real conditions in pump systems differ
Applications
Abstract: At first sight, parallel operation of pumps does not em to po any problems. Under real conditions, however, parallel pumping proves to be rather more complex than typically portrayed in the relevant literature.So as to ensure that pumps and systems produce the required flow rates and pressures detailed information is needed about the real curve of both the pumps and the system. The duty point of the individual pumps oper-ated in parallel will shift and this will affect both efficiency and NPSH per-formance. Only if the changes are taken into account will a parallel pumping system functi
on properly.
Fig. 2: Pump system schematic (2 pumps operating in parallel)
4
H Druckh öhenverlust in der
Sammeldruckleitung
岁的成语H Druckh öhenverlust in der
Sammelsaugleitung H Druckh öhenverlust in der zur
Pumpe individuell geh örenden Druckleitung H Druckh öhenverlust in der zur Pumpe individuell geh örenden Saugleitung
国产电影推荐H
H
H
H Dr.-Ing. Gerhard Mollenkopf
M
H V AS世界上最强的国家
H
V AS
H V AD Head loss in the discharge manifold
H V AS Head loss in the suction
manifold
H V PD Head loss in the pump's individual discharge pipe
H V PS Head loss in the pump's individual suction pipe H V PD
H
V PS
In textbooks and minar materials mostly only general conclusions about the parallel operation of two or more pumps are given.
A formula is shown in which the flow rates of the individual pumps are simply added for a given constant head.
System curve H A
Duty point 0Shutoff head
Characteristic
curve of pump I or pump II
Characteristic curve pump I + pump II B single
Q single
Q parallel
Q I = Q II = Q parallel / 2Flow rate Q
Q parallel Fig. 1: Parallel operation of two identical cen-trifugal pumps with stable characteristic curve
If the efficiency is included in the characteristic curves, it becomes evident that when changing from single to multiple pump operation,the operating point of each indi-vidual pump increasingly shifts to-wards lower load conditions. The choice of pump size for pumps working in parallel is therefore largely dependent on the expected operating time. If it is planned that most often three pumps will be working in parallel, then the best efficiency point (BEP) of the pumps should be chon for this condition (Q tot = 3 •Q opt , individual pumps).However, it must be ensured that if a single pump is operated on its own, the incread flow of that pump still falls within the permis-sible range, in particular with respect to NPSH performance (e Fig. 5).
In the opposite ca, when parallel operation is only occasionally
required, for example to meet peak demands, the pump size should be chon so that single pump operation is near the BEP. During parallel operation the duty points of the individual pumps shift towards low flows with lower efficiency. There is usually no risk of insufficient NPSH.
Pump
characteristic curves
The pump characteristic curves and the system’s mode of operation are of para-mount importance. Various types of pumps, particularly when the impeller is cut down substantially below its largest diameter, tend to hydraulic instability, i.e., the developed head at shutoff (Q = 0) is lower than at some other operating point.
If such pumps are connected in parallel, the cond pump cannot open its check valve against the high operating pressure produced by the first pump and runs with Q = 0 (Fig. 6). The energy input by the motor is almost Applications
Fig. 3: Pump characteristic curve taking into account the loss
Fig. 5: Shift of operating point when pump working in parallel is stopped
5
芽菜是什么菜Flow rate Q
H e a d H
H A
Q I Q I+II Q I+II+III
3 pumps in parallel
Pump I
2 pumps in parallel
苍蝇幼虫
loss, the interction with the system characteristic and therefore the total duty or operating point and the operating point of each pump can be determined (e Fig. 3).The form of the system characteris-tic must be carefully considered when an existing facility is to be ex-panded, for example by installing further pumps in parallel. As shown in Fig. 4 (prenting the sys-tem curve of a cooling circuit), the total flow rate cannot be signi-ficantly incread by installing more pumps.
Duty point lection
Each pump’s duty point is lected according to the mode of opera-tion, considering the efficiency of each pump and the pump system,as well as the NPSH of the pumps.
completely converted to heat within the pump, and a substantial increa of the temperature of the enclod fluid can result.
≠Pumps
= Shutoff heads
For reasons of better utilization of the pumps, a small pump (for
example OMEGA 300/560) is installed next to veral larger pumps (for example RDLO 500/790) in some pump systems to achieve a more complete coverage of the flow range with good efficiency (e Fig. 7). By properly choosing the speed and diameter
both pump sizes can be designed to have the same shutoff head . It should be kept in mind, however,that depending on the motor, the actual operating speed can cau significant head differences. The can lead to a major shift of the operating point , particularly for the smaller pump.
For the smaller pump, which usually has a higher operating speed and correspondingly higher inlet and discharge velocity , the so-called individual loss must be carefully calculated.
≠Pumps
≠Shutoff heads
In some rare cas pumps can be run in parallel when they do not have the same shutoff head. As shown in Fig. 8, satisfactory operation can only be achieved when the shutoff head of the smaller pump is not exceeded. For smaller flow rates the same problem occurs as for a per-formance curve instability, i.e. the
smaller pump is running but cannot open its check valve, and the motor energy heats up the fluid in the pump.
A similar mode of operation and risk are prent when a cond pump is started up with a vari-able speed drive in parallel to a pump already running.
Throughflow in the cond pump will only start when its shutoff head exceeds the operating pres-sure of the first pump (interction with the system curve). Except for this problem, a variable speed pump is an ideal partner to a fixed speed pump for parallel operation.Sufficient instrumentation with flow meters and pressure gauges is helpful in this ca.
Applications
6
Fig. 7: Parallel operation of two pumps with equal shutoff heads
Shutoff head: Head developed by the pump for zero flow rate (operating against a clod valve).
Shift of operating point: When veral pumps are operating in parallel against the same system curve, the total flow rate is divided among them according to their individual flow rate for the head impod by the system. If the actual characteristic curve of a pump deviates from that of the expected characteristic, for example due to design tolerances or variation of the speed, the actual operating point of the pump can vary substantially from the expected one.
Inlet / discharge velocity: The average flow velocity (Q/A) at the pump inlet or discharge
can be calculated using the actual inlet or outlet diameter.
Fig. 8: Parallel operation of two dissimilar pumps
Fig. 6: Parallel operation of two pumps with unstable characteristic curve
Flow rate Q
H e a d H
H 0 II
H 0 I
dlorH II
H I
H A
H I+II
Q I Q min Q II Q I+II
A = cross-ction = 1.π.D 2
4
S,D
Flow rate Q
H e a d H
葡萄是酸性还是碱性H I H I+II
H A
H B H B =Duty point with only pump I running
H 0 =Shutoff head is lower than H B : pump II will run
with Q = 0; check valve cannot open.
H 0
Flow rate Q
Pump I = smaller pump Pump II = larger pump B IP = Duty point of pump I when operating in parallel
H e a d H
H 0 (Shutoff head)
H I
H II
B IIP
B IP
幼儿咳嗽B IIE
B IE
H A (System curve)
H I+II
Q I Q II Q I+II
