Pumps for Hydrocarbon Service

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Engineering Design Guide: GBHE-EDG-MAC-1508

Pumps for Hydrocarbon Service Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Engineering Design Guide: Pumps for Hydrocarbon Service

CONTENTS SECTION 1 SCOPE 3 2 HYDROCARBON PROPERTIES 3

2.1 General 3

2.2 Pure Hydrocarbons 3

2.3 Associated Compounds 4

2.4 Crude Oil 5

2.5 Toxicology 6

2.6 Cavitation 7

2.7 Velocity of Sound 7 3 FLAMMABILITY HAZARDS 7

3.1 General 7

3.2 Definitions 8

3.3 The Electrical Area Classification 8 4 CHOICE OF PUMP TYPE 22 5 LINE DIAGRAM (PROCESS) 22 6 LAYOUT 23



7 SHAFT SEALS 23

7.1 Selection 23

7.2 Engineering of Seals 24 8 CONSTRUCTION FEATURES 25

8.1 General 25

8.2 Effects of Low Density 26 9 MATERIALS OF CONSTRUCTION 26

9.1 Process Wetted Parts 26

9.2 Mechanical Components 27

9.3 Non Metallic’s 27



APPENDIX A - BARNARD & WEIR SEAL THEORY FIGURES 1 VAPOR PRESSURE OF HYDROCARBONS 2 VAPOR PRESSURE OF LIGHT HYDROCARBONS 3 VAPOR PRESSURE OF GASOLINES 4 SPECIFIC HEAT OF HYDROCARBON LIQUIDS 5 SPECIFIC GRAVITY OF OLEFINE, DI OLEFINES AND PARAFFINS 6 SPECIFIC GRAVITY OF AROMATICS 7 VISCOSITY - TEMPERATURE CHART FOR PARAFFINS, AROMATICS

AND PETROLEUM FRACTIONS 8 VISCOSITY - TEMPERATURE CHART FOR MINERAL LUBRICATING

OILS TABLES 1 PURE HYDROCARBON PROPERTIES 2A CRUDE OILS PROPERTIES 2B NINIAN: PROPERTIES OF CRUDE OIL, NAPHTHAS AND KEROSENE 2C NINIAN: PROPERTIES OF GAS OILS AND RESIDUES 3 PURE HYDROCARBON FLAMMABILITY PROPERTIES



BIBLIOGRAPHY DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE



1 SCOPE This Engineering Design Guide provides guidance on the selection of suitable pumping equipment for handling hydrocarbons and specifies appropriate construction features. The principles are relevant to other flammable process materials which have similar physical properties. 2 HYDROCARBON PROPERTIES 2.1 General The properties of pure hydrocarbons that affect pumping vary widely, see Table 1. Crude oil contains a mixture of Hydrocarbons ranging from gases to tars and oil refining separates out products within a boiling range. Olefin plants convert light distillates to olefins; Aromatics plants catalytically reform naphtha and gasoline; and then these are separated by distillation. Consequently it is common to pump mixtures with intermediate properties. For density and viscosity a mean value is appropriate but for Nett Positive Suction Head (NPSH) and sealing considerations the lighter components govern. 2.2 Pure Hydrocarbons Hydrocarbons are commonly described by series which exhibit a continuous trend of properties or by descriptions which classify by similar properties, e.g. C3 's, C4's, light distillate, liquefied petroleum gases (LPG). In general hydrocarbons are not corrosive and if heavier than kerosene exhibit reasonable lubricating properties. Pure hydrocarbons are grouped in the following homologous series: (a) Normal paraffins (Cn H2n+2)

Methane (C = 1), Ethane (2), Propane (3), Butane (4), Pentane (5), Hexane (6), Heptane (7), Octane (8), Nonane (9), Decane (10), Undecane (11), Dodecane (12), etc, etc.



(b) Iso-paraffins (Cn H2n+2)

Isobutane (4), Isopentane or Methylbutane, Neopentane or Dimethylpropane (5), Isohexane or Methylpentane, Neohexane or 2.2 Dimethylbutane, Di-isopropyl or 2.3 Dimethylbutane (6), Isoheptane (7), etc, etc.

