Discussion on the design speed of cryogenic liquid pumps
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There are only a few manufacturers that produce liquid oxygen pumps and liquid nitrogen pumps at present. As users, when the operational requirements are met, one would prefer pumps with a low rotational speed. So the question arises: Do users have the right to choose the rotational speed? Could you let the equipment manufacturer know that the rated speed of the pump I want to purchase should not exceed 3000 rpm? Has the manufacturer met the same operational requirements, offering the user two or more pumps with different rotation speeds, impeller diameters, and numbers of stages to choose from?
Under conditions that permit it, operating at low speeds is certainly better as it helps to reduce power consumption; this requires manufacturers to improve the operational efficiency of low-temperature pumps, although achieving high efficiency presents certain challenges. However, during actual operation, in order to ensure the stable functioning of the cryopump and avoid the effects of pump vaporization, it is often necessary to leave a certain degree of opening in the reflux valve, which to some extent affects the pump’s efficiency. Put simply, the manufacturer goes to great lengths to improve the pump’s efficiency by a small amount, but the opening of the return valve during operation cancels out this improvement.
Thank you! What basis should the owner use to specify this rotational speed? For the liquid oxygen pump, with a design pressure of 40 kilograms and a flow rate of 60,000, what is the maximum allowable rotation speed?
Your statement applies to centrifugal pumps, as the rotational speed is directly related to the outlet pressure. Also, what is the unit for your 60,000 data volume? When you need to reach a certain flow rate and pressure, the energy consumption of any centrifugal pump is the same, unless you change the type of pump; for example, if you switch to a positive displacement pump, the energy consumption might be lower due to differences in efficiency.
For variable-frequency pumps, specify your pressure and flow rate requirements. The maximum speed should be slightly higher than what is needed for those flow rate and pressure levels; for example, setting the frequency converter to around 90% can achieve full load operation. So, by stating your process requirements, they will be able to design a pump for you.
Thank you for the reply from the guy above; 60,000 is based on standard conventions in air separation technology, and it refers to the amount of oxygen supplied per hour in cubic meters, after the liquid nitrogen pump has vaporized the oxygen. If the oxygen pump’s flow rate is 75 cubic meters per hour, that’s equivalent to 1250 L/min. I’m sorry for not expressing myself clearly! If, for a liquid oxygen pump with an outlet pressure of 4.0 MPa at a flow rate of 75 cubic meters per hour, there are options of 3000 rpm with 4 stages of compression, or 5000 rpm with 2 or 3 stages of compression, we should choose the first option. So what is the relationship between the rotation speed and the number of compression stages in the design?
”Regarding this pump, once it reaches its rated speed, in order to avoid the effects of vaporization on the pump, the reflux valve is left slightly open. Generally, high-pressure pumps are equipped with a vapor return valve; however, during calibration, the manufacturer still advises leaving some degree of opening in the reflux valve. At our site, we adjust the setting based on the vibration level of the valve when it is closed – if there is significant vibration, we leave some opening; if the vibration is minimal, we close the valve completely and then observe how the pump operates. So regarding whether the return valve of the pump can be fully closed, I think it depends on the specific conditions of the pump in question. Do anyone have different opinions?
It is recommended to leave some opening in the reflux valve to ensure fluid flow within the reflux line~~
In practical production of cryopumps, it is better to leave the reflux valve slightly open; of course, in theory it can be closed completely. Regarding the vibration issue with the check valves in your company, vibration can be caused by various factors such as pipeline stress and valve selection. Given this, could the vibration in your case be due to the use of valves that are too large for the application? The opening degree of a check valve is usually very small. If a valve with an excessive flow capacity is selected, it will have to operate at a low opening degree frequently. In such conditions, the valve experiences high flow resistance, a narrow throttling gap, high flow velocities, and severe shock forces. When these factors exceed the valve’s stiffness, its stability is compromised, and vibration is likely to occur. Additionally, with a low opening degree, the sealing surface of the valve core is close to the throttling orifice, which also damages that sealing surface. Just my personal opinion, for reference.
I strayed from the topic; let’s get back to it! What are the requirements for the pump speed?