Mody and Marchildon: Chemical Engineering Process Design
Chapter 12 IN-PLANT TRANSFER OF SOLIDS AND LIQUIDS
P:/CEPDtxt/CEPDtxtCh12
12.1. Liquids:
When it comes to moving liquid around a plant the choice usually falls to the venerable
centrifugal pump and the pipe system. The centrifugal pump is so widely used that
complete descriptions of all the variations of this pump could fill volumes. In this course,
we’ll stick to a brief overview of the centrifugal pump, how to specify one, and when
alternatives should be considered.
All Centrifugal pumps share, as the name suggests, the fact that energy is imparted to the
fluid through the application of centrifugal force. The impeller in the pump spins and
increases the pressure of the fluid. The things that limit the use of a centrifugal pump are:
- the inefficiency with fluids of higher viscosities. Correction factors to
horsepower and differential head begin when viscosities are greater than 10 cP.
Capacity is affected when viscosities reach around 45 cP (which is about the
viscosity of typical motor oil at 100 °F )
- the limited range of differential pressure,
- the inability to deal with flashing fluids, and
- although there are many centrifugal pump designs that can handle solids, the
garden variety pump does not fare so well when solids are present.
So, if you have a fluid that is clean, not close to the boiling point, has a viscosity under 10
cP and your need is only for moderate pressure increases, the centrifugal pump is king.
Variations on the centrifugal pump have been developed to try to overcome the various
limitations (i.e. multistage pumps for high pressure), but it’s worth looking at other pump
types when you have a situation other than that above.
Higher Viscosity Fluids:
Centrifugal pump designs have not been able to overcome the inefficiencies that come
with handling high viscosity fluids. Other pump designs (generally positive
displacement) have inherent advantages in handling high viscosity fluids.
One such design is the rotary gear pump (typified by the products of such companies as
Liquiflo www.liquiflo.com, or Gorman Rupp www.grcanada.com or
www.gormanrupp.com ).
Gorman Rupp gear pump
Liquiflo – www.liquiflo.com
These pumps handle higher viscosity fluids with pressures in the same range as
centrifugal pumps (0 - 400 USGPM, 0 to 600 psig).
Rotary gear pumps generally lose some capacity on low viscosity fluids when the pump
differential pressure climbs as is typified by the graph below.
Rotary Gear pump handling low viscosity fluid (water)
courtesy of Liquiflo (www.liquiflo.com)
Molten polymers (or other very high viscosity fluids) that require pumping at high
pressures and temperatures have successfully used rotary gear pumps from companies
such as MAAG (www.maag.com) or Waukesha.
www.maag.com
www.gowcb.com (Waukesha Pumps)
When High Differential Pressures are Required:
This is one field that centrifugal pumps have in fact been successfully modified to
accommodate. The multistage centrifugal, which can be thought of as just being a bunch
of centrifugal pumps all in series, and mounted on the same shaft, can be made to provide
high pressures (> 8000 ft of head). Refer to ‘barrel’ pumps from companies such as
Sulzer (www.sulzerpumps.com).
Sulzer Barrel Pump
www.sulzerpumps.com
The alternative to the centrifugal pump for high pressure applications is to us a positive
displacement pump. If viscosities are low, then the diaphragm pumps provided by such
companies as Milton Roy or American Lewa are recommended.
American Lewa Diaphragm Pump
(www.americanlewa.com)
Milton Roy Diaphragm Metering Pump (www.miltonroy.com)
Liquids Containing Solids:
A centrifugal pump can be used in these applications up to a point. Trash pumps as
they’re sometimes called use open impellers and open internal designs to handle large
size solids (up to 3”, but pump is limited to low differential pressure applications of about
110 ft of head).
Trash Pump Impeller - Courtesy of Gormann Rupp
Perhaps more commonly used is the diaphragm pump as supplied by companies such as
Sandpiper and Gormann Rupp that can provide 150 ft of head and solids handling.
