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Pump selection criteria
5

Pump selection criteria

Pump selection criteria

(OP)
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,

RE: Pump selection criteria

First, you want to confirm with the operation for the requirement of the backup pump for the existing system. For example,
- For 100% capacity back up pump, you may select the pump size the same as the existing pump. It can be easier for the operation and maintenance.
- To improve or upgrade the operation performance, select a higher capacity pump for the operation and change the existing pump as a backup.

RE: Pump selection criteria

(OP)
Dear Mike,

We are not able to find a Oil cooled Pump of 12 HP submersible pump with 30 Stage in India.

Any help possible ?

Thanks,

RE: Pump selection criteria

Where does the requirement for 96m head come from?
Why do you need an oil cooled 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.)

RE: Pump selection criteria

Have a look at the recommended spacing between pumps in the Hydraulics Institute standard to see if the sump can accommodate both pumps.

RE: Pump selection criteria

George, can't see any problem re spacing - only one in operation at anyone time (supposedly)

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.)

RE: Pump selection criteria

(OP)
Hi Artisi,
We would like to replace water filled/ cooled pump with Oil filled/ cooled pump to increase the efficiency.
Current water filled pump specs i mentioned in the beginning. Since the distance between sump and overhead tank is about 400 meters we are using multistage 100 meter pump. Excuse me if I am unable to present what you would like to know. I can clarify based on your reply.

Thanks,

RE: Pump selection criteria

Do not see oil cooling any more efficient than water cooling, pump efficiency comes from hydraulic design.
Do not understand the need for 100m pump, what is the height difference between the sump and the overhead tank and what is the pie diameter.
Suggest you sketch the installation with the relative heights, distances etc.

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.)

RE: Pump selection criteria

About 15m is nowhere 100m.
The pump data sheet is meaningless if the pump duty is about 15 metres - where does the 100 requirement come from?

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.)

RE: Pump selection criteria

(OP)
Dear Artisi,

I have asked your specific question to our engineering team. Their reply is below:

" The site is on an elevated terrain. Every bend adds to certain resistance to water flow and that is translated into pressure requirement of the pump."

Any specific queries should i ask them ? Please let me know.

Thanks,

RE: Pump selection criteria

Back to your original question, forget oil cooled pump, I see no advantage what so ever for oil cooled clean water pump plus the second pump selection appears ok.

I still cannot accept a head loss over 80 metres coming from pipe / pipe fittings unless you are using very small diameter pipe or there is something missing from the information you have given.

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.)

RE: Pump selection criteria

(OP)
Dear Artisi,

As per the attached spec in the drawing

Pipe dia is 2.5 " - maximum dia of pump inclusive of cable guard and motor.

Is this too small and because of this the head loss is happening ?
Please advice .

RE: Pump selection criteria

(OP)
Please see the attached drawing.

RE: Pump selection criteria

Every bend adds to certain resistance to water flow and that is translated into pressure requirement of the pump."


Yes, that is true ....... but we would like to see the pressure drop calculation at rated flow.

I am having a lot of trouble understanding how a pump for a properly sized piping system can have 15m elevation head and 85m of friction head.


Did someone purchase and install a piping system that is too small ???

Was an MBA somehow involved with this pipe sizing ?

MJCronin
Sr. Process Engineer

RE: Pump selection criteria

you are wasting our time and your time, going round and round in circles trying to help in understanding why you have 100m total head.

Supply the design drawings or sketch out the system, there is nothing to show how the system fits together, take some photo's tell us about the installation -- crystal ball gazing doesn't work.

However, your original question was answered a few posts back:

"Back to your original question, forget oil cooled pump, I see no advantage what so ever for oil cooled clean water pump plus the second pump selection appears ok." (assuming the total head loss is 100m as you have told us, which is hard to believe).

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.)

RE: Pump selection criteria

vdvdasa,

Some questions on this?

Is this QF10-24 pump existing and in operation?
Or is it the proposed one?

