There are a number of gases that can be delivered in portable cryogenic vessels for gaseous or cryogenic use but for the purposes of this article we will be discussing the operation of a portable liquid nitrogen vessel.
First, it is the responsibility of the end user to be thoroughly familiar with the operation of the vessel and the limitations of the vessel performance. This starts with ordering the vessel. Some vessels are intended for liquid use only, some will deliver gas and/or liquid and some are intended for higher pressure gas use. The end user must know the pressure and flow requirements of the application and what they expect to be using, gas or liquid. This information should be passed on to the person the liquid vessel is being ordered from so they can deliver the correct vessel.
As an example, if you know the application requires nitrogen gas at 25 cubic feet per hour and a pressure of 100 psi, order it as such and the correct vessel will be delivered to handle the application. If you require the vessel for liquid withdrawal at 22 psi only, order it as such and again, the correct vessel will be delivered. It sounds simple but there are nuances to be aware of with the operation of these vessels.
Adjusting the Pressure
All liquid nitrogen vessels will be delivered to the client site at 22 psi for safety purposes. If the application requires a higher pressure gas to be delivered, the operator must open the pressure building valve on the top of the vessel. (All valves are clearly marked with a stainless steel tag.) Opening this valve will allow liquid nitrogen to enter a copper tube that is soldered to the inside of the outer wall of the vessel, warming the liquid and turning it into a gas. This warm gas is returned to the head space above the liquid nitrogen in the tank and creates a higher pressure inside the vessel. The pressure inside the vessel is indicated on the pressure gauge on the tank. The gas should not be used immediately after the gauge indicates it has reached the desired pressure. The gauge is reading the head pressure only and using it immediately would quickly drop the pressure. Leaving the pressure building valve open for 30-40 minutes allows the gaseous head pressure to saturate the liquid in the vessel to a higher pressure as well. This results in a vessel that will deliver the high pressure gas for an extended period of time. The pressure building valve can be closed at this point. If you notice the pressure in the tank is dropping to an unsatisfactory level, you can use the pressure building valve like a throttle by opening it again to build additional pressure.
A note of caution, once the cryogenic vessel has been pressurized in the above manner it cannot be used for liquid withdrawal.
Dispensing liquid nitrogen at high pressures is dangerous and can cause significant damage to equipment as well as serious personal injury. Once the pressure in the vessel has been increased it cannot be reduced to the safe level of 22 psi for liquid nitrogen withdrawal. Do not attempt to vent the vessel down to a safe pressure as the liquid is saturated at a high pressure and will not be suitable for liquid nitrogen withdrawal.
Valves and Fittings
As indicated, there are a series of well-marked valves at the top of the vessel. They are as follows; Liquid Valve (CGA-295 valve configuration), Gas Valve (CGA-580 valve configuration), Vent Valve and the Pressure Building Valve. There is anti-tampering hardware on the fittings to ensure the proper equipment is used for the withdrawal of the gas or liquid.
Fittings should never be removed from the vessel to make it fit ones application or for any reason.
"How do I know how much product remains in the tank?" This is a question that has been answered with some uncertainty until recently. Customarily there is a float tube inside the vessel that sits in the liquid in the tank. As the liquid goes down the float tube goes down and indicates the product level on the top of the tank. This float tube technology has been around for a long time and has been subject to common failure as sometimes the tube gets stuck in the full position, giving the user a false sense of security. New technology has emerged in the form of a digital gauge called CylConnect that monitors the product levels with accuracy within 2%.
At 22 psi the safety relief device on the vessel will often only hiss quietly but at higher pressure gas use the safety relief device may sound off quite loudly, enough to startle those in proximity of the vessel. The higher the pressure in the tank, the louder the report from the safety relief device. There is a device that will silence the noise; it is called the Whisper Valve. It is secured to the vent valve on the top of the vessel. When the vent valve is opened with the Whisper Valve installed it takes over as the safety relief device and relieves excess pressure quietly. The Whisper Valve also saves a considerable amount of product as the silent venting does not agitate the liquid nitrogen in the tank. The common cryogenic agitation experienced without the Whisper Valve creates additional pressure within the tank. Pressure equates to heat and heat and cryogenics do not play well together. Heat causes more pressure, more safety relief activity and thus more product losses.
It is imperative that everyone working with cryogenics is fully familiar with the safe handling of the product and that the appropriate personal safety equipment be available for use. (Safety glasses, headgear and full face shield and waterproof cryogenic safety gloves are recommended as a start.) Liquid Nitrogen sits at -321 degrees F and can cause serious and permanent physical injury.
Caution must be taken to avoid confined space situations as nitrogen gas that is venting from the portable dewar is an asphyxiant. Oxygen depletion monitors should be considered is such cases.
If you have questions about liquid nitrogen or other gases at your facility, contact our technical staff. We are here to serve and we take it very seriously so, call us at Middlesex Gases & Technologies where integrity and service have carried us proudly since 1949.
For more information on any of our services or products, contact Ron Perry, Sales & Marketing Director.