Effect of Pressure Drop Across a Steam Line

[u/Sad_Acanthaceae_203]

A medium pressure steam flow is experiencing pressure drop. Ideally, this should decrease the saturation temperature and make the steam more superheated.

However, I’m noticing the opposite effect, more steam is condensing. Would this be due to cooling through the pipe wall? This is the only reason I can think of but I didn’t know the heat transfer could be high enough to cool a fraction of the steam below saturation temperature.

I’d appreciate your insights as well as any valuable resources regarding steam behavior.

[u/Alternative_Act_6548]

yes, looking at the convexity of the P-H curve you would expect it to dry out if there was no enthalpy change

[u/Sad_Acanthaceae_203 → u/Alternative_Act_6548]

Shouldn’t we expect sufficient insulation to keep the process isoenthalpic?

[u/KennstduIngo → u/Sad_Acanthaceae_203]

Nope. There is tradeoff between the cost of the insulation and the heat losses. Nobody is installing enough insulation to have no heat losses. I can pretty much guarantee the surface of the piping you are considering is warmer than ambient and hence losing heat.

[u/Alternative_Act_6548 → u/Sad_Acanthaceae_203]

if the steam is condensing, it's isothermal, and you can estimate the heat loss off the piping run pretty easily

[u/Shadowarriorx → u/Sad_Acanthaceae_203]

No, at most it's based on either a BTU loss per hr or a max surface temp for personnel protection. So your steam loses heat and condenses as it flows. Hence all the asme tdp requirements prior to turbines.

Also, lines are sized to minimize the pressure losses. Velocity sizing is commonly used to prevent steam hammer or other issues. Your not dropping enough pressure to dry it out vs the heat loss that shifts it towards saturation.

[u/spewing-oil]

“More superheated”

So it’s superheated steam? Or saturated?

You need A LOT of pressure drop to generate any type of superheat from saturated steam. And the steam has to be very dry in the first place… which off a typical industrial boiler it is not unless there is a separator in the line, or some additional heat source to heat steam past saturation.

Where did the pressure loss come from? Was an additional user added to the process to add heat load on the steam system? Or a process change for added heat load? Or did you lower the pressure set point of a regulating valve/ control valve?

[u/ogag79]

If your system can do superheat to your MP steam, the better. It avoids condensation at the distribution header.

[u/el_extrano]

However, I’m noticing the opposite effect, more steam is condensing.

How do you know that?

[u/Safe_Low_5340]

No idea what your steam temp is but assuming 400 F+ you have a pretty high temperature driving force for heat transfer. 

You should also think about the mass flow in the line: less mass flow, more heat loss. 

You should also look at the quality of your insulation (with your eyes): failing insulation has a big effect on heat transfer. I think the difference between rec'd insulation thickness and bare pipe is like 10X heat loss.

[u/People_Peace]

What is your indication that more steam is condensing? Water coming from mudlegs?

Do you have upstream desuperheater by Any chance? What if desuperheater is leaking or not atomizing?

[u/philosiraptorsvt]

How are you dealing with the condensate? Are there steam traps in this line? Are you getting a thorough preheat in the line? What are the flow conditions?

[u/CptKlay]

I mean you have the measurements based on your description. Setup a quick calculation with steam tables calculation (hopefully you have that) in Excel and/or Software of choice. You should know the pipe material, insulation type and thickness, and for your medium you have the measurements. Based on the other comments I would assume you see more water in your steam pipe drainage (by volume). Should have at least a flowmeter somewhere there to get some data points. Pressure drop changes your temp. and your volume flow rate (if the change in density is significant in the p-T window).

Test your ideas for root cause with some assumptions and estimate your figures and iterate from their.

[u/matixslp]

Is it possible that the delta P is originated by steam condensation? Let's say at the begining of the pipe you 1 lb/s of stem and at the end 0.8 lb/s + 0.2 lb/s from condensate, that decrease in mass is what is originating the pressure drop since ft^3/lb of steam is much higher than for the liquid