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DIFFICULT PUMPING APPLICATIONS
SELECTING SEALLESS PUMPS AND CIRCULATION SYSTEMS
Ron Forsberg, Sr. Project Engineer, Sundyne Corporation
A sealless pump is generally used when there is a need to contain toxic,
dangerous or valuable fluids or where specific applications warrant their use.
The application of the pump may be dictated by environmental, safety, noise or
space concerns.
Where Are Sealless Pumps Used?
• Hazardous/Lethal Fluids
• Heat Transfer Fluids
• Expensive Fluids
• Fluids Requiring High Purity
• Fluids With Foul Odors
• Volatile Fluids - Solvents
• Molten Solids
• Monomers & Polymers
• Replacement of Expensive Multiple Mechanical Seal Systems
The rotating members are driven by a rotating magnetic field that is transmitted
through a primary containment shell or liner. Two drive methods are used;
Magnetic Drive Pumps (MDP) and Canned Motor Pumps (CMP). In both methods, a
portion of the pumped fluid (pumpage) or an externally supplied flush fluid is
used to cool the drive mechanism and lubricate the bearings.
What Are Magnetic Drive Pumps And When Do They Apply?
The Magnetic Drive Pump (MDP) utilizes an outer ring of permanent magnets or
electromagnets to drive an internal rotating assembly consisting of an impeller,
shaft and inner drive member (torque ring or inner magnet ring) through a
corrosion-resistant, non-magnetic containment shell. They are used when primary
containment is a must and secondary control is desired. See figure A.
• Mild Acids
• Solvents
• Heat Sensitive Liquids
• Liquids Containing Moderate Solids
• Fluids Presenting Difficult Sealing Challenges: Dissolved Solids That
Precipitate Out Of the Solution.
• High Melting Point Liquids & Heat Transfer
What Are Canned Motor Pumps And When Do They Apply?
The Canned Motor Pump (CMP) has a common shaft to link the pump and motor in
a single contained unit. The pumped liquid or an external flush is circulated
through the motor but is isolated from the motor components by a
corrosion-resistant containment shell. The canned motor pump is used when both
primary and secondary containment of the process fluid is a must. See figures B
& C.
• Lethal Services
• Extremely Hazardous Services
• High Suction Pressure Services
• When Advanced Diagnostics is required
• Where secondary containment vs. secondary control is a must!
Two industry organizations, the Hydraulic Institute and the American Petroleum
Institute have recognized the sealless centrifugal pumps as a unique pump type
and have adopted a unique standard or specification for them.
Reference: ANSI/HI 5.1-5.6 American National Standard for SEALLESS CENTRIFUGAL
PUMPS. API-685, SEALLESS CENTRIFUGAL PUMPS (Unpublished)
Both of these organizations have recognized the importance of the drive
circulation systems and have devoted a significant portion of their standards to
describing the types of systems and the importance of each. Both have identified
these circulation systems similar to their seal support and flush plans.
However, neither specification goes into much detail about these plans. They do
supply some graphic representations as shown on the attached figures, but they
do not provide a detailed description or direction. Caution must be used in
referring to these plans as they do not use the same identification system and
not all plans are defined in each standard. The following are excerpts from
those standards.
EXCEPTS FROM HI 5.1 – 5.6 2000 (Used with permission of HI)
5.3.3.3 Circulation piping plan selection
Selection of an appropriate circulation piping plan depends upon knowledge of
liquid properties such as: cleanliness, volatility, specific heat, viscosity,
specific gravity, toxicity, melting point, temperature, corrosive-ness and any
tendency to form solids. The following should also be considered are: flow
rates, NPSHA, frequency of starts, cooling or heating availability and potential
loss of suction liquid. Typical circulation plans are shown in Figure 5.9 and
are grouped by application type considering pumped liquid cleanliness,
volatility and temperature. The manufacturer may also offer additional plans
specific to design and application requirements. A detailed analysis should be
conducted for each application.
5.3.3.3.1 Clean liquids
Clean liquids are those with no solid particles, non-volatile, moderate
temperature, sufficient NPSH and a moderate degree of hazard. This description
fits the majority of sealless pump applications and can be handled by various
circulation piping plans.
5.3.3.3.2 Dirty liquids
Dirty liquids include solid particles. Centrifugal separation, mechanical
filtration, or a separate clean external flush liquid may be used. Also,
volatility and temperature shall be considered.
