This document discusses liquid containment and mechanical shaft seals for pumps. It describes the importance of liquid containment to prevent pollution. It then discusses different types of shaft seals including packing, lip seals, single and dual mechanical face seals, and cartridge seals. It provides details on selecting the proper seal based on factors like liquid viscosity, temperature, pressure, and compatibility with elastomers. API specification 682 for mechanical seals in refinery pumps is also summarized.

1. Liquid Containment
By Joseph M. Sutula
 The Importance Of Liquid Containment
 Supplying Information To Your Pump Representative
 Types of Shaft Seals
 Match The Seal To The Environment
 Identify Potential Leak Causes
The Importance Of Liquid Containment
The overall effects of industrial pollution may be controversial depending upon our
personal beliefs, but there is concrete evidence that life on earth can suffer from
pollution in air, ocean, or ground water. In light of the increasing emphasis on
decreasing leakage of pollutants in the industrial setting, there is a need to become
informed about sealing equipment under our control. Because pumps are generally
considered to be more expensive than the piping, valves, and related accessories
surrounding them, pumps are higher profile when they leak. Seal manufacturers are
gearing up to meet the challenge but have thus far focused on centrifugal pump
principles because their numbers dominate the market. Positive-displacement (PD)
pumps may pose special sealing problems because they have been designed to
handle viscous, non-Newtonian, or other problem fluids under a wide range of
physical conditions. Many factors must be considered in order to arrive at the
proper sealing solution.
Supplying Information To Your Pump Representative
Liquid Name and Synonyms
It is important to give the proper name of the liquid, as well as any synonyms, to
your manufacturer's representative. If the name must remain proprietary, then
supply at least a family description; for example, acidic or alkaline, starch or salt.
Oftentimes furnishing initials alone can be confusing to nonchemists. PVA is
interpreted by most to mean polyvinyl alcohol while polyvinyl acetate is generally
represented by PVAc.
Elastomer Compatibility

2. Time and money can be saved if elastomer and liquid compatibility is known at the
outset. Typically, when little is known about the liquid, more expensive
components will be recommended for sealing than may be warranted simply as a
safety measure. For example, perfluoroelastomers can be applied to liquids pH 2
through 13 at ambient to moderate temperatures and on some heat transfer liquids
to 315°C / 600°F. Fluoroelastomers, at approximately one-tenth the cost, can be
applied to many acids at moderate temperatures and can handle most heat transfer
liquids up to 205°C / 400°F.
Chemical attack on elastomer frequently manifests itself as o-ring swell and/or
shrink. O-ring grooves are usually designed with some extra volume built in to
compensate for swell, but the physical forces available from excess swell cause the
confined o-ring to extrude beyond or break the confining wall. Many flat gaskets
contain elastomers as binders which are also subject to modification or failure
when they come in contact with the wrong liquid environment.
Viscosity
Newtonian liquid viscosity varies with temperature. In general it will become less
viscous as temperature rises and more viscous with decreasing temperature.
Viscosity can vary widely depending upon temperature conditions imposed by
transport, storage and processing.
Mechanical seal efficiency is affected by liquid viscosity. As a rule of thumb, hard
faced (both rubbing faces) should be used when viscosities exceed 5,500 cSt /
25,000 SSU. Performance of other seal components, such as springs and set
collars, can also be adversely affected by high viscosity. Low viscosity, on the
other hand requires different solutions. Water is typically handled very well with
one hard face and one soft. Solvents are less lubricating than water and often
require the use of harder faces to maintain seal face integrity.
It is important to know both ambient and operating temperature viscosity from the
standpoint of transfer as well as sealing of a liquid.
Temperature
Solidification Point
Liquids which are normally solids at ambient temperature must be maintained
within a narrow range of temperature above their solidification points to avoid
damaging seal components.
Physical Phase Change

