Stainless Steel and Bronze impellers

[21121956]

Hello everybody:

In the Technical Specifications for the Drainage System of a small hydroelectric plant, it can be read:

“In the case of submersible pumps, the pump and motor shall be contained in the same enclosure and designed as a package, with built suction filter. For over 30 kW pumps, impeller shall be of stainless steel. For less than 30 kW pump impeller may be bronze”.

According to these Specifications, the material change of impellers can be done if there is a change in motor power (depending on the head and flow, which are causing a shift in power), but how is justified the change of material of the pump impeller if the wastewater being pumped remains the same?

Thanks in advance for your comments about this subject.


El que no puede andar, se sienta.

 

[Artisi]

Probably a spec. written by an someone who doesn't know his / her elbow from their XXXXhole or maybe clouded by what is available from various manufacturers or it sounds / looks good when trying to justifies their position or a consultancy fee.

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.)

 

[EnergyProfessional]

I suspect someone 40 years ago determined the extra expense of SS is not worth for smaller pumps. Or maybe availability was difficult at the time.

Then generations of engineers in that consulting firm just copied/pasted the spec without ever thinking about it.

Probably the same reason why VFD pumps still get specified with balancing valve..... old habits die slow.

 

[miningman]

Unfortunately these comments are true, and if anything underestimate the problem. I have had to endure specifications that prohibited the use of copper never seize on piping systems on pumping systems but specifically called for copper never seize on the pumps. All specs provided by the same group of consultants.

All younger engineers should be encouraged to query the reasons for ANYTHING before being allowed to use cut and paste

 

[rotw]

I have a previous experience with booster station. Means high flow duty pumps typically 3 or 4 pumps working in parallel. Number of pumps has been stretched to the minimum so to have a competitive costs. As result each pump was operating quite to the right of the curve but still ok. In this case the impeller material was determined according to the water analysis but also checked against the cavitation. Typically we had to account for permanent cavitation and increase the NPSH required accordingly (that extra margin depends from material selection). If the NPSH margin is not comfortable then lifetime of the impeller may suffer due to permanent cavitation. In reality the NPSH curve of a pump includes some permanent cavitation that is by convention 3% losses of head (do not remember if this acc. to Hydraulic Institute).

I am not saying that this is the explanation to the OP problem. But Impeller material, in my experience, is not defined strictly according to the result of the water / pumped medium analysis but also is a parameter to play with to set the total NPSH_required.

 

[hydtools]

I think the key words are shall and may. Shall be of stainless steel, indicating must conform. May be of bronze, allowing the use of material other than bronze.

Ted

 

[Artisi]

Ted, think you are probably spot on with shall and may, however it doesn't really address the question of why allow 2 different materials for the same application.

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.)

 

[BigInch]

The stresses within the impeller when transferring the power and torque from pump to fluid from those higher power pumps may preclude use of lower stress materials such as bronze in those particular high power pumps. If the design of the impeller and its dimensions remained more or less the same, yet the power increased considerably, radial stresses might be considerably higher than what bronze could resist.

Independent events are seldomly independent.

 

[rotw]

There is heat rejection from the pump to the medium due to losses. For a given pump efficiency, the higher the duty (kW) the higher the heat rejection from which some amount is transferred to the pumped fluid. As the application is wastewater, not sure if the heat will build up but I wonder if locally the temperature could rise.

Point is that an increase of temperature of the medium is in general not wanted from a material corrosion point of view.

This is speculative but a food for thought...

 

[EnergyProfessional]

rotaryw: good thoughts, and this is why dead-heading of pumps is not recommended since the heat can casue cavitation.

but I don't think whne there is flow (even a little) this is an issue. Especially not with most likley cold wastewater.
you could do the math at what flowrate the pump would even measurably heat up the water. I really doubt this is an issue. If you have 1000 gpm pumps, and only run 1 gpm, thsi might be a problem.
thsi is even less a problem in variable speed pumps, since less rpm mean also less heat.
unless you run 200°F boiler water, i don't see a concern.

 

[21121956]

Hello everybody:

I am very grateful for your points of view and your comments. Thanks.

El que no puede andar, se sienta.

 

[z00b]

You are good to go with SS impellers.