Department Editor: Rebekkah Marshall
general considerations [ 1 ]
data collection
Preventing Runaway Reactions thermal stability criteria [1 , 4 ]
A process is considered to be thermally The following data are especially relsafe only if the reactions can easily be evant in avoiding runaway reactions: As a guideline, three levels are sufficient controlled, and if the raw material, the • Physical and chemical properties, ig- to characterize the severity and probproducts, the intermediates and the renition and burning behavior, electro- ability of a runaway reaction, as shown action masses are thermally stable under static properties, explosion behavior in the Table. the considered process conditions. Check and properties, and drying, milling, Defining high, hi gh, meDium into the process equipment, its design, its and toxicological properties anD low risk [1] sequence of operation and the control • Interactions among the chemicals Severity Probability strategies. In addition to the engineering aspects, get detailed information on • Interactions between the chemicals High ΔT TMR ad ad > 200K ad < 8 h and the materials of construction thermodynamic and kinetic properties Medium 50K < ΔT ad < 200K 8 h < TMR ad ad < of the substances involved, such as the • Thermal data for reactions and de24 h reaction rates or heat-release rates as Low ΔT TMR ad composition reactions ad < 50K and ad > 24 h the boiling point a function of process conditions. Deter- • Cooling-failure scenarios cannot be surmine the physical and chemical properpassed ties, as well. Understanding of thermal-hazard po- design options [2 ] a u tential requires knowledge of various If a reaction is has the potential for The adiabatic temperature rise is calculated skills and disciplines [3]. These include: by dividing the energy of reaction by Operating mode: The mode of opera- runaway, the following design changes the specific heat capacity as shown in should be considered: tion is an important factor. For instance, Equation (1). a batch reaction, where all the reactants • Batch to continuous. Batch reactors (1) ad = 1,000Q r r/ C p require a larger inventory of reac- ΔT ad are charged initially, is more difficult to tants than continuous reactors do, so where: control than a semi-batch operation in the potential for runaway in continu- ΔT = adiabatic temperature rise, K which one of the reactants is charged ad ad ous systems is less by comparison progressively as the reaction proceeds Q r r = energy of reaction, kJ/kg (for more, see Design Options). • Batch to semi-batch. In a semi-batch Engineering: Design and layout of the reaction, one or more of the reactants C p = heat capacity, J/(kg)(K) plant and equipment and its built-in conis added over a period of time. Theretrols impact the entire process. The cafore, in the event of a temperature or t xu (tmr) TMR ad pacity of the heating or cooling system pressure excursion, the feed can be TMR ad (the time to maximum rate, adiabatic) is is important in this context. Process enswitched off, thereby minimizing the a semiquantitative indicator of the probability gineering is used to understand the conchemical energy stored up for a sub- of a runaway reaction. Equation (2), defining TMR ad trol of the chemical processes on a plant sequent exothermic release ad in hours, is derived for zero-order reaction kinetics: scale. It determines which equipment • Continuous, well-mixed reactors to should be used and how the chemical (2) ad = C pRT o o2 /3,600qo E a plug flow designs. Plug-flow reactors TMR ad processes should be performed. In adrequire comparatively smaller volumes where: dition, take into account technical failure and therefore smaller (less dangerous) R = gas constant, 8.314 8.314 J/molK of equipment, human errors (deviations inventories for the same conversion from operating instructions), unclear T o o = absolute initial temperature, K operating instructions, interruption of • Reduction of reaction inventory via q = specific heat output at To, W/kg o increased temperature or pressure, energy supply, and external influences, changing catalyst or better mix- E a = activation energy, J/mol such as frost or rain (for more, see Deing. A very small reactor operating The sign Options). TMR value provides operating personnel The TMR at a high temperature and pressure with a measure of response time. Knowledge Chemistry: The nature of the process and may be inherently safer than one of the TMR allows allows decisions to be based on the behavior of products must be known, operating as less extreme conditions an understanding of the time-frame available not only under reaction conditions, but because it contains a much lower infor corrective measures in case heat transfer also in case of unexpected deviations ventory [3 ]. ]. Note that while extreme is lost during processing. (for example, side reactions, instability conditions often result in improved of intermediates). Chemistry is used to reaction rates, they also present their References gain information regarding the reaction Venugopal, Bob, Avoiding Runaway Reacown safety challenges. Meanwhile, a 1. Venugopal, pathways that the materials in question Chem. Eng., June 2002, pp. 54–58. tions, compromise solution employing modfollow. 2. Smith, Robin, ”Chemical Process Design,” erate pressure and temperature and McGraw-Hill, New York, 1995. Physical chemistry and reaction kinetics: medium inventory may combine the 3. Kletz, T. A., “Cheaper, Safer Plants,” The thermophysical properties propertie s of the reac worst features of the extremes [ 3 ]. ]. IChemE Hazard Workshop, 2d., IChemE, tion masses and the kinetics of the chemiRugby, U.K., 1984. cal reaction are of primary importance. • Less-hazardous solvent 4. Gygax, R., Reaction Engineering Safety, Physical chemistry is used to describe the • Externally heated or cooled to inter- Chem. Eng. Sci., 43, 8, pp. 1759–71, Aureaction pathways quantitatively. gust 1998. nally heated or cooled
Hazardous Area Classifcation
Department Editor: Rebekkah Marshall Guidelines by location Over the years, hazardous area classification requirements for the U.S. have evolved around a single area-classification system known as the Class/ Division system. Today, the system addresses establishment of boundaries of hazardous areas and the equipment and wiring used in them. Meanwhile, European countries, as well as some other countries around the world, have developed their own area classification systems to address hazardous locations safety issues. This independent development has resulted in systems for these countries or groups of countries based on the International Electrotechnical Commission (IEC) Zone system, with deviations to meet each country’s national codes. While other countries do accept and use the Division system (most notably Canada and Mexico), the majority of the world’s hazardous locations are classified using the concepts of the IEC Zone system. The U.S. National Electrical Code (NEC; NFPA 70) also recognizes the Zone system and allows its use in the U.S. under article 505 of the NEC. ATEX requires the use of I EC-type hazardous area classifications.
