Centrifugal Pump Suction Lift 8m

[Joseph1988]

Dear,

I'm currently working on a Sea Water system whereby a centrifugal pump will be utilised to take suction from a tanks.

The distance between the bell mouth of the Tank, to the center line of centrifugal pump is 2m.
However, due to space constraint, there will be a vertical lift of 8m above base line in mid way of the pipe line, followed by a horizontal pipe of 3m, vertical down 6m, horizontal 3m and finally to the suction nozzle of the pump.

Assuming that the system is always filled with water, will the centrifugal pump be able to lift through 8m suction lift?
Since we will be gaining back the static height of 6m after the lift, NPSHr =3.2m for the pump will always be meet.

Thanks.

 

[pennpiper]

How about giving us a sketch of the suction profile?

Sometimes its possible to do all the right things and still get bad results

 

[bimr]

The system is looking at that 6m of downpipe as additional suction pipe, not as energy recovery, so it is not going to help.

10.3m is the maximum theoretical height water can be lifted under perfect conditions at sea level. That does not take into consideration altitude, friction loss, temperature, suspended particles or the inability to create a perfect vacuum. All
these variables affect pump performance and reduce the theoretical suction lift.

The key here is that the pump has adequate NPSH or Net Positive Suction Head. A centrifugal pump can lift water to a maximum of the available NPSH in the system before it begins to cavitate. The practical maximum suction lift for any centrifugal pump is about 8.5 of water. This is based on atmospheric pressure, which is required to act on the surface of the liquid and move it into the pump suction once the impeller vanes have accelerated the liquid that was in the pump inlet and moved it away. NPSH is expressed in absolute terms, so in a closed system it is the pressure available in the suction piping and system, less the vapor pressure of the liquid.

Centrifugal pumps suction lift is further reduced if a significant length of suction pipe is involved.

Problems are almost certain to be experienced in priming the pump, retaining its prime, etc. If water flows back through the pump, it may possibly damage the pump.

Don't think it will work as described.

 

[MJCronin]

Agree with bimr, if the system works at all, you are near the absolute limits for suction lift !

A sketch will certainly help here.... It sounds like you cannot run the suction piping straight because of some kind of administrative issue ????

Note: a suction side priming system will be certainly be required.

Have you talked with the pump vendor and explained your problem ? If not, why not...? Are you installing used equipment ??

MJCronin
Sr. Process Engineer

 

[LittleInch]

The key thing you haven't described is where is the minimum liquid level in all this. I agree with most of the others. Any vertical lift in the inlet pipework more than about 7-8m above the liquid level would be difficult to work as you will start to boil the liquid at that much reduced pressure. A sketch of your profile would help a lot.

I'm not sure I understand what a "bellmouth" of a task is. Nozzle I understand.

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

 

[bimr]

Example of suction bell:

 

[itdepends]

If the tank liquid level is designed to operate below the top of the u tube then your run the risk of it draining back to the tank when the pump is stopped. In addition- unless your suction velocities are high enough you won't be able to get the air out of the u-bend if the tank level is too low- normally you could bleed a suction line to atmosphere but that won't work in this case (if the tank level is too low).

To start the pump up after a shutdown with the tank level low you would need to connect a hose to prime the suction pipe (with the valve to the tank closed), bleed the air from the high point, open the suction valve (hopefully you're discharge valve is air tight), start the pump, turn off the prime and open the discharge valve.

It'll be a pain in the backside- don't do it.

As to your original question- as long as the bell mouth is submerged by a couple of metres (reducing the lift to 6m and preventing the suction line draining down by sealing the inlet), the pump should be able to generate the lift(subject to it meeting it's NPSH requirements).

As a chem eng/metallurgist the first part of any answer I give starts with "It Depends"