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Chemical Engineering Plant Design
Lecture 18 Piping Systems
Instructor: David Courtemanche
CE 408
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Piping Systems
• At this point we have designed most of our unit operations
• Flow rates
• Pressures
• Temperatures
• The vessels are connected by lines on our Process Flow Diagram
• We need to start thinking about what those lines actually are
• Start with a layout of the equipment
• How large are the pieces of equipment?
• How far apart should they be?
• Piping is usually run overhead if it is going long distances
• Do we need different elevations for different pieces of equipment?
• Need for static pressure legs to maintain different pressures in different pieces of
equipment
• Does flow enter in the top, the bottom, or somewhere else in the equipment?
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Piping Systems
• Here is a more detailed flow diagram courtesy of Larry Coleman
• This one assumes everything runs at vacuum
• Dimensions are not shown here, but there is some indication of what would be at different elevations
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Piping Systems
• Start by doing a plot layout
• Overhead View
• (Not drawn to any particular scale)
• What are dimensions of each piece of equipment
• How far apart do they need to be?
• Maintenance and operating concerns
• Process Safety Concerns
• Site limitations
• Other equipment/buildings/topography
Raw LA storage Prepolymer Reactor
Hold Tank Lactide Reactor
Distillation Column
X feet
X’ feet
X’’ feet
X’’’ feet
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Piping Systems
• Elevation View
• Do you want to gravity drain from one piece of equipment to the next?
• Do you need to build up pressure before pump inlet?
• Do you need to isolate vapor space of one vessel from the next?
• These Evaporators are fed from the top
• The combination of the plot layout and the elevation plan leads to lengths of the
piping runs
• Also numbers of elbows and fittings, etc
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Piping Systems
• After we have a layout and an idea about the piping paths we need to determine a few more things:
• Pipe diameter
• Pumping requirements
• These are very much connected to one another
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Piping Systems
• Pressure drop in pipelines
• Frictional Pressure Drop
∆𝑷𝒇 = 𝟖𝒇 ൗ𝑳 𝒅𝒊
𝝆𝒖𝟐
𝟐
• Where:
• ∆𝑷𝒇 is the frictional pressure drop, Τ𝑁 𝑚2 𝑁 is𝑘𝑔 𝑚
𝑠2
• 𝒇 is the friction factor, unitless
• 𝑳 is the pipe length, m
• 𝒅𝒊 is the pipe inside diameter, m
• 𝝆 is the fluid density, 𝑘𝑔
𝑚3
• 𝒖 is the fluid velocity, 𝑚
𝑠
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Piping Systems
• Friction Factor, 𝒇• The friction factor is a function of the Reynold’s number and the pipe roughness
𝑹𝒆 =𝒖𝒅𝝆
𝝁
• 𝝁 is the fluid viscosity
• Reynolds number is dimensionless
• Pipe roughness
• Absolute roughness is a characteristic of the given pipe category
• Relative roughness
𝑒 =𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑟𝑜𝑢𝑔ℎ𝑛𝑒𝑠𝑠
𝑝𝑖𝑝𝑒 𝑖𝑛𝑠𝑖𝑑𝑒 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟
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Piping Systems
• Pipe Fittings
• Obstructions in the line cause turbulence and generate additional pressure drop
• Velocity Heads, K Factors
• Add in the following pressure loss to the total from equation on Slide 7
𝒐𝒏𝒆 𝒗𝒆𝒍𝒐𝒄𝒊𝒕𝒚 𝒉𝒆𝒂𝒅 = ൗ𝒖𝟐𝟐𝒈
• The number of velocity heads per fitting can be found on next slide
• Total up the velocity heads for all of the fittings in the line
• The pressure drop is:
∆𝑷 = # 𝒉𝒆𝒂𝒅𝒔 ∗ 𝝆𝒈 ∗ ൗ𝒖𝟐𝟐𝒈 = #𝒉𝒆𝒂𝒅𝒔 ∗ ൗ𝝆𝒖𝟐
𝟐
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Piping Systems
• Pipe Fittings
• Equivalent Pipe Diameters
• Total up the number of equivalent pipe diameters for all of the fittings
• Multiply by the pipe diameter
• Increase the length of pipe by the calculated length
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Piping Systems
• Valves
• Gate Valve (slide valve)
• Used as a shut off valve
• Takes several turns of valve to position gate in place
• Better for infrequent use
• Low pressure drop
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Piping Systems
• Plug Valve
• Used as a shut off valve
• Quarter turn to open or close
• Opening in plug is either in-line with pipe or 90° from flow
Opening
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Piping Systems
• Ball Valve
• Used as a shut off valve
• Quarter turn to open or close
• Opening in plug is either in-line with pipe or 90° from flow
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Piping Systems
• Globe Valve
• Used for flow control
• Design is such that resisitance to flow changes smoothly
as the valve is opened or closed
• Somewhat linear when not at the extremes of being
opened or closed
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Piping Systems
Positive Displacement Pumps
• Tighter tolerances than centrifugal
pumps
• Generate a set volumetric flow
• Good for metering flows
• As opposed to just moving
material from one location to
another
• Can generate “as much pressure