5.3.3.3.3 High temperature
The temperature of motor windings or magnetic drive components can be controlled
by using a variety of circulation piping plans. Volatility and cleanliness
should be considered in selecting a plan.
5.3.3.3.4 Volatile liquids
Circulation to suction vessel or pressurized circulation may be used to avoid
thermal effect of drive heating on pump NPSH requirements. Consideration of
vapor pressure increase with temperature and of specific heat of liquid shall be
required. Use of a separate low-volatility drive external flush liquid is also
possible. Liquid cleanliness and temperature shall also be considered.
5.3.3.3.5 Liquids that solidify
Jacketed pumps may be required for high melting point liquids and easily
polymerized or crystallized liquids. External flush liquids may also be used.
Figure 5.9 does not include plans for jacketed pumps.
5.3.3.3.6 High viscosity
Viscosities that would cause objectionable drag losses in the drive section or
inadequate bearing cooling flow (generally above 200 mPa-S (200 centipoise)) may
be handled with external flush liquid. Start-up and operating viscosity should
be considered.
5.3.3.3.7 External flush
External flush should be used where there is potential loss of suction, zero
flow, or entrained vapor. Separate compatible external flush liquid supply with
appropriate cooling may be used to provide lubrication and cooling of the drive
section. Precautions still apply for normal centrifugal pump operation.
EXCERPTS FROM API 685 (Unpublished)
U.2: CIRCULATION PLAN SELECTION AND APPLICATION (See Figures D-1 & D-2)
It is recognized that the product lubricated bearing design and application
considerations are essentially the same for canned motor pumps and magnetic
drive pumps. Selection of an appropriate circulation plan depends upon knowledge
of fluid properties such as cleanliness, volatility, specific heat, toxicity,
melting point, and tendency to form solids or polymerize. Also to be considered
are intended operation, flow rates, NPSH, frequency of starts, and cooling or
heating availability. Factors internal to the unit design such as pressures,
temperatures, flows and heat transfer characteristics within the drive section
as well as hydraulic performance of the pump end must be understood in order to
properly select circulation plans and assess application questions. Possible
advantages and limitations of available plans must also be understood. The
circulation plans shown in Figures D-1 and D-2 coupled with detailed knowledge
of individual unit design allow for the handling of most applications. Comments
on individual considerations are as follows::
U.2.1: Clean, non-volatile, moderate temperature fluid with sufficient NPSH.
This description fits the majority of sealless pump applications and can be
handled by variations of circulation plans shown.
U.2.2: High Temperature: Temperature of motor windings or magnetic drive
components can be controlled by a variety of circulation plans shown in the
grouping for high temperature.
U.2.3: Volatile Fluids/limited NPSH Available: Reverse circulation and
pressurized circulation plans may be used to avoid the thermal effect of drive
heating on pump NPSH requirements. Consideration of vapor pressure increase with
temperature and of specific heat of fluid is required. Use of a separate low
volatility drive buffer fluid is also possible.
U.2.4: Venting and Cool Down: When pumping cold fluids which are volatile at
atmospheric temperature use of a separate vent line back to the supply vessel is
necessary to cool the pump and piping to near pumping temperature prior to
start-up.
U.2.5: Fluids Containing Abrasive Particles may cause objectionable wear.
Centrifugal separation, mechanical filtration, or separate, clean buffer fluid
may be used to remove particles from the circulation fluid.
U.2.6: Jacketed designs may be required for high melting point fluids and easily
polymerizing or crystallizing fluids. Buffer fluids may also be used.
U.2.7: High Viscosity: Viscosities that would cause objectionable drag losses in
the drive section or inadequate bearing lubrication (generally above 100 CP) may
be handled with an external source of circulation fluid. (CPS = CS X SG, SSU =
4.64 x CS). Start-up as well as operating viscosity must be considered.
CIRCULATION SYSTEM APPLICATION: CLEAN PUMPAGE
API 685 1-S/HI 101: An internal system where a portion of the process fluid
is diverted from a high pressure region of the pump, circulated through the
drive section, and returned to a lower pressure region of the pump, usually the
suction. This system is typically the most reliable as it requires no
maintenance or external systems. The application engineer and user must evaluate
the effects of process fluid temperature rise through the drive section
throughout the complete operating range of the pump. For volatile fluids, with
little NPSH margin, the temperature rise combined with a return to suction
pressure could result in flashing of the fluid. This Vapor Pressure Margin
Analysis is supplied as a standard by some manufactures and will be a
requirement of the API standard.
