Pump Curve and Minimum head 1

[nabeel3]

Dear All
I am working on a dewatering assignment for a seawater-filled chamber that is 13 meters deep from ground level. I need to dewater it. For that, I received a data sheet from a pump supplier, which is attached here. I am scratching my head to understand below two data.

1) If my static head is 13 meters and the dynamic head requirement is approximately 3 or 4 meters. Can this pump work for me since its minimum head is given as 20.2 meters and max is given as 32.3 meters? My question is what if the maximum head required is less than the minimum head of the pump?

2) In the pump curve of the proposed pump in addition to the pump curve in blue, there are two graphs in black color. What is it? I couldn't understand it. I contacted the supplier. But unfortunately, he is not a technical person. He couldn't help me more

 

[EdStainless]

I don't know why there are two Efficiency and Power curves.
Another thing that we don't know is what the minimum head requirement for the pump is.
If you pit is 13m deep you may only be able to get down to a couple of meters of water left.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[LittleInch]

Nabeel,

A drawing would help and also what are your variables?

Does the pit start off full of water or what?
Where is the pump located? - This is v important to know.
Where is the end of the discharge pipe? Does it just pump out to the ground? If so what is that level (height) compared to the pump outlet nozzle
By "Dynamic Head" do you mean frictional pressure losses in the discharge piping due to flow?
What flow did you use?

But as a first pass
1) This pump would appear to be oversized in terms of head as you say, this would be at the max flow of the pump. You can add some sort of control valve on the end to control the flow to avoid the pump going off the end of the curve. If you didn't do this, the pump would flow more water until the discharge head equalled the static head plus frictional losses. If this is "off the curve" then the pump consumes more power and often fails due to excess vibration and power. If the pit start full of water then you will need to control the flow to the 29l/sec max flow as there will not be any static head and your pump will just fly off the end of the curve and prob trip on excess amps.

2) On the first page the black lines are the pump efficiency over the flow rate and the blue lines are the pump shaft power.
not quite sure what P1 and P2 are, but my guess is P1 is the power and efficiency at the max impellor size and P2 is at your chosen size (150mm) as the rated power for your motor is shown as P2. Note that this is very close to the max power of the pump.


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

 

[nabeel3]

Dear Little inch

This is a seawater screening chamber. We have to dewater it for concrete work and Mechanical work. Downstream of the chamber there is another chamber, now separated by coffer dams. both chambers are at the same level. We use a flexible hose to lift the water up to the ground level which is 13 meters high from the bottom of the dewatering chamber and discharge it into the downstream chamber. The hose is tied to the ground level. Pump sits at the bottom level. The end of discahrge hose is at +6.5 level. Flow is not a big concern. We have to empty a minimum of 1300 cubic meter of water. I have 3 or 4 days to do it.

Please see attached file. In the drawing I have marked " dewatering pump here, flexible hose and water discharged here." . Do not consider the vertical turbine pump shown. We are planning to use a 6-inch flexible hose.

 

[nabeel3]

Dear EdStainless

At the initial stage of dewatering, a maximum of 7 to 9 meters of water is available.

 

[nabeel3]

Dear All
Sorry, i forgot to mention that this is a submersible pump

 

[LittleInch]

nabeel,

That's kind of what I suspected.

your issue is that you have two different operating scenarios
One - initial water level - a vertical lift of 4-5m
Two - Final water level = a lift of 13-14m

Pumps only like one.

Your options in my opinion are:

Use this pump, though it is pretty meaty - 11kW and install a globe valve on the end of the hose with some sort of flow meter and limit the flow to nor more than 30 l/sec. You will need to adjust this regularly as the water level decreases and the pump starts working harder you will need to open the valve more. You should be able to remove all the water in less than 24 hours. The biggest issue will be someone just opening up the valve because they think it will go "better" or faster and then you either trip the pump or break it.
OR
Use some sort of PD pump - screw or diaphragm? - as this will just pump the same amount of water regardless so long as it will meet the final vertical lift, but 13m isn't that much.

Your vertical lift is too high to put a pump at ground level so you do need some sort of submersible.


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

 

[Artisi]

not clear, do you need to dewater once or is it an ongoing requirement.

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.)

 

[TugboatEng]

Sounds like this is a one time job. I would suggest renting a 3 inch air operated double diaphragm pump and a 50hp diesel compressor to go with it. That will clear things out in 24 hours and be a heck of a lot cheaper than buying anything. I suggest going big to give yourself some cushion. If you run an AODD pump too hard it will ice up.

 

[nabeel3]

Dear Artisi

This is a one-time Job.

Dear Tugboat

I am looking for a cheaper option. The above pump with the Control panel cost us around $ 5330/-. You mentioned 50 Hp compressor.
If I go with my option I calculated Pump KVA as 11/(0.9 X 0.5)= 24.4 KVA. ( Power factor= 0.9, Efficiency= 0.5 from data sheet attached)
Generator KVA = Pump KVA / 0.6 - Is it okay

 

[TugboatEng]

The 50hp (185cfm) compressor is the smallest trailer mount diesel powered unit commonly available to rent. They typically cost $200 USD per say. The pump will likely be about the same. You'll need to figure out a hose situation. You can do this job in 24 hours for ~$1500 assuming you're going to work 8 hours per day.

 

[LittleInch]

Nabeel.

The 50% efficiency on the pump data sheet is already allowed for in the choice of the motor. Motor efficiency is noted as 90%, but it already gives you max amps - 35.3A

You have an 11kW electrical motor. A quick calc gives you a 14.5kVA, so A 20 KVA generator should be more than sufficient for running purposes, but watch the start current. Start against a closed valve, but motor start currents can be difficult with a generator without some sort of soft start. This is noted as a star/delta starter which reduces your start current, but still somewhere betweenn1.3 to 2.6 x FLC

But basically your pump is the wrong choice for this operation without constant monitoring and control.

Drop an AODD in, plug an air compressor in and walk away.

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

 

[Artisi]

visit your closest pump hire company if there is such a place where you are located or your local construction company and borrow / hire a small pump for the job, you have already spent more time here talking about it than it would have taken to pump out the pit.

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]

Artisi has it right, but to be fair, nabeel needs either some sort of submersible pump or one which can be lowered close (2-3m) to the water level as the water level goes down.

But yes, a 50m3/hr / 220 USGPM pump would pump this out in just over a day.

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

 

[nabeel3]

Little inch

Based on your advice, I speak with the Pump supplier. He send me another Pump, attached here. Little better than the previous one.

 

[LittleInch]

Well it's better, and it will probably do the business to be fair as although it will go off the end of the curve, the power flattens out and that's what will trip the pump. But would be much better if you contorl the discharge to <25 l/sec. If you want to go this way note that this one is a DOL starter so will draw a load of current starting, even against a closed in head. With a generator you have real problems as the generator may not be able to cope, but you don't want a massive one which is gulping at 20% load for hours on end.

We're all telling you not to do this. Why don't you get what tugboateng suggests. It will work much much better.

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

 

[nabeel3]

Little inch and Tugboateng

I couldn't find the rental pump which he suggested. We are not in USA

 

[LittleInch]

You're showing as SA - South Africa?

E.g.

https://www.aqsliquidtransfer.co.za/product-category/aodd/

https://www.rotopumps.co.za/air-operated-double-diaphragm-pumps

and many more.

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

 

[nabeel3]

Little inch

We are in Middle East

 

[LittleInch]

Well look locally. Someone will have these or similar pumps.

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