Geared Up For Power
EP Editorial Staff | May 12, 2010
New, proven gearmotor technology is helping power plants boost their competitiveness, despite increasing numbers of regulatory and public-relations challenges.
The power industry is experiencing unprecedented change. The biggest change is in the restructuring (deregulation) of electricity markets, resulting in a much more competitive business environment. Increased competition, in turn, is spurring greater investment in energy efficiency, improved operating performance and overall reliability. At the same time, owner/operators are being asked to modify plant systems to meet emission standards and/or grid demand. If passed, climate-change legislation will also have a huge impact on power utilities. The additional parasitic load created by carbon capture and scrubbers for emissions control, as well as substantial public resistance to adding new fossil power, have opened a Pandora’s box of challenges. Thus, maximizing plant efficiency and increasing existing capacity is the path of least resistance and most cost-effective route for operations that want to remain competitive.
Most thermal power plants utilize large-horsepower, low-speed pumps for plant circulating-water service. This equipment will most likely require upgrading in order to comply with pending climate legislation. In addition, a majority of these plants have been operating for 40+ years—and are badly in need of major overhauls. This article highlights an innovative solution for improving efficiency, reliability and performance of the existing circulating-water pump/motor /control systems in such plants (Fig. 1).
The startup and shutdown sequences of any pumping system are very tough on the pump. This is due to the time it takes for the “system” to achieve hydraulic stability, specifically in the case of a long discharge from the water source to the condenser. This transient condition can and will reduce the pump life by as much as 50%.
Conventional start-up procedure…
- Motor Operated Valve (MOV) at pump outlet is fully closed or 10% open (full open is not an option).
- Valve at source is fully closed or 10% open.
- Start motor. Allow pump to come up to speed and near shutoff pressure (30 seconds to one minute).
- Slowly open the MOV to full open position (precise time depends on distance from pump to condenser).
- Position MOV to desired control-flow rate (this could require a partially closed valve at pump or at destination, depending on system requirements).
The procedure outline here is the conventional method for starting a circulating-water pumping system. By maintaining a closed or partially open valve for any period of time the pump and system (valves, piping, supports and structure) are subject to high levels of vibration and potential damage. If the system is not properly vented, water hammer—an anomaly causing severe damage to the pump and system—also will occur. The results of these events can be seen as:
- Piping failure
- Broken pump shafts
- Motor damage
- Structural damage
- Broken pipe hangers and/or supports
Proven technology, in the form of the CST/Gearmotor system, is now available to deal with such issues. It consists of a 4-pole AC induction motor direct-coupled to a planetary gearbox, complete with an internal thrust bearing, hydro-viscous wet clutch and circulating lube-oil system to drive the circulating-water pump.
- As the motor reaches full speed, the sun gear rotates the planet gears.
- The planet gears rotate the free-floating ring gear.
- The output shaft is integral to the planet carrier and does not turn.
- Hydraulic system applies pressure to the annular piston.
- The stationary and rotating plates of the clutch pack are compressed together on their free-floating splines.
- This results in application of torque to the ring gear, braking and decreasing its speed.
- This causes the planet carrier/output shaft to increase rotational speed while the planet gears rotate about the ring.
- Ring-gear speed reaches zero, the output shaft speed will be at 100% of normal operating speed.
Key features of the technology
The motor will be directly mounted to the gearbox structure and be connected to the gearbox with a flexible coupling. An integrated wet clutch will allow the motor to achieve motor base speed unloaded. The clutch will then be engaged to gradually bring the pump up to full speed.
The 4-pole motor/gearbox can be up to 30% smaller than an equivalent high-pole-count, low-speed direct-drive motor construction. This configuration allows for significantly reduced size and weight, as well as reduced cost for the pump driver support structure.
As shown in Fig. 2, the internals consist of two distinct sections: the planetary output and the clutch. The planetary gearbox and hydro-viscous wet clutch provide for the speed adjustment of the driven pump during startup and shutdown. (With the CST controlling the rate of discharge [flow] from the pump, hydraulic instability is minimized.)
This CST/gearmotor technology also allows for a stable, controlled startup—ramp time is dependent on distance from pump to condenser—and shutdown of the circulating-water system, which protects the pump from potential damage due to obstructions and reduced NPSH conditions.
Startup procedure with new gearmotor solution applied…
- MOV is fully open.
- Valve at source is fully open (open discharge).
- Start motor is unloaded (low inrush motor design).
- Engage CST gearmotor and begin applying power to driven pump.
- Slowly accelerate and increase driven pump speed until pipeline is packed and is stable.
- Adjust pump speed to final desired flow and pressure.
- Monitor/control speed and flow are at DCS.
The typical circulating-water system is friction-dominated, making it well-suited to a controlled start—especially during extended startup sequence and/or low-water-level, high-back-pressure operations, where system demands may fluctuate and NPSHa may be an issue.
Another concern is prevention of load surge through the system. Just as a drive system can input excessive torque into the system under a sudden start, a load surge going through the system can also overload the mechanical drive components. The wet clutch provides an adjustable torque-limiting feature to protect the drive system.
An additional benefit of the controlled-start technology is that the motor can be started unloaded. The no-load start minimizes the duration of damaging locked-rotor current starting a loaded motor. The CST drive system also gives the operator the flexibility of stopping the system without stopping the motor. This avoids multiple motor restarts and subsequent rotor overheating.
In order to control heat generated across the hydro-viscous clutch, the CST is configured with a cooling system sized to remove the dissipated heat from the relative motion of rotating and stationary plates in the disc pack. Either an oil/air or an oil/water heat exchanger, this cooling system is operational only during acceleration.
When considering replacing or upgrading your circulating-water system, look at the big picture during the evaluation process and consider the total system cost. Uptime availability and reliability are the primary drivers in the power industry. Why replace equipment with the same old technology that has placed your plant at risk for so many years? MT
Baldor’s CST/Gearmotor System
- Electromechanical soft start:
- Motor starts unloaded
- Load accelerated at desired
rate (seconds to minutes)
- Stable speed control via wet clutch
- Fast-response control system
- Standard communications
- Standard communications
- Inherently torque limiting (overload protection) for possible intake obstructions
- Gear ratios selected to optimize pump speed for system efficiency
- Gearbox flange designed to mate to existing pump structure
- Electromechanical soft start:
- Minimizes impact on power system
- Lowers duration and amplitude of locked rotor current in accelerating the pump
- Possible extended acceleration and deceleration times
- Capability to completely utilize breakdown torque capacity of motor from 0 to operating speed
- Fewer exerted stresses, leading to increased motor life
- Repeated system starts possible without restarting the motor
- 4-pole motor use at any pump speed:
- Lower motor cost
- Improved motor availability
- Reduced maintenance costs
- Simple bearing design (thrust handled in gearbox)
- Smaller, lighter, easier to handle
- Possible efficiency improvements
- Possible power-factor improvement
- Pumps can be designed to run at any desired speed by changing gear ratio (versus operating at a conventional motor’s full-load speed)
Bill Livoti is a fluid power and power industry engineer with Baldor Electric Company. He also is vice chair of the Pump Systems Matter (PSM) initiative. Telephone: (864) 281-2118; e-mail: firstname.lastname@example.org