From the data sheet you've provided, for the flow and head you mention ( 18m3/hr @ 96m) you appear to be operating at the far end on the curve at low efficiency (50%). The 63.1 % is the best efficiency, but is at 12.5 m3/hr

The back up pump would appear to be slightly bigger with better efficiency so IMHO, this should be the main pump, not the back up. The reported parameters ( a data sheet would be nice) appear to be similar so what is the issue here?

HOWEVER, the points being made above are valid and that is to work efficiently and not break down, trip or wear excessively, the pump duty point at close to its best efficiency need to be matched to the hydraulic requirement of the downstream system.

All we know at present is that there is a pipe 400m long of unknown size or material, with a 15m lift in elevation from your sump to your tank to arrive at an unknown pressure ( open to atmosphere? level control valve?~) via an unknown number of bends.

If you ask your site team for the relevant information then we could estimate the back pressure / frictional losses better to se eof you pump is the best fit or not.

At present the pumps appear to be creating excessive head and are not matched to your flow demands ( 20m3/hr) properly.

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

RE: Pump selection criteria

(OP)
Dear All,

I am extremely sorry for the inconvenience caused by me for providing insuffcient and unscientific data.

After visiting the site hurriedly today and after meeting the technicians, I heard as below:

Tank height = 20 meters
As the site is situated in hill kind of terrain differential elevation from sump to Tank = 20 meters
Two fileters, Sand filter and Carbon filter. head loss = 4 meters

Flow rate = 20 cubic meter/ hour
Pipe dia = 2 inches
Pipe length = 400 meters

From the above they calculate Total Dynamic Head as 96 meters.
The friction losses are approx. = 40 meters. Calculation they will give me asap.

However I have got good advice here from Artisi that by increasing Pipe Diamater We can have a lesser Head requirement.

And from LittleInch I learnt that Flow rate and Head should be operated properly so as to get good efficiency.

I am very thankful for these advice.

Is there any formula relating Power and Head ?

Once Again thank you members for their valuable time.

Thanks,

RE: Pump selection criteria

OK, So is you maximum static head now 40m? (20 from sump to bottom of tank and then 20 more to fill the tank?).

A 2" pipe at 20m3/hr is around 2.75 to 3 m/sec, which is a little fast for an economic flow rate - somewhere between 1.5 to 2 m/sec would be more efficient.

At this level of output head / pressure you also need to know how is the tank operated. Empty to full or maintained at a certain level? If empty to full then make sure you look at the worst case and best case of pressure requirement. Head loss through your filters looks quite low. If these get dirty what is the pressure loss / how often are they monitored or changed?

"hydraulic" power is Q X H / 367 for water, Q m3/hr, H in metres, power in KW.

A better explanation Is here https://www.engineeringtoolbox.com/pumps-power-d_5...

The trick with best matching of pump to requirement is to not add too many extra factors into the required hydraulic power requirement and then choose a pump with your rated flow and differential head close to the Best Efficiency Point.

Also your pump curve you sent looks more like an axial flow turbine to me as the shut in head (250m) is a lot more than the end duty point ( 100m). This will mean that your pump flow rate is relatively insensitive to changes in head requirement of 20-30m ( full to empty or dirty filter), but the shut in pressure is a lot more than normal flowing so your pipe and valves need to be capable of that pressure rating. Or maybe that's what multistage centrifugals look like?






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

RE: Pump selection criteria

L.I. Looking at their catalogue I would say radial tending towards mixed flow design.

Still too many unknows to make any real sense but the 96 metres is starting to look like the magic number with 2" pipe dia. / 400 meters plus filters etc - needs 2.5" (marginal) 3" is better.

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.)

RE: Pump selection criteria

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.

RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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.

RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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,


RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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,

RE: Pump selection criteria

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.

RE: Pump selection criteria

(OP)
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,

RE: Pump selection criteria

(OP)
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,

RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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,

RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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,


RE: Pump selection criteria

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.)

RE: Pump selection criteria

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.

RE: Pump selection criteria

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.)

RE: Pump selection criteria

(OP)
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

RE: Pump selection criteria

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.)

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