(c) Olefins (C n H2n)

Ethylene (2), Propylene (3), Butene-I, cis-Butene-2, trans-Butene 2, Iso-butene (4), Pentene (5), etc, etc.

(d) Diolefins (C n H2n-2)

Propadiene (3), Butadiene 1.2, Butadiene 1.3 (4), Pentadienes, ethylbutadienes (5), etc, etc.

(e) Acetylenes (C n H2n- 2)

Acetylene (2), Methylacetylene (3), Butyne or Ethylacetylene (4), etc, etc. (f) Olefins-Acetylenes (C n H2n-4)

Vinylacetylene (4), Allylacetylene (5), etc, etc. (g) Aromatics (C n H2n-6)

Benzene (6), Toluene (7), ortho-, meta-, para-Xylene Ethylbenzene (8), Propylbenzene Trimethylbenzenes, lsopropylbenzene or Cumene (9), etc, etc.

(h) Cycloparaffins (C n H2n)

Cyclopropane (3), Cyclobutane (4), Cyclopentane (5), Cyclohexane, Methyl Cyclopentane (6), Dimethylcylopentanes, Ethylcyclopentane (7)



2.3 Associated Compounds Simple organic compounds of carbon, hydrogen and oxygen are associated with hydrocarbons - some of them are: (a) Alcohols (C n H2n+1 OH)

Methanol or Methyl Alcohol (1), Ethanol or Ethyl Alcohol (2), Propanol-l or normal Propyl Alcohol, Propanol-2 or Isopropyl alcohol (3), Butanol or Butyl alcohols (4), Pentanol or Amyl alcohol (5), etc, etc.

(b) Glycols & Glycerol (Glycerin)

Ethanediol or Ethylene glycol (2), Propanediol or Propylene glycol, Propanetriol or Glycerol (3), Phenol (C6H50H) or Carbolic acid

(c) Ethers (d) Aldehydes

Formaldehyde, Acetaldehyde (e) Ketones (C n H2n 0)

Propanone or Acetone (3), Butanone or Methyl Ethyl Ketone (4)



2.4 Crude Oil Crude oil properties vary widely, even within an oil field, so although general terms, (such as Middle East, Venezuelan, North African, light, heavy, sour) are used as descriptions these are generalities and insufficient for use in design. Crudes can be alike in nearly all properties but an abnormal variation in one aspect can make them behave very differently. The important properties that affect pumping are discussed further in clause 2.4.2 and these should be obtained for the material under consideration. An evaluation of a North Sea Crude is given as an example in Tables 2B and 2C and comparison of physical properties in Table 2A. 'Light crude' can mean low specific gravity (40° API and higher) in the Oil Industry but in the Petrochemical Industry it may mean medium gravity with higher than typical light and heavy ends (more accurately known as 'gassy' rather than 'light'). 'Sour' normally indicates a positive reaction to the Doctor test. This is related indirectly to sulfur content. However, the term is used to indicate a crude high in sulfur such as typical heavier Middle East crudes. Initially most crudes contain C 3 's and nearly all contain C4. Subsequent handling may change the composition and the method of sampling needs to be appropriate to the intended analysis. 2.4.1 Some Properties and Terms Used in Analysis: RVP (Reid Vapor Pressure) is the total vapor pressure and is much influenced by any C3 or C4's present. IBP (Initial Boiling Point) - ASTM D86 method: Distillation flask temperature when first vapor condenses, at defined temperature in the region of 0°C, and forms a drop. C4 and lower do not affect this. True Boiling Point requires a lower condensing temperature. Flash point: if not quoted is likely to be very low. End point: Temperature of column when changing cut, can mean final boiling

point.