defininG hazardous areas
Table 1. Hazardous Areas* North America Class — Division
Class I — Gas or vapor
IEC (Europe) Zones
Division 1: Present or likely to be present in normal operation
An area in which an explosive atmosphere is Gas/Vapor Zone 1 (Gas) / Zone 21 (Dust) likely to occur in normal operation Division 2: Not or Dust Class III — Fiber present in normal Zone 2 (Gas) / An area in which an explosive atmosphere is or flying (no group operation, could Zone 22 (Dust) not likely to occur in normal operations and, designation) be present in abif it does occur, will exist for only a short time normal operation * This table represents a corrected version from that in the original printing Class II — Dust
Table 2. Relationship Between Divisions a nd Zones North America Europe Division Zone method method IEC standard Ignitable mixture present Zone Zone 0 continuously (long periods) Division 0 (Zone 20-Dust) 1 Ignitable mixture present Zone Zone 1 intermittently 1 (Zone 21-Dust) Zone 2 Ignitable mixture is not Division Zone (Zone 22-Dust) normally present 2 2
Table 4. Gas and Dust Groups Hazardous locations are grouped according to their ignition properties Typical gas Acetylene Hydrogen
IEC gas North Amerigroup can group IIC A IIC + H2 B
Minimum ignition energy 20µJ 20µJ
Ethylene
IIB
C
60µJ
Propane
IIA
D
100µJ
A hazardous area is designated as any I — location in which a combustible material *Methane Metal dust — E is or may be present in the atmosphere in sufficient concentration to produce an Coal dust — F ignitable mixture. The North American Grain dust — G method identifies these areas by Class, Fibers — — Division and Group or optionally by *Mining application under jurisdiction of U.S. Mine Safety and Class, Zone and Group, while the IEC and CENELEC designate these areas by Health Administration (MSHA) Gas/Dust, Zone and Group. The likeliTable 5. Information Required For Establishing hood that the explosive atmospheres are Extent of Hazardous Area present when the equipment is operating Gas/Vapors Dust are designated in Tables 1, 2 and 5. • Flash point • Flammability limits • Auto-ignition temperature equipment selection • Minimum ignition energy, MIC or MESG – for equipment For equipment selection purposes, hazardous area classifications also consider: selection purposes • Gas/Vapor group • The maximum surface temperature of • Vapor/Gas density the equipment under normal operat- • Area ventilation conditions ing conditions (see the Temperature • Location of gas/vapor release points. Frequency and rate of Code designations in Table 3) release
• The ignition-related properties of the explosive atmosphere (see the Group designations in Table 4) • The protection method(s) used by the equipment to prevent ignition of the surrounding atmosphere (see the Protection Method designations in Table 6)
An area in which an explosive atmosphere is continually present or present for long periods or frequently
Zone 0 (Gas) / Zone 20 (Dust)
• A/B classification • Minimum explosible dust concentration • Minimum ignition energy • Minimum ignition temperature (cloud/layer) • Electrical resistivity • Dust group • Area ventilation conditions • Location of dust release points. Frequency and rate of release
Table 3. Temperature Codes The Temperature class defines the maximum surface temperature of the device. Ratings are given with reference to 40°C ambient T1 450°C T3A 180°C T2 T2A T2B
300°C 280°C 260°C
T3B T3C T4
165°C 160°C 135°C
T2C
230°C
T4A
120°C
T2D
215°C
T5
100°C
T3
200°C
T6
85°C
The additional temperature classifications highlighted above are for USA and Canada only
Table 6. Types of Protection for Electrical Equipment (IEC/ATEX and NEC) Technique
IEC PermitPermitted Dested DiZone cription vision
Ex o Ex p Ex q
1&2 1&2 1&2
— 1&2 —
Explosion Proof
Ex d —
1&2 —
— 1&2
Increased safety
—
—
—
Ex ia
0,1 & 2
1&2
Oil immersion Pressurization Powder filling Flameproof
Intrinsic safety Intrinsic safety
Ex ib
1&2
—
Encapsulation
Ex m
1&2
—
Special protection
Ex s
0,1 & 2
—
—
—
2
Nonsparking
Ex nA
2
—
Enclosed break
Ex nC
2
—
Energy limited Simplified pressurization
Ex nL
2
—
Ex nP
2
—
Ex nR
2
—
Nonincendive
Restricted breathing
Table 7. Types of Ignition Protection for Mechanical Equipment (ATEX) Method To ensure that ignition sources cannot arise To ensure that ignition sources cannot become active To prevent the explosive atmosphere from reaching the ignition source
Description Construction safety “c”, Inherent safety “g”, Control of ignition sources “b” Inert liquid immersion “k”, Inert gas pressurization “p”, Flow restricting enclosure “fr”
To contain the explosion and prevent flame propagation
Flame proof enclosures “d”, Flame arresters
Acknowledgement and references We would like to thank Vladimir Stetsovsky of Chilworth Technology, Inc. for his contributions to this page 1. National Electrical Code-2005-NFPA 70, National Fire Protection Association. 2. NFPA 497-2004, Recommended Practice for the
Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas. 3. NFPA 499-2004, Recommended Practice for the Classification of Combustible Dusts and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas.
4. IEC 60079-10-2002 Electrical apparatus for explosive gas atmospheres — Part 10: Classification of hazardous areas. 5. IEC 61241-3-2005 Electrical apparatus for use in the presence of combustible dust — Part 3: C lassification of areas where combustible dusts are or may be present.
Sovnt Sction Mthodoogy Department Editor: Rita L. D'Aquino A STEPWISE ProcEdurE
Table 1. Some well-known databases and solvent selection tools
Organic solvents have been used in many industries or centuries, but the methods and tools to select optimal solvents while minimizing their adverse environmental, health, saety and operational concerns are still evolving. The appropriate selection o solvents depends to a large extent on the application — more specifcally on what needs to be dissolved, and under what conditions. This article presents a our-step approach to solvent selection based upon Re. 1*, where the reader will fnd a list o additional resources on this topic. Identify the challenge and solvent characteristics.
Databases ChemFinder Solvents Databases
NIST Webbook DIPPR and TAPP CAPEC Database Selection Tools
The frst two steps are: 1) identiying the actual problem and technology or unit operation required to solve it; and 2) defning the requirements that must be met by the solvent, using criteria related to its physical and chemical properties ( e.g., pure-solvent properties, such as normal boiling point, the Hildebrand solubility parameter at 300 K, the Hansen solubility parameters; solventsolute properties, such as the solubility o the solute as a unction o the composition o the mixture; and unctional constraints, such as solute loss in solute). Obtain reliable values of solvent properties and narrow down selection. There are several alternatives or this
third step. For example, one can measure the required properties, use a database o properties o chemicals (or solvents), or, use property models to estimate them. For solvent-selection problems not involving chemical reactions, the pattern o the desired solvent is established through analysis o the solute, application type, and other constraints. Once this is established, a database o known solvents can be used to identiy the solvents that match the necessary pattern (Table 1). On the other hand, when chemical reactions are involved, the approach is based on transition-state theory and requires consideration o the solvation energies o the reactants, products and transition states, and thus, knowledge o the reaction mechanism. When the crucial values have been ound, the solvent search could be such that frst, solvent-pure properties are used, ollowed by solvent-EHS, then solvent-solute, and fnally solvent-unction. Narrow down the list by removing the compounds that do not match desired properties. A protocol derived by Britest Ltd. (www.britest.co.uk) seeks to use mechanistic principles to guide solvent selection (Figure). The objective is to ollow the arrows according to the problem defnition and a search criterion until an end-point is reached, thereby obtaining the characteristics o the candidate solvents. These characteristics are used to identiy the group to which the solvents belong using solvents database (see Table 2). The corresponding group-types are evaluated and a fnal selection is made.
