A Lower Cost, High Performance Variable Frequency Drive
Alternative for Paper Mill Machines
by Kim A. Lovejoy, Lovejoy Controls Corporation February 19, 2010
A push has been in progress for several years advocated by both the US
Department of Energy (DOE) and the manufacturers of Variable Frequency Drives (VFD)
to replace the small mechanical steam turbines driving paper roller machines
with electric motors coupled to VFDs.
This replacement certainly qualifies as a control improvement, but comes at both
a high installation cost and much poorer overall plant energy efficiency. Let's
examine the real pros and cons in detail, not just advertising claims, and then
review a lower cost alternative plan.
The small mechanical drive steam turbines used in paper mills are mechanically
sturdy prime movers which were designed to operate on the extraction steam (low
pressure) of a plant generation turbine. This extraction steam is tapped at a
low pressure point between stages of the turbine. Several generating turbines
may feed a common extraction steam header, which in turn feed several plant
mechanical drive turbines. This arrangement is very advantageous in energy
efficiency, the US DOE even admits this, see:
Since the majority of the extraction steam energy comes at very little cost in
boiler steam generation nor generator electrical output, the overall energy cost
is far less than motors even with VFD. This is so because the generation turbine
blade rows between the extraction points and the condenser are the least
efficient of the unit and are far less efficient than the mechanical drive
turbines which are specifically designed for low pressure steam operation.
Compound this advantage with electrical losses to replace the steam driven
devices with motors and there is no question of optimal efficiency.
The problem with the mechanical drive turbines is more one of maintenance
headaches focusing on the speed controls. To maintain good paper quality, the
roller speeds must be correct and stable. The original mechanical/hydraulic
governors and speed changers supplied for these turbines, usually Woodward,
belong in a museum, not an operating plant of this century. The delicate
mechanical "flyball" governors and contamination prone hydraulics are often root
causes of speed oscillations and control limitations. Further, when plant
modernization projects install distributed control systems, these old governors
cannot easily be adapted for remote operation. Outside the plant DCS, the
machines become both operational chores and often neglected orphans.
A Better Approach
The problems of the paper drive mechanical steam turbines can be solved at a
cost of less than 5% that of VFD Motor replacement while retaining the inherent
energy efficiency of the original design. The solution is to apply modern
technology, in fact nearly the same technology as VFD to replace the obsolete
mechanical/hydraulic turbine speed controls. By using modern digital governors
integrated in the same package with digital servo vector drives operating motor
driven electric actuators all of the maintenance headaches and performance flaws
of the antique speed controls are eliminated. The LCC TEG-10 Turbine Governor
Servo Drive and a properly sized electric actuator provide stable speed control
and ease of startup and operation through a DCS serial link. In the end, the
plant retains the energy efficiency of the extraction steam and mechanical
turbine drive system while gaining control capabilities and reduced maintenance.
The TEG-10 has a local control panel and a serial link compatible with all DCS
to permit the DCS workstations to control all aspects of paper machine drive
Quicker, Cleaner Installation
Unlike the VFD retrofit which requires turbine, reduction gear and steam piping
tear outs, high current power wiring, motor base platform foundation work and
motor/VFD installations the TEG-10 upgrade is minimal in project scope and plant
down time. The TEG-10 retrofit consists of removing the existing mechanical/hydraulic
governor and valve operating cylinder, installing a blanking and support plate,
mounting the new electric actuator, and installing the single electronics
enclosure. The full retrofit can easily be accomplished in one day, as opposed
to weeks of VFD retrofit.
The TEG-10 contains both a small mechanical turbine digital governor, a
vector servo drive, and a local operator panel all in one enclosure.
External networking connections are provided for plant distributed control
Pedigree on Nimitz-Class Nuclear Aircraft Carriers
The design basis of the TEG-10 is the LCC NavGov 900 unit developed, installed,
and in very successful operation on US Navy Nimitz-Class Nuclear Aircraft
Carriers. The NavGov 900 is a military hardened version, but shares the same electronics
control design. The US Navy chose the LCC NavGov 900 to replace failing
Woodward mechanical/hydraulic governors on its shipboard mechanical drive
turbines in what has resulted in a very successful retrofit. The TEG-10
now makes this technology available to commercial users.
Contact LCC for a discussion of your paper mill drive turbine controls.