Final Boiling Point (ASTM D86): highest temperature of distillation flask neck: all material boiled or residue starts to crack to lower boiling components. Mercaptan sulfur test: presence of sulfur may indicates odorous properties. The indicate potential for corrosion. AET (Atmospheric Equivalent Temperature): the actual distillation may be conducted under reduced pressure in the later stages. In this case the actual temperatures are corrected for pressure to put them on the same basis as the atmospheric pressure results. CFPP (Cold Filter Plugging Point): used to assess suitability for automotive diesel fuel. Pour point: at its pour point a hydrocarbon will not start to flow within 5 seconds when a (defined) cylindrical container is tipped to horizontal. Aniline point: temperature at which hydrocarbon is completely miscible in aniline - an indication of Aromatic content. Copper Strip Test: results of immersion in comparison with standard samples. 1A, B, C are discolored, 3 definitely corroded. A similar silver strip test is used for aircraft fuels. 2.4.2 Petroleum Fractions. These are commonly defined as follows: These are commonly defined as follows: Product Boiling Range,

Deg. C No. of Carbon Atoms

Gas and liquefied gas up to 25 C1 - C4 Gasoline (petrol) ca. 20 - 180 C4 - Cll Kerosene ca. 175 - 275 C4 - C16 Gas oil, diesel oil ca. 200 - 380 C15 – C25 Lubricating oil C20 – C70 Fuel Oil very variable C10 upwards To C70+ Bitumen, Coke large



2.5 Toxicology The Threshold limiting values (TLV) are the vapor concentrations in air Representing conditions to which nearly all persons can be exposed repeatedly without adverse effect [3]. Prolonged skin contact should be avoided as the light hydrocarbons remove the natural oils by solvent action while heavier hydrocarbons can cause Dermatitis. N-Hexane and Methyl-butyl Ketone cause loss of sensation of the fingertips. Poly-Nuclear Aromatics (PNAs) found in Tars shale oils and some reformer products are carcinogenic [10]. Apart from those specifically noted above and in Table 1 the TLV of hydrocarbons are typically in the hundreds, ego 100 for White Spirit, 200 for Nonane, 300 for Octane Benzene has particularly severe toxic properties and is dangerous on skin contact as well as a vapor. 2.6 Cavitation There is evidence that pumps handling some liquids with a high vapor pressure such as certain hydrocarbons or high temperature water require less NPSH than would be required for cold water. The Hydraulic Institute publishes a chart relating NPSH reduction to vapor pressure at pumping temperature. These reductions apply to pure liquids without entrained air or non-condensable gases present and were derived from laboratory tests on Propane, iso-Butane, Butane, Refrigerant R-11, Methyl Alcohol and water. The effect of variation in composition, temperature and transient effects and the cautionary remarks given with the chart are such that these corrections are not normally applied when specifying the pump duty. API 610 6th Edition also excludes them. The correction is limited to 3 m and should not be greater than 50% of the NPSH required on cold water. Values are as follows (meters).



°C V.P. bara 0.5

1.0 2.0 5 10

0 0.2 0.5 0.9 2.4 3 40 0.15 0.3 0.7 1.8 3

100 0 0.24 0.5 1.2 2.5 200 0 0.2 0.4 0.9 1.8

2.7 Velocity of Sound Representative values are: (meters/sec at 20 - 30°C) 1050 n-Heptane 1085 n-Hexane 1295 Benzene 1278 Cyclo-Hexane 1315 Kerosene 1670 Ethylene glycol 1930 Glycerol (Glycerine) 3 FLAMMABILITY HAZARDS 3.1 General Most flammable materials are hydrocarbon based. The key properties are the initial boiling point, which indicates the quantity of flammable vapor generated, the flash point, which indicates the temperature above which sufficient vapor is released to permit ignition, and the ignition temperature, which determines whether leakage fires spontaneously. The flammable range is a secondary consideration. 3.2 Definitions Highly flammable materials are taken to be those with a flash point below 32°C or those being pumped at a temperature above their flash point. Flammable is taken to refer to materials with a flash point below 66°C. Liquefied flammable gas (commonly described as Liquefied Petroleum Gas) is a flammable material handled as a liquid, which at 17.5°C and atmospheric pressure is a flammable gas.