SMSwin
Address and comments Searchable data and hyperlink index: http://chemfnder.cambridgesot.com Solvent substitution data systems at http://es.epa.gov/ssds/ssds.html; “Handbook o Solvents” rom www.chemtec.org/cd/ct_23.html; and SOLVDB at http://solvdb.ncms.org/index.html Source o physical and chemical data at http://webbook.nist.gov www.aiche.org/TechnicalSocieties/DIPPR/About/Mission.aspx; and www.chempute.com/tapp.htm Pure as well as mixture properties data, including solvent-solute database: www.capec.kt.dtu.dk/Sotware/ICAS-and-its-Tools Address and comments
A specialized sotware or property estimation and solvent classifcation: www.capec.kt.dtu.dk/documents/sotware/SMSWIN.htm Activity coefcient method based on segment contributions. Predictive based on a small set o solubility data. Useul or crystallization solvent selection and extends to LLE and VLE: www.aspentech.com
NRTL-SAC and eNRTL-SAC
Table 2. Well-known solvents together with their related properties
Solvent Name
Molecule type
Group type Charge
1-Methyl-2-pyrrolidinone Acetonitrile Dimethyl sulphoxide Dimethyl ormamide Dimethylacetamide Diisopropyl ether Dimethyl ether Methyl tertbutyl ether Tetrahydrouran Chlorobenzene m-xylene (also o -; p -) Toluene Acetic acid Propionic acid Suluric acid
Amide Nitrile S-oxide Amide Amide Ether Ether Ether Ether Chloride Aromatic HC Aromatic HC Acid Acid Acid
1 1 1 1 1 2 2 2 2 3 3 3 4 4 4
Propanol Ethanol Butanol Ethylene glycol Dichloromethane Heptane Hexane Pentane Methanol Water
Alcohol Alcohol Alcohol Alcohol Chloride Alkane Alkane Alkane Alcohol Aqueous
5 5 5 5 6 7 7 7 4, 5 4, 5
Verify selection. The ourth step is to veriy that the solvent
works as expected by perorming a computational validation by simulation. Experimental validation o a solvent candidate is required at all stages o process development.
NE/EPD E/NPG E/NPG NE/NPG NE/NPG NE/EPD NE/EPD NE/EPD NE/EPD NE/P NE/P NE/P PG E/PG E/PG
NBP (K) 475.15 354.75 462.15 426.15 438.15 341.65 248.35 328.35 338.15 632.35 412.27 383.95 391.05 414.25 610
NMP (K) 249.15 229.35 291.65 212.75 253.15 181.35 131.65 164.55 164.85 404.9 225.3 178.25 289.81 252.45 283.46
23.16 24.05 26.75 23.95 22.35 14.45 15.12 15.07 18.97 19.35 18.05 18.32 19.01 19.41 28.41
E/N E/N E/N E/N NE/EPD NE/I NE/I NE/I E/N E/N
370.35 351.35 390.81 470.45 313.15 371.65 341.85 309.22 337.85 373.15
147.05 159.05 183.85 260.15 178.05 182.55 177.85 143.42 175.47 273.15
24.45 26.13 23.35 33.7 20.37 15.2 14.9 14.4 29.59 47.81
ordered
Sol. Par.
NE = non-electrolytic solvent; E = electrolytic solvent; P = polarizable; EPD = electron-pair donor; I = inert; PG = protogenic (proton donor); N= neutral (donor & acceptor); NPG = non-protogenic (proton acceptor); NBP = normal boiling point; NMP = normal melting point; Sol. Par. = Hildebrand solubility parameter at 300 K (MPa 1/2)
Stability, solubility of reactants, products Two-phase or liquid-liquid (polar phase is water)
Single phase or solid-liquid Homogeous catalysis by Pt group complexes Moderate polarity
SN1/E1
Condensation
Aromatic hydrocarbon (xylene)
Group 2
Fast, low temp, but recovery difficult
Slow, hightemperature, easy recovery
Group 3
Group 1
Group 3
DPA ethers, aromatics Group 1
SN2/E2
High polarity Dipolar aprotic
Water, immiscible solvent
Substrate/product hydroxyl sensitive Yes
Water, carboxylic acids, inorganic acids, lower alcohols Group 4
Consider solvation
Dipolar aprotic ethers Group 1 Group 2
No
Choose ‘polarity’ based on substrate and reagent solubility. May need phase-transfer catalyst Group 3
Water, alcohols Group 5
Group 6 Group 7
Contollng Cystal Gowth Department Editor: Rita L. D'Aquino he ormation o crystals requires the birth o new particles, also called nucleation, and the growth o these particles to the nal product size. The driving orce or both rates is the degree o supersaturation, or the numerical dierence between the concentration o solute in the supersaturated solution in which nucleation and growth occurs vs. concentration o solute in a solution that is theoretically in equilibrium with the crystals. In a batch crystallizer, the crystal size distribution (CSD) is controlled by rst seeding the initially supersaturated batch with a known number and size distribution o crystals, and then controlling the rate o evaporation or cooling ( i.e., rate o energy transer) so as to achieve a level o supersaturation that supports adequate crystal growth and an acceptable rate o nucleation. The relationship between supersaturation and growth is linear, but that between nucleation and growth is raised to a power that is usually greater than one, making it dicult to grow large crystals when nucleation is occurring. The ollowing procedure describes how to achieve the optimal growth rate: 1. Screen the seeds at the beginning o the experiment to determine the cumulative number o crystals that are greater than a given size N’ . Estimate N Li , the number o crystals o a given size (L av ) obtained rom the screening:
T
Li
=
∆W i
(1)
3
Lav k v ρ c The parameters are dened in the table o nomenclature. To convert rom µm to t, multiply by 3.28 x 10 –6. 2. Continue to measure the number and size o crystals as the cooling or evaporation program is in progress. Prepare an inverse cumulative plot o the number o cr ystals greater than a given size vs. size o the crystal (Figure 1). The crystal growth rate depends on the energy transer rate, so modiy the rate o energy transer until a desirable product is obtained. 3. Repeat the rst two steps at intervals throughout the batch cycle and plot the results as shown in Figure 1. The amily o curves resulting rom data plotted under the selected conditions indicates that the number o crystals is not increasing with time. Thus, no additional nucleation is occurring yet. 4. Proceed to collect crystal samples, anticipating the onset o nucleation. Figure 2 indicates that the number o crystals is signicantly increasing with time. In this gure, t 1 (not to be conused with t 1 in Figure
1) represents the start o this new set o batch dynamics. It is sae to assume that signicant nucleation is now occurring and that the rate o energy transer is too high. 5. By taking the slope o the curve representing the estimated number o nuclei present at the measured point in time (N t )i vs. time (t )i , one can determine the nucleation rate. Using your representation o Figure 3, create a dashed, horizontal line across the lower portion o the graph depicting the selected, cumulative number o crystals (N i’ ), and their sizes (L 1–L 4) over time (t 1–t 4). 6. For a selected cumulative number o crystals (N i ’ ), plot the crystal size ( L ) vs. time (t ), as demonstrated in Figure 3. The slopes represent the crystal growth rate ( G ). I the level o supersaturation changes during the run, the growth rate also changes. Non-parallel lines would indicate that the larger crystals are growing at a aster rate, due to a reduced diusional resistance [layer] at the crystal surace. With larger particles, the resistance layer may be smaller, allowing the solute to more readily reach the crystal surace and incorporate itsel into the lattice. These actors collectively contribute to the accelerated growth rate o the larger particles. Parallel lines indicate that the growth rate is not dependent on crystal size. 7. Increase the rate o cooling or evaporation until additional nucleation occurs, upon which you can saely assume that the growth rate is too high. 8. Develop a seeding and evaporation prole that will yield a growth rate that is lower than the value ound in Step 6. When determining the growth rate, keep in mind the dierence in mixing characteristics between a laboratory-scale vessel and a commercial conguration. A small tank generally oers a higher relative pumping capacity, shorter blend time, and higher average shear rates within a narrower range.
