Does a PRV reduce Temperature on Steam? 4

[zimonmayo]

I am looking to install a Pressure Reducing Valve into a Steam System, where the System Design Inlet Pressure is 11.5 bar @ 330 degrees C and the Outlet Pressure is 3.8 Bar. Will the Temperature of the Steam drop when it passes through the PRV as the Steam Tables show 3.8 Bar Steam to have a temperature around 180 degrees C, or will the Steam on the Outlet be Superheated?

I have had a PRV sized to suit the above parameters and a 3" valve will do the trick. The recommended inlet pipework size I have been given is 4" (which is no problem) whilst the recommended outlet pipework size is 8" (but I wanted to keep as 4"). Would there be any problems with having 4" Inlet and Outlet Pipework?

 

[TD2K]

The temperature will drop. The flow through the PRV is a constant enthalphy process. You can use the steam tables to estimate the outlet temperature.

 

[Montemayor]

Zimon:

First and foremost, you fail to state if your pressures are barg (gauge) or bara (absolute). I have to assume you mean gauge.

Secondly, you fail to state that you have 140 oC of superheat in your 11.5 barg steam.

As TD2K, states, the flow across the Pressure Reducing Valve will be Isenthalpic (the enthalpy value remains constant through the pressure reduction). If you refer to steam tables (or better yet, a Mollier diagram) you will see the following:

Initial conditions:
Pressure = 12.5 bara
Temperature = 330 oC
Enthalpy = 1,338.0 Btu/lb

Final conditions:
Pressure = 4.8 bara
Temperature = 322 oC
Enthalpy = 1,338.0 Btu/lb
(Superheat = 322-150 = 172 oC)

The information you’ve given has no bearing on the size of the piping involved. The pipe size is determined by the steam capacity (flow rate). There is no problem in utilizing a larger-than-required pipe – except the added costs involved.


Art Montemayor
Spring, TX

 

[zimonmayo]

Thanks for the information guys.

Regarding the Superheat you mention that is already present, this system is a long way from the Boiler House so whilst the Design Pressure is 11.5 BarG @ 330 degrees C the actual Pressure / Temperature is going to be a lot lower and will more than likely match those given in the Steam Tables. The system must be designed to cope with the Boiler Outlet Pressure though.

Looking at the calculations, is the final temperature 322 degrees C or 172 degrees C? I am no scientist, but I have scoured the internet looking for a reason why and how the temperature would drop when the pressure does - and what the knock on effects are (e.g. large amounts of condensate)

 

[Montemayor]

zimonmayo:

You may not be a scientist; but more importantly: are you an engineer?

The question I ask is important because we forum members address queries with conventional, commonplace engineering responses - like adiabatic, isenthalpic expansions. Do you have thermodynamic processes in your academic background and well understood? If not, please state so and we all can try to help in another, more basic manner.

Your re-qualification of the steam conditions entering the PRV helps to better understand your application, but it still doesn't identify the PRV inlet conditions. Until you identify the actual inlet conditions it is next to impossible to guess what the outlet conditions will be.

There are no calculations involved in arriving at the 322 oC outlet temperature. The enthalpy is constant, so the expansion follows according to the steam tables or Mollier chart - whichever you use. (Note that the final, expanded steam product at 3.8 barG is superheated[/] - with 172 oC of superheat; therefore, no condensate is formed as a result of the expansion or pressure reduction) You are not going to find your answer in the Internet. The answer is in a Thermodynamics book explaining the isenthalpic expansion of steam.

I hope I have been of some help by answering your specific questions and I also hope you are able to understand what I refer to thermodynamically. I'm not trying to be academic or pedantic; unfortunately, thermodynamics is unavoidable in this application in order to come up with the correct engineering answer and design.

Art Montemayor
Spring, TX

 

[JStephen]

How and why the temperature would drop when the pressure does- one of the consequences of the expansion. This is how air conditioners work- you compress the refrigerant and it gets hotter. You cool it back down, then release the pressure, and it gets cooler than it started.

Ever let the air out of a big truck tire that is fully inflated? On a hot summer day, it will form frost around the outside of the valve stem.

 

[25362]

To zimonmayo, I understand that industrial enginering, aka management engineering, is more concerned with integrated systems of people, materials and equipment, and less with thermodynamic concepts.

Thus I recommend you look for the Joule-Thomson (J-T) effect on gas adiabatic free (isenthalpic) expansion to get an idea of the subject in hand. For a tutorial web site visit

http://www.nd.edu/~ed/Joule_Thomson/background_free_expansion.htm

At ordinary temperatures all "real" gases, except hydrogen and helium, show a cooling effect on such free expansions. At significantly lower temperatures, also hydrogen and helium behave like all other gases, ie, cool by expansion, and can eventually be liquified by their own free expansion.

You probably have heard of making liquid air by use of this phenomenon.

The ratio [Δ]T/[Δ]P when [Δ]P approaches zero is called the J-T coefficient [μ]. The temperature at which [μ]=0, ie, the gas is neither heated nor cooled by free expansion, is called the inversion temperature.

Interestingly enough, gases that at ordinary temperatures are cooled by free expansion show such an inversion at higher temperatures.

As for steam, there are situations in which free -isenthalpic- expansion can bring about moisture, but this happens at higher pressures than those you have indicated.

 

[quark]

There is a little deviation with steam expansion from JT effect IMHO. The process is essentially isenthalpic but temperature effect is not exactly as per JT effect.

