BIODIESEL PUMP. 1

[gorkus]

Could anyone give any advice:

We have to pump biodiesel at 80ºC from a tank under vacuum of 0.9 bar.
The pump is 1 m under the tank.There is not any elbow(in suction line)

Does liquid flow from the tank to the pipe ," by gravity" ??

or
this situation is equal as when the pump is above the supplier tank and it has to lift the liquid.??
then
I have calculated NPSHa and the value is about 2.3m and tne NPSHr of the pump is 2m. so the pump is not suitable in this case.

many thanks in advance

 

[pmover]

well . . .

Does liquid flow from the tank to the pipe ," by gravity" ??
yes, provided the pipe removing the liquid is lower in elevation than the tank.

without knowing flow or head requirements of the pump, you need to find a pump that safely operates with sufficient npsha. techniques used to increase npsha are to increase inlet pressure (likely not an option), increase elevation difference between pump and min tank level, use a low npsh impeller, or add a booster pump upstream of main pump.

good luck!
-pmover

 

[BigInch]

"I have calculated NPSHa and the value is about 2.3m and tne NPSHr of the pump is 2m. so the pump is not suitable in this case."

The pump is suitable when npshA > npshR = OK


BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[davsy]

Gorkus

Is this an existing installation that has pumping problems or are you designing something new.

I assume the NPSHa was calculated based upon worst case, you are very close here between NPSHa and NPSHr so it would be worth spending a bit of time and effort to ensure you have everything right before you commit to purchase the pump (if it is not too late).

When you say there is "no elbow in the suction line" what type of pump is this and where is the outlet connection of the tank. How does the piping run from the tank to the pump, you have said there is 1m of height but no elbow, is this possible. Are there any valves or strainers in the suction line.

I take it that as you are under vacuum and at 80C then you have removed all of the Methanol, if there is methanol in any quantity then you could well have NPSH problems caused by the methanol.

Also I take it that this is a continuous process where the level is maintained in the tank at a point at or above where you calculated the NPSHa and does not drop below.

Is the suction line sufficiently large enough diameter, as you are that close on NPSH an undersized suction line or a restriction of some type will be sufficient to put you out.

I have been to two biodiesel plants in the last 18 months where they have installed a low level stripper operating under vacuum without any consideration for the NPSH of the pump, the easiest solution has been to lift the vessel up a metre or two to give sufficient NPSHa for the pump.

 

[gorkus]

The suction pipe is from the bottom of the tank ,the length between the level of liquid in the tank and the pump centerline is 1 m.

Tank is under vaccum of 0.9 bar
there is not any strainer or valve in the suction line.´
the pumps is a positive displacement pump ( "trirotor pump"
flow : 4.000 l/h
viscosity 4,7 cps.
product : biodiesel at 80ºc
then, I mean:
NPSH= Hss-h+Hsp+Hv
Hss=1m
h= 0 (negligible)
HSp= 1.3 m ( 10.3-9)
Hv = ( I m not sure about the value of vapor pressure asolute of biodiesel at 80ºc,is about 5 mm hg depend on the process and used oils.)

the value is over 2 m

it is ok??
then
NPSHd > NPSHr +0.5 m (0.5m a safety margin.)

NPSHr of the pump at flow of 4000 l/h is 2m.

 

[davsy]

Gorkus

Where are you getting your 1.3m for for the vessel pressure.
You state that the vessel is under a vacuum of 0.9 Bar, do you mean that it is operating at 0.1 Bara or 0.9 Bara there is a big difference.

Anyway, 1 Bara = 10.2m of water therefore 0.1 Bara = 1.02m while 0.9 Bara = 9.18m I will assume you mean 0.1 Bara as this will give you the worst case.

Also vapor pressure of Methyl Oleate is approximately 0.00007 psia (0.000049m) at 80C

So for NPSHa = pressure above liquid + Static Suction Head - (fricion loss + Vapor Pressure)
therefore NPSHa = 1.02 + 1 - (0.0115 + 0.000049) = 2.008m

If your pump has an NPSHr of 2m then you are really too close to call and need to make some changes to the system.

I have assumed 0.0115m for the friction loss based upon a quick guesstimate at 5ft long 2" diameter suction line, if the line is smaller than 2"(50nb) or has any kind of fitting then it is likely to be even worse.

 

[gorkus]

DOES VACUOMETER GIVE US AN ABSOLUTE MEASURE OF THE PRESSURE.??

MANY THANKS FOR YOUR HELP

 

[davsy]

Gorkus

I think you need to confirm to us which it is, Gauge or Absolute.

Just as a matter of interest, when your vessel it at atmospheric pressure, i.e. there is no vacuum, what does the pressure/ vacuum gauge read.

An Absolute gauge normally would read zero at absolute vacuum so it would read just over 1 Bar (1 Bara) at atmospheric pressure, a gauge pressure instrument would normally have a negative scale to the left of zero where full vacuum would be shown as -1 Bar (-1 Barg) and at atmospheric pressure it would typically read zero.

If it reads zero and then you switch on the vacuum pump to pull down the vacuum what does it read, does it start at zero and then go down 0.1, 0.2, 0.3 all the way down to 0.9 where it stops or does the gauge start at 1 and when you pull down the vacuum does it go 0.99, 0.98, 0.97 etc down to 0.9 and then stop there. If it is the first one then you are operating at 0.1 Bar absolute while if it is te second you are operating at 0.9 Bar absolute. A big difference between the two.

 

[gorkus]

IT IS A GAUGE MEASURE ..

 

[davsy]

OK so your gauge starts at zero when it is at atmospheric and reads back down to 0.9 on the scale when it reaches the operating vacuum, if that is the case then you are operating at -0.9 Barg which is 0.1 Bara.

If you are at 0.1 Bara then you are likely not to have sufficient NPSHa for the pump you are talking about based upon the numbers I gave earlier.

 

[pumpking]

Apart from the NPSH issue, it may be worth looking at at an alternative pump type. If its on a transfer duty as you say at that flowrate, that Viscosity then a Centrifugal pump will be a better option - they tend to last longer as there are fewer wearing parts - certainly NPSH figures can be much lower that 2m also !!

 

[gorkus]

we are thinking about to rise the length between the tank and the pump 1.5 m and increase pipe diameter from 1.5" to 2" ore more

would be enough in order to pump the liquid.

any advice??..
Thanks