Doubling the pipe dia and its effect on pump

[vij36]

Respected members,

I have this query regarding improving flow of water in an agricultural fields setup.

Current system is:
Pipe length: 700 m
Pipe dia: 2.5"
Pump: 10 HP

Can the setup be changed like this below?
Pipe length: remain same (700 m)
Pipe dia: 4"
2 Pumps of 10 HP

What factors will be affected (pros and cons) and are there any design resources available.

Could please guide me.

Kind Regards,

 

[LittleInch]

The flow rate for the same pressure drop will be substantially more so if you add more pumping capacity then you will get a lot more flow.

But everything depends on matching your pump to the pipe.

Power means nothing without the pump curve if these are centrifugal pumps or what the maximum pressure if these are pd pumps of some sort.

If you provide the data we can help

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

 

[Artisi]

Data required.
Pump curve.
Actual pipe diameter or pipe standard.
Schematic of pipeline, inlet lift or flooded suction, elevation change etc.
A straight forward cslculation with necessary data.

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.)

 

[fel3]

To follow up on Artisi's point about actual pipe diameter (you may or may not know the following, but I am providing this discussion to help others as well as yourself):

Your 2.5" and 4" diameters are almost certainly nominal sizes, not actual diameters. Many pipe standards show significant differences between nominal sizes and actual diameters and these differences are not the same for each size or for each pipe class. Also, different pipe types (based on their particular standards) have different differences between nominal sizes and actual diameters.

Because these differences can be significant, your final hydraulic calculations should be based on actual diameters, not nominal sizes. Depending on circumstances, I sometimes use nominal sizes for preliminary calculations to get close to the desired pipe size, but I always based final calculations on actual diameters.

Here is an example of this "chaos":, for 4" nominal pipe size:

4" AWWA C900 PVC (typically used as water pipe)
- 305 psi (DR 14): nominal ID = 4.07" (for 6" and larger, nominal ID is less than nominal size; because of manufacturing tolerances with plastic pipe, "nominal ID" is used instead of "actual ID")
- 235 psi (DR 18): nominal ID = 4.23" (for 8" and larger, nominal ID is less than nominal size; for a 30" pipe, the nominal ID is 28.23", which is almost 2" smaller than the nominal size)
- 165 psi (DR 25): nominal ID = 4.39" (for 16" and larger, nominal ID is less than nominal size)
C900 pipe is sized to match the ODs of cast and ductile iron pipe so they can use the same fittings (this makes it easier to sell PVC). The IDs are smaller than the same nominal size iron pipes because iron is stronger than PVC.

4" ASTM D3034 PVC (typically used as sewer pipe)
- DR 26 (PS 115): nominal ID = 3.891"
- DR 35 (PS 46): nominal ID = 3.975"

4" HDPE DIPS Pressure Pipe, PE 4710
- DR 7 (333 psi): average ID = 3.346" (the average ID for a 20" pipe is only 15.058")
- DR 9 (250 psi): average ID = 3.670"
- DR 11 (200 psi): average ID = 3.876"
- DR 13.5 (160 psi): average ID = 4.045"
- DR 17 (125 psi): average ID = 4.202"
- DR 19 (111 psi): average ID = 4.264
- DR 21 (100 psi): average ID = 4.315"
- DR 26 (80 psi): average ID = 4.408"
- DR 32.5 (63 psi): 4.486" (the average ID for a 20" pipe is 20.190 and the average ID becomes less than the nominal size only for 30" through 48")

4" HDPE IPS pressure pipe has different dimensions than 4" HDPE DIPS pressure pipe. These two pipe standards are sized to match two different iron pipe standards. As above, the IDs are smaller because iron is stronger than PVC.

Ductile iron pipe has its own set of IDs for different pressure classes. If you specify double lining thickness, subtract a little more.

Steel pipe has it's own set of IDs for different classes and can be had with different lining thicknesses. Larger diameter spiral weld pipes with cement mortar lining can be made so that the actual diameter matches the nominal diameter.

