Analysis and Treatment of Common Failure of the Turbine Driven Auxiliary Feedwater Pump in One Nuclear Power Station

YANG+Jie-hai　XＩＡＯ+Yi-ting

１Introduction

Since the commissioning of the turbine driven auxiliary feedwater pump, within three years operation, a series of failures have occurred, such as failure of over-speed trip gear, low output, high vibration, trip gear reset failure, and so on. According to the manufacturers advice and on-site maintenance experience, we carried out modification on the structure of the pump, optimized the acceptance criteria for test. In this paper, the analysis and treatment of major defects are briefly introduced, in order to facilitate the analysis of similar problems in other power plants.

As we know that the auxiliary feedwater system serves as a back-up system for supplying feedwater to the secondary side of the steam generators upon loss of the main feedwater supply. The system is classified as an engineered safeguard system. It is composed of 2 motor-driven pumps and 2 turbine-driven pumps.

The auxiliary feedwater pump is a type of TWL 45S supplied by Clyde. The pump supplies feedwater to the steam generators in the following cases:

（１）During temperature increase of the reactor coolant system.

（２）During hot shutdown; to keep the reactor in the cold shutdown condition, permitting actuation of the Residual Heat Removal System.

The Type TWL 45S pumps are of horizontal, two-stage centrifugal design driven by a steam turbine contained in a turbine casing integral with the pump casing. The turbine wheel, has a single row of blades, the pump impellers, turbine wheel and inducer are mounted on a common shaft which is supported on two water-lubricated journal bearings. The bearings are housed in a central water chamber between the turbine and pump casing, and are lubricated by water taken from the discharge of the first stage impeller and led to the bearings through a self-cleaning filter. The pump is supported on the pedestals of a fabricated steel baseplate by feet formed on the pump casing and central water chamber, while the mono block construction of the pump eliminates the need for alignment between the pump and the turbine.

The operating speed of the pump is governed by the turbine control gear which regulates the quantity of steam to the turbine. The main elements of the control gear are the steam stop valve, the throttle valve and the pressure governor. The pump is also provided with electrical and mechanical over speed trip mechanisms which close the steam stop valve when the speed reaches predetermined levels. Speed measurement is provided by an electronic tachometer.

２Failures of the pump

２．１Failure of over-speed trip gear

In June 2014, the overspeed test being carried out, when we stopped the pump manually, however, it was still running with a speed at 8400rpm. We had to stop the pump by closing the steam isolation valve. During the inspection, we found that the stop valve lock nut on the bobbin had sheared, damaged with all of the threads holding the nut. At that time, we believed that the failure was an isolated incident, perhaps due to mishandling or fitting. Then we just replaced a new component. However, a few months later, the similar problem came to the other units.

If the stop valve failed in the open position, there would be no method to trip the machine during an overspeed event. The machine would be able to accelerate.

As previously introduced, two separate types of overspeed mechanisms are provided for the safety of the turbine driven pump namely; a primary electronic overspeed trip which operates via an electronic tachometer; a secondary mechanical overspeed trip which is actuated by a trip bolt contained in the pump/ turbine shaft. In each case, the trip operates by releasing steam pressure on the downstream side of the trip device piston, allowing the steam pressure on the upstream side of the stop valve bobbin to close the steam inlet, thus stopping the turbine/ pump.

The trip device consists of the following components: bobbin, piston, piston rings, lock washer, lock nut, orifice, guide bush, liner.

The root reason is locked in a design flaw of the stop valve lock nut after a series of investigations by the manufacturer. In order to ease the assembly of the components, the nut which holds the piston on to the bobbin has been reduced from an 1.3/4" thread to an M30 thread. This change gave additional space between the piston bore and the lock washer, easing the tabbing of the lock washer, as previous designs were difficult to correctly secure. However, the ability to resist shear weakened.

It has been concluded that the failures were a result of the incorrectly machined nut threads coupled with the reduced nut size (from 1.3/4” to M30) being unable to accommodate the loadings in service. So, the manufacturer increasing the diameter of the nut from M30 to M42 - making the diameter very similar to the 1.3/4” nut used before. The nut design has been modified, removing the closed end. This makes the nut easier to manufacture, control and inspect. At the same time, it also increases the available thread engagement and hence reduces shear stress.

