pumps and pumping stations
DESCRIPTION
Pumps and Pumping Stations. Pumps add energy to fluids and therefore are accounted for in the energy equation Energy required by the pump depends on: Discharge rate Resistance to flow (head that the pump must overcome) - PowerPoint PPT PresentationTRANSCRIPT
Pumps and Pumping Stations
• Pumps add energy to fluids and therefore are accounted for in the energy equation
• Energy required by the pump depends on:– Discharge rate– Resistance to flow (head that the pump must overcome)– Pump efficiency (ratio of power entering fluid to power supplied
to the pump)– Efficiency of the drive (usually an electric motor)
2 21 1 2 2
1 22 2pump L
v p v pz H z H
g g
2
2L f minor f i
vH h h h K
g
Pump Jargon• (Total) Static head – difference in head between suction
and discharge sides of pump in the absence of flow; equals difference in elevation of free surfaces of the fluid source and destination
• Static suction head – head on suction side of pump in absence of flow, if pressure at that point is >0
• Static discharge head – head on discharge side of pump in absence of flow
Total static head
Static suction head
Static discharge
head
Pump Jargon• (Total) Static head – difference in head between suction
and discharge sides of pump in the absence of flow; equals difference in elevation of free surfaces of the fluid source and destination
• Static suction lift – negative head on suction side of pump in absence of flow, if pressure at that point is <0
• Static discharge head – head on discharge side of pump in absence of flow
Total static head Static suction
lift
Static discharge
head
Pump Jargon
Total static head (both) Static suction
lift
Static discharge
head
Static suction head
Static discharge
head
Static suction head
Static suction lif
Static discharge head
Static d t
Total static h
ischarge he d
ead
a
Note: Suction and discharge head / lift measured from pump centerline
Pump Jargon• (Total) Dynamic head, dynamic suction head or lift, and dynamic
discharge head – same as corresponding static heads, but for a given pumping scenario; includes frictional and minor headlosses
Total dynamic
head
Dynamic suction lift
Dynamic discharge
head
Energy Line
Example. Determine the static head, total dynamic head (TDH), and total headloss in the system shown below.
Total static head 730 ft 630 ft 100 ft
pd =48 psig
ps =6 psig
El = 630 ft
El = 640 ft
El = 730 ft
2.31 ftTDH 48 6 psi 124.7 ft
psi
TDH Static head 124.7 100 ft 24.7 ftLH
Example. A booster pumping station is being designed to transport water from an aqueduct to a water supply reservoir, as shown below. The maximum design flow is 25 mgd (38.68 ft3/s). Determine the required TDH, given the following:• H-W ‘C’ values are 120 on suction side and 145 on discharge side• Minor loss coefficients are
0.50 for pipe entrance0.18 for 45o bend in a 48-in pipe0.30 for 90o bend in a 36-in pipe0.16 and 0.35 for 30-in and 36-in butterfly valves, respectively
