Pumps in Series and Parallel

INTRODUCTION

Pumps are used to transfer fluid in a system, either at the same elevation or t o a new height. The needed flow rate depends on the height to which the fluid is pumped. Each pump pump has a headdischarge relationship that is inversely proportional (i.e., if a higher flow rate is needed, then less head or pressure will be produced by the t he pump, and vice versa). This head-discharge relationship, also known as the pump characteristic curve, is provided by the pump manufacturer. manufacturer. Two pumps (or typically more in practice) can be combined in series to increase the height to which the fluid can be pumped at a given flow rate, or combined in parallel to increase the flow rate associated with a given value of head. In theory, if two pumps are combined in series, the pumping pumping system will produce twice the head for a given flow rate. Similarly, if two pumps are combined in  parallel, the pumping pumping system is expected expected to have have twice the flow rate of single pump pump for a given head. head. OBJECTIVES 1. 2. 3. 4.

To demonstrate pump performance when connected in series and parallel To show shut off point of pump in series and parallel To analyse pump network for pipelines operating under pressure To estimate power requirement requirement for a pump as a function of its throughput, pressure increase and efficiency

PROCEDURE 1. The equipment was set up by following some basic procedures. procedures. 2. The main power supply is switched on. 3. The appropriate pump was select and the following valve position is checked. checked. Pump Operation Operati on Single Serial Parallel

Running Pump Pump 1, P1 Both Pump, P1 & p2 Both Pump, P1 & p2

Open Valve 1, 3 1, 4 1, 2, 4

Close Valve 2, 3 2, 4 3

4. Valve V5 (as a variable parameter) is slowly opened until the flow rate reaches to a certain level that is needed by the operation of the pump. Pump Operation Single Serial Parallel

20 20 40

Volume Flow Rate (L/min) 40 60 80 40 60 80 80 120 160

90 90 180

5. The pressure reading on the pressure indicator is observed. Flow rate and pressure is recorded when the stable condition is achieved. 6. The observation is repeated by increasing the flow rate until reaches to a certain maximum level that is needed by the operation of the pump. 7. The pump is turned off. Valve V5 is ensured that it is in fully close position. The main power supply is switched off.

DISCUSSION The objective of this laboratory experiment is to measure and compare the performance of a single centrifugal pump to that of two pumps in parallel and series configurations. THEORY By ssuming steady, uniform, incompressible flow between the inlet and outlet sections of a  piping system containing a pump, the energy conservation equation can be used to illustrate the relationship between kinetic and potential energy of the fluid:

  p2 V 22     p1 V 12      z 2      z 1    H  P   H l      g  2 g        g  2 g   

T 

 

(1)

where the subscripts 1 and 2 refer to inlet and exit sections, respectively [1]. H  p is the 'head' produced  by a pump (in meters); H l_T  represents energy losses (in meters) from friction, turbulence, fittings, etc.; p is the static pressure (in Pa);    is the fluid density (in kg/m3); g  is the gravitational constant (in meters/sec 2); V  is the fluid velocity (in meters/sec); and z  is the elevation of the measurement point (in meters). For this laboratory, the minor head losses, H l_T , may be neglected. One important aspect of pump performance is the pressure, or head, that the pump can produce as a function of flow rate. Generally, the higher the flow rate, the lower the head that the pump can contribute. A parabola is often used to fit this performance data:

 H  p

  H   AQ

2

0

 

(2)

where Q is the volumetric flow rate, A is a constant determined empirically from the data, and H 0 is the head delivered at zero flow rate. For the two pumps in series, the flow Q through the first pump must equal the flow through the next,  but each pump adds pressure head. For nominally identical pumps the total head added is

 H  series



 2  H   AQ 0

2

.

(3)

For identical pumps in parallel, the pressures at the two inlets and outlets are i dentical and the maximum head the two pumps can deliver is no greater than that of one pump. The flow rate, however, is doubled for two identical pumps in parallel:

 Q  2  H  pa rallel   H 0    A     2 

(4)

 In practice, these performance curves will not be met because of losses in piping systems and nonidentical pumps. An important objective when selecting a pump for an engineering system is maximizing the efficiency for the desired flow conditions. For a pump, the efficiency is defined as h = P o /P i

(5)

where Po is the power output from the pump (in Watts), and Pi is the power imparted to the fluid from the pump (in Watts). Output power is determined experimentally with the following equation

 P o= ρg*Q*H  p.

(6)

The input power to the pump is the output power from the motor. Pi varies as a function of flow rate. The purpose to connect pump in series and parallel is to extend the total pump performance in a system. The shutoff pressure is the maximum pressure a pump will develop under zero-flow conditions, which reflects a fully blocked outlet. Pumps combined in parallel can increase the flow twice for t he same given head. In practice, this would be done if a pump provides the right head, but provides small flow. Parallel arrangement is also used if the demand of flow is different. One pump can be used at lower flow than the second  pump. The advantage of arranging pumps in parallel is the redundancy in case failure occurs. On the other hand, applying pumps arrangement into series can twice increase the head. In practice, this would be done if the pump provides the right flow rate, but small head. When pumps are connected in series or parallel, the power input to the pump increases. The effect of increasing motor speed can be estimated using the similarity rules. The operating point of the pump is at the point where the head of the pump is the same as the  pipe resistance. The operating point is the intersection of the pump characteristic curve and the pipe system characteristic curve. The most common reason for choosing parallel pumps is because you want to have a spare in case the single operating pump fails. Pumps in series are generally not advised. This is because the maximum shutoff head of pumps in series is additive, and will result in higher design pressures in the downstream piping/equipment. Generally in pumps increasing the flow rate will decrease the provided head. The efficiency of the pumps was not calculated due to lack of givens (input power) CONCLUSIONS Thanks to Allah The Almighty for His blessing, we manage to finish up the experiment and the reports in the given time. Because of Him we are able to achieve the objectives of this experiment. In a series arrangement, each pump handles the same flow rate, but t he total head produced by the combination of pumps will be additive. For pumps configured in parallel, the flow rate Q is split  between the pumps at the inlet into Q1 and Q2. The commonality of head across parallel pumps is the most important feature of pumps installed in parallel. If the pump heads are not matched, pumps in  parallel will not function properly. REFERENCES  



White, Frank M. Fluid Mechanics; 3rd Edition, McGraw –  Hill; 1994

http://www.cheresources.com/invision/blog/4/entry-322-multiple-centrifugal pumps-in-series-and-parallel/ Lab Manual, Fluid Mechanis.

INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA FLUID MECHANICS BTE 2222

EXPERIMENT 2 PUMPS IN SERIES AND PARALLEL 15 / 11 / 2013

SUHADAHAFIZA BINTI SHAFIEE 1123746 GROUP MEMBERS : -

SINGLE

Pressure different vs. Flow rate 12 10 8    e    r    u    s    s    e    r    p

6 4 2 0 1

2

3

4

5

flow rate

SERIAL

Pressure different vs. Flow rate 14 12 10    e    r    u    s    s    e    r    p

8 6 4 2 0 20

40

60

flow rate

80

90

PARALLEL

Pressure different vs. Flow rate 12 10 8    e    r    u    s    s    e    r    p

6 4 2 0 40

80

120

flow rate

160

180