3. Some chemicals tend to become more viscous, solidify, or precipitate onto seal
faces with changes in liquid temperature. The amount of change in viscosity with
change in temperature will affect decisions regarding seal design selection, as will
the tendency for abrasive precipitates to form.
Pressure
Pressure within a pumping system is a factor in determining whether a balanced or
unbalanced mechanical seal should be used. Single unbalanced friction drive seals
are appropriate for use to about 17 BAR / 250 PSI discharge on lubricating liquids
165 cSt / 750 SSU and are limited to about 6.9 BAR / 100 PSI on viscosities as low
as one cps. Single balanced seals can be applied with nonlubricating liquids to 35
BAR / 500 PSI. Higher discharge pressures can be attained using positive-drive,
single-balanced seals depending upon liquid lubricity, seal face combination, and
seal drive mechanics. The manufacturer engineering department should review all
the conditions before pumps are put into high-pressure service.
Density
Generally liquids whose specific gravity is 0.6 or lower should be handled with a
balanced seal. Liquids in this class are normally extremely volatile and tend to boil
as rubbing friction raises the liquid temperature adjacent to the seal faces. Contact
surface areas (i.e., seal faces) of balanced seals are mathematically designed to
decrease frictional heat, thus increasing application range and seal life.
Vapor Pressure
Vapor pressure at operating temperature is helpful information, especially when
the suction side condition on the pump is a vacuum. A mechanical seal operating at
pressure near the liquid’s vapor pressure, especially at startup, is subject to
flashing. This breaks down the lubricant film and leads to premature seal failure.
The application of a recirculation tube from pump discharge to the seal chamber is
frequently used to alleviate problems of flashing associated with low pressure at
the seal faces.
NPSH Available
Pumps forced to operate under insufficient NPSHa conditions usually exhibit less
than rated flow volume and pressure. They frequently cavitate and vibrate. If a
pump does not operate smoothly due to insufficient liquid, there will probably be
insufficient lubrication at the seal faces as well. High friction and cavitation, or
vibration, at the seal faces can lead to early seal failure.
API Specification 682

4. If API standards are to be the benchmark for specifications, API Standard 682 is
the applicable document to use in specifying mechanical seals for PD pumps. The
most recent document is aimed at the purchase of new pumps for refinery service.
It defines three generic standard seal arrangements and an engineered seal to be
used with an engineered support system for extremes in duty requirements in some
services. API 682 specifies that standard seals, regardless of arrangement, will be
of the cartridge design.
Experience
What has been the user’s experience with the mechanical seal currently in service?
How long has it been in use? If service life has not been satisfactory, what
symptoms have been exhibited? An early seal failure in a new installation may
have been caused by choosing the wrong seal materials or seal arrangement but
might also be a result of damage caused from construction residue left in the
piping.
When there is previous experience, successful or not, the sealing technology
should be reviewed and updated if necessary.
Environmental Controls at Site
Are temperature controls required for the process? Above or below ambient
temperature? Is steam or other energy available for heating? Water or refrigerant
for cooling?
The liquid phase of many tars and asphaltic products can be sealed using low-
pressure steam as a quench or blanket across the atmospheric side of the
mechanical seal faces. A steam quench flushes away weepage and contributes heat
to the seal faces to ensure that product on the faces is in a liquid state while
running or at startup.
Products for heat transfer are handled through a wide range of temperatures.
Cooling may be required to avoid overheating the pump bearings above 290°C /
550°F .
The existence of mechanical or electronic devices for disposal or detection of
weepage or catastrophic leakage should be noted so that any update in sealing can
be interfaced with existing technology.
Types of Shaft Seals
Packing

5. A remarkable number of pumps sealed with packing are still in use today. Modern
materials used in manufacture of packings include inert substances like PTFE and
carbon graphite, metal foils, and flax. Manufacturers will provide
recommendations for tailoring the packing type to the application. Keys to
successful operation include the use of a hardened or coated shaft through the
packing box area, a proper break-in period, and good adjustment of the packing
gland during run time.
Packings pick up dried material and other particles and act as an abrasive wheel
against the shaft. Once the shafting is grooved, packing failure is imminent.
Hardened shafting resists wear and prolongs packing life.
Packings "leak" to allow the product to lubricate the interface between shafting and
packing, especially during initial pump startup. Initial adjustment varies with
packing type, but generally means to run the pump for 30 minutes with the packing
gland loose enough to allow several drops per minute to leak. Every fifteen
minutes thereafter, tighten each gland bolt one-sixth of a turn until leakage is
minimized and the packing box temperature is stable. Ideally, product should
accumulate under the packing gland but should not drip.
Lip Seals
Hydraulic gear pumps must be
considered to be unidirectional pumps
when they use lipseals to retain fluids.
The chamber in which the lipseal is
mounted is vented to suction port
pressure, generally atmospheric
modified by the liquid head in the
reservoir above or below the pump.
When these same pumps are applied to
industrial applications, they must also
be unidirectional or employ more
sophisticated sealing principles.
Recently, multiple lipseals mounted in a gland on a sleeve have been applied
against a variety of viscous nonabrasive liquids successfully. Lip design and
reinforcement allow operating at pressures to 10 BAR / 150 PSI without
supporting fluid pressure behind the lips. Gland ports provide access to interior
passages for cooling the lips or to pressurize internally (Figure 1).
Mechanical Face Seals
Single Seals
Figure 1. Multiple Lip Seals.

