Is it okay to modify the water pump inlet pipe like this?
Thread Content
We encountered a problem during the project construction: the rigid waterproof sleeves installed by the contractor for the circulating water cooling tank during the civil engineering work were too small (as shown in Figure 1). The inlet pipe has a diameter of DN300. Now we want to reduce the diameter of the pipe at that sleeve, from DN300 to DN250 (as shown in Figure 2). The water pump is a double-suction centrifugal pump with an inlet diameter of DN200, an outlet diameter of DN150, a flow rate of 350 m3/h, a head of 30 m, and a power rating of 45 kW. Is it possible to make such changes? Is it likely to cause cavitation? If possible, should concentric reducers be used instead, or equal-teeth reducers?
Based on my personal experience, a double-suction pump has two inlets; on average, their diameter is DN250, with a flow rate of 150 cubic meters. I think this diameter is sufficient, but I don’t recommend going with this approach. It’s better to have a design firm do the calculations and give confirmation, in order to avoid any future issues. If changes are made, it’s necessary to ensure that the connections are made in a way that allows for even pressure distribution, either at the top, at the bottom, or concentrically; otherwise, gas can accumulate before the diameter changes, which can lead to cavitation.
For this issue, it is first necessary to determine whose responsibility it is. This mistake can be corrected; it is necessary to construct in accordance with the original design. It is recommended to carry out local demolition and replace the pipe with an inner diameter of 400 mm. The construction method involves breaking apart the areas on both sides along the embedded pipes; there should be a water-stop ring in the middle, outside the embedded pipes. It’s best if this water-stop ring can be reused if it is sufficient; otherwise, it needs to be enlarged. Sufficient grouting space should be left for the pipes embedded at the demolition site and the holes in the concrete walls; rapid-setting grouting material should be used. Shields are installed at both ends of the embedded pipe, along with grouting holes. The embedded pipe should extend 5 cm outside the wall to facilitate future pipe replacement, cutting, and connection. (For reference)
I agree with what was said on the 3rd floor: for new projects, it’s sufficient to demolish the existing structure and reinstall the embedded pipes; there’s really no need to take any risks.
The designs prepared by the design institutes include a margin of safety. But if you reduce that margin, and if the pump manufacturer specifies an even lower net positive suction head, along with some fluctuations in the process conditions, all these factors combined can turn the impossible into possible:D
Agree with the third floor’s suggestion; a water-stop ring must definitely be installed in the middle, outside the embedded pipes
Why change it? The water inlet pipeline can pass through, and once the waterproofing is done, it’s ready to use.
Agree with the opinion from the third floor; using straight pipes also avoids a lot of trouble.
My opinion is also to chisel out the existing parts and reinstall the waterproof sleeves, but the manager is worried that pouring concrete again in those areas might lead to leaks, so he wants to change the pipe diameter. Sigh~