Regarding rotameters with needle valves

The needle valve in a rotameter is used precisely for regulating the flow rate; I’ve seen them mostly located at the inlet. The precision of the rotor is just that level – what is it used for?

Some are equipped with valves only at the inlet (needle valves, globe valves, etc. can be used)~~to regulate flow~~; others have valves at both the inlet and outlet (some also come with a bypass). This allows for flow regulation, and in case of problems, it is possible to replace the flow meter~~

What the original poster means by a rotameter with a needle valve is that the meter and the needle valve are integrated into one unit, rather than having a needle valve or stop valve installed in front of or behind the rotor for easy maintenance and replacement. If the original poster has used the air-insufflation method to measure liquid levels, they will understand the function of this needle valve: it serves to regulate the level.

It is indeed a combination of a needle valve and a flow meter. The installation location of the needle valve – whether at the inlet or the outlet – matters, and it depends on the properties of the medium

The needle valve is used to regulate flow, with a precision of generally around 4%

This post was last edited by HEJIYUER on 2016-4-3 at 18:51, version 1. The pressure loss of a rotameter is less than 10 KPA; if the pressure difference between the upstream and downstream sides is not significant, then a needle valve is not necessary, similar to the usage in orifice plates. 2. For N2 purging, with 6 BAR upstream and 1 BAR downstream, a throttle valve must be installed in the pipeline to reduce the pressure; if flow monitoring is not required, the process can simply use a throttle valve to achieve this. If a flow meter is installed, the throttle valve can be provided by the process, but in most cases it is part of an integrated set with the flow meter, because the flow capacity of the integrated needle valve matches the full scale of the flow meter. The needle valve is placed downstream of the gas flow meter, as the pressure remains relatively stable = the density is stable = the readings are reliable. The backflow of instrument air is this type of operating condition. 3. The reason for using a needle valve for the liquid is the same as in 2, and it can be placed either upstream or downstream. If the liquid passes through a throttle, cavitation may occur; it is recommended to place it downstream to increase the back pressure. Accuracy: roughly 1.6/2.5. @jiaguoyun @Big Horse with Big Bow