6. Single seals are classified as pusher or nonpusher
types with single-spring or multiple-spring
subclassifications. They can be friction driven or
positively driven. Figure 2 is an example of a single
nonpusher mechanical seal. Nonpusher seals are
characterized by a shaft gasket or bellows that
remains stationary with respect to the shaft. A
tightly fitted rubber bellows is spring loaded against
a soft face. All components are enclosed within a
metal retainer and turn with the shaft against a
stationary face. As the soft face wears, the flexible
rubber bellows extends to maintain sealing contact
with the rotating face.
Figure 2. Single Nonpusher Seal.
Figure 3 is an example of a single pusher
mechanical seal. Whereas in a nonpusher seal the
shaft gasket does not move in relationship to the
shaft, in a pusher seal, the shaft gasket must
move. As the seal faces wear, a spring pushes the
shaft gasket forward to maintain contact with the
faces. Pusher type seals can begin to leak when
normal product leakage combined with face wear
builds up to cause the shaft gasket to "hang up".
Figure 3. Single Pusher Seal.
Friction drive seals can be applied to 17 BAR / 250 PSI on lubricating liquids and
are capable of handling viscosity to 3,300 cSt / 15,000 SSU.
Positive drive seals are locked to the shaft
by set screws. They utilize o-ring, vee-ring,
or similar configuration to seal along the
shaft and against the rotating face. The
stationary face is usually fitted with an
antirotation device, depending upon face
configuration and material of secondary
seal. Mechanical links between the seal and
the shaft, and among components of the
rotating portion, put the seal in the positive
drive classification (Figure 4). Set screw
drive can be used in viscosities to 5,500 cSt
/ 25,000 SSU.
Figure 4. Positive Drive Seal.
Component Double Seals

7. Classifications of the double seals are
the same as for the single seals. They
are simply configured with two
complete units mounted back to back
(Figure 5). Because they are positioned
back to back, the annular space internal
to the two sets of seal faces must be
hydraulically pressurized in order to
close both sets of faces. For best results, the pressurization liquid must be a
lubricant and a barrier to the product. Internal pressure between seals is normally
maintained at .7 to 1.0 BAR / 10 to 15 PSI higher than adjacent atmospheric or
product side pressure. Thus, the film on the seal faces necessary for lubrication will
be the barrier liquid. This arrangement can be used to seal noxious liquids,
hazardous liquids, or those too viscous for a single seal to handle; because all the
seal components are rotating in a "friendly" environment. The barrier liquid
protects the seal faces while excluding product and atmosphere. Viscosity limits
are not known. Friction drive double seals have been successful in 33,000 cSt /
150,000 SSU viscosity liquid, and set screw drive-type doubles can be applied to
55,000 cSt / 250,000 SSU. Pressure on a good lubricating barrier liquid between
the seals can be up to 17 BAR / 250 PSI.
Abrasive Liquid Seals
Pusher: A pin-driven, single-spring, pusher
seal with silicon carbide faces is available
with standard fluoroelastomer or optional
perfluoroelastomer secondary seals (Figure
6). This positively driven seal is cataloged
for paints and inks with viscosity to 16,500
cSt / 75,000 SSU and has successfully
handled caulking compounds which are far
more viscous.
Figure 6. Pin-Driven Abrasive Seal.
Figure 5. Component Double Seal.