API 685 1-SD: An internal system similar to 1-S above, but including an
auxiliary impeller and returning the circulated fluid to discharge. Again a
Vapor Pressure Margin Analysis must be performed. This system is susceptible to
fluid flashing if the pump is operated at the end of curve.
API 685 11-S/HI 111 & 112: Circulation from pump discharge through the drive
section back to suction. This system is also susceptible to problems at end of
curve operation. A drop in differential pressure would reduce flow and cooling
through the drive section.
API 685 13-S, 13-SE/HI 113: A reverse system where a portion of the process
fluid is diverted from a region of high pressure within the pump cavity,
circulated through the drive section, exits the pump through a restriction
orifice and is returned to either the pump suction or the suction vessel. This
system is useful when the temperature rise through the drive section prevents
the return of the fluid to the pump suction. The fluid can be returned to the
suction line at a sufficient distance to allow flashed vapors to re-condense, or
returned to the vapor area of the suction vessel. The restriction orifice must
be sized to maintain the fluid as a liquid throughout the drive chamber.
API 685 21-S/HI 121: An external system where a portion of the process fluid
back pressure is diverted from the pump discharge, through a heat exchanger, and
circulates it through the drive section, to pump suction. These systems are
typically used when the process temperature is only moderately high, but too hot
to provide adequate cooling for the drive section or for volatile fluids where
temperature rise through the drive section could flash the fluid. The affects of
cooling the process fluid must be carefully considered. With many fluids,
cooling can result in a significant increase in viscosity or the precipitation
of solids. As with the API 685 1-S/HI 111 & 112 plans above, these systems are
also susceptible to problems at end of curve operation.
API 685 23-S/HI 123: This system also uses an external heat exchanger. However
it re-circulates a single mass of fluid using an auxiliary impeller. Its
advantages are that it restricts the amount of fluid transferred between the
pump and drive section, and it only has to remove slightly more than just drive
section heat. This can greatly reduce the cooling water requirements. It helps
prevent particulate from being circulated through the drive section, and it can
be used on very hot applications. Again, the affects of cooling on the process
fluid viscosity must be carefully considered. In addition, this system will
typically require a venting sequence during startup.
DIRTY FLUIDS
The objectives of these plans is to provide cooling and lubrication to the
drive section, yet prevent the introduction of contaminates that can damage
bearings, penetrate containment liners or block flow passages.
API 685 31-S & 31-SE/HI 131. A system where a portion of the process fluid is
diverted from the pump discharge through and external centrifugal separator or
filtration system, circulated through the drive section, to the pump suction. As
with the API 685 1-S/HI 111 & 112 plans above, these systems are also
susceptible to problems at end of curve operation. Contamination and blockage of
flow with these systems is always a consideration.
API 685 32-S/HI 132. Similar to API or HI Plan 32 this full flush system uses an
external supply of clean, product compatible, fluid to cool the drive section
and lubricate the bearings. The customer must be able to accept this continued
dilution of the process fluid by the buffer fluid. Particular care must be
exercised in applying multiple pumps to a single buffer supply system. Balancing
of the required buffer flow to each pump can be difficult and this type of
system is not recommended. In addition to its use with dirty fluids, this system
is often used where there is a potential of operating the pump dry, such as, in
tank unloading and transfer operations.
API 685 41-S. This system is similar to the 31-S with the addition of a heat
exchanger. The application is for mildly hot dirty fluids. The same limitations
apply with the added consideration to the affects of cooling on the viscosity
and precipitation potential.
API 685 53-S/HI 153. This system is essentially the combination of the API 685
23-S/HI 123 and the API 685 32-S/HI 132. This system uses an internal auxiliary
impeller to circulate the cooling fluid through an external heat exchanger, with
the addition of an external pressurized buffer fluid. Some systems use a
reservoir tank in addition to the heat exchanger or use cooling coils within the
reservoir. This system is particularly effective where it is desirable to reduce
the amount of process fluid dilution by the buffer. Some systems use close
clearance throttle bushings or internal mechanical seals to restrict the amount
of buffer transfer between the drive section and the pump.
