Sewer Pumps in Tandem

[dellbs]

I posted a similar question in the general civil site.

What sort of porblems or special considerations need to be taken into account when designing pumps in tandem?

In our area of NW Washington, I don't know of any installations?

The particular project is proposing two 18HP pumps in tandem for 480gpm at 177 TDH (135'static). One in the wetwell the other in the valve vault. Is cooling an issue for the pump in the valve vault?

I can do simple pumps, but the tandem arrangement is new and I want to make sure it's done right.

David

 

[oxilume]

Dellbs,

Operating pumps in series (tandem) is not a major issue. I do it all the time in oilfield applications whenever I don't have the "right" pump. However, you are asking for double maintenance. In addition, ensure that the 2nd pump housing pressure ratings will not be exceeded. The two 18 hp pumps appear to be more on the light commercial duty types in leau of heavy industrial type pumps. In fact, it appears that the pumps are gasoline operated engine type trash pumps.

I really don't understand the reason behind two pumps, when you can simply order a pump with a larger impeller and turn it with more HP in order to achieve the higher head requirements. This is especially true for lift stations. However, most lift stations have two pumps, but they are operated in parallel and not in series.

Also, personally, I am completely against placing any type of rotating equipment within the wet well. Bearings, impellers, etc. don't last!



Todd

http://www.oxilume.com

 

[dicksewerrat]

It is much easier to maintain two pumps in one station. One controller, shorter lines to pumps. only one extra float, one pump station. I have never seen your scenario. but then i only started in sewers in 1970.

 

[semo]

I've not placed wastewater pumps in series before; but, know it is done.

Gorman-Rupp does it as standard practice on many of its above ground stations. Sometimes with a flat pump curve, placing pumps in series can get higher flows than pumps in parallel.

Cooling can be an issue depending on the pump. If you are talking about submersible pumps, some of the manufacturers use cooling jackets that circulate some of the pumped water around the pump. I've used this with Flygt pumps on wetwell/drywell retrofits.

I would at least talk to the pump manufacturer to see if they routinely do it and what suggestions they have.

Don't know how the controls would be configured; but, would think there are some tricks to keeping power cost/O&M low.

 

[electricsub]

David,
We have for varius reason had to do tandem pumping of sewage several times over the last few years.
Main reasons were low flows, say around the 4 or 5 l/sec with very high total delivery head, last one was 95 mtrs.
We used a submersible macerator pump in wet well sump ( they were ABS Piranha's ) feeding an end suction pump located in what would normally be the adjacent valve chamber, however this was turned into a slightly larger chamber into which we fitted the end suction pumps, all valves etc, lighting & heating.
One potential problem which did arise the last time was client wanted Eex rated motors, this was'nt a problem but did give us some headaches for the lighting & heating, eventually ( at a cost ) we over came this "small problem"

Under automatic operation using Ultrasonic Level controller via wet well levels, we started the submersible pump first, then via a timer the end suction kicked in after 2 / 3 seconds.
We also had the piping / valves arranged so either of the two submersible could feed either of the end suction pumps, handy for breakdowns.
Now been operating nearly 12 months, without ant problems.
We have others which have been working for 4 years with only minimal maintenance & no pump breakdowns & seals etc are ok, checked them last Feb.

Good luck
Bob

 

[Maury]

Dellbs,

Does Washington require a spare pump(s)? If so would you have to double this arrangement, i.e. four pumps (two tandem setups)? I don't see the benefit from this arrangement. It's a lot more complicated than a single pump, it's bound to be more expensive initially and to maintain, and there's no energy savings.