8. Nonpusher: Hard-face metal bellows seals
fitted with solvent-resistant secondary o-
rings have extended the life of tacky
adhesive pumps in the plywood fabrication
industry (Figure 7). Similar seals are being
used for metallic oxide coating found
on videotapes.
Figure 7. Hard-Face Metal Bellows Seal
Dual Seals
The term "dual seal" is actually the
modern description of two single
mechanical seals mounted on the same
pump shaft and should be qualified by
adding the mounting description
"double" or "tandem." Tandem seals are
oriented in the same direction
hydraulically, and double seals are mounted back to back. Both arrangements
require the use of barrier/lubricants, but only the double must be pressurized. For
tandem dual seals, only the product side seal rotates in the product, but product
weepage across the seal faces will eventually contaminate the barrier lubricant
(Figure 8). The consequences of this contamination upon the atmospheric side seal
or the surrounding environment depend upon the nature of the product.
Cartridge Seals
Cartridge seals are complete single,
double, or tandem mechanical seals
contained within a gland and built onto
a sleeve (Figure 9). In the past, most PD
pumps had to be modified to
accommodate cartridge seals. Recently,
seal vendors have manufactured
cartridges to fit the special shaft
diameters and specifically designed
stuffing boxes of PD pumps.
Figure 8. Tandem Dual Seal.
Figure 9. Cartridge Single Seal.

9. Single cartridge seals will operate within roughly the same physical parameters as
component positive-drive mechanical seals. They can handle viscosities to 7,700
cSt / 35,000 SSU, pressures to 20 BAR / 300 PSI, and temperatures to 205°C /
400°F.
Most dual cartridge seals can be treated
as either true double seals or as tandem
seals, because each is a mathematically
balanced seal (Figure 10). The use of a
barrier lubricant between seals is
required. Lubricant pressure applied
internally dictates whether product or
barrier liquid will be on the product side
seal faces. Pressure is in general limited
to 20 BAR / 300 PSI. It is safe to
assume that dual seals can be
successfully applied to viscosities approaching 9,900 cSt / 45,000 SSU at reduced
speeds and temperatures to 205°C / 400°F. A review of components, secondary
seals, and metals may allow application of the cartridge seal at higher
temperatures; however, the proximity of antifriction bearings may call for cooling
of the barrier/lubricant to assure acceptable bearing life.
API Specification 682
To paraphrase a John Crane International publication, API 682 is a refinery
specification. It defines three generic arrangements:
 Arrangement 1 - Single Seal
 Arrangement 2 - Unpressurized Dual Seal (Tandem)
 Arrangement 3 - Pressurized Dual Seal (Double)
Three types of mechanical seals are available to satisfy these arrangements: a
rotating pusher, a rotating metal bellows (i.e., nonpusher), and a stationary metal
bellows. For extremes in duty services, reference is made to an engineered seal
(ES) and an engineered sealing support system for some services.
Gas Barrier Seals
Gas barrier seals, the latest technology in mechanical face seals, are undergoing
extensive lab and field testing. These are cartridge-style dual seals with faces
especially designed to be pressurized using an inert gas as a barrier between
product and atmosphere. The gas replaces traditional lubricating liquid.
Figure 10. Cartridge Dual (Double) Seal.

10. In one version, the seal faces separate as soon as the pump shaft starts to revolve.
The faces do not come in contact with each other again until the rotation stops. In
another version, seal faces remain in light contact during normal operation. Current
gas barrier seal designs allow a small amount of gas to escape in the product and
for some to escape to atmosphere.
Mag Drive Coupling Units
Magnetically driven (mag drive)
pumps offer positive sealing of
hazardous or difficult-to-contain
liquids. Magnets mounted
radially around the pump drive
shaft are surrounded by a close
fitting canister which contains
the product circulating through
the pump. Magnets attached to
the inside wall of a hollow
cylinder affixed to the drive shaft
and situated around the canister
set up a magnetic field. When the
drive shaft rotates, the field
compels the pump shaft to rotate.
The canister wall is not
penetrated by either shaft and is
statically sealed at its interface
with the pump housing (Figure
11).
Product must be circulated
through the canister to cool the
magnets. Thus, consideration
must be given to the chemical
compatibility of the product with the magnets, canister, and canister gasket. O-
rings are normally used as canister gaskets to allow for a wide range of material
compatibility. Canisters can be formed from corrosion resistant alloys and coupling
magnets are encapsulated with inert materials.
Mag drive pumps are designed to handle corrosive and hard to seal liquids. For
example, sulfuric acid and sodium hydroxide are common corrosives handled with
magnetically driven pumps. Hard to seal applications include isocyanates.
Match The Seal To The Environment
Corrosive Liquids
Figure 11. Mag Drive Coupling Units

