New procedure speeds cold start, protects turbine09/01/1995

New procedure speeds cold start, protects turbineSystem dispatch from today`s power plants must consider availability of purchase power (buy or sell), fuel prices, and unit availability and efficiency. To gain the best combination of these factors, steam units must be capable of quick removal and return to service.However, unit startups are expensive, time consuming and operationally demanding. For example, excessive thermal stresses can be catastrophic to a unit. With those factors in mind, Jacksonville Electric Authority (JEA) developed the "valve open start" procedure.This simple new cold start concept meets manufacturer criteria for unit protection. The critical requirement is to have only superheated (dry) steam introduced to the turbine in a manner that minimizes temperature mismatch, i.gif., the difference between steam and turbine metal temperatures. Turbine manufacturers are most concerned with mismatch, rotor rupture and position, and blade ductility in their technical procedures for startup.

Operators tried to meet these requirements in the past by producing superheated steam at high pressure and high temperature with the turbine throttle valves closed. Only after reaching steam conditions of 700-1,000 psig and 750 F were the throttle valves used for gradual admission of steam to the high pressure (HP) turbine. Bearing vibration often reached "alarm" limits as the turbine accelerated through critical speeds and "rotor long" (where the rotor expands much faster than the casing) conditions were normal. This was especially true if speed holds were required in the 3,000-rpm range.

Valve open start (Figure 1) calls for a new way of thinking (a paradigm shift)--that superheated steam can be produced in a vacuum at low temperatures. The condenser pulls a vacuum throughout the steam system if main steam throttling valves are left open. Superheated steam produced at low temperature may be used to warm the turbine while boiler temperature and pressure increases.

Valve open start at Unit 3Experience at JEA`s Northside Unit 3, which has a 518-MW Westinghouse turbine and a Riley Stoker drum boiler (3,548,000 pounds per hour), illustrates what happens during starting with open valves. Initially, turbine metal temperature is ambient (approximately 90-100 F). Steam is applied to the steam seals. Circulator pumps activate and a vacuum of approximately 28 inHg is produced in the condenser. Because the main steam throttle valves are wide open before firing begins, saturated steam is generated in the boiler drum under 15 inHg vacuum at approximately 178 F.

Steam absorbs more heat energy as it progresses through the superheater. At the same time, the pressure drops because steam is moving toward the condenser, which has a greater vacuum. As the temperature increases and pressure drops, the steam becomes more superheated. Steam temperature is near 200 F when it reaches the throttle valve inlets. The entering steam has almost 60 F of superheat because the saturation point of steam is approximately 150 F at the 22 inHg vacuum present. The turbine manufacturer specifies 100 F superheat, but its procedure calls for throttling across partially opened valves. There usually is a 50-75 F drop in superheat when throttling leaves a traditional startup with a safe 25 F superheat, but if the valves are wide open there is very little throttling. Therefore, if there is 60 F superheat without throttling, there is more superheat than on a traditional startup.

After the new procedure was used on Unit 3 for two years, an October 1994 steam audit found no ill effect from water damage. This indicates that the first steam to go through the turbine before the turbine rolls is superheated steam. Anywhere from 20 to 45 minutes after firing up a furnace, the turbine starts rolling off turning gear with an acceleration rate of 25-50 rpm per minute and a near-perfect temperature match. Note that turbine manufacturers haven`t endorsed this type of startup because the turbine valves are back seated. Manufacturers emphasize that they do not want the boiler controlling their turbine.

This procedure does not totally exclude the turbine controls. The control system (P-2000 DEH) constantly tracks speed and an operator can put it back into primary control at any time. The computer is used to synchronize turbine speed for generator breaker closure.

Lower temperature mismatch

JEA`s staff sees a major advantage in the valve open start. As shown in Figure 2, there is much less temperature mismatch when introducing 210 F steam instead of bleeding 700 F steam to a turbine sitting at ambient temperatures. Manufacturers require mismatches of less than 100 F on startups, which is not practical on cold startups. The wide open valve start produces a much better match. As steam progresses through the system, main steam and turbine drains are left open and the condenser is at 28 inHg of vacuum. Any moisture formed in the headers drains to the condenser.

Normal considerations are made for turbine lubricating oil temperature. The oil should be above 70 F for turning gear operation, 80 F to roll the turbine, 90 F to exceed 1,000 rpm, and 100 F before exceeding 3,000 rpm.Failure of oil temperature to track rpms is one of the major causes of terminating the valve open start. The valve open start must be terminated and cannot be resumed if steam admitted to the turbine is throttled at any time. This event then would require converting to a conventional start procedure.

Unit 3`s system now meets the turbine mismatch criteria, but there are other standard criteria on the turbine. These include holds for rotor temperature versus speed, temperature versus ramp rates, and reheat temperature. There is uncertainty on why some criteria were originally established. JEA`s staff agrees with William A. Sanders of Turbo-Technic Services Inc. His approach is that the basis of the criteria is first for superheat, second for ductility of turbine blades, third for prevention of rotor rupture, and fourth for prevention of rotor expansion or contraction (rotor position).

