4 replies to this topic

[ayan_dg]

I have a API 650 storage tank , holding a liquid X . The liquid X need to be transferred to truck by truck loading pump. After truck loading operation is over , the liquid in the pipe line need to be transferred to the tank by pushing it by Nitrogen through the pump minimum flow line. Please see attached file for pictorial description.
The pump discharge line is about 800 m long and is 4" in size.
Please descriobe a method to calculate the pressure of N2 required for the operation and the avaerage rate of N2 required in Nm3/hr.


 Scheme.pdf   13.15KB   122 downloads

[ankur2061]

Nitogen Pressure:

a. Calculate the friction or head loss for the flow rate you desire to empty out the line. The flow rate may be estimated by finding out the volume of the liquid in the stagnant section of the pipe divided by the time to empty out.

b. Calculate the static head upto the nozzle for the minimum flow connection.

c. Add head loss & static head & provide a margin of say 20% to account for uncertainities in calculations. This becomes the nitrogen pressure required.

Nitrogen flow rate:

1. It will be the same as the volume of the liquid displaced in a given time. If the liquid transfer rate is x m³/h then the N₂ flow rate will also be x Nm³/h.

Hope this helps.

Regards,
Ankur.

[fallah]

Lack of adequate information! But as an approximation:

The pressure of the N2 would almost be a little bit higher than summation of the corresponding pressure of: 1)static head between N2 injection point and min flow line's connection to the tank,and 2)existing pressure in the tank.The volume of the N2 at this pressure almost be equal to the whole piping volume from injection point to tank connection with a reasonable margin.This volume could be converted to Nm3.

[Zauberberg]

It's somewhat strange requirement and a little bit risky in the sense that you could overpressure the tank by introducing excessive quantities of Nitrogen. Is there a procedure in place for this particular operation?

[Art Montemayor]

Ayan:

I fully agree with what Zauber has brought out and want to draw your attention to it.

This type of transfer back into an API atmospheric storage tank is potentially very hazardous. That is not an opinion. This is empirical fact and experience. What you are proposing is OK -- as long as you fully realize the hazard(s) you are incurring and mitigate them all.

The transfer of liquid back into the tank is a simple and basic operation. That is not the problem. The BIG PROBLEM is the potential for over-pressuring the storage tank with the unavoidable gas pressure surge that will occur when this gas pocket breaks through into the tank.

I request that you avoid applying what Ankur and Fallah have tentatively proposed before you go forward with any attempt to apply what you propose. I have done what you are proposing and it can be carried out successfully. But you must design the system such that you do not expose the tank to over-pressure.