Discussion on issues related to centrifugal pumps
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I work in the environmental protection industry. I was recently looking at a chemical process design manual, which states that a check valve should be installed at the outlet of centrifugal pumps. However, in the desulfurization industry, check valves are basically not installed at the outlet of centrifugal pumps; they are only added at the outlet of water pumps. Is it necessary to install a check valve at the outlet of a centrifugal pump? If it must be set, then what’s the situation in the desulfurization industry? Are there special requirements for pumps in the desulfurization industry? Pumps are generally started under pressure to prevent excessive starting current. However, in the desulfurization industry, no shut-off valve is installed at the outlet of the circulation pump (high flow rate, low head) in the absorption tower, preventing start-up under pressure. So why is that? Is it the electrical configuration that solves the problem of high current? Do centrifugal pumps basically all require seal cooling water? Are there any pumps that do not require seal cooling water?
Pump manufacturers always recommend installing check valves on the outlets to prevent backflow caused by water hammer, which could lead to unnecessary damage to the pumps. In the desulfurization industry and among environmental protection companies, the entry barriers are generally low; therefore, in order to save costs, these companies omit the outlet valves. Additionally, some pipes are made of fiberglass, and it seems that valves are not easy to install either. I’ve seen many desulfurization sites where there are no valves at the pump outlets. What is transported are slurries such as lime water, alkali solutions, and magnesium oxide, which have a high solid content and cause significant wear; therefore, the pump seals must be equipped with cooling water to ensure their service life. There are also mechanical seals that do not use cooling water, but they are generally not recommended.
I was once at the site where a desulfurization pump with a capacity of 1,000 cubic meters per hour was in operation; I didn’t even dare to watch as the operators started the pump. Whether the pump would function properly depended entirely on luck. We give suggestions, adding this and that – who cares about you? Sigh. . . . . Besides, they don’t use them very often either; they only turn them on during inspections, just to meet the requirements.
1. If, after stopping the pump, the amount of liquid returning is low and it does not affect the system’s operation, it may not be necessary to add anything. 2. According to the characteristics of centrifugal pumps, it is best to start them when the flow rate is at its minimum. But it also depends on the actual setup of the specific process. For example, factors such as high export pressure and export flow limits result in a certain static pressure being present in advance, which prevents tripping due to excessive flow. The key is to look at the actual setup on site. 3. It could be variable-frequency control. 4. Yes, but it needs to be selected based on the process media at the site. For reference.
It ensures that almost no liquid flows back after the machine stops; a check valve isn’t necessary. The pipeline used for desulfurization seems to be designed for discharge at the outlet, so in some cases it’s indeed fine without such a valve. The power curve of the centrifugal pump also changes: from low to moderate speed, the power curve rises; at medium to high speeds it remains relatively flat; and at very high speeds, the power decreases. Therefore, it’s possible to start the pump with the valves open as long as there is no backflow at the outlet. Sealing elements require flushing with lubricating water, though in some operating conditions, the water from within the pump or other fluids can be used for flushing. In other situations, such as when there are too many impurities or the temperature is too high, causing easy vaporization, external flushing and cooling fluids must be used. The API flushing guidelines can serve as a reference
I’m not in the desulfurization industry, but the selection criteria for mechanical seal flushing systems in API-standard centrifugal pumps are generally universal. And is the cooling water you mentioned used to cool the seal fluid? Or should cooling water be used directly as a sealing fluid?
Centrifugal pumps can remove the heat from the mechanical seal by transporting the medium. However, when transporting high-temperature media, external cooling is generally used, which can be oil cooling or water cooling. Before starting the pump, fill it first, close the outlet valve, and start it with a low current to protect the motor.