Steam Pressures 3

[NCDesign]

I am working on a system and I have 60 psig steam available. All of my components, HX's, coils, etc can be operated at this higher pressure. However, my previous experience (every steam system I have come across the steam was transported at a higher pressure and used at a lower pressure) and everything I read on the subject indicates that I should put in the PRV station and reduce the pressure to 15 psig. But none of it really says why, it just says do it. In this particular case I really don't want to because of overall floor space constraints. It also seems (i haven't done the calcs) like it's wasting energy since you lose energy in the pressure reduction without any work being done. However, I feel like I'm missing something big in using the 60 psig steam vs the 15 psig. Can anyone enlighten me?

 

[ione]

Higher pressure means steam lower specific volume and so smaller pipe diameter. Lower pressure means higher specific enthalpy of vaporization that is that part of enthalpy usually exploited in HX.
In a pressure reducing valve the upstream and downstream enthalpies are the same.

 

[NCDesign]

Ok so no energy is lost and the HX's are actually more efficient at the lower pressure.
Are there any additional installation considerations that need to be made because I am using the 60 psig steam? ASME ratings, service issues, etc. None of my literature addresses any of that.

 

[ione]

Not so fast. Is your HX designed to work with 60 psig or with a lower pressure? There are pressure drop issues on the steam side to be considered. Moreover working at lower pressure the steam temperature will decrease as well (at 15 psig the saturated steam temperature is approx 250 F, while at 60 psig the temperature is approx 307 F).

 

[NCDesign]

I am very aware of the temperature differences of the two steam sources.
What I'm really after is if the design properly incorporates using the 60 psig steam (equipment has been selected at the higher operating condition, i can get the required performance, pipe is selected properly, I am properly insulating for the higher temperature, etc) is there a really good reason to add the PRV station and reduce to 15 psig?
My reps are very accomodating but technically in this manner they have not been of much help.
Your first answer seems to indicate that I would be using more energy since the enthalpy of the steam doesn't change and the equipment is actually more efficient at the lower pressure. (I am doing some calcs and revising some selections to quantifiy this in relation to my design). Are there any other issues outside of this i.e. service, safety, etc that I am missing?

 

[steamdog]

I question the need for PRV's all the time, and my main source of sales ARE PRV valves! As long as sizing and pressure ratings are OK at 60 psig, the one thing you should look at is tube failure. If the water in the tubes is lower than 60 psi, should you get a tube failure steam will enter the tubes and cause lots of potential damage downstream (in pumps, air handlers, or if this is domestic HW then in scalding).
Conversely, If the steam pressure is lower than the water pressure in the tubes of the heat exchanger, then in a failure you will get flow out of the tubes, into the exchanger and then through the steam trap (maybe some waterhammer, much less dangerous).

 

[SNORGY]

The most common reason I see people going to 15 pound steam is to avoid having to employ operators with steam ticket qualifications. However that is irrelevant if your boiler can produce the higher pressure steam, since then you need the ticketed operators by law.

Regards,

SNORGY.

 

[NCDesign]

ione, Steamdog, and SNORGY thanks for your information and insight it has helped tremendously.

 

[willard3]

15 psig steam is an operator problem. 15 psig is low pressure according to codes and above 15 psig requires permitted system operators.

15 psig steam problems are a lot less severe than 60 psig problems.

 

[VerneE]

Don't forget that at the end use location, the control valve could reduce the steam pressure in the coils or exhanger to zero or less. The resulting condensate needs to get out of the heat exchanger or coil and into the condensate return system. If the condensate return is still on the 60 psi side, the condensate stays in the exchanger or coil until the pressure builds up enough to be pushed out.

I have found life is easier with the lower pressure, less water hammer and such, condensate receivers and pumps are in a more comfortable operation zone, steam is a gentle mist, not a roaring, invisible safety hazard.

 

[Wilbur55]

Beware of superheat if you use a PRV. See Figure 2.3.4 about half way down the page here:

http://www.spiraxsarco.com/resource...iples-and-heat-transfer/superheated-steam.asp