WHILE EXTERNAL GEAR PUMPS OFTEN CLAIM HIGHER CAPACITY, SLIDING VANE PUMPS PERFORM BETTER IN MULTIPLE
KEY PERFORMANCE PARAMETERS THAT MAKE THEM THE CLEAR CHOICE FOR TRANSPORTING LIQUIDS VIA TRUCK
Lubricating liquids and petroleum products are the lifeblood
of many industries around the world. From fuel to feedstock,
products like diesel, kerosene and light lube oils keep
vital sectors of the industrial landscape running smoothly.
Transport of these liquids is handled through a combination
of pipelines, railcars, barges, and tank trucks that make up a
large and complex network that spans the globe.
More than 100,000 tank trucks per year transport lubricating
liquids and petroleum products in the United States alone, so
finding the best pump for loading and unloading these fuels
and feedstocks is important for any operation. Two of the
most used positive displacement (PD) pumping technologies
for fuel offloading are external gear pumps and sliding vane
pumps. While both pump designs have proven trustworthy
in their own ways, there are several design and operational
distinctions that may lead to sliding vane pumps being the
better choice for liquid loading and unloading via truck.
External Gear vs. Sliding Vane
External gear pumps deliver a constant amount of liquid with
each revolution, while their tight clearances and mechanical
actuation theoretically restrict any liquid from moving
backward, or “slipping,” during their operation. A smooth,
pulse-free flow is the result of external gear pumps’ rigid
designs. This method allows external gear pumps to perform
especially well when transferring high-viscosity liquids.
Sliding vane pumps, like their gear counterparts, minimize
the chance for slippage to occur. Unlike external gear pumps,
sliding vane pumps do this by maintaining a constant force
between the vane and the casing. This robust design means
that variances in pumping pressure will have little effect on
the sliding vane pump’s flow rate. This design also provides a
gentle and shear-sensitive liquid-handling environment within
the pump. Therefore, sliding vane pumps excel at handling
ultra-thin liquids (0.20-100 cP), medium-viscosity liquids (100-
5,000 cP) and liquids with a thickness of as much as 22,500
cP, with no fall off in performance at any viscosity level.
So, which pump is the better choice?
There are several key factors that can determine the answer
to this question, including initial investment cost, pump
longevity and more.
When offloading a truck, optimizing unloading times is one
such concern. The time you spend loading your truck eats into
the time spent on the road transporting fuels and product
to your customers. Another key concern is the price of the
pump itself. Like any purchase, the initial investment should
undoubtedly be worth the money.
For example, Blackmer® competitor Roper™ Pump Company
claims to offer an external gear pump that boasts both a
higher flow rate and lower purchase price than Blackmer
sliding vane pumps, so is Roper the obvious choice? Not
necessarily. Some drawbacks to external gear pumps – that
are not present in sliding vane pumps – aren’t immediately
apparent, but can grow into a lack of reliability, reduced
service life and compromised levels of performance.
Let’s take a closer look at the ways that Blackmer sliding vane
pumps can reliably outperform Roper external gear pump
models, despite some claims to the contrary:
1) Flow Rate and Friction Pressure
Actual flow rates for external gear pumps will
be lower than promoted.
On paper, Roper external gear pumps deliver flow rates of
between approximately 300 and 400 gallons per minute (gpm)
(1,136 and 1,514 L/min). But due to the realities of the liquid-offloading process, that rate is rarely realized to its fullest
extent. In most offloading circumstances, the use of a 2” hose
is preferred. Issues arise in this instance because a hose of
that diameter will ultimately restrict flow rate. In a system with
a pump that can output 300 gpm (1,136 L/min) or more, the
liquid will build up friction pressure well above the capability
of the pump, which can lead to failure.
As a result, a flow rate of 300 gpm (1,136 L/min) is not
necessarily feasible since the transfer system would not be
able to handle it. This means that the actual flow rate would
be lower in practice than in theory – usually around a 10%
reduction initially. Therefore, in many transfer systems, a high
flow rate isn’t always the most important factor in ensuring
efficient and effective liquid transfer
2) Flow Degradation
External gear pumps depreciate over their
For most technologies, pump-performance levels change
dramatically with age. This is true for external gear pumps
that have meshing metallic gears within the pump chamber.
