Bridging the Governor Retrofit Gap with Electro-Hydraulics

By Lynn Revak,

There comes a time in the life of many turbines when a change of governor system is needed. Often, this is caused by plant-wide control system upgrades. Sometimes the need is simply for closer control or maybe a few remote readouts. Upgrading to anything but electronic controls makes little sense.

Until recently, the choice of governor retrofit packages was limited and expensive. Making the necessary changes required extensive turbine modifications and high governor-hardware costs. The electronic/hydraulic hybrid governors, designed specifically for control upgrades, simplify this. They are designed with the same bolt pattern, input shaft and output configuration as the units they are replacing. Total cost, including installation, is less than half that of previously available systems. Full functionality on any machine is available, and retrofit is very easy.

Original governors on older turbines are almost always self powered. Other than remote control inputs, they require no outside utilities'. 'Full electronic' governors usually require outside electrical power. To operate the steam control valve, they also require a separate device, usually air powered, to respond to their 4-20ma output. Electronic/hydraulic hybrids are 100 per cent self powered, including the valve operating device.

Hybrid control logic is 100% digital with self-checking diagnostics, ease of programming, and so on. The hydraulic circuit packages high output power in minimum space with very low complexity to drive the turbine control valve(s). Actuators mount directly to control valves, eliminating linkage problems.

Currently electronic/hydraulic units are designed to retrofit existing equipment in the place of the most commonly used hydraulic governors. Instead of flyweights for speed sensing, they use internal pulse generators. The pulse generator also supplies power to the electronics. A hydraulic pump and accumulator system provides all of the power required to operate the turbine control valve or other speed controlling device.

The new hybrids are NEMA-D governors, the most stringent performance classification currently in effect. They are certified for hazardous duty locations.

In addition to precise speed control, electronic/hydraulic governors provide a wide range of features: multi-line speed and parameter display; 4-20 milliamp or 3-15 psi remote speed setpoint input; 4-20 milliamp output for remote speed indication; droop or isochronous speed control; keypad adjustable governor operating parameters; vertical and horizontal vibration sensors; governor valve closure on overspeed; plant computer interface; temperature display of external inputs such as bearing RTDS; and export of DC power for external devices.

Electronic/hydraulic governors offer one important feature unique in the industry. They record critical operating parameters on several time bases. These records are available for downloading with a PC to a spreadsheet program for analysis. Data is in battery backed-up RAM and can be examined to see why the turbine 'did what it did' after the fact. Records of speed, setpoint, vibration, valve position and several other parameters are kept. Time bases of 1/8 second, one second, one minute, one hour and one day are used. Long-term trends and 'instantaneous' events can be analyzed. Changes made by operators are also recorded.

Through their microprocessor control, electronic/hydraulic governors are easily configurable for special situations. Often, it requires only a software change to accommodate unusual user requirements.

Electronic/hydraulic hybrid governors have finally bridged the gap between older style mechanical/hydraulic units and the higher priced electronic systems. They offer plant operators of mid-range machinery the ability to modernize their controls at reasonable cost. Demand for the new units is high, and their acceptance is certainly set to grow substantially.