UsefUl observations • Most processors will agree that when it comes to crystals, the larger, the better. Large crystals are easier to handle in downstream operations, such as washing, centriugation and drying. • As previously mentioned, it is desirable or the seeds’ size distribution to refect a narrow cut o particles. In this cut, the weight o crystals with sizes ner than Ls should be minimal because these tiny particles add enormously to the number o crystals that
NomeNclature
Crystalsurfacearea,ft 2 Nucleationrate,(numberofnuclei)/ ft 3/s G Crystalgrowthrate,µm/s k v Crystal-volumeshapefactor, dimensionless L Crystalsize,µm L ’ Smallest-measurablesize,µm L av Sizeofcrystalfraction,µm L f Finalsizeofcrystal,µm L s Seedsize,µm N Numberofseeds N i’ Constant,cumulativenumber of crystals in crystallizer N Li Numberofcrystalsofagivensize,
A B °
L av
N ti S S * t, t i, t f ∆W i c
Numberofcrystalnucleiatanytime Rateofsupersaturation Maximumallowablesupersaturation,lb/ft 3solvent Time,h Weightofcrystalsonscreen Crystaldensity,lb/ft 3
compete or supersaturation and growth. • Studies show that milled seeds may not grow as well as unmilled seeds. Furthermore, not all crystals o a given size grow at the same constant rate. This is sometimes attributed to the dierences in the surace characteristics o particles that have equal dimensions. • Fines destruction in a batch system can greatly reduce the eects o secondary nucleation on the CSD, and signicantly increase crystal size while narrowing the CSD. • In practice, not all additional nucleation can be suppressed. Crystallizations carried out at low levels o supersaturation near the metastable zone (i.e., the conditions under which crystals grow, but do not typically nucleate) will display some secondary nucleation, due to crystal-crystal interactions and contact between the crystals and the impeller. Nevertheless, the mean crystal size, shape and distribution are dramatically improved when seeding is ollowed by a programmed rate o energy transer. Reerence: Genck, W., Better Growth in Batch Crystallizers, Chem. Eng. , Vol. 106, No. 8, pp. 90–95, Aug. 2000. E-mail:
[email protected]
4
3
3
2
2
1
2
3
4
1
1 2
3
Size, (m) = smallest measurable size FiGure 1
s l 1 a t s y r c f o r n e a b h t m r u e n g r a l
1 4 1
2
3
1
= smallest measurable size
FiGure 2
2
=
3
Time, (min) FiGure 3
4
Fuel Selection Considerations
Department Editor: Rita L. D'Aquino he selection and application o uels to various combustors are complex. Most existing units have A limited exibility in their ability to fre alternative uels. New units must be careully planned to assure the lowest frst costs without jeopardizing the uture capability to switch to a dierent uel.
T
Natural gas Natural gas has traditionally been the most attractive uel type or combustors because o the limited need or uel-handling equipment (e.g., pipelines, metering, a liquid-knockout drum, and appropriate controls) and the reedom rom pollution-control equipment. Drawbacks include rising uel costs, inadequate gas supplies and low er boiler eiciencies that result rom iring natural gas, particularly when compared to the iring eiciencies o oil or coal. Fuel oil Fuel oils are graded as No. 1, No. 2, No. 4, No. 5 (light), No. 5 (heavy), and No. 6. Distillates are Nos. 1 and 2, and residual oils are Nos. 4, 5, and 6. Oils are classifed according to their physical characteristics by the American Society or Testing and Materials (ASTM) per Standard D-396. No. 2 oil is suitable or industrial use and or home heating. The primary advantage o using a distillate oil rather than a residual oil is that it is easier to handle, requiring no heating to transport and no temperature control to lower the viscosity or proper atomization and combustion. However, considerable purchase cost penalties exist between residual and distillate. Distillates can be divided into two classes: straight-run, which is produced rom crude oil by heating it and then condensing the vapors; and cracked, which involves refning at a high temperature and pressure, or refning with catalysts to produce the required oil rom heavier crudes. Cracked oils contain substantially more aromatic and olefnic hydrocarbons, which are more difcult to burn than the parafnic and naphthenic hydrocarbons obtained rom the straight-run process. Sometimes a cracked distillate, called industrial No. 2, is used in uel-burning installations o medium size (small package boiler or ceramic kilns, or example). Because o the viscosity range permitted by ASTM, No. 4 and No. 5 oil can be produced in a variety o ways: blending o No. 2 and No. 6, mixing refnery by-products, utilization o ospecifcation products, and so on. Because o the potential variations in characteristics, it is important to monitor combustion perormance routinely to obtain optimum results. Burner modifcations may be required to switch rom, s ay, a No. 4 blend to a No. 4 distillate. Light (or cold) and heavy (or hot) No. 5 uel oil are distinguished primarily by their viscosity ranges: 150 to 300 SUS (Saybolt Universal Seconds) at 100°F and 350 to 750 SUS at 100°F, respectively. The (No.) classes normally delineate the need or preheating or proper atomization. The No. 6 uel oil is also reerred to as residual, Bunker C, and reduced- or vacuum bottoms. Because o its high viscosity, 900 to 9,000 SUS at 100°F, it can only be used in systems designed with heated storage and a high enough temperature (to achieve proper viscosity) at the burner or atomization. Notable fuel oil properties include the following: 1) Viscosity indicates the time required in seconds or 60 cm3 o oil to ow through a standard-size orifce at a specifc temperature. In the U.S., it is normally determined with a Saybolt viscosimeter, which comes in Universal and Furol variants. The dierences between them are the orifce size and the sample temperature. Thus, when stating an oil’s
B
FIGURE 1.
This nomograph is used to estimate annual cost savings from reducing combustible losses due to unburned carbon
viscosity, the type o instrument and temperature must also be stated. The Universal has the smallest opening and is used or lighter oils. 2) The ash point is the temperature at which oil vapors are ignited by an external ame. As heating continues above this point, sufcient vapors are driven o to produce continuous combustion. The ash point is also an indication o the maximum temperature or sae handling. Distillate oils have ash points o 145–200°F; heavier oils have ash points up to 250°F. 3) The pour point is the lowest temperature at which an oil ows under standard conditions, and is roughly 5°F above the solidifcation temperature. Knowledge o the pour point helps determine the need or heated storage, the storage temperature, and the need or heating and pour-point depressant. Coal The selection o coal as uel involves higher capital investments because o the need or handling equipment, coal preparation (crushing, conveying, pulverizing, etc.) and storage; ash handling and storage; pollution-abatement equipment; and maintenance. The operating cost savings at current (2007) uel prices o coal over oil or gas justifes a great portion, i not all, o the signifcantly higher capital investments required or coal. Coal-fred steam generators and vessels inherently suer efciency losses due to a ailure to burn all the available uel. The unburned uel is the remaining carbon in the letover ash. The nomograph (Figure 1) may be used to assess how a reduction in unburned carbon translates into energy and cost savings. A sample calculation ollows. Example: The system is a coal-fred steam generator with a continuous rating o 145,000 lb/h; average (avg.) boiler load = 125,000 lb/h; existing combustibles in ash = 40% (measured); obtainable combustibles in ash = 5%; actual operating time = 8,500 h/yr; design-unit heat output = 150 × 106 Btu/h; avg. heat output = 129 × 106 Btu/h; avg. uel cost = $1.50/106 Btu. Analysis: In Figure 1A, the percent o existing combustibles (measured) are shown on the horizontal axis. The curves above it represent the proposed improvement in percent o unburned carbon in ash. From the coordinates in Figure 1A draw a horizontal line to the curve in Figure 1B that represents the design-unit heat output. Drop the line to the appropriate uel-cost curve in Figure 1C. Extend the line rom that point to the let to obtain the corresponding annual uel savings, assuming continuous operation at ull boiler design output. To calculate actual annual uel
This article has been drawn from the work of Wayne Turner and Steve Doty, “Boilers and Fired Systems,” Energy Management Handbook, 6th Ed., Ch. 5, The Fairmont Press, Lilburn, Ga., 2006.