• Minor loss for an expansion is 0.25(v22 v1
2)/2g
Short 30 pipe w/30 butterfly valve
El = 6349 to 6357 ft
El = 6127 to 6132 ft
30 to 48 expansion
4000of 48 pipe w/two 45o bends
8500of 36 pipe w/one 90o bend and eight butterfly valves
1. Determine pipeline velocities from v = Q/A..
v30= 7.88 ft/s, v36= 5.47 ft/s, v48= 3.08 ft/s
2. Minor losses, suction side:230
230
2 230 48
2o 48
,minor
Entrance: 0.50 0.49 ft2
Butterfly valve: 0.16 0.16 ft2
Expansion: 0.25 0.21 ft2
Two 45 bends: 2* 0.18 0.05 ft2
0.91 ft
L
L
L
L
L
vh
g
vh
g
v vh
g
vh
g
h
3. Minor losses, discharge side:
236
2o 36
,minor
8 Butterfly valves: 8* 0.35 1.30 ft2
One 90 bend: 0.30 0.14 ft2
1.90 ft
L
L
L
vh
g
vh
g
h
1.85
2.630.43f
Qh L
CD
1.85
, 2.63
38.74000 2.76 ft
0.43 120 48 /12f suctionh
4. Pipe friction losses:
1.85
2.630.43fh Q
SL CD
1.85
, 2.63
38.78500 16.77 ft
0.43 145 36 /12f dischargeh
5. Loss of velocity head at exit:236Exit: 0.46 ft
2L
vh
g
Static head 6357 6127 ft 230 ft
6. Total static head under worst-case scenario (lowest water level in aqueduct, highest in reservoir):
, ,TDH
230 0.91 1.90 2.76 16.77 0.46 ft
252.8 ft
static L minor f L exitH h h h
7. Total dynamic head required:
Pump Power
• P = Power supplied to the pump from the shaft; also called ‘brake power’ (kW or hp)
• Q = Flow (m3/s or ft3/s)• TDH = Total dynamic head = Specific wt. of fluid (9800 N/m3 or 62.4 lb/ft3 at 20oC)• CF = conversion factor: 1000 W/kW for SI, 550 (ft-lb/s)/hp for US• Ep = pump efficiency, dimensionless; accounts only for pump,
not the drive unit (electric motor)
TDH
CF p
QP
E
Useful conversion: 0.746 kW/hp
Example. Water is pumped 10 miles from a lake at elevation 100 ft to a reservoir at 230 ft. What is the monthly power cost at $0.08/kW-hr, assuming continuous pumping and given the following info:
• Diameter D = 48 in; Roughness = 0.003 ft, Efficiency Pe =80%• Flow = 25 mgd = 38.68 ft3/s• T = 60o F• Ignore minor losses
El = 100 ft
10 mi of 48 pipe,
=0.003 ft
El = 230 ft 2
1
21
2
v
g1p
22
1 2pump
vz H
g 2p
2 Lz H
2 1TDHpump stat L fH z z H H h
TDH stat fH h
El = 100 ft
10 mi of 48 pipe,
=0.0003 ft
El = 230 ft 2
1
TDH stat fH h
230 100 ft 130 ftstatH
2
2f
L vh f
D g
/ 3.08 ft/sv Q A
Find f from Moody diagram
65 2
3.08 ft/s 4 ftRe 1.01x10
1.22x10 ft /s
vD
40.003 ft7.5x10
4 ftD
El = 100 ft
10 mi of 48 pipe,
=0.0003 ft
El = 230 ft 2
1
6Re 1.01x1047.5x10
D
0.0185f
2
2
3.08 ft/s10*5280 ft0.0187 36.4 ft
4 ft 2 32.2 ft/sfh
TDH 130 36.4 ft 166.4 ftstat fH h
3 338.68 ft /s 166.4 ft 62.4 lb/ftTDH918 hp
CF ft-lb/s550 0.80
hpp
QP
E
kW $0.08 hrDaily cost 918 hp 0.746 24 $1315 / d
hp kW-hr d
Pump Selection
• System curve – indicates TDH required as a function of Q for the given system– For a given static head, TDH depends only on HL, which is
approximately proportional to v2/2g
– Q is proportion to v, so HL is approximately proportional to Q2 (or Q1.85 if H-W eqn is used to model hf)
– System curve is therefore approximately parabolic
Example. Generate the system curve for the pumping scenario shown below. The pump is close enough to the source reservoir that suction pipe friction can be ignored, but valves, fittings, and other sources of minor losses should be considered. On the discharge side, the 1000 ft of 16-in pipe connects the pump to the receiving reservoir. The flow is fully turbulent with D-W friction factor of 0.02. Coefficients for minor losses are shown below.
40 ft
6 ft
K values
Suction Discharge
1 @ 0.10 1 @ 0.12
1 @ 0.12 1 @ 0.20
1 @ 0.30 1 @ 0.60
2 @ 1.00 4 @ 1.00
The sum of the K values for minor losses is 2.52 on the suction side and 5.52 on the discharge side. The total of minor headlosses is therefore 8.04 v2/2g.
An additional 1.0 v2/2g of velocity head is lost when the water enters the receiving reservoir.
