piping and pumping
DESCRIPTION
p&pTRANSCRIPT
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Outline• Pipe routing
• Optimum pipe diameter
• Pressure drop through piping
• Piping costs
• Pump types and characteristics
• Pump curves
• NPSH and cavitation
• Regulation of flow
• Pump installation design
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Piping and Pumping Learning Objectives
At the end of this section, you should be able to…
• Draw a three dimensional pipe routing with layout and plan views.
• Calculate the optimum pipe diameter for an application.
• Calculate the pressure drop through a length of pipe with associated valves.
• Estimate the cost of a piping run including installation, insulation, and hangars.
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• List the types of pumps, their characteristics, and select an appropriate type for a specified application.
• Draw the typical flow control loop for a centrifugal pump on a P&ID.
• Describe the features of a pump curve.• Use a pump curve to select an appropriate pump and
impellor size for an application.• Predict the outcome from a pump impellor change.• Define cavitation and the pressure profile within a
centrifugal pump.• Calculate the required NPSH for a given pump
installation.• Identify the appropriate steps to design a pump
installation.
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References• Appendix III.3 (pg 642-46) in Seider et al.,
Process Design Principals (our text for this class).
• Chapter 12 in Turton et al., Analysis, Synthesis, and Design of Chemical Processes.
• Chapter 13 in Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers.
• Chapter 8 in McCabe, Smith and Harriott, Unit Operations of Chemical Engineering.
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Pipe Routing
• The following figures show a layout (looking from the top) and plan (looking from the side) view of vessels.
• We want to rout pipe from the feed tank to the reactor.
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Plan View
50 ft
feed tank
reactor
40 ft
steam header
35 ft
piping chase
60 ft
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reactor
feed tank
piping chase
50 ft 35 ft
steam header
30 ft
45 ft
40 ft
10 ft
Layout View: Looking Down
reactor
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50 ft
feed tank
reactor
40 ft
steam header
35 ft
piping chase
60 ft
Plan View= out
= in
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reactor
feed tank
steam header
30 ft85 ft
20 ft
60 ft35 ft10 ft
10 ft
Layout View
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Pipe Routing Exercise
• Form groups of two.
• Draw a three dimensional routing for pipe from the steam header to the feed tank on both the plan view and the layout view.
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Size the Pump
globe valve
check valve
200 ft
150 ft
1. Determine optimum pipe size.
2. Determine pressure drop through pipe run.
100 gpm
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Optimum Pipe DiameterThe optimum pipe diameter gives the least total cost for annual pumping power and fixed costs. As D , fixed costs , but pumping power costs .
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Optimum Pipe Diameter
Pipe Diameter
Cos
t/(ye
ar ft
)
Total Cost
Annualized Capital Cost
Pumping Power Cost
Optimum
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Example• Two methods to determine the
optimum diameter:
Velocity guidelines and Nomograph.
• Example: What is the optimum pipe diameter for 100 gpm water.
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Using Velocity Guidelines
• Velocity = 3-10 ft/s = flow rate/area
• Given a flow rate (100 gpm), solve for area.
• Area = (/4)D2, solve for optimum D.
• Optimum pipe diameter = 2.6-3.6 in.
Select standard size, nominal 3 in. pipe.
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Nomograph
-Convert gpm to cfm 13.4 cfm.
-Find cfm on left axis.
-Find density (62 lb/ft3) on right axis.
-Draw a line between points.
-Read optimum diameter from middle axis.
3.3 in optimum diameter
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Practice Problem
• Find the optimum pipe diameter for 100 ft3 of air at 40 psig/min.
• A = (s/50ft)(min/60 s)(100 ft3/min) = 0.033 ft2
• 0.033 ft2 = 3.14d2/4
• d = 2.47 in
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Piping Guidelines• Slope to drains.• Add cleanouts (Ts at elbows)
frequently.• Add flanges around valves for
maintenance.• Use screwed fitting only for 1.5 in
or less piping.• Schedule 40 most common.
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Calculating the Pressure Drop through a Pipe Run• Use the article Estimating pipeline
head loss from Chemical Processing (pg 9-12).
P = (/144)(Z+[v22-v1
2]/2g+hL)• Typically neglect velocity differences
for subsonic velocities.• hL = head loss due to 1) friction in
pipe, and 2) valves and fittings.• hL(friction) = c1fLq2/d5
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• c1 = conversion constant from Table 1 = 0.0311.
• f = friction factor from Table 6 = 0.018.
• L = length of pipe = 200 ft + 150 ft = 350 ft.
• q = flow rate = 100 gpm.• d = actual pipe diameter of 3”
nominal from Table 8 = 3.068 in .• hL due to friction = 7.2 ft of liquid
head
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Loss Due to Fittings• K= 0.5 entrance• K = 1.0 exit• K=f(L/d)=(0.018)(20) flow through tee• K=3[(0.018)(14)] elbows• K=0.018(340) globe• K=0.018(600) check valve
Sum K = 19.5
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• hL due to fittings = c3Ksumq2/d4 = 5.7 ft of liquid head loss due to fittings.