Gorman Rupp – Diaphragm Pump for Solids (1” to 3” solids depending on pumps size)
Sandpiper – Air Operated Diaphragm Pump (Solids handling to 3”)
12.2. Pumps that are Designed for Two Phase Flow
It’s important to distinguish between a fluid that has non-condensable gas and liquid
component (either as two phases or a liquid with dissolved gas) from a liquid that is near
its boiling point and will flash if the pressure is reduced. The latter case will quickly
damage or destroy a centrifugal pump, where the former (two phases) can be handled by
a centrifugal pump.
The standard centrifugal pump can handle gas up to the point where it ‘loses its prime’.
Capacity/head correction curves for the % vapour in liquid based are available from
vendors. Generally the pump does not perform as well as simply assuming a ‘mixed’
density.
The liquid ring vacuum pump (see www.sihi.com or www.nash-elmo.com ) is designed
to handle mostly vapour, but some liquid is allowed.
Nash Liquid Ring Vacuum Pump
There are also specific “multi phase” pumps that use either a screw or rotodynamic type
system.
www.bornemann.com
12.3. Specifying Centrifugal Pumps for Liquid Service:
Key things that need to be specified for every pump are:
1. The fluid properties (density, vapour pressure, temperature corrosiveness, size of
solids)
2. The required flow rate
3. The required differential pressure (total dynamic head) in units of ft of liquid.
4. The available net positive suction head
5. The materials of construction
6. The preferred sealing method.
The reader should refer to a typical pump specification sheet (see standard API 610) for
other items that may be relevant to particular applications.
Step 1. Fluid Properties
The fluid properties are obviously different for every application so we won’t dwell on
this.
Step 2. Flow
The required flow rate is usually known from the process engineering material balance or
some other means. Usually, design margins of 0 to 25% are added to the required flow
and if a minimum flow bypass is required a first pass is to assume 15% of the pump total
flow is for the minimum flow bypass. The Rated flow is thus:
Rated flow = Normal flow * 1.25 / (1-0.15)
Step 3. Differential Pressure
The differential pressure must be determined from the combination of piping frictional
losses, static head requirements, control valve losses, and process operating pressure
requirements.
Differential Pressure = Pump Discharge Pressure – Pump Suction Pressure
Pump Discharge Pressure = Final Destination Pressure + Piping Frictional Losses
+ Control Valve Losses + (or minus depending on the situation) Static Head +
Equipment losses as appropriate (i.e. heat exchangers, filters, etc.)
Pump Suction Pressure = Pressure in the Source Tank – Suction Piping Frictional
Losses + (or minus depending on the situation) Static Head
The piping frictional losses can be calculated using standard fluid flow equations
knowing such things as flow rate, pipe size, number of elbows, valves etc. However,
often times a preliminary pump size is required before detailed piping drawings
(isometrics) are available. The process engineer is required to ballpark these values early
on Depending on the amount of information you have, here’s how to do it.
If there are no equipment layouts available:
For pump discharge piping, assume either 15 psi differential pressure between pieces of
equipment, or determine the pressure drop per 100 ft of pipe and assume there’s an
equivalent (includes manual valves, Tee’s, elbows etc.) of about 400 ft of piping.
If you’re doing any amount of piping pressure drop calculations,
treat yourself to a paper version of Crane Technical Paper No. 410
(www.craneco.com/flow_fluids.cfm)
If there are equipment layouts available, but no detailed piping isometrics:
Determine the line size by assuming a liquid velocity between 5 to 7 ft/sec. Use the
layouts to get a rough length of pipe from the pump discharge to the destination, multiply
the length by 4 to account for manual valves, elbows, other fittings, etc. and calculate the
pressure drop using a formula for fluid flow pressure drop (see course notes or refer to
the Crane Tech Paper 410).
If there are detailed piping isometrics available:
Determine a suitable line size by assuming a liquid velocity between 5 to 7 ft/sec.
