Letdown valve

[firefastball]

Hi there,

I'm wondering if there is any energy loss when steam flows through a let down valve.

Example:
Boiler is producing steam at 11 barg. Pressure drop to the end user is around 1 barg. End user only needs 9 barg so a valve was throttled to reach the pressure.
Is there any energy loss going from high pressure to low pressure?

 

[Latexman]

The throttling process is usually considered isenthalpic ([Δ]H = 0).

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[FredRosse]

If energy does not come out of the piping system, then there is no energy loss, as stated by Latexman. Normal heat loss thru pipe insulation will reduce energy (enthalpy) slightly, but throttling process is isenthalpic.

 

[MortenA]

Just in case: This is not the same as no change in temperature.

 

[firefastball]

The current situation is that valves are manually throttled from 11 barg down to 9 barg. And because it was done manually depending on the operator it is usually above than 9 barg, exanple 9.6 barg.
I'm thinking of installing a prv on the line to improve the control of th system
Would it save energy or flow on the line?

 

[FredRosse]

Yes, the throttling process for steam generally resilts in lower temperature, at least in the pressure/temperature regimes we humans typically use. At very high temperatures steam begine to behave similar to an ideal gas, and throttling process here is very nearly isothermal.

When throttling saturated steam from about 400 PSIA or less, it becomes superheated at the downstream pressure, and at a lower temperature than the initial upstream saturated temperature.

When throttling saturated steam from pressures well above about 400 PSIA, to becomes wet at the downstream pressure, and at a lower temperature than the initial upstream saturated temperature.

 

[Latexman]

What do you mean by prv? Pressure relief valve? Pressure regulator (PCV)? Pressure control loop?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[TD2K]

PRV - pressure reducing valve is my guess

The valve by itself in general won't save you any money since the steam has the same enthalpy.

One exception might be if you have steam turbines that exhaust to the 9 bar header. Lower header pressure = low steam consumption by the turbines. There might be some other cases depending what the steam is used for.

 

[georgeverghese]

A pilot operated pressure regulator has less droop /drift on setpoint than a non pilot operated one, so check the flow turndown required on this service to make the right selection.

 

[firefastball]

When upstream pressure increases, does the flow through a letdown valve increase as well due to the higher pressure drop?

Introducing a pressure regulating device would give an advantage of controlling the flow if there is upstream pressure fluctuations right?

 

[Latexman]

Yes.

Yes.

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[willard3]

The second law of thermo predicts there will be a loss at a pressure reducing station.

 

[FredRosse]

willard3 "The second law of thermo predicts there will be a loss at a pressure reducing station."

What do you mean here? And what is the relevance to the posted questions?

 

[davefitz]

There is a loss in "available energy" but there is no loss in thermal energy . The isenthalpic expansion across a valve ( with no heat transfer) leads to an increase in entropy, thus a loss in energy available to do work. See also threads on exergy.

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