API 685 54-S. This partial flush system is similar to the API 685 32-S/HI 132
with the exception that most of the pressurized buffer fluid is returned to the
buffer source. Close clearance throttle bushings or internal mechanical seals
are used to restrict the amount of buffer transfer between the drive section and
the pump.
WHY DO WE HAVE TO BE SO CAREFUL?
For most fluids, the care required is fairly obvious. If the fluid is going
to be circulated through the drive section and bearings, it must be clean enough
not to damage the components. If the fluid is too hot to provide cooling of the
drive section, other cooling means must be provided. If the fluid is corrosive,
then the correct materials of construction must be provided. Let’s consider a
fundamental principle. In order for a pump to operate properly, the fluid must
remain in the liquid state. For years pump manufacturers and users have
experienced the negative effects of operating pumps in the unstable situation
where the process fluid changes phase and vaporizes. This vaporization occurs
when the combination of temperature rise of the fluid coupled with insufficient
pressure rise or pressure drop moves the fluid to the wrong side of the vapor
pressure curve. In sealed pumps, this vaporization can result in several
undesirable conclusions, including, pump cavitation due to insufficient net
positive suction head available (NPSHA) and dry running liquid seals resulting
in seal failure. For sealless pumps, the problem is even more serious. The
vaporization of the fluid can result in significant drive system failure with
resultant parts damage and potential fluid release to the environment. In the
conventional sealed pump, after suction conditions are satisfied, the primary
heat concern is hydraulic inefficiency. That portion of the input power that is
not converted to the head and flow of the product is inefficiency. That
inefficiency becomes heat. For sealless pumps, in addition to the heat from
hydraulic inefficiency, there are several other heat sources which include
rotor/stator inefficiency, liner losses, and fluid circulation parasitic losses.
The parasitic losses include the auxiliary impeller, viscous drag and
re-circulation losses. In order to calculate these losses and evaluate the
condition of the fluid throughout the circulation path, the manufacturer has
developed proprietary equations that require specific knowledge of the fluid. In
order to properly size the sealless pump, select the proper circulation system
and evaluate the vapor pressure margin, the pump manufacturer must have the
specific fluid properties data. This not only includes the standard head, flow,
specific gravity and NPSHA data that customers normally provide, but also
includes suction pressure, suction temperature, specific heat, viscosity and a
vapor pressure curve. The vapor pressure curve is required to evaluate what
happens to the fluid as the temperature increases. A single point is not enough.
Sundyne Corporation has developed a sealless pump sizing program which not only
selects the best pump based on head, flow and efficiency, but uses those
proprietary equations to calculate the fluid temperature rise and pressure
change throughout the circulation path. Appendices A, B, and C computer
generated Vapor Pressure Margin curves of vertical in-line canned motor pumps,
which show the temperature rise, the actual pressure and the vapor pressure at
the elevated temperature at selected locations within the pump circulation flow
path. Appendix A shows an API Plan 1-S circulation system with adequate vapor
pressure margin. Appendix B also shows an API Plan 1-S system, but the
temperature rise and the corresponding vapor pressure exceeds the fluid pressure
with the motor. This results in the fluid flashing to a vapor and eventual pump
failure. Appendix C shows the same pump but with and API Plan 53-S cooled
circulation system with make-up buffer flow. This again provides adequate vapor
pressure margin.
CUSTOMER/VENDOR COMMUNICATIONS:
The selection and application of the sealless pump, with its circulation
system requires an understanding of the pump, the process and the fluid by the
user and the manufacturer. It requires complete exchange of information,
particularly with respect to fluid properties and the effect of the pump, drive
section and circulation system on the fluid. With inadequate or erroneous fluid
information, the wrong circulation system may be applied which can result not
only in performance problems, but equipment damage and potential release of the
fluid to the environment. The goal of the user and the manufacturer is to
address concerns during the selection and purchasing stage and not in the field
when the system has failed.
REFERENCES:
1. ANSI/HI 5.1-5.6
American National Standard for SEALLESS CENTRIFUGAL PUMPS
Hydraulic Institute
9 Sylvan Way
Parsippany, NJ 07054-3802
2. API-685
SEALLESS CENTRIFUGAL PUMPS (Unpublished)
American Petroleum Institute
1220 L Street, Northwest
Washington, D.C. 20005
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