Application of Thermal Mass Flowmeter in Nuclear Power Plant

Flow is one of the important process parameters in the operation process of nuclear power plants. Common flow measurement instruments include orifice flowmeters, ultrasonic flowmeters, electromagnetic flowmeters, and thermal mass flowmeters. Thermal mass flow meters are currently used for the measurement of gas flow in the industry, and are commonly used in flow measurement occasions for ventilation systems in nuclear power plants. In a nuclear power plant using a shaft seal type main pump, the leakage leakage of the third stage of the main pump is a typical small flow condition. Orifice flow meters and ultrasonic flow meters are generally not suitable or do not provide good measurement results. Since its introduction, thermal mass flow meters have been successfully applied in the ventilation system of nuclear power plants, and thermal mass flow meters also have certain applicability in the measurement of liquid micro-flow.

1 Principle of Thermal Mass Flow Meter 1

Thermal mass flow meter is the use of heat transfer principle, that is, through the flow of heat and heat exchange between the heat source to measure the flow of the instrument, can be divided into two categories: the use of fluid flow to transfer heat to change the measurement of wall temperature distribution ( The heat-distribution-effect heat-distribution flowmeter, also known as a calorimetric mass flowmeter; an invasive mass flowmeter that uses a heat dissipation effect (based on Kim's law), also called a thermally conductive mass flowmeter or plug-in type Mass flowmeter.

The working principle of the thermal distributed mass flowmeter is to wind two sets of heating/detection coils on the outer wall and downstream of the measuring tube. The two sets of coils are heated by a constant current. When the fluid is stationary (no flow passes), the two sets of coils are centered. The downstream temperature distribution is in a symmetric equilibrium. The resistances of the two sets of detection coils are equal. When there is a fluid flow, the fluid will carry heat away from the upstream tube wall and pass it to the downstream tube wall, destroying the original equilibrium state, causing a difference in the coil resistance and detecting the difference. The value thus obtains the fluid mass flow:

(1)

Where A - thermal conductivity coefficient;

cp—The specific heat capacity at the constant pressure of the measured medium;

K———constant.

Heat Dissipation Effect (Based on King's Law) The working principle of the intrusive mass flowmeter is to place two temperature sensors (generally platinum RTDs) in the fluid of the pipeline. The temperature of the fluid itself is measured by one of the platinum resistors. Another platinum resistance is heated by a certain amount of electric power. Its temperature TV is higher than T. When the fluid is at rest, its temperature is the highest. With the increase of the fluid flow in the pipeline, the fluid flow takes away more heat to lower the TV and can pass. Temperature difference to obtain the flow value. The hot wire loss rate based on King's law is:

(2)

Where cv———the specific volumetric specific heat capacity of the fluid;

d——— hot wire diameter;

H/L——— thermal loss per unit length;

V—fluid flow rate;

ρ———the density of the fluid;

λ—The thermal conductivity of the fluid.

The invasive mass flow meter is divided into a constant power type, a constant temperature difference type and a constant ratio type according to the heating method of the platinum thermal resistance.

The constant power type is to heat the platinum resistance with a constant power on the heating circuit. When the fluid medium is static, the temperature difference between the heated platinum resistance and the unheated platinum resistance is the largest, with the flow of the fluid medium. As the temperature on the heated platinum resistor decreases, the temperature difference between the two platinum resistors decreases. The constant power thermal mass flow meter obtains changes in fluid medium flow by measuring changes in temperature differences.

The thermostatic type heats a platinum resistor to a constant temperature above the unheated platinum resistor. With the flow of the fluid medium, the heated platinum resistor causes a decrease in temperature due to heat dissipation, and the feedback circuit feeds back the processor to increase the heater current (which may also be a voltage) to maintain a constant temperature difference, and then detect the change. The current (or voltage) to obtain the change in flow rate.