Table 5 gives properties of many process materials. A selection of these is given in Table 2. 3.3 The Electrical Area Classification [3] This is based upon the properties of the liquids contained in equipment and typical leakages from that equipment as follows: (a) Zone 0

Flammable atmosphere exists continuously or for long periods. (b) Zone 1

Flammable material is likely to be released in normal operation in sufficient quantity to produce a hazard [3 clause 2.3.1]

(c) Zone 2

Flammable material is not in contact with the surrounding atmosphere during normal operation, and equipment is constructed and maintained to prevent release of flammable materials in normal operation in sufficient quantity to cause a hazard, and relief valves, vents, etc, release flammable materials to the atmosphere only in abnormal operation.



TABLE 1 - PURE HYDROCARBON PROPERTIES



TABLE 2A CRUDE OIL PROPERTIES CRUDE OIL COMPARISON OF PHYSICAL PROPERTIES

Crude: Arabian Light North Sea Ninian IBP 46 deg. C 92 deg. C 33 deg. C RVP 3.5 psig NA (low) 8.9 psig SG 0.8524 0.9847 0.8459 Viscosity 5.5 cS @

100 deg. C 10,000 Redwood Secs No 1 @ 100

deg.F

Pour Point - 26 deg. C - 4 deg. C - 12 deg. C Flash Point NA 198 deg. F NA EVALUATION OF NINIAN CRUDE OIL Sample evaluated with ASTM D 2892 equipment. Charge distilled to a temperature of 208°C, at atmospheric pressure, then under a pressure of 10 mm Hg to 400°C AET. The gases were collected separately for analysis and the distillate cuts were blended. The material balance was as follows:



TABLE 2B - NINIAN: PROPERTIES OF CRUDE OIL, NAPHTHAS AND KEROSENE



TABLE 2C - NINIAN: PROPERTIES OF GAS OILS AND RESIDUES



TABLE 3 - PURE HYDROCARBON FLAMMABILITY PROPERTIES



FIG 1 - VAPOR PRESSURE OF HYDROCARBONS



FIG 3 - VAPOR PRESSURE OF GASOLINES



FIG 2 - VAPOR PRESSURE OF LIGHT HYDROCARBONS



FIG 2 – SPECIFIC HEAT OF HYDROCARBONS



FIG 5 - SPECIFIC GRAVITY OF OLEFINE, DI OLEFINES & PARAFFINS



FIG 6 - SPECIFIC GRAVITY OF AROMATICS



FIG 7 – VISCOSITY- TEMPERATURE CHART OF PARAFFINS, AROMATICS AND PETROLEUM FRACTIONS



FIG 8 - VISCOSITY - TEMPERATURE CHART FOR MINERAL LUBRICATING OILS



4 CHOICE OF PUMP TYPE Centrifugal pumps are the usual selection, using the principles of GBHE-EDG-MAC-1117. These would normally be used, even on viscous duties where the performance was significantly affected, see Fig 7 because the liquid end is not dependent on liquid lubricating properties or sensitive to solids. (Viscous hydrocarbons commonly are 'bottoms' from distillation columns and may contain thermal degradation products, residues or high melting point material.) In the case of light hydrocarbons (C3 and below) centrifugal pumps may be used in series or with large bypass flows to avoid the need for positive displacement pumps (light hydrocarbon are commonly of low viscosity and/or exhibit poor lubricating properties). Single stage high speed pumps in parallel are commonly used in preference to multi-stage pumps because they do not depend on process liquid lubrication and are more rapidly maintained. Rotary pumps are commonly used on the more viscous hydrocarbons since the viscosity reduces internal leakage though clearances and the lubricating properties permit the use of small running clearances or contact. Low viscosity non-lubricating hydrocarbons can only be handled if material combinations or coatings can be chosen to permit close clearances or contact. Fuel oils although normally handled by rotary pumps sometimes contain solid particles which cause rapid wear. Reciprocating pumps have a similar requirement to rotary pumps but in addition the reciprocating gland seal requires cup type pressure energized sealing rings to avoid the constant drip necessary with soft packed glands. Hydrocarbon crystals, e.g. Para-Xylene, are relatively soft and there may be a requirement to minimize mechanical degradation. In these circumstances comparative testing of closed impeller and inducer flow pumps led to the adoption of the higher efficiency closed impeller design at 25 rps for solids concentration of up to 45% in mother liquor.