• hLsum=7.2 + 5.7 ft of liquid head loss• Using Bernoulli Equation
P = (/144)(Z+[v22-v1
2]/2g+hLsum)
P = ( /144)(150+0+12.9)= 70.1 psi due mostly to elevation. Use P to size pump.
elevation velocity friction and fittings
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Find the Pressure Drop
check valve
400 ft
50 ft
400 gpm water
4 in pipe
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Estimating Pipe Costs
Use charts from Peters and Timmerhaus.
Pipe
Fittings (T, elbow, etc.)
Valves
Insulation
Hangars
Installation
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$/li
near
ft
Note: not
2003 $
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Pumps – Moving Liquids
• Centrifugal
• Positive displacement
–Reciprocating: fluid chamber stationary, check valves
–Rotary: fluid chamber moves
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Centrifugal Pumps
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Centrifugal Pump Impeller
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Positive Displacement: Reciprocating
• Piston: up to 50 atm
• Plunger: up to 1,500 atm
• Diaphragm: up to 100 atm, ideal for corrosive fluids
• Efficiency 40-50% for small pumps, 70-90% for large pumps
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Positive Displacement: Reciprocating (plunger)
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Positive Displacement: Rotary
• Gear, lobe, screw, cam, vane
• For viscous fluids up to 200 atm
• Very close tolerances
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Positive Displacement: Rotary
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Comparisons: Centrifugal
• larger flow rates• not self priming• discharge dependent of downstream pressure drop• down stream discharge can be closed without
damage• uniform pressure without pulsation• direct motor drive• less maintenance• wide variety of fluids
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Comparisons: Positive Displacement• smaller flow rates• higher pressures• self priming• discharge flow rate independent of pressure
– utilized for metering of fluids• down stream discharge cannot be closed
without damage – bypass with relief valve required
• pulsating flow• gear box required (lower speeds)• higher maintenance
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Centrifugal PumpsAdvantages
• simple and cheap• uniform pressure, without
shock or pulsation• direct coupling to motor• discharge line may be closed• can handle liquid with large
amounts of solids• no close metal-to-metal fits• no valves involved in pump
operation• maintenance costs are lower
Disadvantages• cannot be operated at
high discharge pressures
• must be primed• maximum efficiency
holds for a narrow range of operating conditions
• cannot handle viscous fluids efficiently
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Moving Gases• Compression ratio = Pout/Pin
• Fans: large volumes, small discharge pressure
• Blowers: compression ratio 3-4, usually not cooled
• Compressors: compression ratio >10, usually cooled.
– Centrifugal (often multistage)
– Positive displacement
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Fan Impellers
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Two-lobe Blower
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Reciprocating Compressor
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Centrifugal Pump Symbols
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Pump CurvesFor a given pump1. The pressure produced at a given flow rate
increases with increasing impeller diameter.
2. Low flow rates at high head, high flow rates at high head.
3. Head is sensitive to flow rate at high flow rates.
4. Head insensitive to flow rate at lower flow rates.
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Pump Curve
- used to determine which pump to purchase.
- provided by the manufacturer.
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Pump Curve
Pressure increases with diameter
Low flow at high head
Head sensitive to flow at high flow rates
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NPSH and Cavitation• NPSH = Net Positive Suction Head• Frictional losses at the entrance to the
pump cause the liquid pressure to drop upon entering the pump.
• If the the feed is saturated, a reduction in pressure will result in vaporization of the liquid.
• Vaporization = bubbles, large volume changes, damage to the pump (noise and corrosion).
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Pressure Profile in the Pump
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NPSH• To install a pump, the actual NPSH must be equal
to or greater than the required NPSH, which is supplied by the manufacturer.
• Typically, NPSH required for small pumps is 2-4 psi, and for large pumps is 22 psi.
• To calculate actual NPSH…
NPSHactual= Pinlet-P* (vapor pressure)
Pinlet = P(top of tank, atmospheric) + gh - 2fLeqV2/D
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What if NPSHactual < NPSHrequired?
INCREASE NPSHactual
• cool liquid at pump inlet (T decreases, P* decreases)
• increase static head (height of liquid in feed tank)
• increase feed diameter (reduces velocity, reduces frictional losses) (standard practice)
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Regulating Flow from Centrifugal Pumps
• Usually speed controlled motors are not provided on centrifugal pumps, the flow rate is changed by adjusting the downstream pressure drop (see pump curve).
• Typical installation includes a flow meter, flow control valve (pneumatic), and a control loop.
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Typical Installation
FT
FC
FV
operator set-point
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Designing Pump Installations• use existing pump vendor, note spare
parts the plant already stocks.
• select desired operating flow rate, maximum flow rate.
• calculate pressure drop through discharge piping, fittings, instrumentation (note if flow control is desired need to use pressure drop with control valve 50% open).
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• add safety factor to calculated head – 10 psig spec pump for 20 psig, 150 psig spec pump for 200 psig.
• using head and flow rate, select impeller that gives efficient operation in region of operating flow rate.
• vertical location of pump compared to level of influent tank (NPSH).
• if want to control flow rate – spec and order flow meter and flow control valve also.