| Distributed Control of Mechanical Drive Steam Turbines | |
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It's Not Been Easy in the Past to Put Steam Turbines on DCS With the new capabilities of modern Distributed Control Systems (DCS) of advanced displays and multi-screen workstation the natural tendency at process plants has been to integrate as much of plant equipment as possible into the systems. By doing so, operators can maintain control of much more machinery with better efficiency while keeping better informed of conditions than possible with discreet independent systems. One application area which has lagged in DCS link up has been small mechanical drive steam turbines which operate pumps, fans and compressors for a wide range of process control. The reasons behind the relatively low numbers of mechanical drive turbine DCS integration are: | |
 | Ancient Control Hardware... The existing turbine speed controls (or "Governors") are often mechanical/hydraulic devices of very old design and not well suited for remote setpoint control. Many operating components such as "Flyball Governors" may be seen in equipment museums. |
| | Poor Steam Admission Valve Operators... The original hydraulic or pneumatic operators have very poor linearity, repeatability, and are generally suffering from large hysterisis in signal direction change (close-to-open and open-to-closed). They also have a tendency to become contaminated easily, choking on the bearing oil particles. |
| | Design Information Lacking... There's not much in engineering documentation to mechanical drive turbines, with OEMs maintaining propriety over equipment drawings and instruction manuals generally limited to spare parts listings. |
| | Fast Speed Loop Closure Needed... Acceleration capabilities of even small turbines is very fast, typically 1000 RPM/second, requiring speed control loop closures in the 60-millisecond region. Although possible in modern DCS, too many fast loops gobbles a large share of DCS resources. Typical digital governors, while fast enough, have "overkill" feature laden designs that are cost prohibitive for wide scale distribution of small turbines on DCS. |
| New Technology and New Hardware Changes the Situation, Opens the Door | |
| Thanks to several new technology developments this situation has now changed. Foremost is the availability of Roller Screw Electric Actuators. These near-zero hysterisis positioners eliminate the nonlinearity of the ancient hydraulics and when coupled to Servo Drives provide vastly improved control resolution. Combined with the Roller Screw Electric Actuator is the development of a small digital governor specifically designed for both electric actuator control and DCS drop interfacing. | |
| The combination of the new actuator and new governor forms a common retrofit hardware package which is applicable to any small turbine made by any manufacturer available for a fraction of previous offerings at far lower cost. | |
| How low? A small mechanical drive turbine DCS Drop Digital Governor, Roller Screw Electric Actuator, Cables, Actuator Mounting Hardware, and Redundant Speed Probes can be provided by LCC for less than $15K. This is usually less than a full set of spare parts for the old mechanical governors. What else is needed? Just a timing wheel (if one does not already exist on the turbines) and method of mounting the speed probes. | |
| Some Questions You Might Have... | |
| (1) | How do the Roller Screw Electric Actuators work? How are they so precise? |
| A brushless motor spins a planetary gear system of orbiting roller screws to displace an internally threaded cylinder.. Since the motor is direct-geared to the operating shaft there is no delay nor ambiguity in initial motion such as resident in hydraulic cylinder positioning. The position feedback device is a resolver accurate to 4096 steps per motor revolution, typically 0.1 inch. | |
| (2) | Is there any further dependence on the oil system for speed control of the turbine? |
| No. The bearing oil system remains necessary for lubrication purposes of the journal and thrust bearings and stop valve control only. | |
| (3) | What type of power is needed to run the new system? Can it be redundant? |
| The systems use either a 24 VDC or 48 VDC. This may be supplied from a variety of AC feeds with dedicated power supplies or from 125 VDC feeds via DC/DC converters. For vital turbine control applications redundant feeds may power auctioned power supplies or converters. | |
| (4) | How long does it take to install the full retrofit? |
| Assuming the DCS network cables are pre-run, a small turbine retrofit takes about one day. | |
| (5) | How are the actuators sized in terms of force and valve stroke? |
| The force requirement is based on the maximum steam admission pressure and the active diameter of the governor valve. The stroke is matched to the full stroke range of the governor valve plus a small over-travel. | |
| (6) | What is the routine maintenance of the system? |
| The Roller Screw Electric Actuators require re-greasing on an 18-month period for high duty cycle applications, while up to three years on low duty cycle applications. The governors require no periodic maintenance. | |
| (7) | Can I just run the Roller Screw Electric Actuator with the DCS and skip the governor part? |
| Possibly, depending upon the speed control response required of the turbine and the speed loop closure time available from the DCS. LCC can assist in these calculations. | |
| (8) | How many and where has LCC performed this type of retrofit? |
| LCC has retrofit twelve naval (nuclear aircraft carrier) turbines, seventeen boiler feed pump turbines in nuclear and fossil power plants, and small generation turbines in process plants. Reference contacts are available. More installations are being provided monthly. | |
| (9) | If we have several different turbines from different manufacturers will the new control systems share common parts? |
| Yes. Since LCC governors are common electronic hardware configured for each application they are common and interchangeable for all turbine manufacturers. Electric actuators may also be common depending on individual requirements. Sometimes providing a slightly larger actuator on some turbines may provide a common fleet part. | |
| (10) | Are training classes available for operators and technicians? |
| Yes. Power Point presentations with individual hand outs provide full system training in operation, calibration, and maintenance. | |
| (11) | What do I need to provide to obtain a quotation? |
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For each turbine application:
Turbine Manufacturer (Elliot, Terry, Murray, Worthington, etc.)
Total number of turbines.
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