With every turn, the meshing gears wear. Especially during
priming and line stripping cycles. Over time, the shaft wears
to such a degree that the clearances between the pump’s
rotor and gear become too large. As a result, the pump
becomes less efficient and outputs less flow due to leakage,
or “slip,” of the pumped liquid from the discharge side back
to the suction side.
This slippage phenomenon can lead to significantly reduced
productivity if left unchecked. So, for every gallon that is
moved through an external gear pump, its efficiency is
irreversibly lowered, with the eventual solution being to
simply replace the pump altogether. Since all external gear
pumps will experience increased product slippage over
time, this change in operational capability, which will lead to
decreased flow rates, should be a real concern for anyone
considering purchasing an external gear pump system. In fact,
decreased capacity can often lead to slower delivery times by
more than 50%.
3) Vane Pump Resilience
Vane pump design actively resists flow
As mentioned earlier, most external gear pumps have a
higher output on paper than they can reach in practice. And
the 10% reduction in flow capacity is just the beginning. As time goes on, flow reduction becomes even more pronounced
in external gear pumps. Wear causes the pumps to shift, and
their flow capacity decreases steadily, up to as much as 50%
over the course of five years of continuous operation.
The sliding vane design of Blackmer vane pumps addresses
the issue of flow-capacity degradation by using self-adjusting
vanes that sustain flow capacity and maintain fast delivery
times throughout the life of the pump. Performance of the
pumps are consistent during the vanes’ lifespan – something
that cannot be said for external gear pumps.
4) Pump Life and Repair Considerations
External gear pumps have considerably
shorter lifespans and are harder to service.
Knowing the cost and difficulty of pump maintenance is
another critical aspect to consider. In the case of external gear
pumps, it is usually easier and cheaper to replace the pump
altogether when repairs are needed, leading to extended
downtime and system disruption. Sliding vane pumps can be
economically and easily serviced in the field, saving money by
providing optimized uptime.
There’s a clear difference when comparing the wear/age
timelines between gear and sliding vane pumps. Expanding
clearances and wear within external gear pumps can
become noticeable within a few years from the start of use.
Comparatively, it takes around 10 years for sliding vane
pumps to start showing signs of excessive wear. That means
that within the lifetime of a single sliding vane pump, you
could be expected to replace an external gear pump up to
three times. During those replacement times, the pump-mounted truck would need to be put out of service.
The claim that Roper external gear pumps outperform
Blackmer sliding vane pumps isn’t as cut and dry as some
might think. Remember, a higher reported flow rate doesn’t
always lead to more efficient pumping. Since external gear
pumps are susceptible to more pronounced and predictable
wear than sliding vane pumps, Blackmer pumps will continue
to perform long after the competition’s offerings have
Ultimately, sliding vane pumps deliver improved performance
over external gear pumps where it counts, while avoiding
many of the issues that pop up as external gear pumps age
and wear. Although the implied higher flow rates and lower
price provided by Roper external gear pumps might seem
like a good deal, the longevity and performance of Blackmer
sliding vane pumps will provide any truck-transport operation
a better return on investment in the long run.
Click here for a PDF version of this white paper.
About the Author:
Brian Binder is the Application Engineer for Blackmer®,
Grand Rapids, MI, USA. He can be reached at
Brian.Binder@PSGDover.com. Blackmer is a leading brand
of sliding vane, centrifugal, screw and regenerative turbine
pump, and reciprocating gas compressor technologies and
a product brand of PSG®, a Dover company, Oakbrook
Terrace, IL, USA. PSG is comprised of several leading brands,
including Abaque™, All-Flo, Almatec®, Blackmer®, Ebsray®,
em-tec, Griswold®, Hydro Systems, Mouvex®, Neptune™,
Quantex™, Quattroflow™, RedScrew™ and Wilden®. For more
information on Blackmer or PSG, please go to blackmer.com