Determine the pressure drop using the detailed drawings. If the pressure drop is in excess
of 4 psi per hundred ft, increase the line size.
Control Valve and Flow Element Losses:
For normal low pressure (less than 250 psig) the pump can be sized assuming 15 psi
across the control valve. Ref 2 and 3 refer to pressure drops being calculated as a % of
frictional losses. Once a pump is selected, the actual operating curve can be used to
determine the range in head the pump generates as compared to the frictional losses of
the piping system. The control valve can be checked for adequate sizing at that time.
Flow elements are typically sized to have 100 Inches Water (3.6 psi) of pressure
differential across the transmitter. This DP is slightly more than the recoverable DP and
thus using this value is conservatively high.
Static Head:
The fluid pressure exerted by the fluid due to static head should be added (in the case of
pushing a fluid uphill) or subtracted (in the situation where the destination is below the
pump centerline).
The conversion from pressure differential in ft of liquid to psi is P =  g h, in US units
Diff Press (psi) = 0.4452 * ft of fluid * Specific Gravity of the fluid
Equipment Losses:
If at the time of doing the pump sizing the equipment losses are not know, assume the
heat exchangers have 10 psi of pressure differential, the filters have 10 psi, and vessels
that the material is flowing through have none. Once the design of that equipment is
complete, go back and check your assumptions to see if the pump design is impacted.
When Sizing Suction Piping:
If the fluid is close to its boiling point, frictional losses must be kept to as small a value as
possible to maximize NPSH available. It’s common to determine the piping diameter
based on a 2 to 3 ft/sec velocity to meet this requirement.
4. Calculate the Net Positive Suction Head
Net Positive Suction head is a unit of differential pressure. It is the differential pressure
between the actual fluid pressure and the pressure it would boil at. The units of this
differential pressure unit are similar to mm Hg or inches H2O; they are in ft of fluid (the
fluid you’re pumping).
The “NPSH required” by the pump is the pressure above the boiling point that the pump
requires to not have a small drop in static head due to cavitation. Since cavitation will
permanently damage a pump, it is to be avoided at all costs, and the system designers
should ensure that they have “NPSH Available” which is 2 to 3 ft in excess of what the
pump requires.
NPSH available = Absolute Pressure at the pump inlet (minimum expected) – Vapour
Pressure Fluid (at maximum expected operating temperature)
Convert the value to ft (or m) of liquid using the P =  g h equation from above.
5. Decide on the Materials Of Construction
The material choice depends on the fluid being handled. Material selection is beyond the
scope of this course but you can refer to:
- API 610
- the chemical resistance chart provided by Warren Rupp at
(http://www.warrenrupp.com/pdf/CHEMCHART-WR%20Color-Rev.105.pdf).
- Ulrich, G. D., A Guide to Chemical Engineering Process Design and Economics,
Wiley, New York 1984
6. Determine the Sealing Method
Refer to Perry’s on methods of sealing.
References:
1. Crane Technical Paper No. 410
2. F. C. Yu; Easy Way to Estimate Realistic Control Valve Pressure Drops;
Hydrocarbon Processing Aug 2000 pp 45-48.
3. Connel, J. R. “Realistic Control Valve Pressure Drops” Chemical Engineering,
Spet 28; 1987 p. 123.
4. A. G. Godse; All you need to know about centrifugal pumps, Part 1; Hydrocarbon
Processing; Aug 2001 pp 69-84.
5. A. G. Godse; All you need to know about centrifugal pumps, Part 2; Hydrocarbon
Processing; Oct 2001 pp 39-44.
6. Fernandez, Pyzdrowski, Schiller, and Smith; “Understanding the Basics of
Centrifugal Pump Operation” ; Chem Eng Progress; May 2002 pp 52-56.
7. A. Mose and M. Stevens; “Getting Gear Pumps Up to Speed”; Chemical
Engineering; Sept 2001 pp 101-105.
