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MECHANICAL OVERSPEED TESTING OF NUCLEAR SAFETY-RELATED

TURBINES WITHOUT DRIVING STEAM

Little J.1

1ILD, Inc.

Baton Rouge, Louisiana, USA

1. Background

Rotating equipment, particularly steam turbines, generally employ control systems that perform a variety of

functions, including tripping. “Tripping” is the shutting down of a turbine when certain abnormal situations

occur, for example, low bearing oil pressure, high bearing temperature, and rotor overspeed. Rotor

overspeed, if unchecked, could cause a rotor to fly apart, resulting in substantial damage, and in some

instances, catastrophic results including loss of human life. Consequently, most steam turbines are equipped

with electro-hydraulic or electro-mechanical control systems and backup mechanical overspeed trip devices

to prevent rotor overspeed. These devices must be periodically tested to ensure proper functioning. This

periodic testing is typically required by the entity insuring the equipment against loss. In most instances,

testing turbine overspeed trip systems requires driving the turbine rotor to trip set-points, typically set at 103 -

120 % of the normal operating speed. See, e.g.,United States Patents No. 5,133,189 and 5,292,225 covering

modern over-speed protection devices.

2. Typical historical over-speed testing methods

In commercial nuclear power plants, small to medium horsepower turbines are routinely used as prime

movers (source of rotation), and, as discussed above, are periodically tested to ensure proper functioning.

Generally, less risk is involved when overspeed trip testing is performed at a time when the turbine is not

required to be operational, for example, during refueling outages when the nuclear reactor is not critical.

During refueling outages, maintenance and testing activities which, if delayed, would delay the return to

service (productivity) of the power plant are identified as being on “critical path”. By contrast, maintenance

and testing activities that do not increase the outage duration are identified as off “critical path”. Nuclear

power plant management typically prefers that all maintenance and testing activities, including overspeed

testing, be performed off critical path where possible to minimize outage duration and lost production.

However, the costs associated with conducting these tests can be significant because an alternate source of

steam has typically been required to spin the turbine since the reactor can no longer produce steam. These

costs can include the rental of an alternative steam source capable of spinning the turbine rotor beyond its

normal trip set-points, in addition to manpower costs for engineering, maintenance, and operations support.

Furthermore, the logistics of installation, operation, and removal of the required equipment can add

complexity to an already complex refueling outage schedule.

Alternatively, overspeed trip testing could be conducted using steam provided by the reactor once it is again

operational. However, this testing method is generally not preferred because of the losses in productivity that

result. More specifically, when testing a turbine using steam provided by the reactor, the tests are performed

during the plant start-up from the refueling outage. This testing method is generally considered on critical

path because the testing activity becomes a series activity in the start-up sequence and plant return to service

cannot proceed until a successful over-speed test has been accomplished.

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3. Alternative over-speed testing method

An unfilled need therefore existed for a method and apparatus that allows overspeed testing to be performed

off critical path, and without the need for driving steam. Additionally, an ideal method and apparatus should

allow testing without subjecting the tested turbine to any unacceptable stresses.

One option for such a method is to connect the turbine shaft to a source of rotational power and accelerate the

turbine rotor to its mechanical over-speed set-point with only air in the turbine casing. To rotate a turbine

rotor beyond normal trip set-point in this manner requires a high power motive drive systems that is capable

of overcoming windage effects. “Windage” generally refers to a power loss due to fluid drag on a rotating

body. Windage increases are directly proportional to the cube of the speed of a turbine rotor. Windage

effects for rotors spinning in air at high speeds are significant. Such a method was developed in the United

States in 1998, and has been successfully used for testing of small turbines of less than 1,000 horse power

using only ambient air to surround the rotor during testing. This test device comprises an operator control

system and a drive motive power assembly utilizing a standard alternating current induction motor, variable

speed drive electronics and a belt driven power transmission. Once installed, this device is used to accelerate

the turbine rotor to its test velocity without the use of steam. Rotor speed and acceleration are controlled with

a high level of precision, virtually eliminating the likelihood that, in the event an overspeed mechanism

malfunction occurs, the turbine will be damaged. Another, more complex device was developed later and

patented, also in the United States [1], which enables much larger turbines to be tested in much the same way.

For turbines with rotors approaching 1 meter in diameter, and over-speed set-points approaching 6,000

revolutions per minute (RPM) two new problems arise which are of no consequence when testing the smaller

turbines;

1. Windage losses become very large, necessitating delivery of much power to the turbine shaft.

This problem can result in bearing side load issues, as well as very large electrical power

requirements.

2. Turbine rotor tip speed can approach sonic velocity relative to the turbine casing.

In order to avoid these problems, a purge gas assembly is added to the device design. This assembly provides

a purge gas for which sonic velocity is substantially higher than air, thereby eliminating sonic velocity

concerns. Windage losses and power requirements are both minimized by selecting a purge gas with a low

atomic/molecular weight.