11. Mechanical seal metal parts are relatively thin and are more vulnerable to corrosive
attack than thicker pump walls. For maximum life cycle performance, use seal
components that are as inert as possible in corrosive applications. The seal metals
should be at least as noble as the pump, if not more so. Springs or metal bellows
are especially susceptible to corrosion because of their thin cross sections and large
surface areas. Finally, 316 stainless steel metal parts and Hastelloy-C® springs are
becoming the standard for cartridge mounted or component screw drive seals.
The use of silicon carbide as a material for at least one seal face has helped to
minimize corrosion damage to that area. Some silicon carbide grades are resistant
to pH in a range from 1 to 13 at moderate to fairly high temperatures.
Secondary seals receive the most attention of any seal component because
corrosion on them generally leads to failure of other components. Fortunately,
secondary seal corrosion is easy to detect, even in early stages.
Much has been published by the major manufacturers of sealing products rating the
resistance of elastomers and other components when immersed in various
chemicals. Corrosion rating for metals, elastomers, and plastics can be found from
NACE, most seal manufacturers, and many pump manufacturers.
Abrasive Liquids
Abrasive liquids pose difficult challenges for mechanical seals. The abrasive effect
is lessened with small particles or if a viscous matrix surrounds the particles.
Abrasive liquids are commonly handled with PD pumps at slow rotational speeds
with positively driven, hard-faced mechanical seals. Some applications include
caustic precipitates, coatings, inks, and oxide slurries.
Caustic Precipitates: Certain solutions tend to precipitate crystalline solids on the
atmospheric side of the seal faces. These crystals then grow near the atmospheric
interface on the sealing surfaces eventually forcing the seal faces open and
separating the sealing surfaces. For example, sodium hydroxide (i.e., caustic soda)
poses this challenge. And because it is also corrosive, weepage across the seal
faces can extract a costly toll in pump shafts, bearings, and other parts. A dual seal
can be used with low-pressure water flowing through the cartridge, between the
seals, and out to a safe drain.
Coatings: Dried paint forms a hard shield because of abrasive whiteners or other
tints in adhesive vehicles. Secondary seals are selected based on the chemical
formula of the coating and by considering the cleaning solvent. Hard faces coupled
with a positive drive allow a single pusher seal to handle coating viscosity up to
16,500 cSt / 75,000 SSU.

12. Inks: Thousands of gallons of ink per day are spread upon newsprint every day.
Abrasive metal tints and carbon black suspended in natural oils or with fast-dry
solvents are applied by high-speed presses fed by PD pumps. Abrasive liquid seals
similar to those in use in the paint industry are successfully applied in formulating,
transporting, and printing with inks. Printing inks are often pseudoplastic; they are
quite viscous at rest, but thin out in the pipeline as they begin to move.
Oxide Slurries: Slurries of finely ground metal oxides suspended in fast dry
solvents are coated onto mylar tapes to record audio or video signals. Metal
bellows seals with hard faces and perfluoroelastomers offer thousands of hours of
performance in abrasive-fitted pumps.
Viscous Liquids
Single-friction drive seals can handle viscosities up to 3,300 cSt / 15,000 SSU. The
viscosity-handling capability of positive-drive single seals increases to 5,500 cSt /
25,000 SSU by set screw type, double if pin driven. Vendors of cartridge-mounted
mechanical seals currently suggest their products be limited to 7,700 cSt / 35,000
SSU.
Component double seals are successfully applied to viscous resins and caulks. A
buffer liquid must be present on the sealing faces to exclude the viscous product
and provide a compatible environment in which the seals can rotate. Buffer liquid
systems can be equipped with devices which will set off an alarm or shut off the
pump in case a change occurs in the seal chamber, signaling an incursion of
product or a catastrophic leak.
A multiple-lip cartridge seal design is gaining acceptance for handling viscous
materials; for example, polyester resin 44,000 cSt / 200,000 SSU at 150°C / 300°F.
Several successful clean asphalt applications have also been reported. Many
installations have succeeded with no support fluid system necessary. This sealing
technology is not recommended for abrasive liquid service.
Thin Liquids
Many thin liquids, including solvents such as water, can be handled by PD pumps
with unbalanced single seals. The secondary seals must be selected with product
temperature in mind. Also, mechanical seal manufacturers recommend product
temperatures adjacent to the seal faces be maintained at least 10°C / 50°F below
the boiling point of the liquid to keep the liquid from flashing across the seal faces.
Hot Liquids
High temperatures affect seal components. For example, mechanical seal faces
become distorted, uneven expansion between different materials allows

