Lombard Water Turbine Governor
By Robert W. Timmerman PE, CEM, LEED AP, Museum Volunteer
The Lombard Water Turbine Governor was designed to regulate the speed of water turbines of the Francis type, where the water flowed inward from the periphery, and exited at the center of the turbine. These turbines were designed for moderate flows and heads, typical of the operating conditions encountered in New England mills. (For high flows and low heads, the Kaplan propeller turbine is used, while for very high heads at low to moderate flows, the Pelton turbine is used.) Control of flow through the Francis type of water turbine differs from a steam engine, because the steam engine uses a relatively small flow of steam under high pressure, while the Francis water turbine uses a relatively high flow of water. A nominal 100 hp non-condensing steam engine using steam at 150 psi would require a 3” steam line, while a water turbine with the same power output, operating at the 17 foot head of the lower canal system in Lowell would require a 36” water inlet pipe.
There are two ways for the governor to control the speed of a steam engine: operating a throttle valve which regulates the flow of steam to the engine, or regulate the amount of steam admitted to the cylinders by adjusting the travel of the valve gear. Valve gear regulation was more efficient and but more expensive than throttling, so it was used when economy was more important than initial cost—larger engines, and those running more hours per year.
Throttle valve control of a steam engine is shown in Figure 1, below (this is a photo of a display in the Museum collection):
The belt turns the vertical shaft, which causes the red balls to rotate. Centrifugal force causes the balls to move outward, moving further outward as the speed increases. The pivot for the balls is anchored at the bottom, so the outward motion of the balls causes the vertical shaft to drop, which closes the valves in the green painted enclosure.
Valve gear regulation of the speed of a steam engine is shown in Figure 2 (again from the Museum collection):
The flywheel is painted red, and revolves with the engine crankshaft. The governor weight is inside the flywheel, and is painted yellow. As the speed increases, the weight rotates outward. Movement of the weight is resisted by the spring attached to the weight and the flywheel. Movement of the weight shifts the position of the eccentric, seen behind the flywheel, painted red. Shifting the position of the eccentric changes the amount of steam the valve gear admits to the engine.
Both of these methods of regulating the speed of a steam engine are compact, and fairly simple.
By contrast, the amount of water that has to pass through a water turbine requires a more complex regulating mechanism. A cross section of a water turbine, c 1891, is shown in Figure 3:
A series of vanes are arranged along the periphery of the turbine, which direct water onto the turbine rotor (indicated in the cut by the concentric circles connected to the main vertical shaft at the center of the picture). Water flows through the vanes, onto the rotor where it is decelerated and gives up it’s energy to the rotor, and exits down through the center of the rotor. The vanes are rotated by the arms attached to the vertical shafts attached to the vanes. These levers are actuated by the linkage that is turned by the sector gear, and the sector gear is rotated by the pinion and the vertical shaft at the left of the picture. All this machinery takes a fair amount of force to operate, far more force than can be developed by the simple flyball governor.
A servomechanism is necessary to amplify the small force developed by the governor to obtain the large force needed to move the guide vanes. The Lombard Water Wheel Governor is one such device. The one in the Museum’s collection is shown in Figure 4.
The date and model of this unit are unknown. It is missing the hydraulic oil pump, and oil tank. The hydraulic pump would have been mounted on the side of the machine opposite the view. A tank, containing hydraulic oil under air pressure, would have been mounted under the machine.
The main parts of the governor are:
· The hydraulic oil pump and tank (missing from our version)
· The governor, unit with flyballs at the top of the governor
· The operating cylinder (the large cylinder at the left of the picture)
· The piston rod and attached rack
· The gear which drives the output shaft
· The control valve (small vertical cylinder on top of the main cylinder) which controls the flow of hydraulic fluid to the main cylinder
· The feedback mechanism (the rods on top of the rack), which correct errors in the piston rod position
· The handwheel and gear which permit manual operation of the turbine gates for testing.
· In addition, there are a number of auxiliary parts which provide lesser, but important functions.
The governor is a conventional flyball governor, driven from the water turbine by a belt, shown in Figure 5, below:
A belt from the water turbine drives the governor pulley which is flanged to keep the belt from slipping off. The pulley shaft has a bevel gear on the other end, which turns the vertical shaft for the governor. The governor bearings fix the lower end of the governor in place, so movement up and down occurs at the upper end, where the top of the governor presses upon the small rod, causing it to move up as the governor balls move out. The large spring at the left of the governor acts thru the lever to resist the motion caused by the governor. The force from the spring and the force from the governor balance at the set speed. If the speed drops, the governor balls will move in, and the spring will pull the rod up. The reverse happens if the speed increases.
The inside of the governor is hollow, and the rod connected to the top passes all the way thru the governor, down to the top of the valve. The valve and the feedback mechanism are shown in Figure 6:
The rod can be seen just below the bevel gear that drives the governor. The attachment for the feedback mechanism is just below the bevel gear. The rod then enters the top of the valve (the brass dome on top of the valve).
Inside the valve, the motion of the rod directs hydraulic oil to one side of the piston or the other, causing the piston to move in and out. The motion of the piston moves the rack, which then turn the pinion (small gear), which turns the output shaft. The rack and pinion are shown on the previous Figure 4, the overall view.
As the speed of the turbine changes, either due to changes in load, changes in the head of water on the turbine, or due to the action of the governor, the governor will act to keep the speed constant. The action of the governor may not be sensitive or fast enough to keep the governor from overshooting the speed setting. This action is called “hunting” where the speed goes from too fast to too slow, as the governor attempts to get the proper speed.