C
savings, a correction actor (CF) is required that considers actual boiler load and actual run time: Actual savings, $ = Savings rom chart x CF where CF = operating avg. heat output/design heat output × [(actual operating h/yr)/(8,760 h/yr)] Savings or this example = $210,000/yr × [(129 × 106 Btu/h)/(150 × 106 Btu/h)] × [(8,500 h/yr)/(8,760 h/yr)] = $175,200/yr. Note: I the heat output o the unit or the average uel cost exceeds the limit o the fgures, use hal o the particular value and double the savings obtained rom Figure 1C. It is probable that pulverized-coal-fred installations suer rom high UCL whenever any o the ollowing are experienced: a change in the raw-uel quality rom the original design basis; deterioration o the uel burners, burner throats, or burner swirl plates or impellers; increased requency o soot blowing to maintain heat-transer surace cleanliness; a noted increase in stack gas opacity; uneven lame patterns characterized by a particularly bright spot in one section o the lame and a notably dark spot in another; CO ormation as determined rom a lue-gas analysis; requent smoking observed in the combustion zone; increases in reuse quantities in collection devices; neglect o critical pulverizer internals and classiier assemblies; a high incidence o coal “hang-up” in the distribution piping to the burners; and requent manipulation o the air/coal primary and secondary air registers. Techniques used successully to reduce high UCL and/or high-excess-air operation include: modiying or replacing the pulverizer internals to increase the coal fneness; installing additional or new combustion controls to maintain consistent burner perormance; purchasing new coal eeders that are compatible with and responsive to unit demand uctuations; calibrating air ow and metering devices to ensure correct air/coal mixtures and velocities at the burner throats; installing turning vanes or air oils in the secondary air-supply duct or air plenum to ensure even distribution and proper air/uel mixing at each burner; replacing worn and abraded burner impeller plates; installing new classifers to ensure that proper coal fnes reach the burners or combustion; rerouting or modiying air/coal distribution piping to avoid coal hang-up; increasing the air/coal mixture temperature exiting the pulverizers to ensure good ignition without coking; and cleaning deposits rom burner throats. ■
Matals of Constcton
Department Editor: Rita L. D'Aquino
Low-temperature appLications [1 ] One key engineering consideration is the choice of materials of construction for frigid applications. Nickel-chromium (Ni-Cr) type stainless steels are notably versatile at low or cryogenic temperatures. They offer a combination of high impact strength (IS) and corrosion resistance. In the austenitic phase, with face-centered-cubic crystals, the combination of Cr and Ni in the material improves IS and toughness down to temperatures as low as –250°C. For good IS at temperatures down to –45°C, C-Mn-Si steels are recommended. The most preferred grades are fine-grained steels of pressure-vessel quality, such as ASTM A 516 and ASTM A 537 (in all grades). For temperatures between –45 and –100°C (for example, for liquid-ethylene storage), steels containing 2.5–9% Ni are useful. Between –150 and –250°C, the Ni-Cr austenitic steels (300 series, of 18/8 varieties), are highly recommended. In the nonferrous category, Al has excellent properties for temperatures as low as –250°C. Also attractive are Cu and some of its alloys, which can withstand temperatures down to –195°C.
chemicaL resistance CPVC [2]. Many nonmetals do not have the tensile strength to meet the pressure requirements of various process applications, especially at elevated temperatures. But years of testing and actual field performance prove that chlorinated polyvinyl chloride (CPVC) systems can be pressure rated for operation as high as 200°F. CPVC’s high heat-distortion temperature and resistance to corrosion make it suitable for applications such as metal processing, pulp and paper, and industrial wastewater treatment, where harsh and corrosive chemicals are commonly used (see Figure 1). Another advantage of CPVC is that it is lighter than metal, and therefore less expensive to install, from both a material cost and labor perspective. CPVC is not recommended where aromatic solvents and
esters are present in high concentrations. FRP pipe [3]. Composite fiberglass-reinforced plastic (FRP) pipe has been replacing conventional pipe material, such as steel and concrete, in numerous applications because of its corrosion resistance, low design weight (25% of concrete pipe and 10% of steel pipe), high fatigue endurance, and adaptability to numerous composite blends (Table 5, Ref. 3 ) and manufacturing methods. FRP pipe may be divided into two broad categories: gravity pipe (dia. from 8 to 144 in.) and pressure pipe (dia. from 1 to 16 in.). It is not unusual to see FRP pressure pipe handling pressures as high as 2,000–5,000 psi during chemical processing, with the higher-pressure pipe at the lower end of the diameter scale.
heat transfer properties [4 ] Metals, including specialty materials, are the best choice in terms of good heat transfer. In the lined category, glass is used extensively for process equipment where good heat transfer is required. Lined materials, however, often have the problem of uneven thermal expansion, which may weaken the bonding of the lining in due course. While fluoropolymers have excellent compatibility with various chemicals and special surface and physical chemistries, they are generally not used for reaction vessels because of their poor heat-transfer properties. Thermal conductivities for various materials are listed in the Table, and typical applications are shown in Figure 2.
THERMAL CONDUCTIVITY OF VARIOUS MATERIALS OF CONSTRUCTION [ 4 ] Material Carbon Steel (CS) SS 304 SS 316 SS 316 L Hastelloy B2 Hastelloy C2 Tantalum2 Titanium2 Zirconium2 Graphite Hexoloy Glass1 Lead Inconel2 CPVC PTFE (Polytetrafluoroethylene)1 PFA (Perfluoroalkoxy resin)1 ETFE (Ethylene tetrafluoroethylene)1 PVDF (Polyvinylidene fluoride)1 ECTFE (Ethylene chlorotrifluoroethylene)1
0.24 0.23 0.16
References 1. Nalli, K., Materials of Construction For Low-Temperature and Cryogenic Processes, Chem. Eng. July 2006, pp. 44–47. 2. Newby, R. and Knight, M., Specifying CPVC In Chemical Process Environments, Chem. Eng., October 2006, pp. 34–38. 3. Beckwith, S., and Greenwood, M., Don’t Overlook Composite FRP Pipe, Chem. Eng., May 2006, pp. 42-48. 4. Robert, J., Selecting Materials of Construction, Chem. Eng., September 2005, pp. 60–62.
