Dilute nitric acid is an important raw material in many chemical production. The production process of dilute nitric acid has different requirements for its physical and chemical characteristics and measurement accuracy because of different processes. Dilute nitric acid production is a continuous chemical production process, and all kinds of materials are at high temperature and high precision. Under pressure, chemical reactions and physical changes are constantly occurring. At present, the domestic and international production of dilute nitric acid mainly adopts double-pressure method. The main process is first liquid ammonia through liquid ammonia filter, then through ammonia evaporator into gas ammonia, into the ammonia-air mixer and fully mixed with air and then sent to the oxidation furnace for catalytic oxidation reaction to produce nitric oxide gas, nitric oxide gas through the absorption tower. Dilute nitric acid, which is absorbed by water and converted into nitric oxide gas, can be produced by blowing out the dissolved gas through air after passing through the bleaching tower. In order to control the production process of dilute nitric acid well, it is necessary to know the flow of ammonia, air, dilute nitric acid and steam in the production process in time in order to ensure the safe and stable operation of the whole production process.
Flow measurement in dilute nitric acid plant
Vortex flowmeter is to set vortex generator in the fluid, alternately produce regular vortex from both sides of vortex generator, namely Karman vortex street, vortex frequency converted into pulse signal proportional to the measured medium velocity, and then calculate its instantaneous flow rate. Vortex flowmeter is characterized by simple and firm structure, easy installation, no pressure pipe and heat tracing, low maintenance costs, applicable to liquid, gas and steam, high precision, wide range, pressure loss, no movable parts, high reliability. In dilute nitric acid plant, vortex flowmeter can be used to measure ammonia, air, desalted water and low pressure steam flow.
Vortex flowmeter is sensitive to flow distortion, rotating flow and so on. It is not suitable for low Reynolds number measurement. When installing, the upstream and downstream sides must ensure a sufficiently long straight pipe section. At the same time, the vibration of the pipe has a great influence on the vortex flowmeter. It should be installed as far as possible in the courtyard where the vibration source and electromagnetic interference are strong. The application must be considered comprehensively to ensure its accuracy.
The selection of flowmeter is a very important work in instrument application. In actual working condition, 2/3 of the instrument failure is caused by the wrong selection and installation of the meter. In the dilute nitric acid plant, flow timing should be selected according to the following steps:
(1) First of all, considering the characteristics of the fluid, including temperature, pressure, density, viscosity, conductivity and so on, according to the fluid, select the appropriate type of flowmeter.
(2) Secondly, the installation conditions, including the requirements of the straight pipe section, vibration, electrical interference and so on, are considered.
(3) The energy loss caused by flow measuring elements should also be taken into account in the long-term operation of contact meters in production pipelines. Under normal circumstances, multiple pressure loss measuring elements should not be used in the same production pipeline.
(4) When investment permits and accurate measurement is ensured, the type of flowmeter should be determined and the manufacturer with reliable quality should be selected according to the principle of energy saving and high efficiency.
In short, no one measurement method or flowmeter can adapt to a variety of fluids and flow conditions, different flowmeters have their own unique advantages and disadvantages. Therefore, on the basis of a comprehensive comparison of the characteristics of various instruments and combined with the actual situation of the plant, the optimal flow timing type suitable for production requirements, which is safe, reliable and economical, should be selected.