13. interference fits to loosen, secondary seals degrade, and springs tend to relax. A
variety of properties contribute to the difficulty of sealing hot liquids, including:
 The liquid may be a thermoplastic - solid until heat is applied, liquid
once heated, solid again as it cools.
 The liquid may be thermosetting - it polymerizes and eventually
solidifies as it is heated.
 Heat transfer mediums become thinner and their vapor pressures
increase.
 Some liquids, like starches, become more viscous with temperature
increase.
 Solvents may evaporate leaving sticky or solid residue.
Thermoplastics must be maintained at temperatures above their solidification
point. Tars and asphaltic compounds can be sealed with cartridge-mounted metal
bellows seals using a low-pressure steam quench adjacent to the atmospheric side
of the seal faces. If steam is not available, a double seal pressurized with a hot heat
transfer liquid is appropriate.
Thermosetting liquids, such as phenolics, polymerize when heated, then become
solids. Consideration must be given to the flush cycle and its compatibility with the
mechanical seal.
Heat transfer liquids are useful for temperatures ranging from below zero to
450°C / 850°F, depending upon their chemical composition. They vary in makeup
from petroleum based to synthetics. Some are solid salts at room temperature and
must be thoroughly liquefied before pump startup. All have vapor pressures lower
than water at any given temperature, which is the property that most makes them
useful.
Stuffing box-mounted component seals, pusher, or metal bellows with
fluoroelastomer secondary seals, and a clamped-in stationary face are serviceable
to 205°C / 400°F without cooling. The same arrangements for perfluoroelastomers
are serviceable to 290°C / 550°F without cooling. Above 290°C / 550°F a cooling
device is required to ensure the thrust bearing will not overheat because of its close
proximity of the stuffing box. A single component seal, pusher or metal bellows
fitted with perfluoroelastomers and a cooling collar can be used to 450°C / 850°F.
Adhesives
Cooked starches used as adhesives are generally handled at temperatures less than
95°C / 200°F and can be handled with a single nonpusher (i.e., rubber bellows)
mechanical seal, carbon versus Ni-Resist faces, and with nitrile elastomer
secondary seals. Pump shaft speeds should be reduced from motor speeds.

14. Solvent-borne ureas are used in wood fabrication. These adhesives are hot, tacky,
and viscous. Nonpusher metal bellows seals with hard faces perform well at low
pump shaft speeds. Perfluoroelastomer secondary seals are required to resist the
elevated temperatures and fast-dry solvents. Cartridge-mounted stuffing box dual
seals using an unpressurized buffer liquid solvent could also be used, but the
bearings and pump shaft need to be hardened.
Cold Liquids
Component stuffing box seals with PTFE secondary seals and clamped-in
stationary faces are applicable to temperatures of -67°C / -90°F in stainless steel
PD pumps. The stationary seat is held in place with a gland machined with ports
for vent/drain, outboard the seat. A blanket of nitrogen gas, or dry air, in the vent /
drain area will prevent ice from forming at the seal interface. O-rings of
fluorosilicone elastomer can be used to -73°C / -100°F if liquid is chemically
compatible.
Volatile Liquids
Refrigeration ammonia is a noxious nonlubricating gas liquefied under pressure.
Pressurized double seals are required for sealing circulating systems. Petroleum oil
presents a nearly impermeable barrier because oil and ammonia are immiscible.
Select oil viscosity to be compatible at the pumping temperature of the ammonia.
Pump gaskets used to seal liquefied gasses should be o-ring design in order to
contain vapors during operation. The o-ring design will also contain pressures
associated with increase in product temperature. Simple transfer operations can be
accomplished with single seals.
HCFC and CFC refrigerants can be transferred with single seals, preferably a
balanced design, because of the high vapor pressure of these liquefied gasses. Over
time, significant quantities of these solvents can escape through elastomeric
gaskets if the correct formulation is not applied. Circulating systems should be
designed with this fact in mind.
Most solvents, including water, can be handled and sealed within PD pumps.
Remember that vapor pressure and liquid solubility increase as liquid temperature
increases.
Underwriter's Laboratory (UL) Requirements
A number of pumps are constructed to handle certain hazardous liquids requiring
certification by the Underwriter’s Laboratory. Inventories of manufactured and
purchased component parts (mechanical seals, o-rings) are regularly checked by
UL inspectors. Suppliers of purchased components must submit their product to
UL for certification and host random onsite inspections. Each pump assembled

