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# Pressure dropped during hydrotest

## Pressure dropped during hydrotest

(OP)
Hi All,

As you all know, during hydrotest, the pressure might drop dramatically in the pressure vessel when the ambient temperature reduced (eg. in a rainy day) accordingly. Do you guys know the theoretical reason behind?

This is my understanding..

According to the thermodynamic first law, assumed the expansion workdone is zero since the pressure vessel is a closed system, the heat released from the system is completely converted from the internal energy of the water molecules. Therefore, the pressure of the water (system) dropped.

In another point of view, the ambient temperature reduced resulted in the contraction of the volume of water and steel. The contraction of water is more than the contraction of steel and caused the water inside the pressure vessel dropped. The pressure dropped could be calculated by referring to the bulk modulus of water.

Sorry for my poor explanation.. Hope somebody is on the same page with me and can understand what I am trying to explain.

### RE: Pressure dropped during hydrotest

Was it a real case to see the temperature changed during the hydro of the pressure vessel?

Normally, the hydro of the vessel is conducted above a minimum temperature and held for about one hour. The temperature of the testing medium may need to be maintained during the hydro duration

### RE: Pressure dropped during hydrotest

(OP)
HI mk3223,

Yup, It did happen in real case.. I agreed that the temperature shall be maintained.
However, there might be raining while the hydrotest is conducted halfway, which we have encountered this situation before.

### RE: Pressure dropped during hydrotest

The water is contracting faster than the steel, and reducing pressure accordingly.

### RE: Pressure dropped during hydrotest

See FAQ391-224: How to calculate pressure increase in a closed liquid filled cylinder?. However your vessel must be a very small one, to see a significnt change in temperature in an hour or two. Moreover the rain could cause first the cooling of the wall, thus causing initially a pressure rise. Are you trying to explain something you don't want to admit?

prex
http://www.xcalcs.com : Online engineering calculations
https://www.megamag.it : Magnetic brakes and launchers for fun rides

### RE: Pressure dropped during hydrotest

Prex, typically if a hydrotest fails pressure goes to 0 fairly rapidly, it doesn’t just drop slightly.

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