Pump selection criteria 5

[vij36]

Hi All,

We have a submersible pump with the below spec. daily around 300,000 liters of water will be pumped in a time span of 15 hours.
The sump will be continuously filled with water and will be pumped from there to the overhead tank.

Existing pump
QF-10 24 stg with 10 hp sml motor
giving discharge of – 18 Cm / HR @ 96 Mtr head @ Efficiency of 63.1 %

The pump draws water from a sump of dimensions (16 ft x 20 ft x 6 ft) to an overhead tank at a distance of around 400 meters away.
Now we would like to have a backup for the existing pump.
The vendor suggested the below spec.
Cld throw some light on this to see is this the best backup option?

Backup Pump spec:
QF-30 20 stg with 12.5 hp sml motor -
giving discharge of – 20 Cm / HR @ 100 Mtr head @ efficiency of 73.4 %

Thanks and regards,

 

[spartan117]

If you cannot change the pipe design due to cost restrictions, use the exact same pump for a 100% spare. There is no need for the larger pump size. However, I would change the pipe design and you have excessive pipe friction losses. Something is wrong in either the calculation or design.

 

[Artisi]

Worth checking the current running costs of power versus costing to pipe change and power reduction costs over a couple of years.

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.)

 

[vij36]

Thanks for the many advice.

Today I will get the friction head losses. Then I will post that calculation here for Honorable members comments.

Good thing is we have half of the distance a new GI (Galvanized iron) pipeline laid for an emergency back up. So now only another half of the length new pipe is required.
This is a 3" dia pipe. So can be used for overhead tank pumping with lesser Head requirement.

I will compare the current running costs and the new 3" design cost.

Sorry for some member responses I could not individually reply again due to lack of knowledge and experience. Once I get the details I will reply.

 

[Artisi]

Continue with the 3" line, recalculate the head loss, buy a much smaller pump for the greatly reduced head, remove enough stages from the existing pump for the new duty and make it the stand-by pump.

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.)

 

[vij36]

Respected members,

I have attached the Friction head formula. It calculated as 21.18 meters for the below inputs.
Q=20000 liter/hour
L=400 mtr
C=roughness coefficient=140
Dia 2.5", thickness is 7.01 mm ID= 58.98 mm

So Total dynamic head:
Static head = 20 meter tank
Elevation = 22 meter
Filter losses = 4 meters
Friction head = 21 mtr. as said in the beginning. Attached image has the calculation.

TDH = 67 mtr.

As Artisi advised we will lay new 3" pipe and redesign.

One horrible thing our supervisors did earlier was put a High HP motor and the pressure gauge shows reading. They assumed it as the Head.

How to properly calculate Head using Head using pressure Gauge?

Thanks,

 

[Artisi]

Why so you have 20m static and 22m elevation.
What is the difference between supply level and delivery level - that's the static head.
How about a sketck with levels etc. shown.

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.)

 

[vij36]

Dear Artisi,

I can attach the level sketch tomorrow.

Just wanted to update what I did today. We have diverted the existing 2.5" pipe to a 3" line which is laid for an emergency purpose. Just to check how much head can be reduced.

The pressure gauge at the pump is showing 7.5 kg for the flow rate 18,000 cubic meter/ hour. This is the same flow rate earlier also.

Does this mean total head is 75 mtr?

Earlier the gauge used to show 6 kg with 2.5" dia.

I feel it is gauge problem.

Should we calibrate the gauge ?

Thanks,

 

[LittleInch]

vij36.

Be careful what you do as at 18m3/hr you are operating on the end of the curve and if the frictional difference reduces you could damage or trip the pump as it will flow too much.

If your "normal" pressure is only 6 barg, you might already be operating beyond the end of the curve.

how are you measuring flow??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[vij36]

Hi LI,

We are having flow metre at the discharge. I can send you the pic of it. We take 5 minutes reading and multiply by 12 to calculate the flow in cubic metres/ hour.

