Pumps for Hydrocarbon Service

GBH Enterprises, Ltd.

Engineering Design Guide:
GBHE-EDG-MAC-1508

Pumps for Hydrocarbon
Service

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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

23

7.1

Selection

23

7.2

8

SHAFT SEALS

Engineering of Seals

24

25

8.1

General

25

8.2

Effects of Low Density

26

MATERIALS OF CONSTRUCTION

26

9.1

Process Wetted Parts

26

9.2

Mechanical Components

27

9.3

9

CONSTRUCTION FEATURES

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, Isobutene (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
Gas and liquefied gas
Gasoline (petrol)
Kerosene
Gas oil, diesel oil
Lubricating oil
Fuel Oil

Boiling Range,
Deg. C
up to 25
ca. 20 - 180
ca. 175 - 275
ca. 200 - 380
very variable

Bitumen, Coke

No. of Carbon Atoms
C1 - C4
C4 - Cll
C4 - C16
C15 – C25
C20 – C70
C10 upwards
To C70+
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
0
40
100
200

2.7

V.P. bara
0.5
0.2
0.15
0
0

1.0

2.0

5

10

0.5
0.3
0.24
0.2

0.9
0.7
0.5
0.4

2.4
1.8
1.2
0.9

3
3
2.5
1.8

Velocity of Sound

Representative values are:

(meters/sec at 20 - 30°C)

1050
1085
1295
1278
1315
1670
1930

n-Heptane
n-Hexane
Benzene
Cyclo-Hexane
Kerosene
Ethylene glycol
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:
IBP
RVP
SG
Viscosity

Pour Point
Flash Point

Arabian Light
46 deg. C
3.5 psig
0.8524
5.5 cS @
100 deg. C

North Sea
92 deg. C
NA (low)
0.9847
10,000 Redwood
Secs No 1 @ 100
deg.F

Ninian
33 deg. C
8.9 psig
0.8459

- 26 deg. C
NA

- 4 deg. C
198 deg. F

- 12 deg. C
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-EDGMAC-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 subatmospheric 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 GBHEEDG-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
B
C
U

Recommended - little effect
Minor to moderate effect
Moderate to severe effect
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).


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