２．２Low output problem of the pump

When we start the turbine driven pump with discharge valve closed and make it operate on its recirculation line, the output of the pump is always on the low side. Recent test data shows the speed 8450rpm, the flow rate 26.7m3/h, the discharge pressure 128.4bar.g. According to the periodic test criteria which pressure is 132bar.g with ±3% error allowance, the pressure is not acceptable.

As we all know, the speed of the pump will change with the change of steam inlet pressure. If the steam inlet pressure reduces, the speed reduces, and then the discharge pressure will also decrease. It is a characteristic of the turbine driven pump. After calculation, we can get a result that if we want that the discharge pressure reaches bout 132bar.g, the steam inlet pressure must be about 75bar.g. However, most of the test conditions, the steam pressure is about 66bar.g. So the reason of the low output goes to the test criteria.

Later, we changed the test criteria with the discharge pressure value changed to”No drop in performance of pump”. How can we judge that whether the performance of the pump is falling? Each person may have a different way. I have a suggestion. Manufacturer gives some test curves which contain a curve with steam inlet pressure 64bar.g. The operating condition of the test is similar to the curve. In accordance with pump test code ISO 9906:2000, we can translate the result into data based on the curve. All test data obtained at the speed of rotation n in deviation could be translated to the basis of the specified speed of rotation nsp. The measured data on the pump total head H, can be converted by means of the equation:

Also, tolerance would be better in ±5% according to ISO 9906, as the turbine driven pump is a grade 2 pump. After checking, the test data are qualified.

The low output problem of the pump may also be caused by the jam fault of the throttle valve. The throttle valve is operated by the differential pressure across governor piston to control the steam flow to the turbine and thus regulate the output from the pump to satisfy operational requirements. The differential pressure governor controls the steam flow to the pump turbine by operating the throttle valve in the stop valve chest via the linkage connecting the pressure pistons spindle on the water side of the governor with the steam throttle valve spindle on the steam side. It does so in response to forces generated within the governor related to live steam operating pressure, pump discharge pressure and pump flow. Related structure is as follow picture.

Recently, in commissioning of the new unit, turbine driven pump mini-flow operation conditions, there came a problem that the out-put was too low. The speed was 8100rpm, the discharge pressure was 120bar.g, and the steam inlet pressure was 74bar.g. It was obvious that the test results were not qualified. After the inspection, we found that the throttle valve performances jamming phenomenon. Through disintegration of valve, we saw that governor piston wore. After polishing, the jamming phenomenon disappeared. In fact, usually, jam fault appears in the bearing. It leads to that the valve movement is not flexible and the steam is hard to control.

２．３Vibration problem of turbine driven pump

The turbine driven pump differs in structure with other types of centrifugal pumps. As the turbine and pump sharing one shaft and one shell, few vibration problems appear. Since commissioning, there have been twice vibration problems. June 2014, during the period of mini-flow test, the vibration of the pump exceeded the standard. The situ measurement value was 8.58mm/s, beyond the alarm value 7mm/s. However, the pump vibration probe showed that the value was only 2mm/s. At the same time, the pump speed, flow, pressure and other parameters were qualified. So we judged that there was no problem in the internal structure of the pump. After that, we checked the import and export pipeline stent, and founded no loose. Then we checked the torque of anchor bolts and the bearing cap bolts. After examination, we found that two of the bearing cap bolts were loose. The bolt torque was 160N.M, lower than the standard torque 195N.M. After fastening, the vibration value of the pump was qualified. Vibration value decreased to 2.34mm/s. Since then similar problem occurred in September 2015. The reason was that the anchor bolts were fastening. In principle, with regard to the vibration problem, we find the cause from simple to complex, step by step usually.

２．４Failure of the mitre valve

The mitre valve plays an important role in the trip gear. When the ‘overspeed trip is operated, either electronically or mechanically via the shaft mounted trip bolt, the mitre valve opens (see the picture below), the steam pressure on the underside of the piston is suddenly released and the pressure on the upper side generates a force which rapidly closes the stop valve. As the mitre valve will remain open until the trip gear is reset, the steam leakage past the piston is led to the turbine exhaust through the port in the trip gear cylinder, the pressure on the underside of the stop valve piston cannot build up and the stop valve will remain closed. When the mitre valve is closed, the steam pressure will again build up under the piston and the turbine will re-start.