5 LINE DIAGRAM (PROCESS) Continuously operating large plants will normally have one installed spare hydraulically identical to the running pump(s) but possibly with a turbine drive. Small or batch plants may have uninstalled spare pumps: depending on the consequences of the absence of the pumping process for the period necessary to change the pump with the maintenance facilities available. Slip-plating provision (spectacle plate or slip ring for large bore piping, possibility of springing for smaller piping, or blanking) is required for maintenance isolation. Strainer/filter for pump protection from process contaminants and mechanical debris. Unless the process is known to be dirty or there is a permanent source of maintenance debris immediately upstream of the pump, such as a distillation column with bolted internals, this strainer would be a temporary one used for initial commissioning and after major maintenance work upstream. Strainer openings of 5 mm are adequate for mechanical protection. Valved vent and drain connections to fill and empty the pump. These are provided on the piping where the pump is self-venting and/or draining. Where they discharge a local flammable zone may be created. Heat input from the pump or from the surroundings is a potential source of vapor at the pump inlet when liquefied gases are handled at low pressure and sub-atmospheric temperature. Circulation lines to avoid 'dead legs', thermal insulation and chilling circulation may need to be employed. Provision for seal environment auxiliaries such as heating, cooling, quench, fluid, barrier (see Clause 7). Provision for wear ring flush on coking duties. Provision of nitrogen blanketing to prevent air ingress on vacuum duties (particularly standby pumps).



6 LAYOUT There may be a Zone 1, typically 0.3 m radius local to the gland and this can affect the motor classification in the case of close coupled pumps. 7 SHAFT SEALS 7.1 Selection Shaft sealing shall be by mechanical seals in accordance with the following guidelines: (a) A single mechanical seal shall be supplemented by a secondary lip seal or

a throttle bush. (b) Where the pumped liquid is below 0oC or has an atmospheric boiling point

below 0oC an inert dry gas or liquid quench should be provided to exclude atmospheric moisture or remove ice deposits.

(c) Where pumping temperature is above flash point, but not higher than 30°C

above the initial boiling point and the IBP is above 17.5 oC a single mechanical seal should be employed and the space between mechanical seal and secondary seal should be drained by a local drain pipe to limit spray from a throttle bush.

(d) Where pumping temperature is higher than 30°C above IBP or the IBP is

17.5 oC or below:

(1) Where the seal chamber can reliably be maintained at least 10°C below the boiling point at seal chamber pressure, tandem seals should be employed.

(2) In other cases double back to back seals with a barrier liquid

circulating between them at a pressure at least 1 bar above seal chamber pressure should be employed.

(e) When the temperature is higher than 10° below Auto Ignition temperature

a single seal with continuous steam or inert fluid quench to exclude air or cool dilute leakage should be provided.

(f) Hydrocarbon liquids containing waxes or heavy tars should be provided

with a quench to melt off deposits at the seal and with provision to inject a solvent into the seal chamber at pump shutdown.



7.2 Engineering of Seals 7.2.1 Seal Components Seals designed for low viscosity liquids and high pressures may have a greater than normal hydraulic balance, this may lead to noticeable leakage when running on test with water as the face separation is higher. At low temperatures PTFE wedge secondary seals may be prevented from following up due to contraction, so 'O'-ring secondary seals are commonly used. LPG is particularly likely to gas off between seal faces leading to dry running. Seal face materials should be selected on the basis of resistance to dry running, e.g. Carbon/Solid Tungsten Carbide. A method for assessing the required hydraulic balance is described in Appendix A. On high temperature duties metal bellows seals avoid the problem of suitable secondary seal materials. Lip seals require a hardened surface in contact with the lip and a lead in to reduce fitting problems (lubrication and possibly a follow-up fitting sleeve may be required to prevent the lip seal reversing as it is slid along the shaft sleeve). 7.2.2 Seal Systems Barrier liquids for tandem seals should have: (a) Low pour point/freezing point (b) Low viscosity (c) Good lubricating properties (d) Low vapor pressure. Liquids commonly used are isopropyl alcohol, white mineral oil, ethylene glycol/water mixtures, methanol.



