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,

 

[mk3223]

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.

 

[vij36]

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,

 

[Artisi]

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.)

 

[georgeverghese]

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

 

[Artisi]

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.)

 

[vij36]

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,

 

[Artisi]

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.)

 

[vij36]

Dear Artisi,

I refereed the below link for comparing oil filled over water filled:
Link

The height difference between sump and overhead tank is about 4 floors approx. 15 meters.

The entire spec including the data sheet is attached herein.

Request you to comment ...

Thanks,

 

[Artisi]

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.)

 

[vij36]

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,

 

[Artisi]

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.)

 

[vij36]

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 .

 

[vij36]

Please see the attached drawing.

 

[MJCronin]

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

 

[Artisi]

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.)

 

[LittleInch]

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.

 

[vij36]

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,

 

[LittleInch]

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_505.html

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.

 

[Artisi]

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.)