To World Pumps (February 3, 2005)
In the January 2005 issue of World Pumps, Mike Pemberton talks about the Myths and
Legends of Variable Speed Pumping. This article talks about the latest improvements in
VFD technology to help reduce the negative side effects. Mike brought up some of the side
effects which include requiring motors with inverter grade insulation. This is necessary
because VFD's create voltage spikes, requiring systems that operate at 480V to use motors
with at least 1600V insulation. Mike also states that motor leads need to have an armor
sheath and VFD systems need to be fitted with line reactors and RFI filters to help reduce
the reflective wave or harmonics. There are however, several other negative side effects
that were not mentioned, including harmonic vibration at critical speeds, temperature rise
from heat produced by harmonic frequencies, skin effect which causes pits in metal
surfaces, EDM currents that build up in the rotor and discharge through the ball bearings,
and other problems.
There are conditions that can greatly benefit from the use of a VFD. Pumping conditions
which include hot water, solids, hydrocarbons, and shear sensitive materials, as well as
conveyor belt systems, escalators, and air handlers may be worth putting up with the side
effects of a VFD. However, when pumping fairly cool and fairly clean water at a consistent
head or pressure, which is a large percentage of pumping applications, a throttling valve can
have many advantages over a VFD.
One of the "myths" that manufacturers perpetuate is that a VFD can save energy over a
throttling valve. This is not true unless the head produced by the pump can vary with the
square of the RPM. Being able to operate at any lower TDH is rare, as most pumping
applications require a consistent head to buck the static pressure. If the system requires
100 feet of head, then the pump must produce 100 feet of head at low flow as well as high
flow. In Mikes example (Figure 1), the 50 hertz pump dead heads at about 120 feet of
head. This pump can only be slowed from 50 Hertz to 46 Hertz and still produce the 100'
of head required. Taking this into consideration, the power consumption can then only be
reduced to 77% of full load according to the affinity law. People who sell VFD's but do not
understand them, love to use the affinity law to show the power consumption drop to 10%.
This is impossible, because they forget the part of the affinity law that shows that the
head will drop off by the square of the speed. Reducing this pumps' speed by any more than
8% will cause the pump to be completely dead headed, as it takes 100 feet of head or 92%
of full pump RPM to buck the static pressure and push open the check valve.
Simply working a constant speed pump, with a good brake horse power curve, further left on
its' curve can use the same or even less energy than a pump working on a VFD. This is not a
"myth". In (Figure 2) and in the article, Mike talks about the negatives of working a pump
to the left of BEP. THESE are technical issues that are well understood and CAN be easily
mitigated. #1 Discharge and suction recirculation, as well as low flow cavitation, reduce
impeller life through cavitation like wear. Impellers made of materials with a high tensile
strength will resist cavitation like wear. #2 Low bearing and seal life not attributed to
balance or misalignment, is generally caused by the pump shaft and bearing not being strong
enough to withstand radial deflection. #3 Centrifugal pumps have very little inside the
rotating assembly that actually touches and produces heat. When pumping fairly cool water,
the pump must be almost at shut off before any temperature rise is a concern. A small
bypass can easily take care of the cooling requirement. It takes only pennies extra when
manufacturing a pump to supply a heavy duty shaft and bearing as well as an impeller made
of strong material. THIS is what gives a pump longevity, even when used throughout almost
the entire range of the curve.
The biggest "myth" is that VFD technology conserves energy with centrifugal pump
applications. Cost of valve throttling goes largely unnoticed because there is none, or at
least very little. The "magical" property of a centrifugal impeller that causes the power
consumption to decrease as head increases from throttling is counter intuitive. Throttling
with a valve can save as much energy as a VFD, while running the pump on standard AC
voltage with a pure sinusoidal wave. Controlling with something other than a VFD when
possible also eliminates related voltage spikes, EDM currents, reflective waves, harmonic
vibration, complicated computer controls, and other side effects.
Another "myth" is that operation to the left of BEP causes problems for the pump and
system, when instead the damage is caused by inferior pump components. Many pump
companies do not want pumps to last very long, so newer pumps are designed with barely
strong enough shafts, bearings, and impellers for the pump to function. Any deviation from
BEP, with a pump built to minimum specifications, will cause damage that coincidently causes
the purchase of another pump. On the false pretense of increasing efficiency, pump
companies are selling short lived pumps with expensive controls, INSTEAD of building a
pump strong enough to last. With planned obsolescence as a primary design feature, asking
a pump company to create the perfect pump, is like letting the fox guard the hen house.
Companies make more money when they must warranty a large percentage of their product,
than if they build a product well enough that it never needs to be warrantied. A well known
fountain on Shore Line Drive in Chicago has been running the same pumps everyday since
1927. Some of our oldest hydro electric plants have pumps or turbines that have not been
shut down for any repairs in well over 60 years. Pumps that were made in the days before
the era of planned obsolescence were built to last. Can you imagine how good the Life Cycle
Cost looks on a pump that has had zero or very little repairs in 75 years? Manufacturers
are terrified of losing repeat sales to equipment that last, so they limit the quality designed
into their products.
"Myths" abound because the function of a VFD in conjunction with a pump can be
complicated. Many who understand the electronics of a VFD do not fully understand the
mechanics and hydraulics of a pump, and vice versa. Those who do understand pumps as well
as electronics, would never claim a VFD saves energy over throttling with a valve. Many of
us who have used VFD's in the past and not seen the operating and economic benefits that
were promised, have gone back to throttling with a valve. Recent advances in valve
technology, which include "constant pressure valves", have improved variable flow
performance without using any electronics. In this age of computers, most of us expect
improvements in technology to be electronic. This is not always the case. Operators
should also educate themselves on simpler, less expensive, more dependable, and mechanical
ways to reach their objective. Lack of education creates "myths and legends" which can
trap you in a vicious and expensive circle of planned obsolescence. "Wonders and
transforming powers" of electronic controls are sometimes nonexistent in the bright light
of true understanding.
Cycle Stop Valves Inc.