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 DP = r 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 DP = r 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: