Those who understand pumps, come in
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This afternoon I went to a fertilizer plant, where there were 3 stainless steel centrifugal pumps. The labels indicated a flow rate of 50 m3/h and a head of 38 m; the outlet pressure during operation was less than 2 kg. The pumps experienced significant vibration. The workers said that the mechanical seals needed to be replaced every week or so, while the impellers, due to their lack of corrosion resistance, needed to be replaced monthly. I analyzed the situation and concluded that the problem was mainly caused by cavitation, with the impellers probably not playing a major role in the issue. They didn’t understand what cavitation was, and I’m not very good at explaining things clearly either. So I took some measuring tools and went to the warehouse to check the dimensions of the spare parts. The results showed that the diameter of the impeller’s flange was 82 mm, while that of the pump body’s flange was 87.5 mm. It’s obvious what would happen if such defective parts were used in the pumps. Yet they asked me to manufacture the parts according to their original dimensions, as long as the price was lower than that of the original manufacturer. I insisted on refusing to do such business, even though I could still make a profit by selling them at half price. (I drove all day; I have a terrible headache now and can’t continue writing.) . ）
Personal opinion: Stainless steel and acid-resistant steel are two completely different concepts; choosing only stainless steel pumps for handling acids is simply ridiculous. Besides, with such a large gap in the mouth ring, experts won’t say anything – just watch the joke! !
I’m stunned~~~~ A head of 38 meters, and although I still don’t know the density of the medium being transported, the outlet pressure is only 2 KG? ! If the medium density is not less than 0.53 kg/m3, and if we make a rough calculation while ignoring the back pressure exerted by the pump at the highest point, then with an outlet pressure of 2 kg, the pump will stop; opening the outlet valve would allow a small hydraulic turbine to be used, haha~~~~~~~~~~
The impeller mouth ring is 82mm, and the pump body mouth ring is 87.5mm. We can help you with this. Phone: 56091698; Mobile: 13901683559
Upstairs, are you saying this refers to matching sizes, or are it actually two mouth rings that just have different sizes? If it’s the latter case, that’s fine! Also, do you mean the pump outlet or the final pipeline outlet! In short, what you’re saying is too vague! I really don’t know what you want to say? In addition to cavitation, vibration can also occur due to improper pipeline layout; surging is another condition that can arise in such situations! Also, the quality of its shaft processing as well as its assembly quality can suffer from the issues mentioned above! This post was last edited by luo*wei on 2009-3-18 08:57]
Is it acceptable for the clearance between the impeller mouth ring and the pump body mouth ring to be unilateral, namely (87.5-82)/2? Additionally, the pressure gauge is located about 50 mm upward from the pump outlet; I think there should be no issue with the pressure at the pump outlet. What I want to say is that such users are funny (because I promised to help them build a prototype with the same installation dimensions; they could set any price they wanted, and if there were any problems within half a year, I wouldn’t take a single penny from them, nor would they have to return the pump). They won’t agree to this condition; they only want the accessories). Of course, the manufacturer of that pump is even more ridiculous~ Last edited by tzpump on 2009-3-18 09:55]
Although I’m not sure what the medium is, there is no corrosion on any parts other than the impeller; the main issue with the impeller lies in its blades. The pipes for import and export are all made of carbon steel. Based on the gap in the mouth ring, the actual flow rate and head of the pump are only half of what is indicated on the label, or even less than that.
