Pressure Reducing Stations

[niallmacdubhghaill]

Hi all,

I recently posted a question about nitrogen purging calculations in this forum. Im glad to say that I have resolved my earlier question, but I now have something of a follow up!!

On our plant, we are planning on generating nitrogen at 6-8BarG, distributing it at 3BarG and supplying it to our vessels for inertion/purging/blanketing etc at 25mBarG.

My question is this:

Does anyone have any operational/design experience that would allow them to advise me on the appropriate number of pressure reducing stations that would be required to reduce the pressure from 3BarG to 25 mBarG and to maintain an accurate control of the N2 pressure around the set point (25 mbarG).

I have spoken to different people within my department, nad have, in general, been told one of two things;

1)It is not possible to reduce from 3 BarG to 25 mBarG and maintain accurate control around a set point. What I should do is have two stations, one to go from 3 to 1 BarG and then a second to go from 1 BarG to 25 mBarG.

2)There is no reason why I couldnt reduce from 3BarG to 25mBarG in one go and still maintain accurate control around my set point.

The reason that an accurate control is of such concern is that the vessel is not designed to handle much over pressure, and there is a conservation vent set at 30 mBarG.

If anyone has any suggestions as to how I should proceed, I would be most interested in hearing them.

Regards

Niall

 

[JLSeagull]

Look at the tank blanket regulators for very low pressure applications. Tank blanketing has been discussed on several threads. Fisher and Tyco/AGCO are among the tank blanket regulator providers.

Few choices exist with these regulators for the soft goods. Check the regulator elastomeric and plastic parts for material compatibility with the fluid in the vessel. Assume that the regulator will be exposed to that fluid not just nitrogen.

 

[25362]

I've seen inert gas holders of the bell-shape floating type that serve as pressure-controlled gas reservoirs, the pressure being controlled by weights on the (water) floating bell, their volume sufficient for a 24- to 48-hour supply of gas. Breather valve protection, nevertheless, has to be given to atmospheric inert-gas blanketed tanks in line with what JLSeagull says.

These type of gas holders are called 'gasometers' in Europe and used for the distribution of fuel gas.

 

[pleckner]

I am going to be doing something similar. Check with Protectoseal. I didn't even recognize that they supply pressure reducing stations until I got into a discussion with a Rep about conservation vents.

 

[JimCasey]

Protectoseal's arch rival, Groth, make a blanketing regulator. It is a piston-balanced design so that it takes tiny forces to cause the main valve to stroke, although the DP can be fairly high. AND it has a rotary shutter mechanism to blank off some of the capacity-in effect changing the gain of the regulator and allowing for stable control.

http://www.grothcorp.com/3011.htm

>>2)There is no reason why I couldnt reduce from 3BarG to 25mBarG in one go and still maintain accurate control around my set point.<<
I think that would be true but it would be easier with a small valve than a large one. Example: Scuba tank regulators. Admittedly dual stage, but the final stage takes a vastly greater drop than 3 bar. And having it go overpressure migh ruin one's day.

 

[Latexman]

Check with Fisher-Rosemount. They aquired Appalachian Controls Environmental (ACE) who made the best regulators and pad/depad systems in the business.

Good luck,
Latexman

 

[JLSeagull]

Most of these posts are tracking in the right direction. For a self-contained regulator (instead of a control valve and controller) I would begin looking at the Fisher ACE 95 series along with the Anderson Greenwood. The ACE 95-B2E has a Cv around 1.0. Other models have higher Cv. A similar Anderson Greenwood tank blanket regulator would be the model BV119 series such as BV119C2X1N. The comparable Protectoseal could be series number 20 or 30. Regulators of these series are all available with small and larger capacity coefficients. My recnt project used stainless and some exotic plastics such as Kalrez.

Do not forget the tank over-pressure protection. Tyco, Groth and Protectoseal are also among those who sell this sort of stuff.