The external tandem seal may be a gas seal where the shaft speed permits. Where the liquid pumped is LPG the space between the seals requires to be purged to avoid the presence of damp atmospheric air in an area where sub-zero temperatures may be reached due to flashing of leakage. This arrangement avoids the relatively costly and complicated circulation system with its attendant Instrumentation, Maintenance and operator attention demands. Pumps on medium and heavy bottoms duties should have cyclone separators fitted to circulation lines for seals (to separate out thermal degradation products). Vertical pumps on LPG should have a permanently open vent line (with restriction) to vent accumulated vapor. 8 CONSTRUCTION FEATURES 8.1 General The general requirements for the pump are given in GBHE-EDG-MAC-1014. This modifies API 610 which is specifically concerned with (oil) Refinery pumps. Particular attention is paid to: (a) Bearing arrangements since the failure of radial bearings will lead to

disturbance of all shaft sealing. (b) Some form of secondary seal to limit leakage to atmosphere from the

primary seal. (c) Auxiliary piping connections and plugs. API practice is to use simple

threads for small connections. Such connections have historically been a source of trouble because of leakage and mechanical failure and it is GBHE policy to avoid their presence where practicable and where they are present to seal weld where possible. This topic is dealt with in GBHE-EDG-MAC-1014

(d) Venting of seal chamber. Increasing attention to leakage from flanged joints has led to the conclusion that, for spiral wound gaskets, carbon fibre filling, coupled with adherence to the Defined joint face finish, and careful control of tightening, are necessary to make good joints with negligibly small leakage on LPG systems.



Small internal running clearances lubricated by low viscosity process liquids on multistage pumps may not control shaft movements as well as those on higher viscosities. The film thickness is also less, leading to increased possibility of metal to metal contact and to wear, thus decreasing the natural vibration frequency when the support due to hydrodynamic bearing action is lost. In the event of components touching due to thrust bearing failure it is preferable for contact to occur inside the pump and before the seal faces open. Cooling and lack of air, compared to contact in the vicinity of the seal combined with increased probability of leaking due to shaft movement, result in a safer situation until the fault is discovered. 8.2 Effects of Low Density Pumps designed for use on low density when tested on water: (a) Will produce a higher pressure (b) Will require higher power (c) May have significantly different thrust loading (d) If close radial clearances are present inside the pump may have shaft

movements more effectively controlled. These differences may require special test procedures such as: (e) Use of larger motor for test (f) Additional casing design pressure (g) Reduced speed running (h) Testing on low density/viscosity liquid



9 MATERIALS OF CONSTRUCTION 9.1 Process Wetted Parts Pressure Containing Parts are normally of Carbon steel (or Nodular Cast Iron). Where LPG is concerned the material should be suitable for the lowest achievable temperatures on de-pressurizing, i.e. the atmospheric boiling point. Although this temperature is only attainable on de-pressurizing it is considered that rapid re-pressurizing could take place with the casing still cooled. Aluminium is not be used as, although it has good low temperature properties, it is damaged too readily in a fire. Wetted parts other than pressure containing parts are typically Cast Iron, or, if pumping temperature is above 230°C, 12% Cr. 9.2 Mechanical Components Shaft guards, flingers etc should be either considered disposable, if of low melting point material, or of steel, as fires in the vicinity of pumps are not uncommon. Items such as gear cases in Aluminium may be considered disposable compared with the cost of one off steel replacements. 'Non spark' materials are not required. It is more important to prevent contact between stationary and rotating parts since in the event of contact sufficient temperature is generated - up to the melting point of one of the materials - to cause ignition in most cases.



