At present, China has dozens of large and medium-sized hydropower stations, and it is in a new high tide of water conservancy and hydropower construction. As a result, the amount of cement grouting is enormous. For example, curtain grouting needs 323 km, consolidation grouting needs 418.9 km, and backfill grouting needs 135.4 km in hydropower stations.
The unique nature of this industry is that the electromagnetic flowmeter should choose the correct using, maintenance, and construction process requirements. This is to ensure the quality of construction and the meter can operate correctly and safely, and it will bring significant benefits to the actual work in the future.
1. Measurement characteristics of electromagnetic flowmeters
The flow measurement is critical in the construction process of slurry and water in the grouting industry. Our liquid flow detection instruments are different from public enterprises, and They need to facilitate frequent cleaning and withstand the friction of particles in high-speed slurries and harsh environments. The details are as follows:
- The sensor measuring circuit of the electromagnetic flowmeter is simple, without flow blocking parts, and easy to stall the media parts.
- The electromagnetic flowmeter media contact parts material can withstand the friction of the solid phase particles in the cement slurry at high flow rates and the corrosive effect of chemical slurry.
- The electromagnetic flowmeter has fast response performance and high measurement accuracy.
- The electromagnetic flowmeter can adapt to high temperature, humidity, high dust, and harsh construction environment.
In many liquid flow instrumentation products, electromagnetic EMF has measurement independent of fluid temperature, pressure, density, viscosity. The electromagnetic flowmeter is smooth and has a direct electrical size inside; its response time is fast. When the detection part of the electromagnetic flowmeter has no moving parts, a drip phenomenon will not occur. The electromagnetic flowmeter has high accuracy, and its lining can be made of PTFE plastic and alumina ceramic. It also has the advantage of strong corrosion resistance. In recent years，it has become the instrument of choice for flow measurement in grouting projects.
2. Application of electromagnetic flowmeters in grouting projects
The main process flow of the grouting process is shown in Figure 2, and For effective control during construction, we need to measure and control the water and cement slurry during the construction process.
2.1 Drilling and washing
The grouting is first carried out by drilling in sections from top to bottom in the rock layer. Then, the simple pressure test: at the end of the wash hole, the lower orifice tube, and the sealed orifice are fed with water into the hole, and it is possible to determine the corresponding flow rate values and calculate the permeability of the rock from this. The results of the calculations are relevant to the evaluation of the permeability characteristics of the rock and the collation of the grouting results. This measurement point is critical and sensitive. The water has a specific conductivity and meets the measurement requirements of the electromagnetic flowmeter. The crucial consideration is the diameter of the electromagnetic flowmeter, as the pressure water test and the grouting are the same.
After the pressure water test, the grouting pump will pump a certain amount of cement slurry in water and ash to the hole, part of the slurry will enter the fissure and spread, the remaining slurry will return to the spot through the return pipe and flow back to the slurry mixer. Under the specified pressure, when the injection rate is not more than 0.4 L/min, the grouting will continue for 30 min, or not more than The grouting can be finished when the injection rate is no more than 0.4 L/min for 30 min or no more than IL/min for 60 min. Each borehole requires two EMFs to record the inlet and return flow rates. Respectively, the grouting volume is equal to the inlet minus the return volume. See Figure 3 for site pipework and EMF installation layout.
3. Application Notes
3.1 Impact and handling of construction processes
According to the principle of circular grouting, the return slurry should flow back to the mixing drum, so we should use two electromagnetic flow meters and then measure the flow rate of the slurry in the inlet and return pipes, respectively (as shown in Figure 3).
However, some users have removed the EMF from the return pipe, and the return pipe is connected directly to the inlet pipe downstream of the EMF via a tee. If the return slurry is not returned to the mixing drum, an electromagnetic flowmeter is used to measure the grouting volume. However, some users have removed the EMF from the return pipe, and the return pipe is connected directly to the inlet pipe downstream of the EMF via a tee. If the return slurry is not returned to the mixing drum, an electromagnetic flowmeter is used to measure the grouting volume.
3.2 Measuring the effect and handling of the amount of slurry attached to the pipe
After each grouting, the residual slurry in the measuring tube of the EMF should be removed in time. Otherwise, the cement slurry will quickly produce different degrees of cementation in the measuring line and even block the EMF and the connected grouting tube. In addition, the adhesion layer in the measuring tube of the electromagnetic flowmeter causes additional relative errors.
3.3 Influence and treatment of air bubbles in media
Due to the process or the medium itself, the measured liquid often contains some bubbles. The electromagnetic flowmeter is a flow rate type flow method. The percentage of area occupied by air bubbles in the circular cross-section of the pipe is almost equal to the amount of influence of air bubbles on the flow measurement. Due to the frictional process of air bubbles passing over the electrode surface, a spike pulse disturbance potential is generated, much larger than the standard flow signal. Usually, electromagnetic flow converters are not able to handle this interference effectively.
When the measurement value is unstable and severe, the instrument can not work, some inexperienced users only from the requirements of the process out, the installation position of the electromagnetic flowmeter is not well thought out, to prevent the generation of bubbles, some users to install the electromagnetic flowmeter in the grout pump inhalation end, the inhalation end of the slurry is often mixed into tiny bubble-shaped bubbles, so the electromagnetic flowmeter is generally installed in the discharge end of the pump. Electromagnetic flowmeter preferably installed vertically, it is the slurry to flow from the bottom up. The horizontal installation makes the electrode axis parallel to the horizon, not perpendicular, because the bottom of the electrode is easy to cover by sediment, and the top electrode is easy to be the liquid in the occasional bubble rubbed over the electrode surface.
3.4 The impact and handling of harsh construction site environments
The grouting construction site environment is usually harsh, such as high temperature, moisture, high dust, etc.. If the electromagnetic flowmeter shell is poorly sealed, the junction box is exposed. As a result, condensate and dust are prone to accumulate in the electromagnetic flowmeter junction box or through the poorly sealed joint surface into the electromagnetic flowmeter shell due to the weak electromagnetic flowmeter flow signal (usually a few mA). If condensate and dust exist, it will cause the electromagnetic flowmeter converter input circuit impedance drop, attenuating the flow signal to the amplifier, or damage the excitation circuit and signal.
To avoid failures, we can fill the junction box with insulating material. The maintenance and commissioning of the electromagnetic flowmeter must avoid water, keep the junction box dry and clean, and avoid immersion in water or slurry.
At present, China is at a new high point in the development of hydropower. The amount of cement grouting is enormous, and dynamic and accurate measurement is essential. The electromagnetic flowmeter has become the instrument of choice for measuring the flow in grouting projects with its unique advantages. It has been used to good effect in many large and medium-sized hydropower projects throughout the country.