Please describe what the end Client wants to achieve.

PWSlack

Now the liquified nitrogen storage is connected to vaporizer and being operated manually. Flow is being controlled by valve (one of DBB valves) located just downstream of the liquified storage and before vaporizer.
We are going to put a control valve downstream of vaporiser. This control valve will be activated as pressure in the main nitrogen supply system is below the normal operation pressure.
Could you please help me to find if it is any safety related issue associated with our modification?

There is no issue with substituting a self-acting valve instead of the manual one, provided all pressure and temperature criteria have been met. The change, though, ought really to pass through a formal HazOp Study at the plant in question.

Can the nitrogen supplier offer advice, perhaps?

Please provide more detailed information about the nitrogen generation system and the controls currently used.

As far as I understand your question you have actually 2 systems:
1: "Normal operation" Nitrogen generation system (pressure swing adsorption??)
2: Back-up system consisting of liquefied Nitrogen tank plus vaporiser (manually operated)

Now my more specific questions:
a) For which purpose is the Nitrogen being used?
b) How is the malfunction of the "normal" system detected?
c) Are there "typical" malfunctions which require the operation of the back-up system?
d) How long is the maximum tolerable lag between failure detection in the "normal" system and the start-up of the back-up system?

Regards nudnik

Nudnik,

Currently it is membrane type unit.

This just plant header - purging, blanketing and etc.

Sometimes nitrogen unit is out of service because of high oxygen content in the supplied nitrogen.

lag is unpredictable and can be enough long - even 14 shift or longer.

Thanks.

OK, so it sounds as nitrogen availability is not highly safety critical.

I suggest the following schematic:

Place an oxygen sensor in the gas line downstream of the membrane unit (zircon dioxide sensor!). Connect the output signal of the oxygen sensor to a control unit. If O2 threshold level is exceeded, the controls should activate a valve in the supply line between the nitrogen vaporiser and your nitrogen supply line with the pressure regulator. Install a correctly dimensioned pressure relief valve between the vaporiser and the valve operated by the controls so that potentially hazordous pressure build-up is limited in the case that the valve fails to open.

Please: Observe your local saftety codes!! They might require additional saety features!

Regards nudnik

What are the requirments as far as oxygen ppm for the nitrogen and what volume do they need? The normal level of oxygen for cyrogenic system is approaching 1 ppm or less. However there are some systems that can supply 50 ppm or less that only use filtering system. Liquid system = 99.999 % pure, filtered system = 99.995 % pure.

You will need a process control loop that continuously monitors the variable that has been chosen as the control (pressure) against the set-point (control valve position) and when necessary signals the valve to close (pressure has dropped below the set-point).

The loop must be tuned to acheive the desired results. There will be the ability to select "manual" mode in which case you are back to what you have now.

The safety issue is going to be when oxygen in the liquid also boils off. Your equipment will have to be explosion proof unless the nitrogen is pure.