9.3 Non Metallic’s 9.3.1 Resistance of Rubbers to Hydrocarbons Natural rubber: considerable swelling Styrene-Butadiene: considerable swelling Butyl -50 +125°C: severe swelling in some HC's Ethylene Propylene: not recommended Polychloroprene (Neoprene) -20 + 130°C: good resistance to swelling in oils Nitrile -20 +120°C: can have high swelling resistance to

oils if it is high in acrylonitrile Acrylic -20 +120°C: excellent resistance to sulfur

containing oils Polysulfide -70 +90°C: excellent resistance to oils if it

contains high sulfur Polyurethanes -70 +90°C: good resistance to mineral oils Silicone poor swelling resistance Fluoro-rubber (Vi ton) +250°C excellent resistance to oils Perfluoro elastomer (Kalrez) +300°C better chemical resistance than Viton



9.3.2 Use of Elastomers with Hydrocarbons for Pumps

A Recommended - little effect B Minor to moderate effect C Moderate to severe effect U Not recommended

Notes: Properties depend on constituents, whether of manufacture and form of component. Elastomers have a tendency to absorb LPG (e.g. ethylene at -100°C, Propane at -40°C). On depressurizing the absorbed liquid vaporizes causing disruption of components such as O-rings which exhibit a crumbly appearance. Geo. Angus Sil-80 Silicon rubber has proved resistant to this.



APPENDIX A BARNARD & WEIR SEAL THEORY [9] A recently proposed theory, as yet without full theoretical backing, that describes observations on long lasting unfailed seals with some success. The theory is that thermal face distortion leads to a central face band at near critical pressure and temperature and side bands adequate, in combination with thermal properties and geometry of faces to permit heating and cooling, to satisfy the required boundary conditions. The three bands or tracks are as follows: (a) Process side band where, due to viscous shear and increasing heat

loss path, conditions progressively approach the critical pressure and temperature for the sealed liquid.

(b) Central band: a mixed phase region at critical conditions which is stable

under load changes and has the minimum film thickness due to face distortion under temperature profile.

(c) Atmospheric side band where due to reducing heat leak path the

temperature falls until the liquid recondenses to a thin layer whose surface tension opposes leakage.

Key points: (1) Medium hydrocarbons above C7 exhibit the effect most clearly and are

progressively easier to seal. (2) Faces should be of uniform cross-section and have a low coefficient of

thermal expansion to achieve mechanical stability. (3) Hydraulic and spring forces should balance critical pressure applied over

up to 60% of face width. (4) Where critical conditions are not reached a seal may operate as a thick

film leakage path with higher leakage and susceptibility to dirt. (5) Heating may be required to permit critical conditions to be reached on light

hydrocarbons.



(6) If critical conditions vary this will require a different set of conditions for stability.

BIBLIOGRAPHY (1) Hydraulic Institute Standards, 13th Edition 1975, USA. (2) American Petroleum Institute Standard 610: Centrifugal Pumps for

General Refinery Services, 6th Edition 1981. (4) Applied Hydrocarbon Thermodynamics, Wayne C Edmister, Gulf

Publications 1961. (5) Data Book on Hydrocarbons, J B Maxwell, R E Krieger (NY) 1977. Also

has Heat Loss by Radiation and Natural Convection (Section 12). (6) Chemical Engineers Handbook, Perry/Chilton, McGraw. Section 3,

Physical and Chemical Data. (7) International Thermodynamic Tables of the Fluid State, Ethylene 1972.

Butterworth. (8) Chemicals from Petroleum, A L Waddams, Murray 1962. Introductory

survey of Processes, Raw Materials and Derivatives. (9) A Theory for Mechanical Seal Face Thermodynamics, Barnard & Weir.

BHRA Conference on Fluid Sealing, Durham, September 1978, paper HI. (10) Health Aspects of Lubricants, Concawe Report 1, 1983.



DOCUMENTS REFERRED TO IN THIS ENGINEERING DESIGN GUIDE This Engineering Design Guide makes reference to the following documents: AMERICAN PETROLEUM INSTITUTE API 610 Centrifugal Pumps for General Refinery Services (referred to in Clauses 2.6 and 8.1). AMERICAN SOCIETY FOR TESTING AND MATERIALS ASTM D86 Distillation of Petroleum Products (referred to in Clause 2.4.1) ASTM D2892 Distillation of Crude Petroleum (referred to in text with Table 2A. Page 10). ENGINEERING DESIGN GUIDE GBHE-EDG-MAC-1014 Integration of Special Purpose Centrifugal Pumps

into a Process (referred to in Clause 4). GBHE-EDG-MAC-1117 Special Purpose Centrifugal Pumps (referred to in

Clause 8.1).