Exotic
Exotic
Exotic
250
Exotic
Exotic
Exotic
Fluoropolymer, glass lined, exotic
0.19
1. Common choice for lining material 2. Exotic metals
300 C ° 200 , e r u 150 t a r e 100 p m e 50 T
Thermal conductivity, W/(m)(K) 60.59 40.71 14.23 14.23 9.12 10.21 57.5 21.67 20.77 121.15 125.65 1.00 35.30 12.00 0.14 0.25
Fluoropolymer, glass lined, exotic
Glass lined,* exotic
Exotic
Glass lined,* exotic
0 Weak acids Weak bases Salts Strong acids
Excellent
Aliphatics Strong bases
Good
Strong oxidants Halogens
Fair
Aromatic solvents Esters and ketones
Poor
CPVC offers resistance to a variety of harsh chemicals Figure 1.
-50
Exotic
Exotic
-100
Exotic
Exotic
Application:
Storage
Typical equipment:
Tanks, vessels
Transport Pipelines, valves, owmeters
Exotic Agitation Mixers
Exotic (Agitation + heat transfer) Reactors
When looking beyond steel for materials of construction, it is important to consider the intended application and temperature range. Exotic (specialty) metals (see Table) are shown here to serve well in all applications. Another material, equally suited to a specific requirement, however, may be chosen as the more cost-effective option Figure 2.
Heat Transfer Fluids and Systems Department Editor: Rebekkah Marshall
STARTUP
FLUID ANALYSIS
1. Verifycontrolandsafetysystems:Itisvitallyimportanttoverifyallcontrolandsafetysystems arecalibratedandreadyforoperationandare functioningproperly 2.Checkforleakage 3.Remove moisture from the system, using dry, compressedairorothersuitablemeans.Fillthe systemwithheattransferfluid 4.Systemfilling a. Fillthesystemtodesiredlevelavoidingany unnecessaryaerationofthefluid b. Openallvalves,thenstartthemaincirculationpumpinaccordancewiththemanufacturer’srecommendations.Allowforthermal expansion of fluid in determining the cold chargelevel c. C irculatetheheattransferfluidthroughthe systemforabout3to4hourstoeliminateair pockets,and toassurecompletesystem fill beforefiringtheheater 5.Starttheheater a. Bringthesystemuptotemperatureslowlyto helppreventthermalshocktoheatertubes, tube/heaterjointsandrefractorymaterials; andallow operators tocheck thefunctioning of instruments and controls. The slow heat-upwillalsoallowmoisturetrappedin allsectionsofthesystemtoescapeasvapor. Inertgasshouldsweeptheexpansiontankto removenoncondensablesandresidualmoisturetoasafelocation.Holdthetemperature stableabove100°C(212°F)untilnosigns ofmoistureremain(knockingorrattlingof pipes,nomoisturefromvents,andsoon) b. Bringthesystemtooperatingtemperature, putthe“users”online,andplacetheexpansion-tankinertingsystemintooperation c. Thefluidshouldgenerallybeanalyzedwithin 24hofplantstartupandannuallythereafter d. Checkandcleanstartupstrainersasneeded The system should beheated and cooled for at leasttwocycleswiththe filter inplace sincethe resultingexpansionandcontractionwillloosenmill scale.Reinsulateanysurfacesleftbareforleakcheckingpurposes.
Fluid testing helps detect system malfunction, fluid contamination, moisture, thermal degradation,aswellasotherfactorsthatimpactsystemperformance(seeTable).For systemsoperatingnearthefluid’smaximumtemperature,annualanalysisissuggested.
OPERATIONS Heaters: Proper fluid-heater operation will help ensurelonglifeofthefluid.Commonheaterproblems include flame impingement, excessive heat flux,controlovershoot,lowfluidflow,andinterlock malfunctions. Piping and pumps: Aleak-freesystemwillhelpto ensuresafeandreliableoperation.Somekeyfeaturesofaleak-freedesignareasfollows: • Maintainvalvesandpumppackingandseals • Avoidtheuseof threadedfittings(weldedor flangedconnectionsarepreferred) • Realignpumpsandretorqueflangesoncesystemachievesoperatingtemperatureafterinitial systemstartup • Confirmwithyourfluidsupplierwhattheproper elastomersare.Notallelastomersarecompatiblewithallheattransferfluids
Test result
Potential effects
Possible cause
Viscosity increase
Poor heat-transfer rate, de- • Contamination posits, high vapor pressure, • Thermal degradation pump cavitation • Fluid oxidation
4, 5 4, 5 3, 4
Total acid number increase
System corrosion, deposits
• Severe oxidation • Acidic contamination
3, 4 4, 5
Moisture increase
Corrosion, excess system pressure, pump cavitation, mechanical knocking
• System leaks • Residual moisture in new or cleaned unit • Unprotected vent or storage
2 2
• Contamination • Dirt • Corrosion • Oxidation • Thermal stress
1, 4, 5 1, 4 1, 3, 5 1, 3 1, 4
Insoluble solids increase
Poor heat transfer, wear of pump seals, plugging in narrow passages
Low- and high-boiler increase**
Pump cavitation, poor heat • Low boilers transfer, excess system • High boilers pressure, deposits • Contamination
Possible actions*
2, 3
2 4 4, 5
* For detailed guidance on actions, please consult with your fluid engineering specialist. ** For an excellent discussion on low and high boilers, please consult Ref. [ 4 ].
Possible actions 1. Filtration: Small diameter particles sus-
pendedinheattransferfluidcanbeeffectivelyremovedviafiltration.Filterswith 100-mmorlessnominal-particle-removal ratings should be considered for initial system treatment. Continuous filtration through10-mmratedfilterscanmaintain systemcleanliness 2. Venting: Iflowboilerconcentrationand/ ormoistureisallowedtoreachexcessive levelsinthefluid,problemssuchaspump cavitation,increasedsystempressureand flash-pointdepressioncanoccur.Intermittent,controlledventingtoasafelocation isacommonsolutiontominimizethepotentialforproblemscausedbyexcessive lowboilerormoistureconcentration 3. Inerting: Aneffectivemethodofminimizingfluidoxidationistoblankettheexpansiontankwithaclean,dry,inertgas, suchasnitrogen,CO2,ornaturalgas 4. Dilution/replacement: Canbeusedtoremovesomefraction(orall)ofthefluidand replacewithvirginfluidtomaintainfluid propertieswithinnormalranges.Caution is advised when using reclaimed fluid, which can return degradation products and/orcontaminantsintothesystem 5. Cleaning: Ifasystemflushisnecessary, several differentmethodsareavailable. Specialty-engineered,heat-transfer flush fluidsmaybeusedtoremovesludgeor tar from piping/equipment. Hard carbondepositsonheatersurfaces(“coke”) generallyrequiretheuseofmechanical cleaning techniques like sand or bead
blasting,wirebrushing,orhigh-pressure waterjetting.Forprocesscontamination, consultwithyourfluidsupplierforsuggestedcleaningmethods
SHUTDOWN Preventoverheatingoffluidduetoresidual heatintheheater. 1.Shutoffburnercompletelywiththecirculating pump stilloperating.Continueto runthepumpatfullcapacitytodissipate residualheatintheheater 2. Whentheheaterhascooledtothemanufacturer’srecommendedlowtemperature, shutoffthecirculatingpumpandswitch offrequiredheaterelectricalcontrols 3.Cautionmustbeexercisedduringshutdowntoensurethatnoareainthesystempipingistotallyandcompletelyisolated.Thiswillpreventavacuumfrom forming,whichcoulddamage(implode) equipment 4.Operateheattracing,ifneeded References and further reading 1.G amble, C.E., Cost Management in Heat TransferSystems,Chem. Eng. Prog.,July2006 pp.22–26. 2. Gamble,C.E.,CleaningOrganicHeatTransfer FluidSystems,Process Heating ,Oct.2002. 3. Beain, others, Properly Clean Out Your OrganicHeatTransferFluidSystem, Chem. Eng. Prog.,May2001. 4. Spurlin,others,DefiningThermalStability,Process Heating ,Nov.2000. 5. “LiquidPhaseDesignGuide,”Pub#7239128C, Solutia,Inc.,1999.
