The metering pump does not have sufficient pressure
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I would like to ask professionals: A metering pump is a type of positive displacement pump, and when selecting one, the rated pressure depends on the back pressure in the pipeline where the fluid is to be delivered. What will happen if the rated pressure of my metering pump is set too low, lower than the pressure in the pipeline where the chemical needs to be injected? I came up with 2 possible outcomes: 1. Low pressure, leading to backflow of the liquid; 2. Since it is a positive-displacement pump, although its rated pressure is low, the pressure at the pump outlet can keep increasing until it exceeds the pressure in the pipeline where the chemical needs to be added, at which point the chemical can be delivered to that pipeline. However, the flow rate at this point will be lower than the required operating flow rate ? These two possible outcomes have been bothering me, so I hope professionals and experts can help me understand them in detail – which outcome will occur, and what are the reasons behind it. Thank you!
The result will be the second one. The outlet pressure of a positive-displacement pump depends on the back pressure, while the rated pressure of the pump depends on its impeller design and materials. When the pump’s rated pressure is lower than the back pressure, the pump’s flow rate decreases (related to volumetric efficiency). The shaft power increases as the outlet pressure rises, so motor selection needs to be taken into consideration.
2. The positive displacement pump is not related to pressure levels; it depends only on the volume being adjusted. The reason for this could be a malfunction in the check valves at the inlet and outlet, or it might be due to leaks in the diaphragm, piston, or gaskets.
Thank you for your replies. Then let me ask the opposite question: if the rated pressure is higher than the required back pressure, and the rated flow rate is selected based on the operating flow rate, then in actual operation the opening degree of the metering pump also needs to be reduced in order to meet my desired operating flow rate, right?
Both the inlet and outlet of the metering pump are equipped with check valves, so backflow cannot occur; this does not happen in the first case； The second scenario occurs when the rated pressure is lower than the system pressure; in this case, the outlet accumulator is unable to store energy. Only when the flow rate decreases can it exceed the system pressure, so the actual flow rate will be reduced.
A metering pump relies mainly on the check valves at the inlet and outlet, as well as volume changes, to inject various additives into the pipeline. The presence of check valves prevents backflow; only the second scenario can occur. In that case, however, the sealing points at the pump’s packing and the check valves may leak significantly due to the buildup of pressure. Generally, the rated pressure of such pumps is chosen to be higher than the pressure in the pipeline
I would like to ask the experts here: I am looking to choose a metering pump to deliver a 32.5% urea aqueous solution. The pump will be installed nearby and used to send the liquid to a spray gun for spraying. Should the pressure required be determined based on the needs of the spray gun alone? I need expert advice; I’m a complete beginner and don’t understand.
I work in the field of peristaltic pumps and metering pumps; I’m from Baoding Chuangrui Pump Industry. You can get in touch with me at 15175359602
This post was last edited by HSLJHZ on 2017-12-4 at 14:23. I deal in peristaltic pumps and metering pumps; the maximum pressure they can handle is 3 kg. You can get more information from me first. Baoding Chuangrui Pump Industry, Zhu Ziren
The maximum outlet pressure that a metering pump can deliver at its maximum operating flow rate is generally specified in the pump’s selection manual or instructions. Therefore, the actual outlet pressure of the pump must not exceed this maximum allowable value; otherwise, the pump will be damaged after a short time, and its flow rate will be very low during operation. As for the pressure at the pump outlet reaching the spray gun, it is generally sufficient to calculate the pressure drop using standard engineering methods, that is, the pipeline resistance plus the minimum pressure required by the spray gun. One thing to keep in mind: does the liquid sprayed by the gun need to be continuous and stable? If it remains stable continuously and there is a damper at the pump outlet, the liquid discharged will not be of ideal quality; in such cases, a gear pump could be considered, as it ensures continuous stability.