In 2014, during the turbine driven pump commissioning of unit 2, after overspeed trip, the trip gear could not be reset. After trip gear cooling and manual operation, failure disappeared. After analysis, several possible reasons were obtained. It might be due to impurities between the mitre valve core and valve seat, causing the valve cannot be reset. It also might be due to the temperature difference, resulting in a small gap between the valve core and valve seat, leading to a card. At last, it might be that the valve core failed to find the central of the mitre valve after trip. Then we dismantled the valve. We founded the mitre valve core wear and tear, as pictures below shown. We analyzed the design of the valve core existed defects. The existence of the right angle of the valve core increases the risk of the card.

Because other power plants have similar problems occur, the manufacturers have made an improvement. A radius is added in the valve core. They introduced the radii to eliminate the possibility of any further fouling. After the replacement of the new spare parts, the same problem does not happen again.

２．５Failure of the steam isolation valve

The steam isolation valve is located in the upper reaches of the turbine driven pump, although it is not a part of the pump, it controls the inlet steam, and the failure will lead to the failure of the pump. Since commissioning of the valve, it frequently appeared to open a timeout or could not open. As the valve affects the function of the pump, it is necessary to simply introduce the problem of the valve. The valve is pneumatic valve, the actuator is composed of cylinder and spring, and the body structure is a double gate valve with a spring. Solenoid valve excitation to supply air to the cylinder, then push the valve stem to close the valve. When the valve receives the signal to open, solenoid valve degaussing, cylinder exhaust, then the valve opens under the action of spring. The structure shows as below.

The opening time required of the valve is less than 20s. Take unit 2 as an example, in October 2012, the valve could not open; in November, the time was 21s; in December, the valve was automatically opened after 86s. In order to make the opening time qualified, the following measures have been done.

１）The air supply pressure of the cylinder was adjusted to 4.5bar by 6bar.

２）The stroke of the valve is adjusted to 68.5mm by the original 70mm.

３）Add water containing graphite to the packing for lubrication.

After the implementation of the measures, the opening time is about 17.2s. However, the root cause is not clear. We cannot accept that the opening time only be qualified after treatment. After all, a second is very valuable once the accident happens. After a series of analysis, reasons for the slow opening are found. It is the following reasons that lead to the problem.

１）Cylinder driving force margin is insufficient. It may be caused by stiffness of the spring insufficient.

２）The friction is too large, including packing and disk.

３）Exhaust capacity of the cylinder is insufficient: the solenoid valve is small.

So we got a hole which size is Φ6mm in the opening side of the disk, to reduce friction. Opening timeout problems did not appear after the improvement. But the opening time is about 19s, close to the limit value. Then the small solenoid valve V301 was replaced by large solenoid valve MT302, to speed up the exhaust velocity. The time was less than 10s after changing the solenoid.

３Summary

The turbine driven auxiliary feedwater pump plays an important role in nuclear safety. It is used to provide adequate feedwater flow to the secondary side of the steam generators during unavailability of the main feedwater system. Its performance seriously affects the nuclear safety function of nuclear power station. This paper briefly introduces the main defects of the turbine driven pumps of 4 units in one nuclear power plant, and the analysis and treatment of the corresponding problems. From the article, we can see that the defect is mainly manifested in two categories, one is stuck in trouble, and the other is the deviation of the test data. For the card trouble, generally due to the design defects of the structures, so manufacturers have made some improvements. For the deviation of the test data, after analysis, we believe that the test criterion is not reasonable, so we optimize the criteria. The implementations of these measures, have further improved the availability of the pumps, and effectively guarantee the realization of the nuclear safety function. I hope that the introduction of this paper can give some help to other power plants, and the corresponding treatment process can provide some ideas for you.

【References】

［１］Su Linsen.Device & Systems of 900MW PWR[M].Beijing:Atomic Energy Press,2007.

［２］Jiaying.Analysis and Troubleshooting to Steam Feed Water Pump Common Problems[J].Inner Mongolia Electric Power,2011,29,59.