Pristine Processing Equipment
Department Editor: Kate Torzewski rocesses in the pharmaceutical, biotechnology, ood and semiconductor industries must meet a high set o standards to ensure high product purity. Equipment criteria specic to high-purity processes are established to minimize contamination and maintain product integrity. In designing a pristine process, material and equipment style are o upmost importance. Bacteria is the main cause o contamination and is prone to growing in the dead cavities o equipment created by sharp corners, crevices, seams and rough suraces. Another source o contamination is leaking, which allows undesirable chemicals to compromise the quality o the process ingredients, by causing contamination, rusting and particle generation.
P
MATERIALS OF CONSTRUCTION Many actors must be taken into consideration when selecting materials o construction or use in pristine process applications where high-purity and sanitation are paramount. All suraces should be constructed o a smooth material that will not corrode, generate particles or harbor dead cavities. These criteria can be met with three standard materials: 316L stainless steel (SS), polyvinylidene fuoride (PVDF) and polytetrafuoroethylene (PTFE). The advantages and disadvantages o these materials are summarized below to acilitate the material selection process or a given application with consideration o chemical compatibility, cost, and temperature stability. MATERIALS OF CONSTRUCTION Material Stainless Steel
PVDF
PTFE
Advantages • Mechanical strength • Functions at 121°C (steam-sterilization temperature) • Chemically inert • Resistant to corrosion and leaching • Durable and long-lasting • Retains circumerential strength
Disadvantages Vulnerability to corrosion by certain chemicals, which increases with temperature Functions only intermittently at 121°C
• The most chemically inert plastic • Resistant to corrosion and leaching Complex shapes are • Avoids leaching dicult to orm • Suitable or coating equipment
EQUIPMENT STYLE SELECTION Critical actors in high-purity equipment selection include cleanability, cost, fow capabilities and product compatibility. With these considerations in mind, criteria useul or choosing pumps, valves, seals and piping are described in this section. Pumps A undamental requirement o pristine processing pumps is the ability to clean a pump in place without disassembly. Pump seals, gaskets and internal suraces should eliminate the buildup o material and should clean out easily during wash cycles. The most common pump styles or high-purity processes are centriugal, lobe-style and peristaltic pumps, which are outlined below. PUMPS Pump style
Advantages
Disadvantages
Centriugal
• Low cost • Easy cleanability
Eciency and fow decrease with increasing pressure and volume
• Low cost • Easy cleanability • No mechanical Peristaltics seals • Non-damaging to delicate products
Rotary Lobes
Applications best suited for this style • Handling lowviscosity products • Handling h igh fowrates (40–1,500 gal/min)
The need or hoses may cause issues in elastomeric • Small , batch-oriented compatibility, temperature applications and pressure limitations, • Laboratory or pilotand a need to change scale plants hose regularly
• Higher pressure and fow capabilities High cost • Unaected by pressure variations
• Large, continuous duty applications • Steaming and high pressure applications
Valves Valves should not harbor contaminants and must be easy to clean. By these criteria, diaphragm and pinch valves are excellent choices or ultrapure processes, as they have smooth, gently curved suraces that will not harbor contaminants. Ball check, ull-port plug and ull-port ball valves are good choices as well, while butterfy, spring check, gate and swing check valves are all unacceptable, since contamination can collect in the corners that are essential to their design. Though several valves are appropriate or pristine processes, certain valves are better suited or particular applications. Diaphragm valves are the most widely used in high-purity systems or their resistance to contamination and ability to be used as a control valve. Ball and plug valves, on the other hand, are less costly and are not limited by temperature and pressure. Also, in applications using sterile steam and reeze-drying, ball valves are preerred over diaphragm valves because they eliminate the risk o catastrophic seat ailure. Seals As with all pristine processing equipment, high-purity seals should not have any cavities where contaminants can breed. By choosing a seal with gland rings that do not need to be threaded or ported, the areas where bacteria can breed are minimized. In choosing a seal material, it is important to nd a compound that will not swell, crack, pit or fake, thus reducing seal ailure and contamination. To ensure the success o seals, furoelastomers are a top choice in pristine processing applications or their excellent thermal stability, chemical resistance and mechanical durability. Piping The surace o piping, as well as any wetted equipment parts, should have a very smooth surace. When 316 SS is being used, electropolishing is a good method or achieving an ultra-smooth nish. Joining methods should minimize crevices and dead cavities, and all materials should be ree o biological degradable substances, leachable substances, and glues and solvents that may migrate into the product stream. References 1. Smith, B., What Makes a Pump or High-Purity Fluids?, Chem. Eng., pp. 87–89, April 2002. 2. Schmidt, M., Selecting Clean Valves, Chem. Eng., pp. 107–111, June 2001. 3. Wul, B., Pristine Processing: Designing Sanitary Systems, Chem. Eng., pp. 76–79, Nov. 1996. 4. Weeks, D. T. and Bennett, T., Speciying Equipment or High-Purity Process Flow, Chem. Eng., pp. 27–30, Aug. 2006.
Pump Selection and Specifcation Department Editor: Kate Torzewski
PUMP SELECTION n choosing a pump, it is important to match a pump’s capabilities with system requirements and the characteristics o the liquid being processed. These actors include the inlet conditions, required owrate, dierential pressure and liquid characteristics. Generally, the quality o the liquid should remain unchanged ater passage through a pump. Thereore, material compatibility, viscosity, shear sensitivity and the presence o particulate matter in a liquid are important considerations in pump selection. Most engineering applications employ either centriugal or positive displacement (PD) pumps or uid handling. These pumps unction in very dierent ways, so pump selection should be based on the unique conditions o a process.
I
Centrifugal pumps The most widely used pump in the chemical process industries or liquid transer is the centriugal pump. Available in a wide range o sizes and capacities, these pumps are suitable or a wide range o applications. Advantages o the centriugal style include: simplicity, low initial cost, uniorm ow, small ootprint, low maintenance expense and quiet operation. Positive displacement pumps Though engineers may be frst inclined to install centriugal pumps, many applications dictate the need or PD pumps. Because o their mechanical design and ability to create ow rom a pressure input, PD pumps provide a high efciency under most conditions, thus reducing energy use and operation costs. Choosing centrifugal versus positive displacement These two main pump styles respond very dierently to various operating conditions, so it is essential to evaluate the requirements o a process prior to choosing an appropriate pump. Table 1 illustrates the mechanical dierences between these pumps, as well as the eects o pressure, viscosity and inlet conditions on owrate and pump efciency. Range of operation Pump styles range ar beyond simply PD and centriugal pumps. PD pumps encompass many specifc styles, including a variety o reciprocating, rotary and blow-cover pumps. Likewise, centriugal pumps encompass radial, mixed, and axial ow styles, which all PumP ComParison Chart
Mechanics
Cefgl Pp
Pve dplcee pp
The pump imparts a velocity to the liquid, resulting in a pressure at the outlet.