The frictional headloss is: 2 2 21000 ft
0.02 152 1.33 ft 2 2f
L v v vh f
D g g g
Total headloss is therefore (8.04+1.0+15.0)v2/2g = 24.04 v2/2g. v can be written as Q/A, and A = D2/ 4 = 1.40 ft2. The static head is 34 ft. So:
2
22 22
52
TDH 34 ft 24.042
/1.40 ft s34 ft 24.04 34 ft 0.19
ft2 32.2 ft/s
stat L
vH H
g
22
5
sTDH 34ft 0.19
ftQ
0
20
40
60
80
100
120
140
160
180
0 5 10 15 20 25
Discharge, Q (ft3/s)
TD
H (
ft)
Static head
System curve
Pump Selection
• Pump curve – indicates TDH provided by the pump as a function of Q; – Depends on particular pump; info usually provided by manufacturer– TDH at zero flow is called the ‘shutoff head’
• Pump efficiency– Can be plotted as fcn(Q), along with pump curve, on a single graph– Typically drops fairly rapidly on either side of an optimum; flow at
optimum efficiency known as “normal” or “rated” capacity– Ideally, pump should be chosen so that operating point corresponds
to nearly peak pump efficiency (‘BEP’, best efficiency point)
Shutoff head
Rated capacity
Rated hp
Pump Performance and Efficiency Curves
Pump Selection
Pump Efficiency• Pump curves depend on pump geometry (impeller D) and speed
Pump Selection• At any instant, a system has a single Q and a single TDH, so both
curves must pass through that point; operating point is intersection of system and pump curves
Pump System Curve• System curve may change over time, due to fluctuating reservoir levels,
gradual changes in friction coefficients, or changed valve settings.
Pump Selection: Multiple Pumps• Pumps often used in series or parallel to achieve desired pumping
scenario• In most cases, a backup pump must be provided to meet maximum
flow conditions if one of the operating (‘duty’) pumps is out of service.• Pumps in series have the same Q, so if they are identical, they each
impart the same TDH, and the total TDH is additive• Pumps in parallel must operate against the same TDH, so if they are
identical, they contribute equal Q, and the total Q is additive
Adding a second pump moves the operating point “up” the system curve, but in different ways for series and parallel operation
Example. A pump station is to be designed for an ultimate Q of 1200 gpm at a TDH of 80 ft. At present, it must deliver 750 gpm at 60 ft. Two types of pump are available, with pump curves as shown. Select appropriate pumps and describe the operating strategy. How will the system operate under an interim condition when the requirement is for 600 gpm and 80-ft TDH?
TD
H (
ft)
Flow rate (gpm)
Pump A onlyPump B only
System curve
Eff
icie
ncy
, % Pump B
Pump A
10
100
40
50
30
20
90
80
70
60
0
120
110
200 400 120010008006000
70
60
50
40
Either type of pump can meet current needs (750 gpm at 60 ft); pump B will supply slightly more flow and head than needed, so a valve could be partially closed. Pump B has higher efficiency under these conditions, and so would be preferred.
TD
H (
ft)
Flow rate (gpm)
Pump A onlyPump B only
System curve
Eff
icie
ncy
, % Pump B
Pump A
10
100
40
50
30
20
90
80
70
60
0
120
110
200 400 120010008006000
70
60
50
40
The pump characteristic curve for two type-B pumps in parallel can be drawn by taking the curve for one type-B pump, and doubling Q at each value of TDH. Such a scenario would meet the ultimate need (1200 gpm at 80 ft), as shown below.
TD
H (
ft)
Flow rate (gpm)
A onlyB only Two B’s
System curve
Eff
icie
ncy
, % Pump B
Pump A
10
100
40
50
30
20
90
80
70
60
0
120
110
200 400 120010008006000
70
60
50
40
A pump characteristic curve for one type-A and one type-B pump in parallel can be drawn in the same way. This arrangement would also meet the ultimate demand. Note that the type-B pump provides no flow at TDH>113 ft, so at higher TDH, the composite curve is identical to that for just one type-A pump. (A check valve would prevent reverse flow through pump B.) Again, since type B is more efficient, two type-B pumps would be preferred over one type-A and one type-B.
TD
H (
ft)
Flow rate (gpm)
A onlyB only Two B’s
System curve
Eff
icie
ncy
, % Pump B
Pump A
10
100
40
50
30
20
90
80
70
60
0
120
110
200 400 120010008006000
70
60
50
40
One A and one B in parallel
At the interim conditions, a single type B pump would suffice.
A third type B pump would be required as backup.T
DH
(ft
)
Flow rate (gpm)
A onlyB only Two B’s
System curve
Eff
icie
ncy
, % Pump B
Pump A
10
100
40
50
30
20
90
80
70
60
0
120
110
200 400 120010008006000
70
60
50
40
One A and one B in parallel