Vortex flowmeter instructions for use and selection

Project Overview

1. The current technical level of vortex flowmeters and orifice flowmeters:

The basic structure of a vortex flowmeter consists of a vortex generator, a detection element, and a signal processing amplification circuit. Currently, for The research on vortex generators has reached a very complete level, with the triangular generator being the best type. The detection elements include thermistors, strain gauges, piezoelectric crystals, differential capacitors, ultrasonics, etc. Many of the signal processing parts have been computerized. Vortex flow meters are easy to install (can be installed directly on pipelines), small in size, highly interchangeable, and have high long-term operation accuracy. They can be used for measurement of most liquids, gases and vapors. At present, the sales of vortex flowmeters in the world market are increasing by about 30% every year. At present, the technological development level of orifice flowmeter is still based on certain empirical formulas. In 1980, the International Organization for Standardization merged the two standards R541 and R781 into the standard ISO.

Due to its simple structure, low cost, reliability and other advantages, the orifice plate throttling device is suitable for almost all medium measurements, and the matching differential pressure transmitter has developed rapidly, which makes it have its own shortcomings. be compensated.

2. Comprehensive performance evaluation of vortex flowmeter and orifice flowmeter

Orifice flowmeter (referred to as orifice) consists of a throttling device, a pressure sensing device and a differential pressure transmitter Composition, the pressure guide pipe needs heat tracing measures for places that are prone to freezing. There are about 20 static sealing points for a flow measurement loop. There are the following problems during use: easy to freeze, easy to block, easy to leak, and heat tracing can easily cause differential pressure changes. The transmitter components are aging, and isolation fluid needs to be added to the pressure-guiding tube in some situations. Due to unstable heat tracing or process operation, the isolation fluid lines of the positive and negative pressure-guiding tubes are often unequal, resulting in liquid column differences and inaccurate flow indications. The above will cause the flow coefficient to change and the measurement accuracy to decrease. If the pressure guide pipe is shortened and the differential pressure transmitter is installed directly on the pipeline, there will still be a dead zone for flow.

The vortex flowmeter (vortex flowmeter for short) has only 3 static sealing points, which is not easy to leak, has no dead zone of flow, does not require heat tracing and insulation, and is not affected by fluid weight, temperature, pressure, viscosity, etc. Influence, the flow coefficient remains unchanged for a long time. However, when the vortex street is used in vibration situations, the flow measurement will be inaccurate. At present, anti-vibration vortex streets have been launched on the market to overcome the impact of vibration on inaccurate flow measurement.

(1) Initial investment: An imported vortex street is about 20,000 yuan (DN15-DN50), and a throttling device includes a differential pressure transmitter, orifice plate and flange, and pressure guide Pipes, valves, insulation boxes or protective boxes also cost 15,000 yuan. From a long-term perspective, using vortex streets is still cost-effective.

(2) Installation cost: The vortex street installation is simple. You only need to ensure that there is a certain straight pipe section before and after the flow meter, the straight section of the orifice plate, concentricity, pressure pipe, transmitter, and insulated box. All have certain installation requirements, and the installation cost is several times that of vortex street.

(3) Maintenance costs: In addition to periodic calibration requirements for measurement, vortex streets generally do not malfunction., but the orifice plate is not the case. There is a certain amount of maintenance required to eliminate leaks, drain sewage regularly, fill in isolation fluid, replace pressure pipes, valves, insulation, clean the orifice plate, etc. For example, if there are 200 sets of flow orifice plate measurement circuits (requiring insulation and heating), a certain maintenance fee will be invested in the renovation of the insulation and heating system every two years. This does not include the cost of updating the differential pressure transmitter and the orifice plate. Calculated, it is enough to buy a certain number of imported vortex streets.

(4) Operating costs

1. Steam consumption cost: such as 200 sets of flow orifice plate measurement loops (requires thermal insulation and heat tracing ), each tracing point consumes 0.02t/h steam. If the average annual steam consumption is 4,300 hours, the steam cost is 40 yuan/ton, then the annual steam consumption cost is approximately 688,000 yuan, and the annual cost of each circuit is 3,440 yuan. Yuan.

2. Energy consumption cost: The pressure loss of the vortex street is smaller than that of the orifice plate, which is about 1/15 of that of the orifice plate. Therefore, long-term operation consumes less energy for pumps and fans. The orifice plate is 15 times larger than the vortex. When used for gas or steam flow measurement, due to its low density, the volume flow rate of the same pipe diameter is large, the pressure loss is even more serious, and the energy consumption is higher.

3. Leakage sewage charges: The sewage charges depend on the number of sewage discharges, which are generally about 20 times a year. The discharged sewage and materials pollute the atmospheric environment. If the sewage exceeds the standard, the environmental protection department will also impose fines.

(5) Long-term operating accuracy: The design system accuracy of the orifice plate is 1.5%-2.5%. Since the differential pressure and flow rate are linearly related, when the flow rate is lower than 30%, the error increases and the gas is too large. On the other hand, due to the long-term wear and tear of the medium used, the acute angle becomes blunt, causing the flow coefficient to change, which is also an important reason that affects the accuracy. Due to the special structure of the vortex street, when the accuracy is actually determined (about 0.5%-1% of the measured value), the accuracy is almost unchanged.

(6) Interchangeability: The same vortex can measure the flow of gas, liquid and steam. The electronic circuit board and sensitive components are common to flowmeters with different diameters. Signal output usually comes in three forms: pulse, analog and digital signals. It can be switched through the switch on the circuit board, and the user can switch to another output signal at any time according to his or her needs. Changing the range is also easy. For analog output, you only need to change the input pulse frequency. This is not the case with orifice plates. The apertures are designed for specific media, and the interchangeability is poor. It is also difficult to change the measuring range, and the orifice plate needs to be recalculated.

(7) Turndown ratio: Since the output frequency of the vortex street has a linear relationship with the flow rate, the turndown ratio of the flow rate can reach 20:1-80:1, for example: Foxboro company's vortex flowmeter measurement The range ratio of gas and steam is 80:1, and the range ratio of measuring liquid is 40:1. It can adapt to the requirements of larger process flow measurement range, and the accuracy can still be guaranteed. The differential pressure of the differential pressure flowmeter has a non-linear relationship with the flow rate, and the measurement is inaccurate when the flow rate is small, and the range ratio is only 3:1-5:1. If the range ratio is enlarged, we must try to improve the differential pressure transmissionaccuracy of the device.

(8) Reliability: Now there is a more reliable vortex street. It installs two sets of electronic circuits and two sets of sensitive components on one meter body, and they are independent of each other and can be used for important tasks. In flow measurement situations, it is difficult for differential pressure flow meters to achieve this.

(9) Energy saving effect: For example, the steam consumption is 120t.