8. M. Zaher; “Avoid Cavitation in Centrifugal Pumps”; Chemical Engineering June
2003 pp 50-54.
12.4. In Plant Transport - Solids:
Nearly every chemical plant must handle a solid at some point in the process, whether as
a raw material, additive or finished product. Often the handling equipment for solids falls
into the domain of the mechanical engineering department, but it’s not uncommon for
chemical engineers to be involved also. Preferably gravity (via chutes usually) should
be used since it’s the most reliable. Where Gravity can’t be used, two classes of solids
conveying systems are used: pneumatic conveying, and mechanical conveyors.
In first determining the means by which a solid should be transported, the physical
properties of the material should be understood. Properties such as:
- is the material friable (breaks apart easily)?
- is the material sticky, hydroscopic ?
- what is the particle size and is it dusty?
A mechanical conveyor system is suitable when:
- materials require gentle handling
- a high capacity (material flow rate) is required.
A pneumatic conveying system is suitable when:
- flexible routing is required
- sealing the system (i.e. conveying under nitrogen, dry air, or when high levels of
dusts might be harmful)
- there are multiple pickups or discharges.
The basic types of mechanical conveyors are:
-
-
-
-
Belt
o High capacity
o Can handle large particles
o Does not break up the material (low attrition)
Screw (Rigid or flexible helix)
o Screw conveyors can convey under dry air or nitrogen
o Heating or Cooling can be done
o Inclined and vertical orientation are possible
o Can damage the material, but usually not
very much
Vibratory
o Good for short distances
o Slight inclines are allowed. (For large
inclines see spiral vibratory .)
conveyors – www.carrier.com
o Can be sealed at both ends providing dust
tight systems
o Not good for sticky, damp powders
En Masse
o Drag Disk and Aeromechanical
o Drag Chain and Redler
o Bucket Elevator
12.5. Pneumatic Conveying:
Pneumatic Conveying utilizes the principal that energy in the form of air pressure and
velocity will move solids down a pipe. The principal is commonly encountered in your
household vacuum cleaner. Two things need to be present to make solids flow down a
pipe using air. They are:
1. The velocity of the air must be greater than the pick-up velocity of the solids.
That is to say the material must become fluidized before it will move. The pickup velocity can be determined from a theoretical point of view using Drag
Coefficient calculations, or you can measure it using a real system.
2. As the air and solids move down the line, particles impact the pipe, slow down,
and then are accelerated again by the air. This absorbs energy and reduces the
pressure of the air. Thus, there must be enough pressure in the air to offset the
energy losses or the line will stop moving and become plugged.
There are two basic types of conveying systems, vacuum and pressure. Pressure systems
are then subdivided into dilute (high velocity) and dense phase (low velocity) systems.
Within the dense phase category, there are several different variations on the theme
provided from a variety of different vendors.
Courtesy of Buhler Canada (www. Buhlercan.com)
Dense Phase conveying allows you to use lower velocities, which normally means the
material has less attrition by the time it reaches the end of the conveying system. Dense
phase conveying systems are usually more expensive to purchase (partially due the need
for pressure piping, flanges, and screw compressors or use of higher pressure air) so the
advantages offered by reduced velocities need to be justified. The sizing of dense phase
conveying systems is normally left to the vendor and usually vendors that can provide
both dilute and dense phase conveying should be consulted so as to seek out the most
economical system that meets the needs of the process. It’s not uncommon for the
pneumatic conveying vendor to request a sample of the material for testing so that they
can properly design the system and provide performance guarantees.
Dilute Phase conveying can be provided in positive pressure and negative pressure
systems. Negative pressure systems are well suited to situations where there are multiple
pick-ups (i.e. a central vacuum system in your house), and positive pressure systems are
well suited where there are multiple destinations. In both systems, the standard
equipment used is either a positive displacement rotary lobe type blower or in small
systems, sometimes a regenerative type blower.
