High-speed centrifugal pump auxiliary oil pump
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Hello everyone, there is such a question – could you help me analyze it? Thank you. For high-speed centrifugal pumps equipped with an auxiliary oil pump, it is necessary to start the auxiliary oil pump first before starting the main pump. This is because high-speed centrifugal pumps have high operating temperatures, and the lack of lubricating oil can easily lead to overheating. A Bearing speed B Medium temperature C Outlet pressure D Medium density. The standard answer is D, but I have a vague feeling that A is correct; I seek guidance.
The auxiliary oil pump must be turned on before starting ———————————— The rotation speed of the bearings is determined by the motor, and it remains constant before and after starting; there seems to be no need to specifically mention turning on the auxiliary pump before starting. For a pump, the head is determined by the flow rate and is independent of density. When the flow rate remains constant, the higher the density of the liquid, the greater the power required by the pump. At the moment the pump starts, that is, when it pushes the liquid from a stationary state to a flowing state, the power required is especially high. If the liquid has a high density, this will increase the power needed by the motor at startup (at this point, most of the electrical energy is converted into heat energy). Is it for this reason that it is necessary to start the oil pump in advance?
A: High-speed pumps operate at relatively high speeds; generally, manufacturers require that the oil pump be started 10 minutes in advance of starting the pump itself
Reply to 4# fisky: First of all, I would like to thank the moderator for their reply. Then I have a question – when it is said that a high-speed pump has a high rotational speed, what exactly does that mean by high rotational speed? Motor bearings and pump bearings?
This post was last edited by Juliet on 2011-12-10 at 16:55. Reply to 3# zhangdazui: After reading your reply, I agree with one point – that is, at a certain flow rate, the higher the density of the fluid being transported, the higher the power required by the motor. However, there are two points with which I don’t quite agree: First, you said that the head of a pump is related to its flow rate; that’s not correct, right?； Second, at the start-up stage, you said that the mechanical power was particularly high – isn’t that correct? Regarding these two points, could you provide more explanations?
Reply to 5# Juliet regarding pumps: There is no difference between the motor of a high-speed pump and that of a regular pump; the motor is connected to a gearbox via a coupling, which is used to increase the rotational speed
Reply to 6# Juliet: First of all, you said that the head of a pump is related to its flow rate, right? That’s not correct； Second, at the start of operation, you said that the mechanical power is particularly high – that’s not correct. ———————————————————— First question: The characteristic curve of a pump is the H-Q curve; how can the head H have no relation to the flow rate Q? The mechanical power P of the motor is given by P = (H*Q*g + 1/2*Q*g*u^2) * liquid density. Since H has no relation to the density, the parameters within the parentheses in the formula remain unchanged when transporting different liquids. The power required to transport high-density liquids is higher than that needed to transport low-density liquids. The higher the power, the greater the current; and the greater the current, the more heat is generated. The second issue is similar to pushing a cart: at first, it takes a lot of effort to move it, but once it starts moving, one hand is enough to keep it in motion. It requires a lot of work to change an object’s inertia. Although the flow of fluid is slow and its kinetic energy is low when the pump starts up, this is precisely when the motor is most likely to get damaged. That’s why the pump is started first, followed by the valve. When the pump starts, there’s also the added factor of dealing with a high-density liquid; these two factors that increase the load mean it’s necessary to start the auxiliary oil pump in advance.
High-speed pumps come in types with single-stage and two-stage speed variation, and their rotational speed is generally between 5000 and 8000, with some possibly reaching higher speeds. Therefore, before starting the machine, the high-speed bearings of the motor or gearbox must be lubricated first; otherwise, overheating may occur in a short period of time, leading to bearing damage and shaft seizure.
Reply to 8# zhangdazui: Well, according to the pump’s performance curve, the pump’s head is indeed related to the flow rate (decreasing as the flow rate increases), but the maximum head should be independent of the flow rate. However, according to the pump’s performance curve, the flow rate is at its lowest at the beginning of pumping, and consequently the power consumption is also at its lowest. As for the issue of changing inertia you mentioned, I think it relates to overcoming friction; I’m not quite sure how much it has to do with power.
Reply to 9# wsm1988: Hello, is it related to the medium density?