SAFE GOVERNING OF TURBINE SPEED

SECOND WORLD CONGRESS, MONTREAL

OCTOBER 5-8, 1981

Colin D. Bass

SAFE GOVERNING OF TURBINE SPEED

The first step toward safe governing steam turbines is answering the basic question: "Do we need a governor?" In any process industry application, the answer is yes. Even applications like constant head cooling water pumps are subject to conditions like impeller damage that can unload the turbine and damage it. If this unloading is rapid and complete, the possibility of personal injury from turbine debris is significant.

Following this basic decision to govern, is the selection of governor type. The primary purpose of the governor is safe operation of the turbine, particularly during load changes. To accomplish this basic safety related function requires relatively simple controls. However, secondary considerations make the choice of the governor type more complicated.

Four types of governors are predominately applied to steam turbines in service today. These are CONTROL OIL PRESWIE mechanical, hydraulic, mechanical/hydraulic and electric. Figure 1 through 5 show typical

a schematics of each of these governor types. All of these provide speed control by providing the Figure 2. Typical Hydraulic Governor following functions:

1) Speed Setting or Speed Reference 2) Speed Measurement 3) Error Calculation (Difference Between Speed

Setting and Speed) 4) Steam Flow Correction

SPEED YECUANICAL s w n

STEAM ! N E T

Figure 1. Typical Mechanical Governor

DROOP SLIDER

SPEED

+OIL INLET

Figure 3. Typical Simple Mechanical/Hydrauiic Governor

When reviewing the application of each type of governor, keep in mind the first rule of governing: "buy only what you need". The second rule is buy only what you can operate, troubleshoot and maintain". Electric governors will be described with many attractive features, however without specially trained operators, they will be useless.

OMPENSATING

HIGH PRESSURE

OIL SUPPLY

HIGH PRESSURE

CONTROL

DRAIN COMPENSATION OIL

Figure 4. Compensated Mechanical/Hydraulic Governor

r---i I I REMOTE I SEW0 k-1 I J I

HVDIIULIC ACTUATOR MOVER

MAGNETIC PICKUP

HVORAULIC SOURCE POWER SUPPLV

- -

Figure 5. Block Diagram. Typical Electronic Governor

Other major considerations in governor selection are:

1) Accuracy of control/speed of response. 2) Type of speed rangehetting of control. 3) Power output to move steam valves. 4) Options.

Fo r governors , t h e N a t i o n a l E l e c t r i c a l Manufacturers Association (NEMA) has quantified these needs into Table 1 below. Values in the first two columns represent varying degrees of speed

'

control to maintain a process variable.

Column three indirectly specifies the rate at which the governor must close the steam valve for 100% load rejection. It is this characteristic that is critical to turbine operating safety. System factors which influence these values are listed below:

1) Governor speed of response. 2) Servo system slew rate. 3) Turbinehtring dynamic characteristics. 4) Rate of unloading.

Turbine driven systems which can be rapidly unloaded require faster governing systems than similar applications that cannot be unloaded quickly.

For normally operating process machines, all of these values are relatively constant, and therefore relatively easy to control. The real control problem occurs when abnormal loading conditions occur. It is routine to review the off speed of turbine generators following generator breaker opening while operating at full load. Generally speaking, control systems must not allow ihe turbine to trip on overspeed due to these conditions. In the case of turbine generators, the unloading rate is easily defined and the analysis usually is not subject to controversy. For turbine driven mechanical loads. like the fans and pumps, the problem is more difficult to review realistically. Normally there is some time involved in unloading rate which tends to aid to the governing system. For accurate system analysis, that rate should be realistically specified. Review of turbine response during severe load disruptions, like loss of a drive coupling are of interest, however, failure of the governor to keep turbine below trip speed is not usually necessaly. In fact, it is often better to trip the turbine after coupling loss. (Note that in this evaluation, loss of the coupling has significantly reduced the turbine rotating mass. This loss creates essentially a completely new set of turbinetstring dynamic characteristics).

Table 1 Speed Governor System Classification Speed shall be classified as follows:

Column 1 Column. 2 Class of Maximum Speed Maximum Spee Governor Regulation Variation System Percent Percent, +

Beyond the basic ability of a governor to control turbine speed, options can greatly increase the systems operating safety. Two which will be discussed in some detail later are safety trip backup and overspeed test devices.

Mechanical governors probably meet the requirements of the second selection rule best. They are relatively simple devices. Their use on new turbine applications has nearly stopped, however, because of their limitations. They are basically constant speed controls with speed ranges limited to about 20°h of rated speed. They are generally not considered to provide precise control. By design, they provide NEMA A performance. Without lots of maintenance, this performance generally deteriorates with time. Because the steam valve is moved directly by the flyweights, the forces on the flyweight toes are high. This causes wear on the toes which results i n reduced per formance. App l i ca t ion of mechanical controls is limited to small turbines requiring relatively small output forces. They have few options to increase System safety.