Since its development in 1998, this method has been used in over 25% of the US nuclear power units, has

been endorsed by the Electric Power Research Institute (EPRI) and has won acceptance by the US Nuclear

Regulatory Commission (USNRC) [2, 3, and 4].

4. Mechanical details of alternate over-speed testing device

While basic theory is the same for testing of turbines of any size, there are significant differences in both

mechanical and installation details for small, versus large turbine implementations. Typical test

implementations for small turbines, less than 1,000 HP, have been temporary, as they can be set up and

restored in just a few hours. These small turbines can be tested using power transmissions which utilize

synchronous gear belt drives due to the low windage, power requirements and resulting minimal bearing side

loads.

Larger turbines require the introduction of purge gas to the turbine casing, as described earlier. Additionally,

rigid power transmissions are also required to prevent unacceptable asymmetrical bearing loading due to the

application of motor torque during rotor acceleration.

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4.1 Testing of small turbines, less than 1,000 HP

A typical temporary installation for the alternate testing device can be seen below, in Figure 1. The pictured

test set-up is for a model number GS-2N Terry® Turbine used for Reactor Core Isolation Cooling (RCIC) in

a US Boiling Water Reactor (BWR) nuclear plant. Note the synchronous gear belt drive and small drive

motor size which can be used for testing of such small turbines. (turbine under test is to the left and the

driven pump, currently uncoupled, is on the right)

Figure 1 Installation of drive motor assembly on a RCIC turbine in USA

4.2 Testing of larger turbines

A typical permanent installation for the alternate testing device can be seen below, in Figure 2. The pictured

test set-up is for a model number CCS Terry® Turbine used for High Pressure Coolant Injection (HPCI) in a

US Boiling Water Reactor (BWR) nuclear plant. Note the rigid gear-box type power transmission and large,

permanently mounted motor required for testing of the larger turbine models. (turbine under test is to the left

and the driven pump, currently uncoupled, is on the right)

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Figure 2 Installation of drive motor assembly on a HPCI turbine in USA

As discussed earlier, testing of larger turbines requires the addition of purge gas to the turbine casing to

mitigate the effects of windage loss and eliminate rotor tip sonic velocity as a concern. The purge gas is

typically added to the turbine case, and then measured at the turbine shaft glands, as can be seen in Figure 4.

Figures 3 and 4 show schematically how the test device is installed on the HPCI turbine which appears in

Figure 2.

Figure 3 Schematic representation of HPCI turbine /pump set, with coupling spacer removed

HPCI

Turbine

HPCI

Pump

HPCI

Booster

Pump

R

P

M

Trip

G

L

A

N

D

G

L

A

N

D

Stop Valve

Gland Leakoff

P

Gland Pressure

P

Local

Cabinet

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Figure 4 Schematic representation of HPCI turbine /pump set, with testing equipment installed

5. Example testing results

Figure 5 shows typical results for an over-speed test performed on a HPCI drive turbine. Due to the

permanent nature of the drive motor installation the turbine can be uncoupled from its driven pump set, all

testing performed and the system restored in less than one 12 hour shift. As can be seen in the figure below,

actual over-speed testing of the turbine takes less than 35 minutes. The turbine control test device (TCTD)

used to perform the test for which the results are shown in Figure 5 utilizes a micro-processor controlled

variable speed drive. This computer control results in very reproducible rotor acceleration and is responsible

for the narrow band of over-speed test results for sequential tests.

HPCI Turbine Over-Speed Test Results

0

1000

2000

3000

4000

5000

6000

0 20 40 60 80 100 120

Time, in minutes

Turbine Speed, RPM

Turbine RPM

Minimum Acceptable

Maximum Acceptable

Figure 5 Typical over-speed test results for a HPCI drive turbine

Coupling

Coupling

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6. Conclusion

In conclusion, the described alternate method for over-speed testing of nuclear Safety-Related drive turbines

does not require steam to test the mechanical over-speed trip set-point. It allows testing to be performed

quickly and safely, with far more consistent results than were previously possible. The method has been used

in over 25% of the US nuclear power units, has been endorsed by the Electric Power Research Institute

(EPRI) and has won acceptance by the US Nuclear Regulatory Commission (USNRC).

7. References

[1] “Turbine Control Testing Device.”

United States Patent 6,582,184B2. Date 7-17-01.

Taiwan Patent 515861. Issue Date 7-17-01.

International Patent Cooperation Treaty, Application No. PCT/US02/015312. Date 7-17-01.

[2] “Terry Turbine Maintenance Guide, HPCI Application: Replaces TR-105874 and TR-016909-R1,”

EPRI, Palo Alto, CA: 2002. 1007459.

[3] “Terry Turbine Maintenance Guide, RCIC Application: Replaces TR-105874 and TR-016909-R1,”

EPRI, Palo Alto, CA: 2002. 1007460.

[4] “Terry Turbine Maintenance Guide, AFW Application: Replaces TR-105874 and TR-016909-R1,”

EPRI, Palo Alto, CA: 2002. 1007461.