pornlert_wana,

I'm having trouble picturing your situation. You say that your fluid is a gas of low density. Typically, gas piping is modeled as an SG of 0 since it usually contributes nothing meaningful in terms of load compared to pipe/insulation/etc.

So, even if you have coded a very low density for your gas, you ought to get almost identical results for a W case and a WNC case.

My first suspicion, from what you've posted so far, is that you incorrectly coded the specific gravity and your spring is being sized to carry more weight than what you have. In the WNC case, the spring is going to act with the same force, but without the weight to resist it, you can end up overloading your equipment nozzle.

What SG have you coded for the gas?