Hydraulic governors offer an improvement over mechanical controls in several areas. Variable speed range is expanded to about 31, available output forces from the hydraulic servo piston can be suitable for any turbine size and steady state control accuracy is better than mechanical governors. The factors that limit new application of these controls are oil temperature sensitivity and hydraulic system maintenance. Both changes in oil temperature and oil contaminants affect the performance of the control system.

Today, then, the choice of governor systems is generally between mechanical/hydraulic and electronic controls. Either of these can be supplied to provide steady state performance to

Column 3 ,d Maximum

Speed Rise. Percent

Column 4 Trip Speed

Setting Percent

NEMA Standard 11-13-1979

any of the NEMA specifications in Table 1. They can both be interfaced with servo systems sufficient to control any size steam valve@). There is a wide selection of options available to tailor the control to the application. The choice of electronic controls usually hinges on one or more of the following considerations:

1) Electronic governors generally respond more quickly to load changes. "

2) Electronic governors can be mounted comple te ly o f f the turb ine, thereby eliminating the requirement for governor drives.

3) Electronic governors can provide speed ranges of 10:l.

4) Where plant process controls are electronic, electronic governors allow direct use of the process control voltage output for speed setting.

5) Generally, controls and indicators available from electronic goveinors make automation easier.

6) For power generation, isochronous load shar ing is avai lable f rom e lec t ron ic governors.

7) Control of functions in addition to speed is possible with electronic controls.

" This statement is true generally when comparing rnechanical/hydraulic controls with electric controls using electro/ hydraulic actuators. Pneumatic pressure is also used for actuator power. Here the output of the electronic governor is converted to pressure in an I/P transducer. This pressure acts on a pneumatic diaphragm actuator which is attached to the steam valve. Almost any electronic governor can be used this way, however, each application of this type must be

carefully reviewed. Pneumatic actuators are usually slow compared to hydraulic systems and may not provide adequate control response. Generator sets and other loads capable of rapidly unloading should be carefully reviewed before pneumatic actutors are chosen.

Most items in the above list are self-explanatory. In a particular application any one of them may be the key to the choice of electronic governors. Normally electronic controls provide NEMA D or better performance. The single item that seems to warrant further explanation is item 7.

Multiple parameter control is one of the best reasons for considering electronic controls. This feature lets the governor (speed control) expand to control other parameters. This usually results in the elimination of one or more controllers, elimination of additional steam valves and simpiification of mechanical linkage. This can result in safer operation of the turbine. Exhaust pressure control and extraction pressure control are typical of this type governor.

An exhaust pressure control system utilizing a non-electronic speed control typically consists of a speed limiting governor and an exhaust pressure control (Figure 6). Each of these operates a separate steam control valve which is positioned in series. Normally the speed setting of the governor is set above normal operating speed. Therefore, the speed setting is unsatisfied and the speed governor control valve is at fully open. Control of turbine load is done by the exhaust pressure controller. In this situation. it is not uncommon for steam contaminants to accumulate on the inactive governor valve and freeze it in this open position. Once frozen, the speed limiting function is defeated and loss of load will result in turbine raising to trip speed. NOTE: Typically exhaust pressure control can be done on generator sets tied to a large bus or on through- drive turbines which are part of multi-unit string. In these circumstances speed contro l is maintained by the eiectric bus or other units in the multi-unit drive.

By using electric governors, the system described above can be greatly simplied and safety improved. The electric governor can have separate control loops for both speed and exhaust pressure. These two loops each generate an eiectric signal which is compared in a low signal selector to pass the signal calling for least steam to the single actuator and control valve. Figure 7 shows a simplied block diagram of such a system.

Figure 6. Typical Exhaust Pressure Control Scheme

Figure 7. Block Diagram Electronic Exhaust Pressure Control

Electronic control of a single or double automatic extraction steam turbine can result in even greater reduction of control hardware. Figure 8 shows the elements required for a single automatic extraction control using a hydraulic governor and hydraulic pressure controls. This system depends on numerous springs, orifice and over 30 pin type linkage connections. These systems are subject to output variation due to temperature, control oil contamination and to wear of mechanical linkage.

With an electronic control, all control functions are included in one electronic package. Turbine mounted hardware is limited to speed and pressure sensors and the actuators. Figure 9 shows a simplied block diagram of an electronic single extraction control.

