7 replies to this topic

[xyz168]

Is it possible to have a negative value on outlet dP calculation? If it's possible, does it apply to all kind of fluid? Or does it only apply to liquid? Thank you.

[latexman]

A negative value on outlet dP would mean the atmosphere is flowing into your PSV, i.e. that you have a vacuum breaker. Does that make sense in your case?

[xyz168]

No...but what if we have a higher outlet point? Isn't there a potential energy aspect to it?

[djack77494]

...but what if we have a higher outlet point? Isn't there a potential energy aspect to it?

xyz,
It is possible for the absolute pressure at your PSV outlet nozzle to be below atmospheric pressure, which I believe is the answer to your question. This could occur if your outlet piping has a very low dP and the PSV is in liquid service and located above its destination. The gravity (potential energy) would motivate the liquid to flow from an area of lower pressure (at the PSV outlet nozzle) to an area of higher pressure (its destination). The pressure upstream of the PSV could be whatever, since the PSV orifice will take the needed dP.
Doug

[JoeWong]

xyz168,

Is it possible to have a negative value on outlet dP calculation? If it's possible, does it apply to all kind of fluid? Or does it only apply to liquid? Thank you.

I hope you mean the static head gain is more than the frictional pressure and results in-situ pressure at pipe inlet end is lower than pipe outlet end.

For subcooled liquid relief and no flashing along PSV & discharge line (e.g. water), you may have the potential of pressure below atmospheric pressure in the event the PSV discharge line is long and vertically extended downward, small discharge line size result complete liquid sealing and liquid column form in the vertical piping. In this case, it is possible to have in-situ pressure at pipe inlet end is lower than pipe outlet end. This scenario invites a lot of problem like vibration, noise, surging, etc. You may consider to increase discharge line size to avoid formation of liquid column.

For flashing liquid or 2-phases flow, it is hard to experience "negative dP" as you mentioned.

For gas relief, there are several locations where possibly experience pressure recovery e.g. PSV discharge expander, small pipe expanded to large header, etc. In this area posible reverse pressure (e.g. inlet pressure smaller than outlet pressure). But per experience it is hardly to pressure lower than atmospheric.

No...but what if we have a higher outlet point? Isn't there a potential energy aspect to it?

Not clear with this statement. Do you mean the PSV discharge is higher than the discharge pipe end or in opposite ?

For formal case, above explanation may helps.
For later case, the static head would properly sufficient to create positive pressure.

Hope above helps.

[latexman]

Just as cooling water lines can be at a vacuum at their highest elevation. I've seen that.

[rxnarang]

At the cost of stating the obvious, fluid goes from a state of higher energy to one of lower. Pressure is only one form of energy. The right statement to make is that in an adibatic flow ( no heat transfer) the stagnant enthalpy of a fluid is constant. Stagnant enthalpy is based on making the velocity of the fluid zero by an isentropic process.

Stated simply, fluid CAN flow from lower pressure to higher pressure, if the velocity at lower pressure is high enough.

A friend of mine once bet that if he makes a small hole in a pipe flowing liquid at high speeds, the liquid will not come out.

I almost took this bet on, and, like an engineer, wanted to make sure that I am going to win it, so I did an experiment in the field. I lost the bet.

Regards
Rajiv

[fallah]

Stated simply, fluid CAN flow from lower pressure to higher pressure, if the velocity at lower pressure is high enough.

A friend of mine once bet that if he makes a small hole in a pipe flowing liquid at high speeds, the liquid will not come out.

I almost took this bet on, and, like an engineer, wanted to make sure that I am going to win it, so I did an experiment in the field. I lost the bet.

Regards
Rajiv

1-In the SYPHONING EFFECT we can have flow fom lower pressure to higher pressure without having high speed at lower pressure.
2-In your bet loss, certainly the static pressure at small hole had been higher than atmospheric pressure.
Regards