Doing away with Sump Well in two-stage pumping

Published on by in Technology

Doing away with Sump Well in two-stage pumping

We had commissioned a lift irrigation system in 1998 in two stages which is still in working condition till this date.

Now due to some problem of ownership of land where sump well is located, we want to do away with the sump well and put an in-line pump instead.

The general set and other information is as given in the attached sketch.

Can anyone advise how the hydraulic gradients will behave?

Will there be threat to the pipes in the first stage?

Is the proposed system advisable?

Your experience much appreciated.

Media

Taxonomy

7 Answers

  1. Hallo Yashpal,

    i would say that the idea of Anand to replace the Stage 1 pump (~100 HP) and eliminate stage 2 pump is worth to think about. Normaly if you replace a 20 year old pump into a modern one (think about the synchronous motor type) the efford will be amortized within a few years and then you get into a winning-zone. And all the coordination-problems inbetween the 2-pump system will be obsolete. But the indispensable condition is, that your pipe-system in section 1 is in a proper pressure-stage. 

  2. The suggested change to straight series pumps will work but the controls are likely to be more complex and expensive. The problem is that you have to ensure the suction pressure is high enough to meet the NPSHr requirements of the booster pump as the flow from the booster pump to the final distribution chamber will change as the level in the distribution chamber changes.

    Since the current installation pumps from the well to a sump, I'm assuming there is not any excess head available on the well pump. When you put in the in-line booster pump, you may have to increase the well pump's head to keep sufficient suction pressure on the booster.

    A VFD on the well could be employed however retrofitting an assumed constant speed pump with a VFD drive doesn't always work well if the pump is not designed to be VFD driven. In order for a VFD to work, you'll probably need to install a suction pressure transmitter at the in-line pump and send a signal back to the well pump. Then you'd need a PLC or some type of controller to adjust the suction pressure of the in-line booster pump by varying the speed of the well pump. This would likely be a major expense with telemetry costs.

    An old school but less economical solution would be to install a bypass PRV at the well and set the discharge pressure at the well to a constant pressure determined to keep the suction pressure at the booster pump within acceptable limits. This could also be done with a VFD. This recommendation would only work if the pump operating point could be moved to the left to generate higher head to supply the increased suction pressure requirement of the booster pump.

    You'd likely need a flow control valve at the distribution chamber to set the discharge flow at a constant value. The problem to deal with is making sure all the pumps discharge at the same flow.

    Without knowing the elevations of the pumps and tanks along with a full description of the components and operation, it's difficult to make a informed recommendation, along with pump curves. I'd suggest that if you don't have hydraulic modeling software, download a copy of EPANET2 and create a simple model of the system. Then see what happens when you switch to an in-line booster pump from a sump pump.

    Solving the land use problem would be the best solution but with enough money any problem can be solved.

    1 Comment

    1. Thank you, Lyle for the detailed answer with suggestions. I'll post on commissioning of the system. The well pump has slightly more head, which will assure what we call "flooded suction" And as suggested by you I'll analyze the system in EPANET2. tHANKS AGAIN.

  3. Direct inline boosting (no suction sump) is very common and recommended for water distribution networks. The main benefits are energy savings and less possibility of potable water contamination.

    In water transmission pipelines where two stage pumping is required typically to avoid high pressures, direct inline boosting can also be used, but is less common because of higher operational risks.  Water transmission typically involves high flow, high pressure, long pipelines, varying topography and surge protection devices.  Operating two pumping stations is series (direct inline boosting) needs a comprehensive and coordinated control philosophy to deal with specific operational situations such as a pump trip or total power failure in one of the pumping stations.     

    1 Comment

  4. I also believe the two stage will work, provided there is proper balancing to avoid cavitation - it would be simpler to replace the Stage 1 pump (~100 HP) and eliminate stage 2 pump. This would be a simpler system to operate in the long run

    1 Comment

  5. I also believe the two stage will work, provided there is proper balancing to avoid cavitation - it would be simpler to replace the Stage 1 pump (~100 HP) and eliminate stage 2 pump. This would be a simpler system to operate in the long run

  6. Hi, Yashpal. I agree with Andre, it can be done. In fact we have installations here in the Philippines wherein multi-stage pumping is used to overcome space limitation in constructing well sump. Balancing load between pump stations can be easily achieved with proper design using variable frequency drives, programmable controller and instrumentation.

    1 Comment

  7. Hi Yashpal, I can see no reason why the proposed system will not work. It is however not possible to say how the proposed system will behave hydraulically with only the information supplied and in particular, without the proposed pump curves. I would suggest you ask the pump supplier to do a system analysis and recommend the pumps that are best suited for the proposed system.

    Trust your project works out well.

    Regards.

    1 Comment