by Kim A. Lovejoy, Lovejoy Controls Corporation February 19, 2010
 

Introduction

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.

Original Technology

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:

        Steam21_rotating_equip.pdf

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 turbine operation.


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 interface.


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.