Pressure Build Vaporizer Design 1

[kts5313]

Hi,

The pressure build circuit in my situation is as follows: We have two LIN tanks (15,000 gallons each) that share a common ambient pressure build vaporizer (PBU,PBV), the LIN is drawn from the tanks to the vaporizer and the GAN is fed back to the tanks to maintain tank pressure. The pressure build vaporizer controls the pressure in the tanks around 130 psig through a pneumatic control valve on the outlet of the vaporizer.

Under normal operating conditions, the tanks feed about 5000 lb/hr combined (to the plant as well as the PBU). For this reason, LIN deliveries occur twice a day (~ every 12 hours). When the LIN is delivered, the pressure in the tanks naturally drop. The pressure build vaporizer tries to correct for this using the control valve (Cv=20) to build the pressure back to setpoint, but often overshoots. The pressure build vaporizer was recently replaced with a larger unit capable of handling 35K SCFH, which is oversized 1.5x for the plant demand. The issue is ice build up. The PBU is continuously covered in ice, and in the winter it is a hazard. I believe the issue is the deliveries and how often they occur as the control valve sees the drop in pressure and allows the LIN to flog through excessively in an attempt to build the tank pressure back up. My thought currently is to modify the control valve either by reducing the instrument air pressure signal to the controller or change the control valve seat to reduce the Cv.

I do not have much experience in this particular situation so I was wondering if anyone else has experienced something similar? Is this the nature of the process or is there something that can be done to reduce ice buildup on the vaporizer? Any help would be greatly appreciated, I can provide more details if needed. I attached a basic diagram for reference.

 

[georgeverghese]

Pressure overshoots are due to 2phase (V-L)slugging in the ascending return line from the vaporiser. In any case, piping pockets in the return line should be avoided. How does your vaporiser compare to this for equivalent capacity:

https://www.lindedirect.com/gas-supply-modes/ambient-vaporizer

Do you have a thermal design datasheet for this vaporiser ?

For winter operations, a steam or electric bundle heated unit would be required. There are hot water bath heaters also.

 

[LittleInch]

kts,

Welcome to E-T.

A few comments and questions
1) It's always best not to assume everyone is familiar with your abbreviations so first time use spell it out. I'm assuming (always a bad thing to do) that LIN is Liquid Nitrogen?
2) Don't mix units. We have gallons for storage but lbs for usage. Not everyone will be able to rapidly convert
3) A diagram is good, but make it make sense. So where does the gas go for use? What are the pressures and temperatures at each point
4) Data sheets are also useful. Is there a data sheet for your vapouriser? It sounds undersized to me or not designed for continuous flow
5) Is this a new thing or has something changed or was it designed this way?
6) "Overshoots"? What overshoots? Pressure? If so by how much? Is it an issue? Is your control valve too slow?

Remember we can only see what you give us, not what you're looking at every day so the more info, within reason, the better.

Is your issue ice build up? Or something else?

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

 

[pierreick]

Hi,
I believe the equipment belongs to the supplier of liquid nitrogen who provides a skid (storage tank and vaporizer). Did you contact the supplier?
Pierre

 

[Compositepro]

The schematic looks okay. The way the pressure is regulated is that when the control valve opens due to low pressure in the tank, liquid flows into the vaporizer to get vaporized and build pressure in the tank. When the control valve closes, the vaporizing gas will push the remaining liquid in the vaporizer back into the tank and stop vaporizing. Now if you rapidly fill the tank by pumping in liquid from the truck. This will compress the gas in the head space of the tank, which can increase the pressure and require venting of vapor from the tank. There is a required dead band between the pressure set point of the vaporizer and the set point of the vent valve, or you will constantly be oscillating between building pressure and venting. The delivery driver should be connecting a vapor line between the tank and truck,

 

[Caloooomi]

Pressure in tank drops, ambient pressure builder circuit kicks in to bring pressure back up. As cryogen flows through the vap, it gets covered in ice. The amount of ice is liked to the ambient dew point, the duty of the unit and run time of the vap. Ambient vaporizers are normally rated for an 8 hour capacity, so with longer run times (easily down in winter when temp < 0 degC), then the ice will build up further reducing the effective surface area. Options to counter the loss of surface area due to icing are: manual defrost of the unit to keep the vap clean or size the vaporizer based on a longer run time.

I'm surprised there's only one PBU for 2 tanks though, especially since you are withdrawing and refilling a lot of liquid. Is the control valve linked to a common pressure header between two tanks, or is it taking pressure readings from both? Does filling happen simultaneously? I'm more curious on the tank set up to be honest, seems weird.