Booster pump sufficient?

[venkat96]

Hi guys,
Where i work we have these two pumps which transfer raw water at 50-60 m3/hr from tankers to a storage tank that is 18 mtr high through a 6 " pipe that is 240 m long with 13 bends. Sometimes we get muddy water from tankers which needs to be treated before transferring to the tank. So we proposed to install a filter of 40 m3/hr capacity and first take water from the tanker to a 25 KL tank and thereby use a primary pump to transfer the water to the filter and hook the outlet to the 6 " line. Based on the filter supplier data we can't give more than 3.0 kg/cm2 pressure at filter inlet and the pressure drop across filter is 1 kg/cm2. So we get 2.0 kg/cm2 at filter outlet which i think is not sufficient to lift the water to the tank. So we thought of adding a booster pump at the outlet to again increase the pressure by 1 kg/cm2 at the outlet. Is that how it works or am i wrong. Does it affect the existing system ? Please help.
I've attached little sketch for your understanding.

 

[LittleInch]

So is the second pump still running wrist you send water from one tanker to the filter?

Why are you going via this 25kl tank? If the tank is atmospheric then what flow or level control are you using? How do you stop the pump from flowing more than it should?

What is the output of the raw water pump?

But it r r does seem to be quite tight on pressure depending on what your pressure drop is on the 6 inch line. 1 bar drop on the filter is presumably the pressure drop when fully dirty?

 

[venkat96]

Thanks LittleInch,

The booster pump will run only when the first pump starts running.

The filter is not continuous operation. 2 or 3 times a day we get irrigation water that is dirty. Rest of the time we get clear underground water. So when the tanker with irrigation water comes, we want to transfer it to that 25 KL tank and add chlorine to it. We are using level switches to turn on the filter feed pump.

Raw water pump output is 60 m3/hr. It's a 15 KW pump.

The filter is actually 2 filters one multigrade and one activated carbon filter. Filter supplier said the pressure drop will be 0.5 kg per filter during normal operation. But when full dirty it might go to 0.8/1 kg per filter.

 

[goutam_freelance]

Considering the flow velocity in 6 in pipe is small(about 0.6 m/s) the pressure drop in 240 m+13 bends works out to be about 1 mwc.
So the required head for primary+booster pumps is =18 m(static head)+10 m (filter drop)+1 m (pipe drop)=29 m(say 30 m).
And the required pressure after filter is =18m(static head)+1 m pipe drop=19 m

So if you have 20 m already +10 m(booster pump) this should be sufficient. But you may need a throttling valve to control the flow such the booster pump does not get overloaded.

But as @LittleInch indicated, the requirement for a 25 kL tank is doubtful, and if it can be eliminated, the system operation can be simplified.

Also, be careful about dirt deposition in the raw water pipe. This may increase the pipe friction drop. It may be a problem as the velocity in the 6 in pipe is <1 m/s. The extra margin available in the booster pump may be handy for such occurrences.

 

[pierreick]

I think the 25 m3 tank is a buffer tank, necessary to transfer the material from the tanker, to avoid additional logistic costs.
What you call a booster pump is a transfer pump. To me you need a capacity to collect the liquid from the filter prior to the pump. This will mitigate the risk of cavitation. This pump should start on LSH and stop on LSL.
The flow from the filtration unit will change upon delta P increase (dirt accumulation).
My view
Pierre

 

[venkat96]

Goutam,

This plant will run seperately while the two raw water feed pumps are pumping water at 60 m3/hr each so the flow in pipe should be 60+60+40 = 160 m3/hr worst case.
We want to eliminate the 25 Kl tank too and run the plant with only the raw water feed pump and chlorine dosing in the line itself. But this existing pump is of higher capacity (65 m3/hr @ 46 m head).

Thanks.

 

[goutam_freelance]

flow in pipe should be 60+60+40 = 160 m3/hr worst case.

This information was missing in the earlier post.

However, the worst case pressure drop in piping works out to be about 12 m, and with the static head of 18 m the required pump head is about 30 m. As you have 20+10=30 m head already available, this should just suffice. But considering that you are handling dirty water a margin of 5 m head in the booster pump(15 m head) will do no harm.

 

[Snickster]

The OP said there are two filters in series each at normal pressure drop of 0.5 kg/cm2 or 1.0 kg/cm2 and for two in series and total of 2 kg/cm2 max. for two dirty filters in series. Therefore there will be a minimum of 10 m head available upstream of the booster pump, so booster pump needs to have 20 m head, including any losses between pump and connection to 6" header.

This assumes that the connection of the booster pump discharge line to the 6" header is at the beginning of the 6" pipeline so at the connection the entire friction loss of the 6" line is present at the connection point. If the filtration system/booster pump is located at the end of the 6" header at the storage tank, it will basically just have to overcome mostly the 18 m static head of the storage tank.

 

[LittleInch]

What you need to find really is a filter with a higher rating than only 3 bar. That is low.

You could alternatively just install a pressure regulating valve upstream the filter down to 3 bar, pipe direct from the dirty water tanker at slightly slower rate and inject your chlorine into that line. You will still need a booster pump d/s the filter to get it into the end tank and 15m diff head sounds like a good plan.

All depends how much residence time you need for the chlorine before mixing it with the water coming from the other tanker.

 

[venkat96]

Snickster, Normal operating pressure drop across both the filters is 1 kg/cm2 while dirty it goes more than that.

LittleInch, Can i get the same effect by throttling the discharge valve without using pressure regulator? How could i calculate the pressure drop through a butterfly valve while throttling?

One vendor suggested dosing chlorine in the 25 KL tank and give it a residence time of 1 hr, while other vendor suggested online dosing.