Turbine speed

Whenever the process machine operates at the same speed as its driver, the two can be directly coupled. This direct couphng stiU allows for a variable speed, through acuustments of the speed or the driver. Steam turbine speed can be easily adjusted, and electric motor speed can also be varied by the use of special drives that vaiw the frequency of the power applied to the motor. Wdiether the speed is fixed or variable, direci coupling of two machine shafts presents the problem of accommodation of misalignment. To this purpose, machines are coupled through a.flexible coupling. 

Beeause there is eonsiderable design flexibility in aeeommodating optimum turbine speed, standard meehanieal drive steam turbines are often ehosen. The offset aiTangement (Figure 4-41) provides additional freedom for the turboset eomponent layout. 

The gas turbine during the start-up is on an auxiliary drive, initially it is brought to a speed of about 1200-1500 RPM when ignition takes place and the turbine speed and temperature rise very rapidly. The bleed valves are open to prevent the compressor from surging. As the speed reaches about 2300-2500 rpm, the turbine is declutched from its start-up motor, the first set of bleed valves are closed, and then as the turbine has reached near full speed, the second set of bleed valves are closed. If the turbine is a two or three shaft turbine as is the case with aero-derivative turbines, the power turbine shaft will break loose at a speed of about 60% of the rated speed of the turbine. 

A Campbell diagram is frequently used to determine the effect of multiple excitation frequencies in high-speed steam turbines. Figure 7-11 shows a Campbell diagram for a condensing steam turbine. If this partic uiar turbine operates at a speed of 8,750 rpm, the turbine blades would not be excited. But, if the turbine speed is reduced to 7,500 rpm, the turbine blades would be excited at four times running speed. If the turbine were operated at 10,000 rpm, a three-times running speed excitation would be encountered. What this means is that any vibration in the... 

As with the motor driver, the steam turbine must be matched to the compressor. Also, a turbine rating of 110% of the maximum power required by the compressor should be specified. This should be at the compressor s normal speed point. The turbine speed should include a maximum continuous speed 105% of the normal compressor speed. API Standards 611 and 612 cover general purpose and special purpose steam turbines 7, 8]. 

An increase in weight flow over the set point would cause a signal to reach the governor, which would reduce the turbine speed to maintain the desired flow. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

Determine horsepower loss from Figures 14-29A, 14-29B, and 14-29C at the reduced turbine speed and exhaust pressure. 

Fundamentals of Turbine Speed Control, Bulletin H-21A, Elliott Co., Steam Turbine Div., Jeannette, PA. 

Turbine ) Increasing turbine speed. Evaluate the steam level and consider adding an exhaust condenser. 

As shown in Example 9-1, increasing the pump impeller size from 13 inches to 13.5 inches increases the flow by 3.8%, discharge pressure by 7.8%, and horsepower by 12%. Increasing the turbine speed from 3,300 rpm to 3,400 rpm increases the flow by 3%, the discharge pressure by 6.1%, and the horsepower by 9.4%. 

In general, addition of the base to the solvent-rich part of the reaction zone, near the tip of the impeller, resulted in higher levels of Q formation due to higher local concentration of the base. Addition of the base to the upper part of the reactor, where the mixture is water-rich, resulted in local dilution before reaction with C, and in a lower amount of by-product Q. So, the concentration of base is the solvent-rich layer should be low, and the optimal procedure is a high turbine speed and base introduction in the top layer. 

Turbine speed is controlled by two split-range valves, one on the 10 psig inlet to the turbine and the other on the 100 psig steam that can also be used to drive the turbine. Your instrumentation system should be designed so that the valve on the 10 psig steam is wide open before any 100 psig steam is used. 

The control scheme shown in Fig. 17.4 is certainly quite common. But is it the best Figure 17.5 is a copy of the crude charge system in a now-defunct refinery in Port Arthur, Texas. I saw it in operation many years ago. It worked fine. The required flow of crude directly controls the governor. The turbine speed is then always at its optimum. The AP across the process-control valve is always zero, because there is no process-control valve. This design is a direct descendant of the original method of controlling the steam flow to pumps. The steam inlet valve was opened by the operator, so that the desired discharge flow was produced. 

The problem with this new air blower, was that we could not bring the turbine speed above its critical speed. The critical speed of a turbine is stamped on the manufacturer s nameplate. Turbines are typically run well above their critical speed. If, for some reason, a turbine is run close to its critical speed, it will experience uncontrolled vibrations and self-destruct. 

And the turbine speed came back up, well above its critical speed. 

But not for long. After 15 min of operation, the turbine speed slipped back down. Once again, I had lost a lot of vacuum in the surface condenser. Once again, the vapor outlet temperature had dramatically increased. But this time, the condensate outlet temperature had also increased. What was my new problem ... 

Ladies and gentlemen, it is not a good idea to run a turbine with the governor speed-control valve wide open. Why Because you no longer have any speed control. And the turbine speed is then free to wander. The rest of this story is pure philosophy. 

Increase the set-point speed. If the turbine speed fails to come up, the governor was already wide open. 

It is quite important not to operate a turbine-driven pump by throttling the steam flow to the turbine. Let s assume that the operators have set the turbine speed at 3500 rpm, by adjusting the steam inlet gate valve upstream of a malfunctioning governor. Suddenly, the discharge flow-control valve cuts back, and the pump s flow decreases from 2000 to 1200 GPM. The pump speed will then increase, because fewer pounds of liquid are being pumped, and less horsepower is required to spin the pump. 

In this particular example turbine speed control was taking place. The closed loop feedback is turbine speed in rpm as measured by an optical pickup on the generator box (nearly instantaneous). In this test, the experimental speed controller and the simulated speed controller (in both models), used a proportional gain of 0.001 and an integral gain of 0.001 x 0.75 with an input of speed error in rpm and output in fuel valve %. For the experiment presented, the fuel flow rate in grams per second... 

The interaction between the pressure and turbine speed controllers (Figures 2.133 and 2.134) can be decoupled. Figure 2.136 illustrates how a drop in the speed of the shaft can open both the inlet and the extraction valves, and an increase in shaft speed can close them both. Therefore, one way to eliminate interaction between flow and pressure loops is to allow the pressure controller to throttle both the supply and the extraction valves. 

---