15. must pass a physical test established by Underwriters Laboratory before it can
receive the UL Label.
High Vacuum Applications
PD pumps can evacuate still bottoms from a high vacuum if there is enough
physical head afforded by the liquid column on the suction side of the pump to
overcome pipe friction loss and the system vapor pressure. Pressurized double
seals should be used to maintain the integrity of the vacuum. Cartridge dual seals
may be applied when suitable for the product viscosity. PD pumps are not designed
to produce the system vacuum or draw off the condensate.
Identify Potential Leak Causes
Dry Run
A high pressure differential between the product side of the mechanical seal to the
atmospheric forces a thin film of product to spread across the surface of the
rotating face. Rubbing friction of the seal faces generates heat. Surround the seal
with product to help dissipate that rubbing heat. If heat is not removed or if no
lubricating film exists on the seal faces, damage can occur, even in a short period
of time. Thermal distortion of either face promotes uneven face wear, since only
the "high spots" are in contact. Eventually, openings at the product edges allow
entry of any substances in the area, including damaging particles. A rapid change
in temperature of 95°C / 200°F can fracture a seal face from thermal shock.
Dry Startup
Startup of the pump with no liquid at the inlet port may result in seal damage.
Damage is minimized, however, if liquid reaches the mechanical seal within thirty
seconds or if a film left over from shutdown exists between the seal faces.
In some applications, the pump must be drained between runs. To avoid seal
damage from repeated dry startups, fit the pump with a cartridge dual seal and a
reservoir mounted on the pump base pressurized to supply lubricant to the seal
faces. This arrangement allows the pump elements and piping to be drained with
no damage occurring to the mechanical seal.
Inadequate Pressurization
Double seals must be pressurized and the normal pressure level recommendation
should take into account system relief valve and other downstream pressure
variations. Seal chamber pressure should be maintained at a level higher than the

16. pump relief valve setting because an increase in product viscosity or a valve failure
can cause dramatic increases in downstream pressure. Moreover, seal faces must
remain closed, otherwise barrier liquids may become contaminated or lost.
Handling a volatile liquid at a temperature too close to its boiling point may cause
"flashing" of vapor across the seal faces. Vapors do not lubricate the seal faces
adequately resulting in diminished seal life. One solution is to direct a flow of
discharge into the seal chamber to increase the pressure surrounding the seal. If the
pressure increase is high enough, the product’s boiling point may change enough to
prevent "flashing."
System Test With Improper Fluid
New systems are often tested for joint and seal leaks prior to startup with fluids
which are easy to detect. The test fluid must be lubricating as well as chemically
compatible with the mechanical seal components if the pump is to be operated.
Water is relatively inexpensive and considered to be inert, but it is not a good
lubricant and can corrode steel piping. CFCs and ammonia leaks are readily
detectable but are not compatible with all sealing elastomers and are not lubricants.
Double seals must be pressurized prior to the start of testing as if in an actual
production situation.
Avoid tests with nitrogen or steam unless the pump can be bypassed. Neither has
lubricating qualities, and elastomer compatibility must be considered with steam.
Liquid containment is a growing concern in the fluid-handling industry. Please
remember this is a basic introduction and engineers are strongly encouraged to
discuss liquid containment options with the pump manufacturer's representative.
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