One cause of hunting is the inertia of the water or steam in the inlet line to the engine or turbine. A change in power output requires a change in flow. To get this change in flow, the water or steam in the inlet pipe must be accelerated or decelerated, as the case may be. Changing the velocity requires a force, which translates into a temporary change in pressure at the engine or turbine. If the load increases, more flow will be needed, which will require force to accelerate the flow. This force will reduce the pressure available at the inlet of the engine or turbine, effectively reducing the power output, just when more power is required. The engine or turbine will slow down more, and the governor will open the throttle, change valve gear position more, or open the wicket gates more. Eventually the machine will be up to speed, but with the throttle or gates open wider than they need to be for the desired output. The governor will then reduce flow, causing the machine to slow down. In some cases, this cause the machine to oscillate.
A simple example will illustrate the effect of fluid inertia.
Consider a 50% change in output to be done in 5 seconds.
100 hp Steam Engine:
Steam pressure: 150 psi
Line size: 3”
% change in pressure to accelerate flow: 0.07%
100 hp Water Turbine
Water head 17 feet
Line Size 36”
% change in pressure to accelerate flow 14.6%
Clearly the 0.07% reduction in pressure on increase in flow will not have any effect on the steam engine, but the 14.6% reduction in pressure on increase in flow on the water turbine will reduce the power output, just when an increase in power is required. The net effect will be for the governor to open the gates more than necessary to just meet the increased demand for power, causing the turbine to have difficulty in reaching equilibrium.
The Lombard Governor incorporates an additional feedback loop in addition to the one consisting of the governor and the control cylinder operating the wicket gates. This additional feedback loop feeds back the position of the rack to the control valve, in a direction to oppose the motion of the turbine gates—that is, if the governor calls for an increase in gate opening, the feedback will oppose opening the gates. The feedback is also arranged so that there is more feedback on calls for sudden changes in gate opening than on gradual changes. On a sudden change in load, the wicket gates will called upon for a large movement to keep the turbine speed constant, and to accelerate the water in the inlet line (or canal). This large movement, carried on for too long will cause the turbine to miss the speed setting, and oscillate. Slower movements will require a less severe change in gate opening, and so are less likely to cause hunting.
The feedback is accomplished in an ingenious manner. Figure 4 shows a vertical rod attached to the rack. Figure 6 shows a portion of that rod, and the rest of the mechanism. The vertical rod is attached to two other rods that are pivoted at the level of the governor. The final rod is attached to a brass cylinder, the other end of which is attached to a rack which turns a gear on the rod from the governor. The brass cylinder is a dashpot, a cylinder filled with oil with a piston inside. The piston moves inside the cylinder with some clearance, and works similar to an automobile shock absorber. Due to the viscosity of the oil, more force will be transmitted from the linkage to the output end of the cylinder the faster the input rod moves.
The output end of the cylinder is attached to a rack (a rectangular bar with gear teeth cut into it). This rack meshes with a small gear on the governor rod. Inside the governor housing, there is a coupling on the governor rod, with coarse threads. Turning the gear with the rack screws or unscrews the coupling, depending on direction. This changes the length of the governor rod. Since one end of the governor rod is attached to the governor and will not move, changing the length will change the position of the hydraulic valve. The parts are arranged so that movement of this auxiliary linkage opposes the action of the main governor, and causing the governor to return to the neutral position. The action of the dashpot causes more force (and hence more motion) to be transmitted if the linkage is moving fast than if it is moving slowly, causing sharp changes in position of the main cylinder rod to be more attenuated than gradual changes. This reduces the tendency to overshoot on a sudden change in load.
On a sudden loss of load, or in the event the governor belt breaks, the governor will go to full wide open position, overspeeding the turbine. To prevent this, the Lombard Governor incorporates a device to shut down the turbine in the event of overspeed, shown in Figure 7:
The Overspeed Trip consists of a pivoted lever which fits over the governor rod, and which is pulled down by the spring shown at the left of the picture (our governor seems to be missing some parts that attach the spring to the lever). A hooked lever holds the main lever out of action by hooking against the top of the casting. This latch is held at the lower end by a mechanism that also fits over the governor rod, and which has a trip that is released by a small flat lever fitting over the governor rod. If the governor rod rises to the maximum upward position, corresponding to the hydraulic valve causing the cylinder to move the wicket gates to full open, it will trip the small lever. That will release the latch, causing the spring held main lever to pull the hydraulic valve to the position that causes the wicket gates to close. This trip must be manually reset to put the turbine back in operation.
There is another lever which can cause the governor rod to move up and down. It is theorized that the purpose of this is to test the operation of the governor.
In order to operate the turbine for testing without the governor, the large handwheel is provided. This wheel turns a small gear, which meshes with the large gear on the output shaft. See Figure 8, below:
The shaft carrying the handwheel and small gear is free to slide in its bearings, so that the handwheel can be used to drive the large gear on the output shaft. As exhibited, the output flange from the governor (not shown, it is behind the gear) is rigidly connected to the gear. In practice, there is a pin clutch, the handle for which is at the bottom of the gear. It is believed that as installed, this clutch could be used to disconnect the governor from the large gear, but that the large gear would remain connected to the output shaft. This way, when running the turbine manually (after repairs, for example), the governor would be disconnected via the pin clutch, and the handwheel would engage the large gear, to position the wicket gates.