Around 5 minutes in you keep saying how cavitation is due to air bubbles getting sucked into the pump. You've got it wrong. That pump is not going to be harmed by a bit of air so long as you're not running it dry. Cavitation bubbles are not air, it's steam under near vacuum pressures. The whole reason why it's so destructive is because there's not air in the bubbles that form to provide any pressure, it's just vacuum and the little bit of steam pressure isn't going to increase as the collapsing bubble compresses it because that'll be condensing back into water. The important thing to prevent cavitation on a pump is that the pressure at the suction side cannot be allowed to dip too low. If you had a ball valve upstream of the pump and it was closed then that would likely start causing cavitation damage depending on the design of the pump. Air is not going to hurt it so long as there's still plenty of water keeping the seals cool and wet. People sometimes conflate dry running a pump with a pump having too much restriction on the suction side but these are apples and oranges.

@FireSprinklerTech

7 ай бұрын

This is exactly why I started this channel, so I can get better at this trade. I've never bothered to look into cavitation, I just assumed the way it was taught to me was correct. After reading your comment I went to NFPA 20 2022 handbook and found this: "In NFPA 20, the two types of net positive suction head (NPSH) that need to be considered are termed the NPSH required and the NPSH supplied. The NPSH required by the pump is determined by the pump manufacturer and is a function of both the speed and capacity of the pump. Curves indicating the NPSH versus flow can be obtained from the manufacturer. The NPSH supplied is the pressure head at the pump inlet that causes water to flow through the pump into the eye of the impeller. The NPSH supplied is a function of the water supply. For any pump installation, the NPSH supplied must be at least equal to the NPSH required for the operating conditions specified. If the NPSH supplied is less than the NPSH required, some of the incoming water vaporizes and forms bubbles. These bubbles can collapse within the pump, producing noise and vibration and causing significant damage to the pump. This phenomenon is referred to as cavitation." Thanks for taking the time to comment on this.

@AndrewMerts

7 ай бұрын

@@FireSprinklerTech Definitely a complex topic, even my comment is an oversimplification and ignores dissolved gasses but I'm actually surprised the NFPA states that the pressure required is a function of the speed and capacity because that actually ignores temperature. I am a layman, I have no idea if it's common for pump manufacturers to specify on whatever chart they give you for NPSH if it's specified with a maximum water temperature but the hotter the water, the easier it is to have cavitation due to the increased vapor pressure. For a given pump you should have a lower required NPSH for 20C water compared to 50C water. It's probably a moot point in practice but when the pump manufacturer is coming up with the chart for minimum inlet pressure for a given speed they would have to use some value for the vapor pressure.

I think the reason the tanks aren't draining equally is because they were not piped for a balanced flow rate. The tank closest to the pump has the least restriction so it will drain faster. You can see the tanks all drain at a lower rate the further they are from the pump. Multiple tees should be utilized to equalize the piping distances between the tanks. We use this piping arrangement in plumbing multiple hot water tanks to balance flow.

I use to do the inspections at the Shop Vac plants until they were bought out by Great Star Tool, a Chinese company. They've done one inspection since taking over. They've moved 90% of the operation to China in the past year. So i doubt they still make that model vac anymore.

Seems they would have used a larger size pipe from the tanks. One that matches the 3 inch pipe running to the fire pump. FYI, ShopVac went out of business several years ago. I don't know if someone else purchased them or they restarted.

Hi, Were all of the tanks sealed at the top? If the tops of the tanks were all connected together with 1.5 " BlazeMaster it would help keep all of the tanks equalized. As far as the pipe looking better down stream right after the pump, it could be because of the turbulence scouring the pipe. Thank you for your videos. Wishing you all the best, backflowjoe from Cleveland Ohio

@FireSprinklerTech

8 ай бұрын

All of the tanks have vents built into the lids. I think they need to have slightly larger outlets on the tanks, maybe even a 4” “suction line” instead of a 3”?

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Not sure why my comment got deleted? Was that you or was it KZread?

@FireSprinklerTech

8 ай бұрын

It wasn't me. I just checked and there is only one comment that was held for review. Not sure why it was held either.

@FireSprinklerTech

8 ай бұрын

Please try reposting your comment. If you had a link in it, that will cause issues.