4 replies to this topic

[ayche]

Dear All

I was recently given a project to calculate NPSH(A) for a centrifugal pump installed at my workplace.

After some brainstorming I am stuck at a crucial juncture in this project.

The Pump is being used for MS(Motor Spirit)/Gasoline service and the tank type is floating roof.

I wish to know what will be the absolute pressure at the liquids free surface.

Will the effect of floating roof be considered, I think it should be as it rests on the liquid.

Another doubt is had the tank been fixed roof type what will be the absolute pressure in that case.

[fallah]

ayche,

 

To calculate the NPSHA for the pump taking suction from a floating roof tank, the absolute pressure at the liquid surface inside the tank will be the local atmospheric pressure.

 

For the fixed roof tanks, the absolute pressure on the liquid surface will be normal operating pressure (local atmospheric pressure plus a few mbarg of tank design pressure).

[Ajay S. Satpute]

Ayche,

 

A sketch/P&ID should provide a better understanding of your system.

 

What's there in the free-board space in the tank, is it ambient air, pressurized N2 or any other gas? What's the operating pressure for the free-board space (set point for Vacuum Relief Valve, if any)?

 

The weight (kg) of the floating roof may be considerable, but if you take the huge liquid surface area (cm2) into consideration, the pressure due to floating roof (kg/cm2) is normally negligible. But please calculate it yourself for your system.

 

Why not connect a pressure gage at pump suction line (may be in drain) and check the pressure at the suction, if possible? 

 

 

 

Regards.

 

Ajay S. Satpute

[ayche]

dear fallah

my doubt for floating type is clear, it will be equal to local atmospheric pressure only!

but as far as fixed roof goes the absolute pressure should be equal to local atmospheric pressure plus the vapour pressure. isn't it?.

 

dear ajay

the tank is in ms service, in the free board space there will be ms vapour if the tank is of fixed roof type,

the pressure at the liquid's free surface would be equal to vapour pressure or not?. i have this doubt.

 

regards

[fallah]

ayche,

 

For fixed roof storage tank you should take the absolute value of the tank's normal operating pressure. It might be higher than the local atmospheric pressure plus the fluid vapor pressure as the gauge pressure inside the tank may be maintained (e.g.by gas blanketing) at a value higher than the fluid vapor pressure to reduce the loss of inventory by emission to the atmosphere through the free vent...

 

Of course, taking the absolute set pressure of the tank's vacuum breaker, if any, would be most conservative value to calculate the NPSHA as there might be a case in which the pumping out rate causes the tank would go under vacuum due to gas blanketing valve failure or operating of pump at end of curve condition...

Edited by fallah, 11 January 2015 - 03:38 PM.