CHOOSE PUMP FOR FIRE FIGHTING SYSTEM

[Corvo]

I currenty have two engines attached to WORTHINGTON Model 6LN 18 Pumps (375 m3/h). I do not know the NPSHR of this pump but I think that the NPSHA is less then 1 or almost zero since the water reservoir is at the same level as the impeller. What is the best choice for this kind of stituation? And what about if the water reservoir is lower then the pump?

 

[pmover]

i'd contact the mfg and obtain a set of performance curves for the pump in question.

surely, someone designed & reviewed the system to avoid such a potential problem. good insight on your part to investigate!

also, from the google search webpage, type in:

worthington+pump

and the first 4-5 results are worthy of investigating to further assist you.


good luck!
-pmover

 

[TD2K]

The NPSHA won't be close to zero.

In addition to the elevation change, there is also the difference between the vapor pressure of the water and atmospheric pressure in the NPSHA calculation. Depending on the water maximum temp, that can give you close to 30 feet right there. Granted, you need to include any line losses (and the issue of keeping the pumps' suction flooded) but should have a fair bit of NPSHA, just need to find out the NPSHR and hope they were chosen appropriately.

 

[Corvo]

I gather more information on the pumps. A few years ago there was a fire drill that took a lot of water, they told me that the two pumps stopped runing when the water level got too low. There is another pump, build by portuguese company EFACEC, with a different kind of design that runs quite well and soaks every last drop of water.
Another thing I would like to undestand is if the NPSH depends on the design of the pump, since the worthington pump is different then the Efacec pump.

 

[25362]

Sometimes the suction (header) configuration is such as to enable one pump to keep sucking while others receive much air and stop functioning. I'd be interested in reading your comments.

 

[Corvo]

THe WORTHINGTON Model 6LN 18 Pumps is a Horizontal Split-Case pump. Is this means anything in terms of NPSH? Does this kind of pump works betters other some conditions then other types?

 

[abcmex]

Corvo,
when you check the curves from a pump you will notice that it contains a curve for the NPSHr. Even pumps from the same serie or type have specific data.
As already 25362 mentioned there are other factors to consider like the installation, suction line, leaks etc.

 

[25362]

For an example of how suction configuration may affect performance take the case of centrifugal pumps sucking water from cooling tower basins.

Centrifugal pumps for this service are provided with short, smaller diameter, suction pipes to reduce the residence time so as to prevent small air bubbles from coalescing into larger bubbles with consequent suction troubles.

For these cases linear water velocities of 2.5-3.5 m/s have been recommended.