Latent heat of steam at lower pressures is high when compared to that of higher pressures. Upon expansion, the required latent heat is absorbed from steam itself with a subsequent reduction in sensible heat and thus temperature. But I echo Mr Montemayor's comments that it purely depends upon the inlet quality of steam. With saturated conditions at inlet(100% dry) steam at outlet is always superheated.

The link below gives you a steam property calculator. You can use this to get the outlet temperature of steam by trial and error methods by keeping the enthalpy constant.

http://www.chemicalogic.com/steamtab/demo.htm

Regards,

 

[25362]

1. Saturated steam at pressures above 35 barg may generate moisture in isenthalpic expansions (Mollier diagram).

2. Superheated steam cools down by a few degrees on isenthalpic expansion as Mr Montemayor clearly stated. Wet steam cools more due to water evaporation.

 

[krb]

Montemayor:

Do you know of a reference that addresses flow through a PRV or steam station? I am having the same discussion with a coworker. I have told him that (under our inlet and outlet conditions) that the steam coming out of a station is superheated, and he disagrees. I used his Mollier chart to show him, but no luck. Any info would be helpful.

Thanks
KRB

 

[rmw]

Try this link for some starters.

http://www.engineeringtoolbox.com/28.html

Also see

http://www.engineeringtoolbox.com/22.html

rmw

 

[Montemayor]

KRB:

I can't believe another engineer won't accept the results of a thermo process as depicted on the Mollier Diagram, but there are some granite skulls out there. Another way is to use an official, authoritative, and recognized source of Thermodynamic properties. I refer you to the NIST website at:

http://webbook.nist.gov/chemistry/fluid/

This is as authoritative and recognized as you can get in the USA. Let’s take a walk-through on the application cited by zimonmayo in his query to give you an example of how to show and prove to others what is happening in an adiabatic, isenthalpic expansion of steam:

1) At the above site, select “Water” as your working fluid;
2) Select the units you like and then select an “isobaric” process (constant pressure); Press to Continue;
3) Now enter the absolute pressure of the inlet steam – 12.5 barA;
4) Pick a temperature range; I selected 250 oC to 400 oC, in 1 oC increments;
5) Press for data, and then select an HTML table of the results; you can now see the tabulation of steam properties and the corresponding phase. Note that 330 oC puts the steam clearly in the Superheated region. Take note of the Enthalpy value of 1,338.0 Btu/lb.

Now, repeat the above process for 4.8 barA, except that now you will be looking at the Enthalpy column results looking for the 1,338.0 Btu/lb (the enthalpy remains constant, remember?)and - LO and BEHOLD! – this value is reached at a temperature of 322 oC and the Phase column indicates that it is pure vapor. Going back and selecting Saturation properties -- temperature increments instead of Isobaric properties, you will find the saturated temperature (& all other values) corresponding to the 4.8 barA pressure. This checks out & verifies the superheated condition of the valve’s outlet stream.

If the guy still doesn’t believe you, bet him a car load of beer, set up a regulator and run steam through it at pre-determined conditions and check the outlet temperature. And be sure to send me a six-pack, at least, for my effort.

I hope this helps you out and have a nice weekend.


Art Montemayor
Spring, TX

 

[25362]

A couple of additional comments:

1. Quark is absolutely right. Although expanding steam through a control valve (CV) is frequently called a "throttling" process, and throttling processes are equated to J-T expansions, they are not the same. In a true J-T free expansion process flow rates are low and there is no appreciable change in the kinetic (and potential) energy of the expanding gas.

CV steam throttling involves, in fact, two steps:

a. A gain in kinetic energy (a higher velocity) o/a of the enthalpy while passing through the restriction to the lower pressure side. This step is considered quasi-isentropic.

b. An almost complete recovery of the enthalpy o/a of the (isobaric) reduction in kinetic energy while expanding after the restriction at the lower pressure. Almost complete... because of the small enthalpy lost by friction.

A note: depending on the starting degree of superheat, step a. may involve the transitory production of wet steam.

2. Not every isenthalpic expansion of dry or superheated steam ends up in superheated steam. Sometimes it results in wet steam. For example, from data taken from the NIST website:

Superheated steam originally at 100 bar and 320^oC with an enthalpy of 2782.8 kJ/kg, expands isenthalpically to 30 bar.

This expansion results in wet steam: ~ 1.14% moisture (!)
The final steam has the following characteristics:

Enthalpy of dry vapor: 2803.2 kJ/kg
Enthalpy of liquid: 1008.3 kJ/kg
Total enthalpy: 2782.8 kJ/kg

Did I complicate matters ?

 

[quark]

Zimonmayo,

Don't play with 4" outlet piping. In the absence of data about flow rates and if I presume your inlet is thoroughly designed then you do require an 8" outlet piping.

Specific volume of steam at inlet condition is 0.2364 m³/kg and at outlet conditions it is 0.5664 m³/kg. This shows your outlet should be about double the size of your inlet.

25362,

Enthalpy of dry vapor: 2803.2 kJ/kg
Enthalpy of liquid: 1008.3 kJ/kg
Total enthalpy: 2782.8 kJ/2kg??
or
Total Enthalpy : 2803.2(1-0.0114)kJ/kg + 0.0114*1008.3kJ/kg?

am I wrong?

Regards,

 

[25362]

Quark, you're not wrong, neither am I. If you carry out your estimation you arrive at a total of 2782.8 kJ/kg !

 

[quark]

ok, I curse my negligence. Multiple PRVs, is the way to go in such a situation.

 

[krb]

Montemayor,

Thanks to you and everone else who provided input on this. As soon as my coworker gets back to work I'll try again.

Thanks
KRB