Continue to Site

Eng-Tips is the largest engineering community on the Internet

Intelligent Work Forums for Engineering Professionals

  • Congratulations IDS on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!

P vs V of water

Status
Not open for further replies.

idesign73

Mechanical
Sep 25, 2002
28
My company makes liquid level gauges (my area being boiler trim). I am working on calcs for a gauge with a valve at each end. The gauge is pressurized to hydrostatic test pressure (at 70°F) and the valves are closed. Over the course of an hour the gauge loses about 150 psig of pressure (starting pressure = 4500 psig). After performing a hydrogen leak test, it is assured that the gauge is not leaking. My thought is that the gauge glass is lifting a bit, therefore reducing the pressure. I need to quantify how much the glass would be lifting for a given pressure drop, however, I can not find any equation that would correlate pressure and volume with a liquid, specifically water. Any thoughts? Any other thoughts as to the drop in pressure?
Thanks in advance for any help!
 
Replies continue below

Recommended for you

Thanks Guidoo! I'll give it a shot and let you know how it worked out! Much appreciated!!
 
The gauge is held at 70°F. My main problem is trying to justify to the customer why the gauge loses about 150 psig over the course of an hour or so. It must be the relaxation of the glass, allowing a larger volume. Like I said, a helium (I mis-spoke up above) leak test shows that the gauge is not leaking.
I have worked through the calculations using the bulk modulus equation, it seems to be giving me data that is in the realm of believable.
 
Have you checked the valves? Fluid may be leaking through the valves thereby releasing pressure.
 
Relaxation of the glass seems like an unlikely explanation. I'm not sure that's a physical phenomenon I've even heard of. Besides, if it is relaxing a bit over an hour, it might continue to do so over a longer period of time until it breaks!

If it is not a leak, than my guess is that it has to do with a slight change in the temperature of the hydrotest water over the course of the test. A quick calculation shows that a temperature change from 70°F to 67°F would be enough. Even if you were monitoring temperature, this small a change might not be enough to be noticeable.

Tim
 
Status
Not open for further replies.

Part and Inventory Search

Sponsor