SIMPLE NER Method for Checking Cryogenic Liquid Cylinder Vacuum Insulation

How do you know if a liquid cylinder marked "BAD" actually needs to be sent in for repair or if the “problem” is something that can be fixed at your shop? What is “BAD?” Is it venting too much or just doing what is normal? Or is the problem a leak or a stripped fitting?

Ideally, liquid cylinders with bad vacuums should be identified and separated from other “bad” vessels since it is clear that they will need to be sent in for rehab. Segregating those with bad vacuums also allows more focus time to trouble-shoot vessels that you might be able to repair yourself.

The SIMPLE NER test method was developed to provide an easy, clear-cut identifier for determining bad vacuums and procedures can be found in the West LCR Handbook at http://www.westcryogenics.com/Liquid_Cylinders.php. It allows for testing without instrumentation and in many cases shows there to be no problem with a vessel at all. Just a couple of weeks ago, a client called with a near urgent need to get a vessel shipped in and repaired. I walked him through hot-fill procedures in the morning and SIMPLE NER testing in the afternoon. The next morning we found the vessel was “fixed” and able to go back into service.

Cryogenic Liquid Cylinder NER (Normal Evaporation Rate) Testing

Full NER Steps include:

- Make sure your pressure builder and gas use valve are securely closed
- Open the Vent valve and connect a fill source to the liquid connection on the liquid cylinder
- Fill the Vessel with liquid nitrogen to “full” level by weight and leave the vent open allowing the liquid to fully stabilize (ie- until venting stops)
- If the vessel vents continuously for several (3-4) hours after filling and shows no sign of slowing down then check to make sure your pressure building line is not leaking past the valve. If it is then your pressure building valve needs to be rebuilt or replaced. If the PB is fine then stop the test and save whatever product you can as you already know that your vessel needs to be sent in for service.
- Now, top off liquid level to replace initially vented product
- Log the weight of the filled vessel and leave the vent open
- After 24 hours, reweigh the vessel and log this (minus vessel tare weight) as “Net Starting Weight”
- Measure the weight of the vessel in two successive 24 hour periods
- Subtract the final weight from the starting weight to determine the total weight of product lost. In all cases subtract the tare weight of the vessel so that you are just logging the weight of the product. (net weight)
- Divide the net weight of product lost by the net starting weight to get the percentage of loss over two days.
- Divide that figure by two to get the daily NER.