Plate heat exchangers are widely used in many industrial applications because of their efficient heat exchange capacity and compact structure. However, in some cases, the micro-deformation characteristics of plate heat exchangers can cause pressure fluctuations in the lubricating oil system. Here is a detailed analysis of the problem and what can be done about it.
1. The working principle and characteristics of plate heat exchanger
Plate heat exchanger is mainly composed of fixed press plate, movable press plate, upper and lower guide rod, rear pillar, plate, clamping bolt, clamping nut and other parts. The plate is sealed by a gasket and suspended between the fixing platen and the heat exchange plate, and is clamped by a clamping bolt. Protective cover can be selected as required.
Working principle: Two kinds of working fluids flow alternately in a narrow and tortuous channel formed between the plates, separated by a layer of plates in the middle, through which heat exchange is carried out. Plate heat exchangers can achieve an average temperature difference as small as 1℃, and the heat transfer efficiency is as high as 99%. In the case of the same pressure loss, its heat transfer effect is 3 to 5 times that of the traditional tubular heat exchanger, the floor area is only 1/3 of its, and the metal consumption is only 2/3 of its. According to the different process conditions, the plate heat exchanger can form different process combinations through the parallel or series combination of multiple flow channels.
Plate heat exchanger has the characteristics of compact structure, small footprint, high heat transfer efficiency, flexible operation, wide application range, small heat transfer loss, convenient installation and cleaning.
2. Micro-deformation anomaly analysis of plate heat exchanger
2.1 System Devices
B plant of a power generation Co., Ltd. installed two 1,000MW ultra-supercritical coal-fired units, each unit is equipped with two feed water pump steam turbines, each feed water pump steam turbine is equipped with an independent lubricating oil system, including two sets of small plate heat exchangers, one operating, one standby. The thickness of the heat exchanger plate is 0.5mm, and each plate is equipped with a sealing gasket.
2.2 Accident Course
In the process of regular rotation of closed cooling pump, after starting No. 2 closed cooling pump, stop No. 1 closed cooling pump at 11:07, the steam turbines of No. 1 and No. 2 feed pump trip successively, with an interval of 85 ms, and the first MEH trip is "lubricating oil low pressure" (fixed value 0.07 MPa). Through consulting the historical curve, it is found that the alarm of low lubricating oil and low stator cooling water flow of feed pump turbine is issued at the same time as the switching time of closed cooling pump. 11:07:08 Shut down No. 1 closed cooling pump; 11:07:11 The pressure of closed cooled jellyfish tube suddenly dropped from 0.78MPa to 0.56MPa (the lowest), and the fluctuation range of water pressure reached 0.22MPa; 11:07:11 2 feed water pump steam turbine lubricating oil pressure low Ⅱ switch reached the operating value (0.07MPa), feed water pump steam turbine trip; 11:07:11 Low stator cooling water flow protection reaches the action value, but due to the existence of 30 seconds delay, the protection does not trigger the unit trip logic (stator cooling water cooler is a plate heat exchanger).
2.3 Cause Analysis
The core component of the plate heat exchanger is a stainless steel heat exchange plate with a thickness of about 0.5 mm. The heat exchange plate is usually pressed into a variety of groove or corrugated shape by mold, which can increase the stiffness of the plate, prevent the compression and deformation of the corrugated plate, and enhance the turbulence degree of the fluid and the heat exchange area. Because the plates of the plate heat exchanger are thin, when the pressure difference between the two sides of the heat exchanger changes, the shape of the plate will change slightly, resulting in the volume of the heat exchanger on both sides will also change. This feature is often overlooked in practical applications.
In the process of rapid fluctuation of closed cooling water pressure, the heat exchange plate of the plate cooler is slightly deformed by the pressure difference on both sides, the water side space shrinks rapidly, the oil side space expands, and the lubricating oil in the system expands to fill the increased space on the oil side of the heat exchanger, resulting in the lubricating oil pressure of the feed pump turbine fluctuating to less than 0.07MPa instantly. When the operating value of the lubricating oil pressure low II switch is reached, the two feed pump steam turbines trip successively.
3. Handling measures
In order to avoid the trip of the feed pump turbine caused by the fluctuation of lubricating oil pressure caused by the plate heat exchanger, the following measures can be taken:
Adjust the protection logic: In the feed pump turbine lubricating oil pressure Ⅰ connection backup lubricating oil pump protection logic, cancel the delay of 3 seconds to ensure that the backup oil pump can start in time.
Install an accumulator: Install an accumulator in the oil supply line of the lubricating oil system (especially near the suction point of the pressure tube). When the lubricating oil pressure is reduced for a short time, the stored lubricating oil in the accumulator is released to compensate the system pressure and effectively alleviate the impact of the momentary fluctuation of lubricating oil pressure on the system.
4. Closing remarks
Plate heat exchangers are widely used because of their high efficiency and compact characteristics, but special attention should be paid to the influence of micro-deformation characteristics on the safe and stable operation of the system in the design and application. By taking the above measures, the fluctuation of lubricating oil pressure caused by micro-deformation of plate heat exchanger can be effectively reduced and the normal operation of equipment can be guaranteed.
