Not very likely, but you need to check with the pump manufacturer's specifications.
In case you are not familiar with submersibles, it should be noted that the typical submersible pump has a higher maintenance cost for repairs than a centrifugal pump will.
So long as you're placing the submersibles in the wet well part (I assume) then I can't see an issue so long as the pit your pump is in is exposed freely to atmosphere and your pump is at a similar level or lower to that which your current pump inlet line is at. You need to be careful of any screens or filters becoming blocked which can lead to cavitation which as it is submerged is difficult to hear or see the effect until it is too late.
As bimr says - check your new pump NPSHR and all your parameters - vapour pressure, head of liquid above the pump inlet and air pressure above the liquid (whatever it is)
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
Original pumps were dry well, so how do you propose to install wet well submersibles or will the submersibles also we set up for dry well? A bit more information would help your enquiry.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
We know that submersible pump has a higher maintenance cost for repairs than a centrifugal pump but we had problems with vertical mounted bearing frame (vibration, alignment) and the dry-well was flooded with sewage 2-3 times since last 10 years. Initial cost is less though and pump choice is diversified.
Just so you know what you are in for. One would not want the public works manager to be upset at the increased repair cost that he will be seeing.
It is probably typical that the average sewage pump is incorrectly sized.
If the existing pumps were installed in 1979, you got your money's worth out of them. I would hope that you are also planning to replace the electrical system as well, because it is probably obsolete.
Are you planning to replace the pumps in the dry well, or install the pumps in the wetwell?
The decision to change the conventional pumps with submersible pumps were taken based on a detailed analysis (cost (initial and maintenance), durability, efficiency, etc.)
The pumps that we need in replacement are relatively small (e.g. Flygt 60 HP, ABS 58 HP). The pumping station constructed in 1979 but pumps were replaced a couple of time since then.
I was first asking about NPSH calculations for such replacement, but reading you: Did you had "bad experiences" with this kind of replacement? (conventional for dry-well submersible?).
No bad experiences. Most of the time what I have seen are projects that relocate dry well pumps from the dry well to the wet well. Which is ok, but would not be the preferred alternative.
Do You mean borehole submersible pump ? These are also centrifugal pumps (radial or mixed), and the motor is integrated together with the pump impellers in a single assembly casing.
The cooling of the motor is done by the flow of pumped medium (water) itself. So it matters that when the pump is selected the diameter of the well is suitable (i.e. not too big relatively to the pump size/discharge flange) to leave an appropriate gap so that velocity of fluid is high enough to cool the motor but not too small neither so that the heat/energy is also dissipated effectively. If the diameter of the well is already known keep that in mind and give the information to the manufacturer.
It matters also to stay within the 80-120 BEP range. There is great deal of pump investment calculation vs. pump efficiency calculations (price of kW.h) to select the optimal pump as normally you get - say from one given manufacturer - many pump sizes that will all do the job more or less. So an option is to have an Lifecycle cost done by the manufacturer to back up the final choice.
A point of importance is the selection of the pump metallurgy. So a water analysis shall be available to select the correct materials and kept in the record in case of failure or claim in the future. Some good performance and resistance can be achieved with a sort of synthetic (polymer) material which is quite cost effective manufacturer may provide normally such solution.
If not bronze would be the next choice for water without ammonia content. Mind the risk of high temperature as material selection for these pumps is very sensitive to combination of corrosive medium (chloride content) and temperature (e.g geothermal conditions). Above 55-60 degC the corrosion problem can be serious and devastating and you may need to switch to a duplex SS material which would make the pump very expensive.
With regard to NPSHrequired that also has to do with the different pumps that you get as many possible selections and the one you finally select. In addition when the pump is pumping the level decreases or fluctuate so that means less NPSHavailable will be affected - anyway double check.
The design team that I am a member of has done a lot of conventional to submersible DRY conversions as well as submersible pump used in DRY situations. 100‘s of conventional submersible installations as well. Flygt is OK with submersing about ½ of the motor, two or three feet of water. If using a through wall penetration, a 90 degree turn down with flare inside the WW, and a valve and 90 bend upward, I’ve never had a problem with NPSH, do the calc’s. One caution, if this is a recycle installation, as in return sludge, with a 100% duty cycle, try a very hard screw impeller or recessed impeller. As for heat removal, specify a run-dry duty submersible motor. Nice to know that flooding won’t destroy the motor, bearings, or any internal components. If at all possible, mount the protective devices and all other electrical devices above grade or flood. Both Flygt and ABS require seal fail and overtemp monitoring.
Steve
