The hottest s209e combined cycle waste heat boiler

Analysis and treatment of water supply system failure of s209e combined cycle waste heat boiler

Wenzhou Longwan gas turbine power plant stag209e gas steam combined cycle is equipped with two sets of PG9171E Gas turbine generator units made by Ge in the United States, two sets of waste heat boilers made by CMI in Belgium and a set of steam turbine generator units made by GE. The total installed capacity of the power station is 330 MW

the waste heat boiler is a vertical, forced circulation, double pressure, no afterburning boiler; The rated evaporation capacity is 175 T/h. The feed water deaeration system of two boilers is a common main pipe, a common deaerator, and three 100% of the maximum evaporation capacity of the boiler. 2. Spring fatigue test motor: it is a motor-driven feed pump that controls the operation of the components of the spring fatigue test machine. Since it was put into operation, there have been many failures in the water supply system. This paper analyzes the cause of the accident and introduces the transformation scheme for reference. 1 equipment and fault overview

1.1 basic parameters of feed water pump

model: mn508a impeller stage: 8 stages

rated flow: 237.9 m3/h lift: 847 m

speed: 2 970 R/min absorbed power: 645 K w

recirculation valve: pressure type (integral structure with check valve)

manufacturer: Belgium ensival

1.2 fault overview

since the waste heat boiler was put into operation, There have been many abnormal phenomena, such as water leakage between high barrier membranes with clear shell level of the feed pump, damage to the flange gasket of the warm-up pipeline of the feed pump, high pressure behind the balance plate of the feed pump, wear and seizure of the balance plate, etc; In addition, the valve core of the electric valve of the feed water bypass was broken, the feed water regulating valve was leaking, and the water level of the standby boiler drum rose during the operation of one boiler. Therefore, the reliability of water supply system has become a serious hidden danger for the safe operation of combined cycle units

2 failure cause analysis

2.1 the flow margin of the feed pump is too large

according to the requirements of the technical code for design of thermal power plants (sdj1-84), the capacity of the feed pump supporting the drum boiler should be 110% of the maximum continuous evaporation of the boiler. However, since the waste heat boiler is designed with a main steam bypass system with 100% capacity, there is a 25% margin between the flow of the combined cycle water pump and the rated evaporation capacity of the waste heat boiler. The increase of the flow margin leads to the increase of the pressure margin of the feed water system at the same time. Although the flow pressure characteristic curve of the feed water pump is relatively flat, due to the low load rate of the peak shaving combined cycle power plant, the unit operates at low load for a long time, The pressure of water supply system is too high

2.2 the control mode of the feed water pump is unreasonable

the logic setting of the feed water control system provided by the boiler manufacturer is that one feed water pump must be put into operation before the flue gas baffle of a waste heat boiler is opened and heated. If the second waste heat boiler is put into operation, two feed water pumps must be put into operation in advance. Therefore, when the output of the waste heat boiler is equal to 50% of the rated working condition during the startup of the two boilers, the feed water pump has been far from the design working condition

the recirculation valve of the feed pump and the check valve at the outlet of the feed pump are integral, and the valve rod of the check valve drives and controls the recirculation action. According to the manufacturer's information, if the feed water flow is lower than 65 m3/h, the recirculation valve will open automatically. If the feed water flow is higher than 65 m3/h, the recirculation valve will open automatically. However, from the analysis of actual operating conditions, it is difficult to ensure that recycling customers only need to import the desired capacity and process effect into the system flow

during the cold start-up of the waste heat boiler, the feedwater flow is almost zero for about half an hour, and the feedwater pump only relies on the recirculation pipeline for heat dissipation. The efficiency of the feedwater pump is low under small load, and the power transmission loss is converted into the heat of feedwater. The long-term circulation makes the feedwater temperature rise, and the leakage of the balance plate causes partial vaporization through the gap process, causing pressure fluctuations, causing high-frequency cavitation induced vibration, resulting in the wear of the balance plate

2.3 frequent startup of feed water pump

for the balance plate, the instant of constant speed feed water pump startup is one of the worst operating conditions, while the feed water pump of the combined cycle unit as peak shaving starts and stops very frequently, which is a fatal cause of balance plate damage

2.4 the structure of the balance plate of the feed pump is unreasonable

since the feed pump was put into operation, the difference between the pressure behind the balance plate and the inlet pressure of the feed pump has been as high as 0.2 ~ 0.3 MPa. From the field mapping analysis of the disassembly of the water pump, the radial clearance, especially the axial clearance, between the balance plate and the balance seat seriously exceeded the standard, resulting in the reduction of the differential pressure after the first 6.3 of the balance plate if the extensometer is used; In the process of changing the working condition of the feed pump, it is easy to cause low-frequency shaft shifting and wear of the balance plate; Such a vicious cycle leads to insufficient balance force to balance the axial thrust, and finally causes the balance disc and balance seat to bite

in addition, the feed pump lacks necessary monitoring and protection means (such as axial displacement measuring points), and the health status of the feed pump cannot be judged during operation, which brings hidden dangers to the development of accidents

2.5 the pressure difference between the front and rear valves of the feed water pipeline is large

due to the sliding pressure operation of the waste heat boiler turbine of the combined cycle power plant, when the load decreases, the drum pressure decreases, while the outlet pressure of the feed water pump rises, and the pressure difference before and after the feed water regulating valve changes maliciously, which accelerates the scouring of the valve core of the feed water regulating valve, destroys the flow characteristics of the regulating valve, and increases the leakage

when one waste heat boiler is in operation and the other waste heat boiler is in cold state, the valve core of the feed water bypass electric valve is broken under the pressure difference of 10 MPa before and after the feed water electric valve of the standby boiler

3 equipment transformation of water supply system

3.1 changing the operation mode of water supply pumps

"main pipe" water supply system has the advantage of controlling the number of water supply pumps under different load conditions, which can reduce the pressure of water supply system as much as possible and obtain better energy-saving results. Therefore, make the following changes to the feed pump control system:

(1) cancel the logic that only one feed pump operates and the second waste heat boiler starts locking protection

(2) when the sum of the feed water flow of two waste heat boilers is greater than 200t/h

(considering the desuperheating water flow), and only one feed water pump operates, the logic of the second feed water pump is linked

the sum of the feed water flow of the two waste heat boilers is less than 180t/h, and the two feed water pumps are running, an alarm signal will be sent for the operator to confirm whether to stop one feed water pump according to the load

the sum of feed water flow of two waste heat boilers is less than 160t/h, and two feed water pumps operate, and one feed water pump will automatically stop

this can reduce the time of low flow and high pressure operation of the feed pump, reduce the pressure borne by the feed system, reduce the damage of the balance plate of the feed pump, and improve the safety; Moreover, the operation of the pump in the high-efficiency area near the design working condition can not only improve the economy of the system under variable working conditions, but also