Figure 8. Block Diagram Hydraulic Automatic Extraction Control

From the above, it can be seen electronic controls Safe operation of a steam turbine requires, in have great versatility. They also have some addition to a suitable governor, an effective safety limitations. Three major limitations are: trip system. As a backup to the speed control

Uninterrupted electrical power is required. This forces use of redundant power supplies and battery backup systems. Field wiring between the governor and actuators requires special mechanical and electrical protection. Mechanical damage makes the governor inoperable. Electrical noise induced in wires not properly shielded makes the control operation less than satisfactory. Field service requires personnel with good electrical background who are equipped with adequate instrumentation. If the control cannot be serviced to the point of replacing faulty modules, the advantages of electronics are lost. Note that printed circuit board repair capability is not required for successful application of electronic controls.

provided by the governor, overspeed protection must be provided. There are as many overspeed systems as there are governor types. These are mechanical, hydraulic, mechanical/hydraulic and electric.

I t is not within the scope of this paper to discuss mechanical overspeed trips because they are separate from the speed control. However, since electronic overspeed trips are often included in the electronic speed governors, electronic trips will be discussed briefly.

Electronic trips have both advantages and disadvantages when compared to mechanical trips. Advantages usually given are tabulated below. The major disadvantages of electronic trips are the requirement for continuous power supply.

Figure 9. Block Diagram Electronic Automatic Extraction Control

Table 2: Advantages of electrical overspeed trips

Mechanical

Single mechanical component is required to trip. Therefore, a single failure can defeat the trip.

Must raise actual turbine speed to over- speed to test the trip.

Difficult to adjust.

Electrical

May have more than one input. Typical is 1 of 2 to cause trip or 2 of 3 to cause trip."'

Can be tested by simulation equipment, while the turbine continues to operate at rated conditions.

May be readjusted easily.

Generally housed in the shaft which may Except for symetrical pickup gear, all cause additional mechanical problems. hardware is mounted off turbine.

"* This is a somewhat questionable advantage. The redundant inputs may be necessary to protect against loss of a pickup or its connecting wiring. Redundancy may also be necessary because of noise lntroduced on that connecting wiring that could cause a false trip of a single switch.

Whatever the choice of trip system, trip speed sensing and the primary trip actuator should be separate from the governor. The governor should be specified with an auxiliary input that closes the governor valves on trip. However, this should not be the primary action to stop the turbine. The speed sensor (MPU) often supplied in mechanical governors for tachometers should not be the input for the primary electric overspeed speed switch.

When electronic governors and electronic speed switches are supplied, the primary overspeed speed switch and speed control can share primary pickups on the turbine, but should not share electrical circuitry. It is recommended that all circuitry for overspeed protection, including power supplies, be separated from the electronic speed control. Like with mechanical governors and trips, the electric governor should have an input from the overspeed to close governor valves.

I W F START OF PETROLEUM

2%-F OIL D E G l l O I l l D N

START OF SYNTHESIZED OIL DEGRIOAT1OH I

Malntenance and Tmubleshootlng

Safe governing requires the controls be maintained through established maintenance schedules and procedures. Due to varied plant conditions, these must be reviewed for each specific installation. Four items most often considered in maintenance programs are oil change interval, governor overhaul, control linkage and spares storage.

For mechanical/hydraulic governors or self- contained electro/hydraul ic actuators, the following are some general guidelines for hydraulic system maintenance. The viscosity of the oil for most applications should be between 100 and 300 SUS during normal operation. Extreme limits are 50 to 3000 SUS during cold start-up or abnormal temperature conditions. Figure 10 shows some available oils whim can be

IDEAL OPERATING RANGE POUR WlNT

Figure 10. Oils Selection Chart

successfully used with hydro-mechanical governors and electro hydraulic actuators. In order for this chart to be meaningful, some specific oils are listed, however, other oils meeting the same requirements and having equal properties may also be used.

Oil maintenance is essential for long and reliable governor operation. Regular, but not necessarily frequent, oil changes must be made. Once a class of oil is selected, its use should be adhered to. Adding or changing oil of one class to another class without thoroughly cleaning a hydraulic system may cause operational problems such as foaming and sludge formation. Some classes of oil may not be compatible with gaskets or seals.

Any water, regardless of the quantity, in a governor should immediately be removed and the oil changed.

Clean oil is a necessity, whether filling a governor for the first time or whether adding make-up oil. Clean oil cannot remain clean if the container or pouring spout is not clean. Partially used cans of 011 should be covered and stored in a clean area. Cleanliness of oil and the container cannot be overstressed. Most governors with self-contained sumps do not have filters or screens. It is essenttal, therefore, that contaminants are not introduced to the governor through the oil.

