LN2 variable pumping

[110970]

We need to provide variable flow (3,000 – 72000 scfm) of GN2 at 250 psi and 70 degrees F to several modulators in a test room. We need to maintain 220 psig pressure in the test room manifold. Three centrifugal pumps with VFD’s will pump LN2 through (2) water bath vaporizers. The LN2 is stored in a tank at a pressure of 3psig.
1. Could we avoid creating vacuum in the tank by connecting GN2 (downstream of vaporizers) to the tank?
2. Could we generate small GN2 flow (3000 scfm) without running the pumps or do we need to run just one pump at a low speed and re-circulate the excess LN2 flow?
3. What is the right place for the back pressure valve location: at the end of the manifold in the test room or just downstream of vaporizers?

 

[dcasto]

1. yes
2. yes, but the GIN will be at less than 3 psi, you'll need a compressor.
3. I'd set up a regulator on the outlet of the heaters that senses and controls the header pressure.

Comments: Most LIN tanks are rated to 75 psig. Is it really 72000 scfm or 7200 scfm. 72000 is 800 gpm of LIN. Each pump will be a 266 gpm and will have to turndown to 33 gpm? You should look at the ranges of LIN tank preesure and fiquire what the pumps are going to look like too.

 

[110970]

Thanks dcasto
It is 72,000 scfm. Each pump capacity is 270 gpm.
I did not mention that this system will be working only once a day during 20 min test. Is there any concern about the pump start-up? Also, should the pump discharge pipe be insulated? The suction line will be VJ pipe.

 

[110970]

Is there someone that could help to understand how the Net Positive Suction Head (NPSH) equation below applies to the LN2?

NPSHa = p + Lh – (Vp – Hf)
Where:
p – Pressure on surface of liquid in closed suction tank, in feet absolute
Lh – Minimum static suction head in feet
Vp – Vapor pressure of the liquid at maximum pumping temperature, in feet absolute
Hf – Friction loss in feet in suction pipe at required capacity

A liquid nitrogen tank (spherical shape) has capacity of 217,000 gallons. The inner sphere has diameter of 38’9”. The tank is maintained at inner pressure of 2 psig. The Lh is 40 feet.
Would be “p” and “Vp” equal and should I ignore them? The temperature of the LN2 at 2 psig (16.7 psia) will be 318.45ºF. The LN2 will be taking from the tank at this temperature. What would be the “Vp”?

Thanks