Feed
Hopper
To Dust Collection
As Required
Rotary Airlock
Air Pipe
Conveying Pipe
Discharge Cyclone
Pick-up
Box
Blower
Destination
Hopper
Typical Dilute Phase, Positive Pressure System
To Dust Collection
As Required
Feed
Hopper
Air Pipe
Dust Collector
&
Blower Protection
Filters
Conveying Pipe
Rotary Airlock
Pick-up
Box
Discharge Cyclone
Rotary Airlock
Destination
Hopper
Typical Dilute Phase, Vacuum System
Vacuum
Blower
Dilute Phase systems can be sized with reasonable accuracy using straightforward
calculations that are based on an energy balance (ref 1). There is one empirical factor,
the material sliding friction coefficient, which depends on material properties and should
be based on actual experience with the material being conveyed. The equations are:
Material Losses = Ek + Ep + horizontal losses + elbow losses
elbow loss = centrifugal force * arc length * sliding friction coef
or, elbow loss = #elbows * (m/60* (v/60)2 /gravity/radius_1) *
(2**radius_1/4)*fric_coef
horiz_loss = (m) * distance * sliding friction coef
Ep = m * h
Ek = 1/2 mv^2 acceleration of product
Where
Material losses = ft-lb/min (length-mass/time)
m = mass flow (lb/hr)
v = average conveying velocity in line (ft/min) – typically 5000 ft/min is used
radius_1 = radius of conveying elbow (ft) - typically 4 ft
fric_coef = sliding friction coefficient for material (hard plastic pellets ~ 0.7)
Note that the radius of the elbow divides out in the above equations.
Convert the material losses in ft-lb/min to differential pressure by:
Material P (Inches H2O)= Material Losses (ft-lb/min) / 5.187 / Gas Volumetric Flow
(Cu Ft/Min)
1
ft
3
min
 1 in_H2O  5.187 ft
lbf
min
The gas flow can be estimated early on from the phase density for a dilute phase
conveying system which is approximately 20 lb of material per lb of air for pressure
systems and 10 lb of material per lb of air for vacuum systems. The pickup velocity
should be checked once the calculations are complete, see below.
The material losses are not the only the pressure drop in a system. If the chosen line size
is too small, the air losses through the system can be excessive and other items such as
pickup boxes, cyclones, filters, diverter valves should be added. The air losses should be
calculated using compressible flow calculations for the entire length of pipe (air only pipe
+ material conveying pipe).
Total system pressure drop = Material P + Air P + Other Losses
Other Typical Losses:
Item
Pick-up boxes / Entry Losses
Cyclones
Y-diverter Valves
Filters
Differential Pressure (Inches H2O)
1.55
3
0.8 “@ 5000 ft/min (scale for other vel)
24
When the differential pressure has been calculated, the actual pick-up air velocity should
be calculated and compared to that which is required (for polymer pellets the pickup
velocity should be greater than 4200 ft/min).
The smallest line diameter should be selected that:
1) ensures the pickup velocity for the material is met, and
2) the differential pressure of the blower is within practical values (for a rotary lobe
blower the max discharge pressure is usually about 14 psig and minimum vacuum
pressure is –7 psig)
3) Other considerations such as future expansion should be considered.
References:
1. Gerchow, F.; Pneumatic-Conveying System; Chemical Engineering Feb 17 1975
2. Rentz J. , Churchman C.; Streamline Predictions for Pneumatic Conveyors; Chem Eng.
Progress; May 1998 pp 47-54
3. Crouch C.; Conveying: How Dilute Phase Stacks Up Against Dense Phase; Chem Eng
Progress; Aug 1998
4. Mills D; Using rubber hose to enhance your pneumatic conveying process; Powder and
Bulk Engineering; March 2000 pp 79-87
5. Crouch C; Safely handling your powder with a closed-loop pneumatic conveying
system; Powder and Bulk Engineering; March 2000 pp 43-56
12.6. Slurries:
When solids and liquids are flowing together, the fluid is called a slurry. The nature of
the slurry depends on whether the solids have a tendency to drop out of suspension or
not. Slurries where the solids easily stay in suspension (i.e. clay/water or other solids
where the particle size is less than 50 microns), the fluid will flow as a non-Newtonian
fluid. Where the solids are larger and tendency is for them to fairly rapidly settle out
(particle sizes greater than 0.25 mm), the following equations may be applied.
