Calculating NPSHa when 2 streams combines...

[Chengin]

Hi,
I'm a chemical engineer.

I calculated NPSHa to review increasing crude charge rate.
Usually the pump is run from 1 tank(there are 3 tanks), but during switching time another tank's nozzle open and the stream combines together.
Switch starts when the first tank's level goes down 3.7m and it lasts 4hours until its level reaches 2.9m. (The reason of 2.9m is that there could be sludge or other contaminants below 2.9m)
What I would like to know is when I calculate the NPSHa at the minimum pump level(2.9m), should I consider the other tank's level(it might be over 10m) because it services through the same pipeline?
From what I've learned so far when 2 streams combine together it follows the lower pressure so it doesn't seem that I have to consider it.
But one the one hand, once the stream from the lower level's tank combine with another one(crude pipeline is underground) it's NPSHa might be increased because of the high level of tank..

Thank you in advance..

 

[LittleInch]

I don't quite follow your description of what is happening.

Can you elaborate a bit, especially on the apparent four HOURS that you have two tanks open??

If one tank is at 10m and the other at 2.9, you won't be getting any flow fro the tank at 2.9m and unless you've got a NRV in the outlet line the tank with 10m will start filling the tank which is at a lower level.

Your NPSHA calculation should be based on the lowest possible pressure. That seems to be 2.9m, but I can't really follow it.

Also unless you are somehow taking flow from the top of the liquid, the 2.9m bit doesn't make sense. That sounds like an awful lot of fluid you're leaving in the tank.

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

 

[Chengin]

Thank you for reply and sorry for my bad English.
The tanks's ground level is higher than pump's ground level.
So even the first tank's level is 2.9m, its flow goes to the pump (I can read the tank's level) with second tank which has higher level.
(Eg. when 1 tank services its level falls , and during switch time both low and high level of tanks level fall together)

For you better understanding, I put attached drawing.
So what you are saying is calculating NPSHa is based on the lowest head.

 

[LittleInch]

If your two tanks are at the same level then the pressure drop from yur second tank must be quite high from the tank to the junction with the inlet pipe if whn both tnaks are open the level goes down in both tanks.

But it doesn't really matter - I think from your description the worst case is when tank 1 is at 3.7m head, but you need to look at all the possible options to find the worst case. E.g. what is the head when tank 2 gets to a minimum level?

The only thing you can say from your sketch is that at the junction on the header pipe between the two tanks the head or pressure must be the same. You haven't given much information away there so it's difficult to say much more.

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

 

[JJPellin]

All of our crude tanks have primary pumps right at the tank that push the flow to the common header where the crude booster pumps pick it up. Are you sure you don't have primary pumps at each tank? If not, then I suspect that your pumpers are pinching on a valve from the tank with the higher level to balance the flow from each tank to the rate that they want. Either way, LI is right. The tank with the lower level is the limiting factor. Only consider that tank in your NPSH evaluation.

In our crude tanks, the lowest level for an operation like this would be the point where the floating roof sits down on its legs. Going below this level would open a vent which would trigger environmental considerations. There could also be limits related to the tank mixers, assuming you have tank mixers.

Johnny Pellin

 

[georgeverghese]

There is no response to @LI's first supposition - tell us if you have a check valve at the outlet of each of these tanks.

 

[Chengin]

To LittleInch
Thank you very much. I understand what you meant.
I was worried about that if I calculated the NPSHa was wrong. Because I calculated current situation, but NPSHa is lower than NPSHr even there is no problem now!
Maybe it's because our crude oil's actual vapor pressure isn't that high or actual NPSHr could be lower(because NPSHr from pump's performance curve is derived from operating water case.)
What I just wanted to know is when two streams go together I calculate NPSHa with a stream having lower head(or pressure). Or I should adjust something because there is another stream having head(or pressure).

To JJPellin
We don't have that pumps. There is only static pressure boosting crude oil to the CDU Unit.
Even if one tank's level low the pump in the CDU Unit is lower than tanks' ground level, so crude oil doesn't flow back to the tank having lower level.
(I don't know how low tank manage it, I need to figure it out)

To georgeverghese
From what I remember, we don't have any check valves at the outlet of each tank.
Because CDU Unit's crude oil pump's suction is always having lower pressure than tanks.