The pump captures confined amounts of liquid and transfers them from the suction to discharge port.
Pressure is created and flow results
Performance Viscosity
Efficiency
Inlet conditions
Flow varies with changing pressure Efficiency decreases with increasing viscosity Efficiency peaks at the best-of-efficiency point. At higher or lower pressures, efficiency decreases Liquid must be in the pump to create a pressure differential. A dry pump will not prime on its own
Flow is created and pressure results
Flow is constant with changing pressure Efficiency increases with increasing viscosity Efficiency increases with increasing pressure
Negative pressure is created at the inlet port. A dry pump will prime on its own
belong to a greater category o kinetic pumps. A simple way to narrow down pump styles is to determine the required capacity that your pump must handle. Based upon a required capacity in gal/min. and a pressure in lb /in.2, the pump coverage chart below can help engineers ocus their selection to a just a ew pump styles.
Adapted from Perry’s Chemical Engineers’ Handbook
PUMP SPECIfICaTIONS Based on the application in which a pump will be used, the pump type, and service and operating conditions, the specifcations o a pump can be determined. • Casting connection: Volute casing efciently converts velocity energy impacted to the liquid rom the impeller into pressure energy. A casing with guide vanes reduce loses and improve efciency over a wide range o capacities, and are best or multistage highhead pumps • Impeller details: Closed-type impellers are most efcient. Opentype impellers are best or viscous liquids, liquids containing solid matter, and general purposes • Sealings: Rotating shats must have proper sealing methods to prevent leakage without aecting process efciency negatively. Seals can be grouped into the categories o noncontacting seals and mechanical ace seals. Noncontacting seals are oten used or gas service in high-speed rotating equipment. Mechanical ace seals provide excellent sealing or high leakage protection • Bearings: Factors to take into consideration while choosing a bearing type include shat-speed range, maximum tolerable shat misalignment, critical-speed analysis, loading o compressor impellers, and more. Bearing styles include: cylindrical bore; cylindrical bore with dammed groove; lemon bore; three lobe; oset halves; tilting pad; plain washer; and taper land • Materials: Pump material is oten stainless steel. Material should be chosen to reduce costs and maintain personnel saety while avoiding materials that will react with the process liquid to create corrosion, erosion or liquid contamination References 1. “Perry’s Chemical Engineers’ Handbook,” 7th ed. New York: McGraw Hill, 1997. 2. Petersen, J. and Jacoby, Rodger. Selecting a Positive Displacement Pump, Chem. Eng. August 2007, pp. 42–46.
Avoiding Seal Failure Department Editor: Kate Torzewski
S
eals are assemblies o elements that prevent the passage o a solid, liquid, gas or vapor rom one system to another. When a seal allows leakage o material, ailure has occurred. This guide provides an overview o common seal types and a discussion o seal ailure to aid in choosing the most eective seal and avoiding uture ailure.
seal types Seals types can be classied within two broad categories: static and dynamic. Static seals have no relative motion between mating suraces, while dynamic seals do have relative motion between a moving surace and a stationary surace. Seals do not have to t into one category or the other; rather, seal types can all anywhere on a spectrum between static and dynamic, and ew seals are strictly one type or the other. Table 1 describes the applications and requirements o several common seal types.
seal failure Seal ailure is caused by a wide variety o circumstances, including improper installation and environmental actors such as temperature, pressure, fuid incompatibilities, time and human actors. Most causes o ailure can be described as mechanical diculties or system operations problems. Examples o mechanical diculties include strain on the seal ace caused by improper installation and vibration caused by improper net positive suction head. Meanwhile, system operating problems can include conditions that are outside o a pump’s best perormance envelope, such as upsets, dry running, and pressure or temperature fuctuations. Changes in the fuid being processed can cause problems as well, especially with fuids that fash or carbonize. Common visual indicators o ailure include short cuts, V-shaped notches in the seal, skinned surace in localized areas, or thin, peeled-away area on the seal. Table 2 describes causes o some o the most prevalent types o seal ailure with recommended methods o action. In some cases, the cause o ailure may be dicult to determine due to the complexity o the seal construction. These unique ailure modes can result in faking or peeling o the seal ace, corrosion, faking or pitting o the carbon aces, degradation o the elastomer energizer seals, and spring or bellows breakage. It is likely that these rapid degradations are a result o contamination, which can be avoided with careul installation or using pre-assembled, cartridge-type mechanical seals.
TABLE 1. COMPARISON OF COMMON SEAL TYPES Type
Applications
O-ring T-seal U-packing V-packing Cup-type packing Flat gasket Compression or jam packing
References
Static
Dynamic
Periodic Adjustment Required?
Moving friction
Tolerances Gland Space required (mov- adapters requireing seals) required? ments
X X — — —
X X X X X
No No No Yes No
Medium Medium Low Medium Medium
Close Fairly close Close Fairly close Close
No No No Yes Yes
Small Small Small Small Medium
X X
— X
Yes Yes
— High
— Fairly close
No Yes
Large Large
1.
Ashby, D. M. Diagnosing Common Causes o Sealing Failure, Chem. Eng. June 2005, pp. 41–45.
2.
Netzel, J., Volden, D., Crane, J. Suitable Seals Lower the Cost o Ownership, Chem. Eng. December 1998, pp. 92–96.
TABLE 2. SOLUTIONS TO COMMON CAUSES OF SEAL FAILURE Failure type Definition
Causes
Solutions
Compression A lost o resiliency caused by the set ailure o a seal to rebound ater it has been deormed or some period o time. The seal will exhibit a attened surace corresponding to the contours o the mating hardware Nibbling and A seal starts to appear to be torn extrusion away in little pieces until it loses its overall shape and ows into whatever void area is available
Exposure to excessive temperature or incompatible uids Excessive deormation o the elastomer at installation An incompletely vulcanized seal
Choose proper deection or the seal Choose appropriate elastomer material or the application in terms o thermal stability and compression set resistance
Spiral ailure
Explosive decompression Wear
Excessive clearance gaps Improper seal material Excessive volume-to-void ratio Inconsistent clearance gaps
Increase bulk hardness o the sealing element Decrease clearance gaps Redesign volume-to-void ratio Add anti-extrusion devices A seal rolls within its gland, resulting Applications where a seal is Use an elastomer with a higher bulk in cuts or marks that spiral around the used in a slow, reciprocating hardness circumerence o the seal ashion For male-type installation, increase the Irregular surace over the mating installed stretch on the seal parts causing the seal to grip to Speciy a smoother, more uniorm fncertain contact points ish on mating hardware Change the type o seal to a lip-type confguration Seal exhibits blisters, fssure, pock Gas entrapment within the Use an elastomer material that is more marks or pits, both externally and elastomer during high-presresilient to explosive decompression internally sure cycling, ollowed by rapid Use polymeric or metal seals i depressurization 0possible Smooth burnishing o a sealing Relative motion o the seal Use a harder material surace against the mating surace Use a polymeric solution