The constant ratio type is based on the principle of constant temperature difference, by adjusting the heating current applied on the heating resistance of the heating end to ensure that the resistance of the heated platinum resistance becomes a constant ratio with the resistance of the unheated platinum resistance. Like the constant-differential mass flowmeter, the constant-rate mass flowmeter also obtains the flow value of the fluid in the pipeline by detecting the changed current.

2 Measurement of Micro Flow in Nuclear Power Plants and Application of Thermal Mass Flow Meters

2.1 Typical Microflow Measurements in Nuclear Power Plants

In a nuclear power plant using a shaft-sealed reactor coolant pump (main pump), the tertiary seal leakage flow of the main pump is a typical small flow condition. The shaft seal system is a key component of the shaft seal type main pump. Its long-term reliable operation not only relates to the normal operation of the nuclear power plant, but also directly affects the safety of the nuclear power plant. The function of the shaft seal is to ensure that the leakage of the reactor coolant from the primary circuit of the pump system along the axial direction of the main pump in the normal operation of the nuclear power plant is substantially zero. It is required that the coolant leakage condition be maintained for a long period of time and maintain the controllable leakage rate. . Once the shaft seal fails, the integrity of the pressure boundary of the primary circuit of the nuclear power plant will be lost. If the leakage rate exceeds the replenishment capacity of the chemical-capacity system, it will lead to the occurrence of a potential small-break water loss accident and affect the safety of the power plant. A typical fluid dynamic pressure type shaft seal system usually consists of 3 series mechanical seals and 1 stop seal. The pressure relief system distributes the primary system pressure across the series seals, reducing the pressure on the single seal. Increase the long-term reliability of the main shaft seal. With three levels of pressure reduction, the pressure in the third seal of the main pump is reduced to atmospheric pressure. Considering that if the leakage flow of the third stage of the main pump is too large, it means that the seal may have been worn or damaged, and the flow rate is too small, which will adversely affect the heat dissipation and lubrication of the sealing surface and affect the life of the shaft seal. Therefore, in the normal operating conditions of the nuclear power plant, the leakage flow of the third stage seal of the main pump is about 5L/h. When the flow rate reaches 50L/h, an alarm is required to alert the operator.

2.2 Commonly Used Main Pump Third Stage Seal Leakage Flow Measurement Method

At present, in a number of nuclear power plants (such as Qinshan Nuclear Power), the method of volumetric flow measurement is adopted, that is, the leakage rate is calculated by measuring the rate of change of the leakage flow of the main pump seal in the container through a level gauge. The measurement diagram is shown in Figure 1.

Fig.1 Schematic diagram of volumetric flow measurement device

The volumetric flow measurement device is generally composed of a container (allowing inflow and outflow) and a level switch. When in normal operation, the liquid level in the container changes as the rate of liquid inflow and outflow changes:

(3)

Where a—the area of ​​the container’s vent;

A—The cross-sectional area of ​​the container;

f (t) --- instantaneous flow of leakage flow;

h(t)———the liquid level in the container.

When the flow rate increases, the flow into the container is greater than the flow out of the container downflow opening, and the liquid level in the container starts to rise. When it rises to the set point in the container, the level switch triggers and an alarm occurs, indicating that the flow of the third stage leakage flow is excessive. If the liquid level measuring instrument is changed from a liquid level switch to a liquid level transmitter and the data acquisition and processing system is used, real-time flow of leakage flow can be obtained.

Trunnion mounted ball valves are designed for upstream sealing. The load generated by the line fluid on ball is absorbed by the trunnion bearings and is directly transmitted to the valve body, and the upstream seat driven by medium pressure drifts towards the ball to creater reliable sealing. The seat design has a built-in automatic cavity relief mechanism.


Trunnion mounted ball valves have a bottom entry stem design, where the stem is inserted from inside the body. The higher the line pressure, the tighter the seal. An elastomer seal in the form of an o-ring is located at the lower end of the stem, which provides the first level of protection to the atmosphere. Valves are provided with vent and drain connections for venting/draining of the valve cavity. Vent and drain connections can also be used for on-line affirmation of valve sealing.

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