I have probably gone on too long, and there are more pipe types out there that have other dimensional standards. So, long story short, use actual ID's for hydraulic calcs.

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[bimr]

The rule of thumb for economical pipe sizing is 3-5 ft/sec water velocity. When you exceed this velocity, the friction loss is higher resulting in a waste of pumping power.

Here is a website that has an online headloss calculator:

http://irrigation.wsu.edu/Content/Calculators/General/Pipeline-Pressure-Loss.php

Fluid velocity is determined by the economics of pumping, not some arbitrary factor.

Reasonable pipe velocities depend on the application. There is no correct velocity for all applications. Here is a general guideline.

Reasonable Velocities for the Flow of Water through Pipe (from Cranes Technical Paper 410):

Boiler Feed.............8 to 15 ft/sec
Pump Suction ............4 to 7 ft/sec
General Service.........4 to 10 ft/sec
City.......................to 7 ft/sec
Transmission Pipelines...3 to 5 ft/sec

Go to a basic hydraulics book. Try Cranes Technical Paper 410 as a reference for the above velocities.

 

[vij36]

Dear Sirs,

Thank you all for the valuable inputs. Sorry for the delay in replying.
I was not able to find the pump curve from the manufacturing website nor i could locate the dealer.

All I found is this.
Make: Texmo
10 hp
4 stage
6 KG pressure
Model: evam 70/01

The Texmo website showed the following detail.
Link
evam 70/04
Head range (m): 37/85
Discharge range (1pm): 600/180
Deli size: 2.5"

Existing system is vertical Open-well submersible pump with about 40 m head

As per the honorable member's suggestions these below point I noted down:

1) ID, OD and make of pipe have significant impact on flow and pressure
2) Ideal flow range is: 3-5 ft/s

I have to ask the company for Q or CV value so that I can calculate from blmr given online calculator: for 2 pumps (10 HP) and 4" Dia pipe(HDPE PE100 PN8 DN110 ) combination what is the velocity(ideal: 3-5 fps) and to verify if this is the best combination to improve water flow.

I was referring 10 HP in particular because this is the government imposed regulation on the max HP can be used for a pump

Excuse me for any inconvenience caused for my technical illiteracy.

Regards,

 

[LittleInch]

Great start.

Now assuming your second pump is in parallel to the first then your flow rate should go up by a factor of between three to four.

How much will depend on your current pressure at the top of the pump shaft and flowrate. What is the pressure at the start of your pipe??

What is the current flowrate??
If you don't have a meter just time how long it takes to fill a certain measured volume.

Your only concern is that you could pump too much, but some of your pump power is being used to get the water out of the ground so this should avoid excess flow.

Do you know how far the water is below ground level??

All these things can help to estimate how much more water you can get with a 4" PE pipe in place of type current 2 1/2" pipe.

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

 

[1503-44]

Without specific pump head and flow details, the easiest way to determine new capacity is to assume that
1. Velocity in each pipe size remains the same, so
Pipe capacity Q is increased by the ratio of diameter squared, so
(D2)^2 / (D1)^2 = 4^2 / 2.5^2 = 2.56
Pipe capacity is a bit more than doubled.

2. N identical pumps in parallel will give N x pump capacity = 2 x Q

Take the lowest number of those =2
It is pretty safe to assume your new capacity will be about twice++ what it is now.
You may get more, or less, depending on other still unknown factors mentioned above, but 2X looks to be a safe reasonable expectation, Maybe 3X is possible, but that depends very much on pump flow and head curve details. If your 1 pump is working at maximum conditions now, don't look for much more than 2X.

 

[Artisi]

Still a lot of guess work as to what the flowrate is likely to be, of course what is unknown at this stage - is the inflow capable of supporting the increased capacity.

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.)

 

[1503-44]

Well, pun intended, that's beyond the scope of the subject at hand.