The best time to change the oil is just before the oil is worn out, and before any damage to the governor has occurred. This condition is best determined by the oil analysis. However, since the cost of doing this exceeds the cost of a quart or two of governor oil, it is not a practical solution on a continual basis. Analysis can be used to set up a maintenance schedule which should remain in effect as long as the original conditions do not change. Experience wi th other hydraul ic equipment similar to governors can also be used as a guideline to establish an oil maintenance schedule. Anytime a known contaminant gets into the governor, it should be drained, flushed, and refilled with clean oil as soon as possible.

Particles of dirt and water in the oil are the greatest causes of governor or actuator failures. Particular care should be taken to keep dirt and moisture out of opened or stored governors and opened control lines. The presence of sludge. varnish, or sediment are good indicators that an oil change is required. It is also an indication that perhaps a different oil should be used, especially if it has only been a short time since the last oil change.

A varnish buildup is an indication that governor operat ing temperatures are exceeding the capability of the oil. This problem can usually be solved by going to an oil with good high temperature characteristics or by installing an oil cooler. Low operating temperatures may lead to the formation of sludge. Sludge is a complex mixture of products from sources such as water, carbon and oxidated oil that has agglomerated and is no longer soluble in oil. Sludging may be controlled by raising the governor operating temperature, increasing the frequency of oil changes, or by changing to a different type of oil. Fluids, such as automatic transmission fluias. mav ~, ~~, prove to be more resistant to sludge than some engine oils.

The following are some specific rules to be used to determine when to change governor oil:

Appearance is different than when new. Oil feels gritty when rubbed between fingers. Oil smells different than when new. (Caution: some oils may smell burned and still be acceptable. Check wi th o i l company representatives. If in doubt, change!) Any water or other incompatible material contaminates the oil. Viscosity has changed, increased or decreased. Excessive wear of parts occurs. If governor has been run at tem~eratures exceeding the recommded limit for the type of oil in use. If governor operating temperatures have changed, bringing oil viscosity outside of ideal operating range. .

The second maintenance consideration is how often should a governor be overhauled or what is governor life. Anytime a hydro-mechanical or pneumatic system becomes contaminated, overhaul should be considered. It is the author's belief that normal overhauls should be in the two to five year range, coinciding with other normal turbine maintenance.

If the environment is such that contaminants cause more frequent overhaul cycles, steps should be taken to keep the contaminants out of the control system.

With proper maintenance and overhaul intervals. the life of a control system can be indefinite. There are many hydro-mechanical governors that have been operating in excess of 20 years.

Improperly adjusted o r worn l inkage can completely offset all the safety precautions D described above. Linkage must be free to move, yet have tightly fitting pins and pivots. It must be adjusted so that the governor valve comes completely closed before the governor reaches the end of its stroke. Since the governor normally operates with the valve at least partially open. linkage is sometimes adjusted to give good response at minimum load. If this minimum load is lost and the governor valve cannot completely close, all speed control is lost. This, coupled with malfunction of overspeed trips, has caused the loss of more than one turbine.

The care and storage of spare controls is also of major concern. Most manufacturers do not ship controls packaged for long-term storage. As in the case for an operating control, water is of major concern. Not only is direct exposure to water such as rain a problem, but the breathing of water vapor and subsequent condensation also is a major potential problem. Where any piece of hardware, mechanical or electrical, is stored for an extended period of time, the use of desicants must be considered. Hydro-mechanical devices may be filled with oil, however, they must be inspected

eriodically because leakage is possible. Unless he entire enclosed volume is filled with oil, water v' may still condense and settle to the bottom. Mamtenance should also include the overspeed trip. Testing of all components of the trip system should be done on a regular basis. Although complete testing can only be done during maintenance when the turbine is out of service,

several systems are available to check parts of the system with the turbine running. During complete system testing, the test should be done under governor speed control. For this function. governors should be specified with provision to temporarily extend the normal range up to trip speed.

Electronic overspeed trips lend themselves easily to partial checks of the trip system with the turbine in service. While temporarily defeating the speed switch output to the actual trip solenoid, test equipment can simulate an increase in turbine speed. By monitoring the switch output, the trip point can be proven. Removing the test equipment and solenoid override returns the system to normal. Note that with two of three trip systems. each switch may be checked individually allowing the two remaining to function normally and continue to protect the turbine during testing.

In conclusion, lets review the major points discussed above. First, any industrial steam turbine requires some sort of speed control. That control must be such that i t will satisfactorily control the turbine followma loss of load. Secondly, the turbine should be equipped with a seaparted overspeed trip device. The overspeed trip should also input to the governor to close the governor valves. Thirdly, safe operation requires proper malntenance of both governor and safety systems. Maintenance should include governor overhaul and proper l inkage adjustment. Maintenance should include regular testmg of the complete overspeed trtp system. For this testing the governor should be provided with an overspeed test device or high speed override.

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