as necessary”
• Safety concerns
• More expensive, more maintenance
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Piping Systems
Centrifugal Pumps
• Curved radial vanes cause the fluid to rotate
and add kinetic energy to the fluid
• The fluid decelerates as it discharges from the
tangential discharge tube
• The kinetic energy is converted to high
pressure
• The pressure at the discharge pushes the fluid
through the piping system
• The actual volumetric flow rate depends upon
the piping system as well as the pump
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Piping Systems
• Power Requirements for Pumping Liquids
• Mechanical energy balance accounts for:
• Pressure drop
• Flow through pipe
• Fittings in piping system
• Pressure drop through process equipment
• Changes in elevation
• Changes in pressure between the source and the destination
𝒈∆𝒛 + Τ∆𝑷𝝆− ൗ∆𝑷𝒇
𝝆−𝑾 = 𝟎
• 𝑾 is the work done by the fluid, ൗ𝑱 𝒌𝒈
• ∆𝑷 is the difference in pressures, 𝑃1 − 𝑃2, Τ𝑁 𝑚2
• ∆𝑷𝒇 is the pressure drop due to friction, etc
• 𝝆 is the density of the fluid, ൗ𝑘𝑔𝑚3
• 𝒈 is the acceleration due to gravity, Τ𝑚 𝑠2
• ∆𝒛 is the difference in elevations 𝑧1 − 𝑧2, 𝑚
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Piping Systems
Pump Efficiency
• Actual power required is:
𝑃𝑜𝑤𝑒𝑟 = 𝑊 ∗𝑚 ∗ ൗ100𝜂𝑝
• Where:
• 𝑚 is the mass flow rate, Τ𝑘𝑔𝑠
• 𝜂𝑝 is the pump efficiency, %
• Typical pump efficiency chart
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Piping Systems
Characteristic Curves for Centrifugal Pump
• The curves (a), (b), etc are for different impeller sizes
• Note that for a given impeller size the flow rate (x axis) increases as the head (differential pressure
between pump outlet and inlet) decreases
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Piping Systems
Pump Head
• Mechanical Energy balance
𝒈∆𝒛 + Τ∆𝑷𝝆− ൗ∆𝑷𝒇
𝝆−𝑾 = 𝟎
• 𝑾 is the work done BY the fluid• We are interested in 𝑾𝒔, the work done by the pump ON the fluid (per unit volume)• Lets rearrange the equation:
𝝆𝒈 𝒛𝟏 − 𝒛𝟐 + 𝑷𝟏 − 𝑷𝟐 − ∆𝑷𝒇 = 𝝆𝑾
• Shaft work is:
𝑾𝒔 = −𝝆𝑾
• Therefore,
𝑾𝒔 = 𝑷𝟐 − 𝑷𝟏 + 𝝆𝒈 𝒛𝟐 − 𝒛𝟏 + ∆𝑷𝒇
• Pump Head is:
𝑯𝒑𝒖𝒎𝒑 =𝑾𝒔
𝝆𝒈
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Piping Systems
Cavitation and Net Positive Suction Head (NPSH)
• The inlet to pump will generate a lower pressure
• If the pressure at the inlet drops below the bubble point of the liquid then bubbles will form
• This is called cavitation and will lead to erratic flow and damage to the pump
• As the bubbles form they create micro-explosions
• The net positive head required 𝑵𝑷𝑺𝑯𝒓𝒆𝒒𝒅 is a function of the pump design and will be specified by
the manufacturer
• Typically 3 m for pump capacities up to 100 ൗ𝑚3
ℎ and 6 m above that capacity
• You need to calculate the available NPSH, 𝑵𝑷𝑺𝑯𝒂𝒗𝒂𝒊𝒍 :
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Piping Systems
Available NPSH
• This is the more or less difference between the pressure at the pump inlet versus the vapor pressure
𝑵𝑷𝑺𝑯𝒂𝒗𝒂𝒊𝒍 =𝑷
𝝆𝒈+𝑯−
𝑷𝒇
𝝆𝒈−𝑷𝒗
𝝆𝒈• Where
• 𝑷 is the pressure above the liquid in the feed vessel, Τ𝑁 𝑚2
• 𝑯 is the height of the liquid level above the pump suction, m
• 𝑷𝒇 is the pressure loss in the suction piping, Τ𝑁 𝑚2
• 𝑷𝒗 is the vapor pressure of the liquid at the pump suction, Τ𝑁 𝑚2
• 𝝆 is the density of liquid at the pump pumps suction temperature, ൗ𝑘𝑔𝑚3
• 𝒈 is acceleration due to gravity
The suction piping must be designed such that the 𝑵𝑷𝑺𝑯𝒂𝒗𝒂𝒊𝒍 > 𝑵𝑷𝑺𝑯𝒓𝒆𝒒𝒅
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Piping Systems
System Curve
• A pump is working against two components to generate flow
• Static pressure from differences in height and pressure of source and target locations
• Independent of flow rate
• Dynamic pressure losses due to friction
• Function of flow rate
• A curve can be generated to show what the pressure head (static plus dynamic) will be as a
function of flow rate
• This curve (the System Curve) can be plotted on the same graph as the pump curve
• The location of their intersection will show you what flow rate you would obtain with this pump in
this piping system
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Piping Systems
• For this example
one would
obtain a flow
rate of 42 m3/h
• The head would
be around 17 m
• The pump would
be working at
about 79%
efficiency
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Piping Systems
Pipe Size Selection
• Economic Pipe Diameter
• Typical Pipe Velocities for for systems under positive pressure
Velocity Τ𝒎 𝒔 ∆𝑷 ൗ𝒌𝑷𝒂𝒎
Liquids, pumped (not viscous) 1-3 0.5
Liquids, gravity flow - 0.05
Gases and Vapors 15-30 0.02% of line pressure
High pressure steam, > 8 bar 30-60 -
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Piping Systems
Pipe Size Selection, Vacuum Systems
• For vacuum systems below 60 mm Hg allow max of 5% of absolute pressure
• You may see 50 m/s listed as a guideline
• You can start there but it is recommended to use pressure drop as your guiding principle
• Not that the low density of vapor under vacuum means that the mass flow rate is very low for a
similar velocity
• This means that vacuum lines are LARGE
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