Characteristics, installation and operation of differential pressure flowmeter

The differential pressure flowmeter is based on the throttling principle of fluid flow, and uses the static pressure difference generated when the fluid flows through the throttling device to achieve flow measurement. It is composed of the throttling device, pressure pipe, differential pressure gauge or differential pressure transmitter and display instrument.

Measuring principle
When the fluid flows in the pipeline and flows through the throttling device, due to the sudden decrease of the flow area, the flow rate will inevitably produce local contraction and speed up. According to the law of conservation of energy, dynamic pressure energy and static pressure energy can be converted under certain conditions, and the result of speed up will inevitably lead to the reduction of static pressure energy, thus generating a static pressure difference between the upstream and downstream of the throttling device. The value of the static pressure difference is related to the flow rate of the fluid flowing through the pipe

Standard throttling device
The elements arranged in the pipeline that can make the fluid produce local contraction are called throttling elements. The so-called standard throttling devices mean that their structural form, technical requirements, pressure tapping methods and service conditions have unified standards. In the actual use process, as long as it is processed according to the standard requirements, it can be directly put into use.
At present, the commonly used standard throttling devices include orifice plate, nozzle and Venturi tube.

1. Service conditions of standard throttling device
(1) The fluid must fill the pipe and throttling device and flow through the pipe continuously.
(2) The flow beam (flow state) in the pipe must be stable, unidirectional, uniform, and not change with time or change very slowly.
(3) No phase change occurs when the fluid flows through the throttling element.
(4) Before the fluid flows through the throttling element, its flow beam must be parallel to the pipeline axis, and there must be no rotary flow.

2. Selection principle of standard throttling device
(1) Venturi tube and Venturi nozzle can be used when the allowable pressure is small.
(3) Under the condition that the flow rate and pressure difference are equal, the opening interface ratio of nozzle is smaller than that of orifice plate. In this case, the nozzle has high detection accuracy, and the required straight pipe length is short.
(4) In terms of processing, manufacturing and installation, orifice plate is the simplest, followed by nozzle, and Venturi tube and Venturi nozzle are the most complex, with high cost and short straight pipe length.

3. Installation of throttling device
(1) The opening of the throttling element shall be concentric with the axis of the pipe, and its end face shall be perpendicular to the axis of the pipe.
(2) There should be no obvious roughness or unevenness on the inner wall of a section of pipe with the front and rear length of throttling element being twice the pipe diameter.
(3) The upstream and downstream of throttling elements must be equipped with straight pipes of a certain length.
(4) Standard throttling devices (orifice plates and nozzles) are generally only used in pipes with diameter D greater than 50 mm.
(3) Differential pressure detection and display

The throttling element converts the flow of the fluid in the pipeline into a differential pressure, which is led out of the conduit and sent to the differential pressure gauge for measurement. There are many types of differential pressure gauges used for flow measurement, such as double bellows differential pressure gauges, bellows differential pressure gauges, differential pressure transmitters, among which the differential pressure transmitters are the most widely used.
Since there is a square root relationship between flow and differential pressure, for the convenience of indication, a square root extractor is often added behind the differential pressure transmitter to make the output current and flow become linear, and then sent to the display instrument for display

4. Operation of differential pressure flowmeter
The differential pressure flowmeter can be put into use after it is installed on site and checked to be correct. Before opening the meter, the impulse pipe must be filled with liquid or isolating liquid, and the air in the impulse pipe shall be discharged through the vent valve and the instrument vent hole. In the process of opening the meter, pay special attention that the elastic elements of the differential pressure gauge and differential pressure transmitter shall not be subjected to sudden pressure impact, let alone under unidirectional pressure. Operation steps of differential pressure flowmeter.
(1) Open the pressure inlet stop valve of throttling device
(2) Open the balance valve and gradually open the shut-off valve at the positive pressure side to make the positive and negative pressure chambers of the differential pressure gauge bear the same pressure.
(3) Open the shut-off valve at the negative pressure side, and gradually close the balance valve, and put the instrument into use. When the instrument is shut down, the operation steps are reverse. During operation, if it is necessary to calibrate the zero point of the instrument online, just open the balance valve and close the shut-off valve.

Post time: 15-11-22