What should be noted when using positive displacement flow meters?

1. Filters must not be skipped under any circumstances! 2. The operating temperature for positive-displacement pumps is generally not very high; be careful with fluids at high temperatures! 3. Remember, do not use steam or inert gas for purging! 4. During the first use, the bypass line must be used first; wait until all the gas in the pipeline has been discharged before activating the flow meter!

(1) The newly installed pipeline must be cleaned before operation, and it is often subsequently flushed with actual flow to remove residual welding slag, scale, and other debris. At this point, the shut-off valves at the front and back of the instrument should be closed first, allowing the fluid to flow through the bypass pipe ； In the absence of a bypass pipe, a short pipe should be installed in place of the instrument. (2) Usually, after liquid cleaning, there is still a considerable amount of air remaining in the pipeline. As pressure is applied, this air flows through the flow meter at high speeds, which may cause the moving measuring elements to operate too rapidly and thus damage the bearings. Therefore, the flow rate should be increased gradually at the beginning to allow the air to escape slowly. (3) When the fluid flow is directed from the bypass pipe to the instrument, the sequence of opening and closing must be correct, and the operations should be carried out slowly, especially on high-temperature and high-pressure pipelines. After startup, use the least significant bit pointer or the dial along with a stopwatch to verify that there is no excessive flow; the optimal flow rate should be maintained at 70% to 80% of the maximum flow rate in order to ensure the instrument’s service life. (4) The filter screen of the new line is most prone to damage; it should be checked promptly after trial operation to ensure it is intact. At the same time, when the filter screen is clean and free of debris, record the two parameters of pressure loss at normal flow rates; in the future, there is no need to remove the screen to check for blockages – instead, it is sufficient to determine whether cleaning is required by observing the increase in pressure loss. (5) Used for high-viscosity liquids, which are generally heated to enable flow. When the instrument is not in use, the liquid inside cools down and becomes thicker; therefore, before using it again, it must be heated first so that the viscosity of the liquid decreases and it can flow through the instrument. Otherwise, this may damage the moving measuring elements of the instrument. (6) Before putting a positive-displacement flow meter for measuring gas into use, lubricating oil must be added, and the level gauge for monitoring the lubricating oil level should also be checked regularly during operation. (7) When using a gas gear flow meter, care must be taken to avoid sudden changes in flow rate (such as the use of quick-opening valves), as the inertia of the gears means that such sudden changes will generate significant additional inertial forces that can damage the rotor. When used as a sensing instrument in a control system, if flow is suddenly stopped downstream, the rotor cannot stop immediately, resulting in a compressor effect that causes the pressure downstream to rise; subsequently, backflow occurs, generating erroneous signals.

Precautions for using positive displacement flowmeters: Positive displacement flowmeters are flowmeters with high accuracy, but improper use will not yield ideal results; only through timely maintenance can the flowmeter operate properly. Proper use and timely maintenance of flow meters are responsibilities that flow meter operators must uphold. For a flow meter measurement system that has been newly designed and selected or reinstalled, a trial run should be conducted after installation checks confirm that everything is in order. The commissioning process is carried out as follows: First, close the valves before and after the flow meter, then slowly open the bypass valve for a horizontally installed metering system (for vertically installed systems, open the main pipeline valve), allowing the fluid to flow through the bypass pipe in order to flush out any remaining debris in the pipes and balance the pressures at the inlet and outlet of the flow meter. If there is no bypass pipe, a short tube can be used in place of the flow meter to allow the fluid to pass through; once the pipe is cleaned, the short tube can be removed and the flow meter reinstated. Use it in accordance with the instructions provided in the flow meter manual. After the metering system is put into normal operation, operators should pay attention to the following aspects: (1) Expelling gas: Usually, after purging with liquid, there is still a considerable amount of air remaining in the pipes. As pressure is applied, this air flows through the positive-displacement flowmeter at high speeds, which may cause the moving measuring elements to rotate too fast and damage the shafts and bearings. Therefore, the flow rate should be increased gradually at the beginning to allow the air to escape slowly. (2) Bypass pipe switching sequence: When the fluid flow is directed from the bypass pipe to the instrument, the opening and closing should be done slowly, especially on high-temperature and high-pressure pipelines. When enabled, in step 1, valve A is gradually opened, and the liquid flows through the bypass pipe for a while first ； Step 2: Slowly open valve B ； Step 3: Slowly open valve C ； Step 4: Slowly close valve A. When shutting down, follow the operations in the reverse order mentioned above. (3) After startup, use the least significant bit pointer or the digit wheel along with a stopwatch to verify that excessive flow has not occurred; the optimal flow rate should be maintained at (70–80%) of the maximum flow rate in order to ensure the instrument’s service life. (4) Check the filter: The filter mesh is most prone to damage when a new line is put into operation; it is necessary to check whether the mesh is intact after the trial run. At the same time, when the filter screen is clean and free of debris, record the two parameters of pressure loss at normal flow rates; in the future, there is no need to remove the screen to check for blockages – instead, it is sufficient to determine whether cleaning is necessary by observing the increase in pressure loss. (5) Measuring high-viscosity liquids: For high-viscosity liquids, they are generally heated to make them flow. When the instrument is turned off, the liquid inside it cools down and becomes thicker; when it is turned back on, it must first be heated so that the viscosity of the liquid decreases before the liquid can flow through the instrument. Otherwise, it may damage the moving measuring elements of the instrument. (6) Avoid sudden flow rate changes: When using positive displacement flow meters, care must be taken to prevent sudden changes in flow rate (such as those caused by quick-opening valves). Due to the inertia of positive displacement flow meters, such sudden changes generate significant additional inertial forces that can damage the rotor. (7) It is prohibited to allow steam to pass through positive displacement flow meters when flushing pipes with steam.