Thanks,

 

[vij36]

Dear Artisi,

Please find attached level drawing. Sump is 97.6 meter level and tank top is at 138 level. Pipe length as per survey drawing is 438 Meters.

Thanks,

 

[vij36]

Please find attachment.

https://files.engineering.com/getfi...3fa0e&file=NEgate_To_over_head_tank-Model.pdf

 

[Artisi]

vil36 - seems you might have finally got the necessary data together to make some informed calculations;

Lets assume static head is about 40 metres, 3" HDPE friction charts for 6 l/sec at 483 meters is about 5 meters allow 2 meters for bends etc, total head is something like 47 meters - suggest you size the pump at 6 l/sec at 50 metres - now about half of what you was considering initially.

please feel free to check and comment.


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.)

 

[vij36]

Dear Artisi,

Thank you for the calculation.

Here is the doubt. With 2.5" GI pipe the pressure gauge showed 6.4 kg. Now with this 3" HDPE pipe the pressure gauge is showing 7.5 kg. We presume 75 Meters is the head.

Is the pressure gauge meter reading is wrong ?

Please correct me if I am wrong.

Thanks,

 

[Artisi]

No comment and cannot comment on the gauge reading, is it a certified gauge - dead weight tested prior to and after taking the head pressure reading, if not it's guess work?

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.)

 

[vij36]

Dear Artisi,

We did the calibration. It has no defect.

One well wisher suggested the below. Just copy pasting exactly ...

My query: I am little surprised why the 3" dia HDPE pipe showing more head (7.5 kg) compared to 2.5" GI pipe?

Reply --

There are two curves... System resistance curve and pump head curve.... These are graphs... System resistance curve is with respect to pump installation site..Here when the flow increases head also increases...

Pump head curve is w.r.t the pump vendor... It is drawn for the efficiency speed flow and head of the pump depending upon the vendor... Here when the flow increases head decreases and vice versa...

Now the selection of pump is based on the best efficiency point of intersection of both the curves.

Thus we cannot simply increase the diameter... We should also consider the pump curve for the increase in diameter which may increase the head instead of decreasing it.

-- end of reply

Request your advice.

Thanks,

 

[Artisi]

No idea what all that was about.
But, increase the internal pipe diameter without any other change and the resistance to head for the same flow will reduce - end of story.
Actually the change to head loss is the change of diameter to the 5th power, but beware, the calculation needs to be on actual dia. not nominal diameter.

Seems it might be time for you to employ a consultant who is capable of sorting out the very minor problem you are involved with. The time and money spent so far would have paid for the 2 hours or so the consultant would have needed to come up with the answer.

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.)

 

[LittleInch]

vij,

The other poster is just quoting the basic way you should be approaching this issue, which is to match the pump to the system.

Now how your two pipe differ I don't know. But please investigate what the actual ID of the two pipes are.

A 2.5" GI pipe is probably about 2.5" ID, but a 3" PE pipe is probably 3" OD - ID??

So your "bigger" pipe might actually be smaller inside??

Also where is the gauge relative to the pump (height and distance from the pump discharge nozzle?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Artisi]

My previous comment stands, otherwise this discussion will continue for another 2 weeks without any resolution:

"Seems it might be time for you to employ a consultant who is capable of sorting out the very minor problem you are involved with. The time and money spent so far would have paid for the 2 hours or so the consultant would have needed to come up with the answer."

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.)

 

[vij36]

Dear Artisi,

Yes as you advised we have consulted Kirloskar company to review all our existing pump installations and also for upcoming new projects.

Nevertheless I have personally gained a lot of knowledge and I have to work on the site more to get ground level experience.

Thank you everyone for all your kind replies.

Thanks,
Vijay

 

[Artisi]

You can't beat hands-on in the field experience, but ensure you work from a known point - guess work doesn't do it for you. Know what flow, head, and power you have, know exact pipe size and length, know what static head is involved, understand the suction / inlet conditions and understand what you are trying to achieve.

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.)