V. What is cavitation in pumps? 1. Cavitation phenomenon: At a certain temperature, when the pressure of a liquid is reduced to its vaporization pressure at that temperature, bubbles form within the liquid. This phenomenon of bubble formation is called cavitation. The bubbles formed during cavitation shrink in size as they flow to higher-pressure areas, eventually bursting. This phenomenon of bubbles disappearing in the liquid due to rising pressure is called cavitation collapse. While the pump is in operation, if the absolute pressure of the liquid being pumped in a certain local area of its flow path – usually somewhere slightly downstream of the inlet to the impeller blades – drops to the vaporization pressure of the liquid at that temperature for some reason, the liquid begins to vaporize there, generating large amounts of steam and forming bubbles. When this liquid containing many bubbles moves forward through the high-pressure areas inside the impeller, the high-pressure liquid surrounding the bubbles causes them to shrink rapidly until they burst. As the bubbles coalesce and burst, liquid particles fill the voids at high speeds; this creates a very strong water hammer effect, which strikes the metal surface at a high frequency. The impact stress can reach several hundred to several thousand atmospheres, while the impact frequency can be in the tens of thousands of times per second. In severe cases, this can cause the wall thickness to be breached. The process in which bubbles are formed and burst in a water pump, causing damage to the flowing components, is known as cavitation in water pumps. After cavitation occurs in a water pump, in addition to causing damage to the flow-through components, it also generates noise and vibration, leading to a decline in the pump’s performance; in severe cases, it can even interrupt the flow of liquid within the pump, preventing it from functioning properly. 2 Basic equations for pump cavitation: The conditions under which cavitation occurs in a pump are determined by both the pump itself and the suction system. Therefore, to study the conditions under which cavitation occurs, it is necessary to consider both the pump itself and the suction system. The basic relationship for pump cavitation is NPSHc ≤ NPSHr ≤ NPSHa. NPSHa = NPSHr / (NPSHc) – this is the point at which cavitation begins in the pump; when NPSHa > NPSHr / (NPSHc), there is no cavitation in the pump. Here, NPSHa represents the net positive suction head available, also known as the effective net positive suction head, and a higher value means it is less likely for cavitation to occur ； NPSHr —— pump net positive suction head, also known as the required net positive suction head or the dynamic pressure drop at the pump inlet; the lower this value, the better the pump’s resistance to cavitation ； NPSHc —— Critical net positive suction head, refers to the net positive suction head at which the pump’s performance declines by a certain amount ； ——The allowable NPSH is the NPSH used to determine the operating conditions of a pump, and it is usually taken as = (1.1～1.5) NPSHc. 3. Calculation of the device’s net positive suction head available NPSHa = Ps/ρg + Vs/2g – Pc/ρg = Pc/ρg ± hg – hc – Ps/ρg 4. Measures to prevent cavitation To avoid cavitation, it is necessary to increase NPSHa so that NPSHa > NPSHr. The measures to prevent cavitation are as follows: (1) Reduce the geometric suction height hg (or increase the geometric backflow height) ； (2) Reduce the suction loss hc; to this end, it is possible to increase the pipe diameter, minimize the length of the piping, as well as reduce the use of elbows and other accessories ； (3) Prevent operation at high flow rates for extended periods ； (4) At the same speed and flow rate, a double-suction pump is used, as it reduces the inlet flow velocity and thus makes cavitation less likely to occur ； (5) When cavitation occurs in the pump, the flow rate should be reduced or the pump should operate at a lower speed ； (6) The conditions of the pump suction tank have a significant impact on pump cavitation ； (7) For pumps operating under harsh conditions, to avoid cavitation damage, cavitation-resistant materials can be used.
Based on the poster’s description, it seems that the insufficient pressure at the pump inlet is causing the fluid inside the pump to vaporize
Is the size of these mouth rings correct? Is the outer diameter also measured without the pump body collar? A difference of 5.5 mm in diameter is not going to affect the thickness of the mouth ring; can a gap that large withstand a pressure of 2 Kg?
The main reason is likely too low pressure at the pump inlet, caused by vaporization. It seems there are some issues with the selection of system equipment, plus the operators don’t understand it. It’s better to find someone with higher expertise to take a look on site first!
If you replace your mechanical seal every week, is that even still called a mechanical seal?
Is it acceptable for the clearance between the impeller mouth ring and the pump body mouth ring to be unilateral, namely (87.5-82)/2? Additionally, the pressure gauge is located about 50 mm upward from the pump outlet; I think there should be no issue with the pressure at the pump outlet. What I want to say is that such users are funny (because I promised to help them build a prototype with the same installation dimensions; they could set any price they wanted, and if there were any problems within half a year, I wouldn’t take a single penny from them, nor would they have to return the pump). They won’t agree to this condition; they only want the accessories). Of course, the manufacturer of that pump is even more ridiculous~ Now I understand what you mean by the mating ring! But an impeller can have an inlet ring at the front, and it can also have an inlet ring on the back! Did you make it clear at that time? Also, you didn’t make it clear about “export 50MM” at that time! You know, you need to be precise in what you say, right? Besides, since the outlet pressure is less than 2 kg, if the user can still use it, you need to tell them that this isn’t a pump with a 50-meter head at all – it’s the user who made a mistake in selecting the pump! Even if the pump manufacturer is at fault, it’s only remedied afterwards!
The vibration is severe; could it be that the outlet valve is closed too tightly?
Actually, LZ still hasn’t explained it clearly. I’m not sure what issue LZ wants to address? What issue would you like to discuss? Thank you. Could you please explain the pump’s installation piping, the medium used, and the process conditions?
It still depends on what kind of medium you are transporting; only then can a judgment be made. . . As for the gap between the impeller and the pump body, I don’t think there’s anything wrong with it; it is feasible to manufacture such a design
After cavitation, the mechanical seal often experiences dry grinding, so it tends to break easily. You can try reducing the size of the outlet valve; if that doesn’t work, then the layout of the inlet piping needs to be improved.