Pumps In Series

Dear Mr W.A Snow,

The second pump in the line has got on sunction the pressure given by the first (discharge).

So the reason is to have higher "head" by the system consisting of two units. The total head of the system will be 2 x Head of the single unit.

In my opinion we should avoid this in the projects due to:

- demand of the spare parts for 2 Units

- one pump has got malfunciton - the whole system is out of the work.

In my opinion in this case you should use 2 pumps, each with proper head (new head = 2 x head of installed pump in series) . One of the pumps is working, second is on stand by in case of failure of the working unit.

If there is a place i think the installation were done like this due to economical reasons. I think investor whanted to take the money and get lost. In the future after the warranty this can be the problem for the user due to stops in case of fauilure / repairs /.

Or

The user has got one unit in the workshop which can be put directly to work.

Hope this help,

BR,

SAM

A multi stage pump is actually two or more pumps in series. It is perfectly acceptable.

Sometimes due to a variable discharge head including or excluding a further pump is also acceptable.

Being electrical you must consider pumps equivalent to batteries and the voltage equivalent to head.

What are they pumping? It could be that the first pump is a jockey/booster pump to ensure the second, main pump has enough Net Positive Suction Head. This could be the case for a low vapour pressure liquid from a tank whose level varies greatly. That is, when the tank level is low, there is not a lot of head for NPSH, and thus the jockey pump is required.

Are the two pumps identical? Same size? Same type? .....

The second pump is for the RO system; it will boost the water pressure to a level where the permeate recovery rate will be high enough to be economically worthwhile.

I think you've got it.

Same reason you see two engines pulling a freight train. It gets more work done.

Series or parallel will "get more work done", obviously. Series will boost pressure, parallel will increase flow rate.

Only if the piping is properly sized.

"Series will boost pressure, parallel will increase flow rate."

Yes, but in parallel they must both be turned on, or water will flow around the one turned off causing inefficiency (assuming only one output pipe). In series the second one can be turned on if there is more demand and left off if low demand. Either way there should be more flow if both are turned on.

• like your comment nt, last sentence untrue, really, ask an expert.

Understanding that the application is in Reverse Osmosis Plant. The need of this plant would be high pressure for the membrane. If during mid phase of the project where the pumps for the project have been ordered the project scope is changed, then one duty and one spare pump decision becomes two duty and order one spare for future. Upsizing the pump to high pressure and same flow becomes very difficult. This would explain why the two pumps are close by and in series.

Ideal scenario would be to have correctly sized, one duty and one standby installation. In this case the pumps would be in parallal. If space is an issue than the installation would be one duty and one spare in store.

In the installation you describe, to maximise reliability you may have to consider the option of changing both the pumps together, if one fails for reason such as wear due to pitting or bearing failure. The cost impact is two pumps to be serviced and two pumps to be kept as spares.

If the organisation can absorb the cost, it is better to correctly size the pump. Install one duty and one spare.

Prem

Just some corrections to some incorrect assumptions that have been made:-

This is not on an RO system. It is on a recirculation loop for a DI bed.

I suppose my basic question was why use two pumps instead of just one. I asked the mech engs why this was and the answer that I was given was "that these pumps are off the shelf units".

Kind Regards

Mr W.A Snow

Sorry 'Hole in the Snow" & with due respect,

I am unable to accept that because the pump is off the shelf item, we can put two pumps in series.

Before we do that, an informed decision has to be made and understood that by installing the pump in series, the plant will incur higher maintenance and inventory storage costs.

The overall systems reliability is also reduced as there are more rotating parts likely to fail.

This would probably lead to question as to in which application one would use pumps in Series. Well the line supplying water to gold fields (say) 500 km from the reservoir have lots of pumps in series.

Prem

(Mechanical Engineer)

If that is the case, then the product output may require a supply flow or pressure that will require boosting, in which case the second pump is a booster pump.

Hi HoleInTheSnow

Your initial feeling that it is wrong was on the mark.

It seems that a quick fix solution became the permanent one. It is quite possible that they will keep on repairing pump units indefinitely.

Booster pumps (on a pipeline) however sometimes make sense.

i am a water plant operator and by no means an engineer, but the reason this is done is due to the huge question what if. if there was a sudden demand for an extremely lagre amount of water more than 1 pump alone could deliver then the second one is there in operation to assist with the sudden new demand. for example a fire and you now have a fire pumper truck to your system via a fire hydrant and most of the new ones are capable of sucking 1100 to 1200 liters per second some even higher. also it is there for future expansion.

i did forget one small detail what type of plant is this and second which end of the treatment process, before or after the treatment line?? first answer is for after the treatment line, second question are they a high or a low lift pump?? in my plant there are 5 pumps. 2 are for the raw water influent and 3 are for the treated water effluent.

I think there might be more than one reason to do it such as pressure and cost.

I tend to believe what some have already said 'to act as a booster' and to reduce the need for a repeater pump station.

This way pressure definately can be improved by (?) factor if it is important.

Hi All,

I actually got an answer to my question today. The reason that we have two pumps installed is that (now wait for it, its a real gem)..............

They had one pump and it did not deliver the required pressure so a second pump was installed in series. It would raise questions about the design of the system in the first place. I suppose, looking on the bright side, we only have two pumps and not three .

I was hoping to see the design parameters and the formulae used to work out all the pressure drops and the pump charts used for the selection of the pumps but I was disappointed.

Thanks for all your answers. I am off to buy a book on pumps.

Kind Regards

Mr. W.A Snow

Did you work it all out through reasoning?

You must be a smart guy could become a programmer even they're usually smart, so I was told.

See what all those cells in series can do!

Check out this very practical FREE book before purchasing others!!:

http://www.lightmypump.com/pump_book.htm

The reason you were unable to find the "engineering" calculations for your installation is because they were never done!

The system, as you found out did not work, so it was McGivere'd with an additional pump.

happens all the time!

See ' DR FREON '.