In horizontal pipes, the turbulence of the liquid flow can maintain the solids in suspended
form. The pressure gradient in a pipe carrying a slurry is usually greater than that for a
liquid alone, except in the situation of very high velocities and small particles where the
solids interact with the turbulence. As the velocity in the pipe line is reduced, the
pressure gradient goes through a minimum. Velocities less than the minimum allow the
solids to settle out.
Liquid
Alone
Pressure Drop / Length
Slurry
solid interaction
with turbulence
(small solids only)
Velocity of Slurry in Pipe
The minimum velocity to keep particles that are less than 1 mm in size in suspension is:
V12
g DS
L
S  L
V D  
 0.0251  1 t m 
 L 
0.775
V1 = velocity to keep < 1mm particles in suspension, ft/sec
g = acceleration due to gravity = 32.3 ft/sec
Ds = particle diameter, ft, such that 85% by weight of the particles are smaller than Ds
Dt = Pipe Diameter, ft
s = particle density, lb/ft3
L = liquid density, lb/ft3
Dt = pipe diameter, ft
m = slurry density, lb/ft3
L = liquid viscosity lb / ft sec
The minimum velocity to keep particles that are greater than 2 mm in size in suspension
is:
 2 gDt   S   L  

VC  1.35 

L


0.5
VC = velocity to keep > 2 mm particles in suspension, ft/sec
g = acceleration due to gravity = 32.3 ft/sec
Dt = Pipe Diameter, ft
s = particle density, lb/ft3
L = slurry density, lb/ft3
Typical Velocities for Minimum Pressure Gradient (Source Perry’s)
Pipe Dia, in
Velocity, ft/sec, in 25% by vol water
Coal (spec grav = 1.4)
Gravel (spec grav = 2.6)
1
1.5
3
3
3.5
7
6
5
10
9
6.3
13
12
7.3
15
18
8.8
17.5
Perry’s reports that there is no single correlation that calculates the pressure drop of
slurries in horizontal piping systems. The one recommended for particle diameters from
0.01 to 4 in with concentrations up to about 30 % vol and pipe diameters from 1.5 to 28
in is:
im  iL
iL
 D g    L  

 150 c  t 2 m
 V  C 
L
D 

3/ 2
Where:
im = pressure gradient for mixture = (hm/L)(m/L), ft of liq per ft of pipe
iL = pressure gradient of liquid only, at velocity V, ft of liq per ft of pipe
hm = head loss for mixture, ft of mixture
L = length of pipe, ft
m = density of mixture, lb/ft3
L = density of liquid, lb/ft3
Dt = Pipe Diameter, ft
c = Concentration as volume fraction of solids
g = acceleration due to gravity, 32 ft/sec2
V = mixture velocity, ft/sec
CD = Drag Coefficient, see Perry’s
The equation is for closely sized particles and for liquid velocities greater than the onset
of deposition (i.e. over about 3 ft/sec).
For the situation where there is a wide particle distribution and where finer particles
remain in suspension forming a somewhat homogenous medium that transports large
particles that have a tendency to settle, the reader is referred to Ref 2.
For all slurries, it’s suggested that for critical systems laboratory measurements be taken
with the actual slurry and the system be scaled up to the required size.
Pumps that can move slurries are generally either a type of screw conveyor or of the type
documented in the liquid transfer above.
References:
1. Perry’s Chemical Engineers Handbook, Fifth Edition
2. Darby, Ron; “Pressure Drop for non-Newtonian Slurries: A Wider Path”;
Chemical Engineering May 2000 pp 64-67.