The valve open start addresses all of these concerns in the procedure. When the turbine goes above 3,000 rpm, it is held until the temperature rises sufficiently on the blades. Turbine blade ductility is protected by raising the intermediate pressure (IP) blade temperature above 250 F before the unit is taken to line speed. Incidentally, IP blade temperature can be measured best in the cross-over to the low pressure (LP) turbine.The relatively long and thin blades in the IP should exceed 250 F by the time the crossover steam is up to 250 F. By the time blade temperature rises, the calculated rotor bore temperature also is above the minimum. A new turbine rotor`s temperature should be above 125 F and an older rotor could be 50 F higher. Therefore, the JEA staff is conservative and requires a rotor temperature above 175 F before taking the unit up. That temperature is less than the 250-300 F specified by the manufacturer. The staff uses

Westinghouse calculations for rotor surface metal temperature and the rotor bore temperature. JEA finds that, at the slow acceleration rate (25-50 rpm per minute), the rotor bore temperature lags the rotor surface metal temperature by 7 F. Therefore, when the blade ductility is met, the rotor rupture is automatically resolved and not a major concern.

Rotor position always is correct when there is no temperature mismatch in rotor versus shell differentials. Rotor long is a concern during traditional startups, especially during speed hold requirements in the 3,000-rpm range. The valve open start procedure produces zero differential expansion in the HP/IP turbine section and in the dual LP turbine sections.Because the turbine metal and lubricating oil temperature requirements are satisfied at this point, the turbine can accelerate beyond 3,000 rpm. In the 3,000-3,300-rpm range, control transfers to the turbine P-2000 computer. The computer controls the speed and holds for ramping temperature and pressure prior to synchronizing the turbine at line speed and closing the generator breaker. The manufacturer requires that heat soak be complete before the turbine reaches sync speed, but JEA disagrees. Sanders states that the turbine soak should be completed before exceeding 50-percent load. Plant operators never exceed 250 MW on the 518-MW unit before Westinghouse calculations for heat soak time are acknowledged by the computer.Boiler firing rate

The initial boiler firing rate limit of 2 F/minute (120 F/hr) is well below the manufacturer`s lim it of 150 F/hr for the boiler and turbine. The firing rate is not exceeded during the pressure and temperature ramp prior to breaker closing, nor during the initial load in crease from 25 MW to 150 MW. The main stream limit of 500 F/hr will not be exceeded with the drum temperature rate of change held to 2 F/minute and the load rate held to 2 MW/minute. The valve open start firing rate had been controlled manually, but was automated in January 1995. Automatic firing should further decrease start time.

After coming on line, it is slightly more difficult to control drum level at lower pressure, but operators have become proficient at it. Initial loading is generally about 17 MW. The load increases to 25 MW when the turbine megawatt control loop is placed in service. As soon as the boiler drum level is stable, operators increase load to 50 MW (minimum stable load) to make the unit available for service. Normal loading proceeds and the turbine soaks are completed before going above half load. That ends the startup.

Even though the actual firing rate is slower than in a traditional start, overall on-line time is reduced. The unit has been on line in as little as five hours. Before firing the boiler, two of the four circulators, one of the two condensate pumps and the polishers are started. Slightly more than three hours is required from firing the boiler to going on line. The other circulators are put in during those three hours. Total time required is five or six hours. To allow some contingency, the published time used for dispatch is eight hours to minimum stable load.There was no change in rotor position during the first valve open startup on Unit 3, and vibration points usually experienced during a traditional startup were almost undetectable. These are reasons why four operating units in three different power plants now use the procedure, including the 275-MW G.gif. turbine on Northside Unit 1. This unit has a Babcock & Wilcox (B&W) universal pressure boiler, once-through without a drum (1.8 million lb/hr). The boiler was modified with a separator to start as a drum type boiler, and then convert to once-through operations at approximately 95 MW. The startup works well on this unit.Kennedy Unit 10, a 134-MW Allis-Chalmers turbine with a B&W boiler (1 million lb/hr) accelerates slowly after firing off. Temperatures come up quickly and the firing time has been reduced by four hours.

The main feature of valve open start is its simplicity. It brings the whole unit up together. Superheat is assured and there is little mismatch between the rolling steam temperature and metal temperature. "Rotor long" no longer is a problem. In addition, less transient vibration is experienced. These cold starts are faster than many hot starts where traditional throttling is needed because of the temperature mismatch.JEA dramatically reduced the time and fuel required to start four steam generating units using the new procedure. Fuel savings for all four units is projected to be $148,000 in 1995. This assumes that there will be 112 cold starts.

After two years of trials using the new valve open start procedure, an October 1994 steam audit found no ill effects from water damage.

ReferenceSanders, William P., Turbine Steam Path Engineering for Operations and Maintenance Staff, Turbo-Technic Services Inc., 1988.Authors

Richard E. Mallard is the deputy director of operations for the three power plants wholly owned by Jacksonville Electric Authority, Jacksonville, Fla. He has more than 32 years experience in power plant operations, maintenance and management.Cedric A. Jordan is an instrument and control technician working for Jacksonville Electric Authority. He is retired from the U.S. Marine Corp. as an aviation instrument technician. He has 18 years experience on gas and steam turbine controls.

A valve open start procedure at Jacksonville Electric Authority`s Northside Unit 3 has

dramatically reduced thermal start up stresses on the unit`s steam turbine.