Pressure reduction during contraction
Pressure reduction during contraction
(OP)
Hello,
I was asking how to accurately estimate pressure reduction during contraction. I saw some experienced engineers using the formula P2=(rho1/rho2)*P1 to estimate the pressure after contraction of 40 barg saturated steam to saturated water but i cannot find where it comes from...Is it correct?
Thanks in advance
I was asking how to accurately estimate pressure reduction during contraction. I saw some experienced engineers using the formula P2=(rho1/rho2)*P1 to estimate the pressure after contraction of 40 barg saturated steam to saturated water but i cannot find where it comes from...Is it correct?
Thanks in advance
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."





RE: Pressure reduction during contraction
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pressure reduction during contraction
So?
considering Z and that:
State 1 is saturated steam at 40 barg
state 2 is saturated liquid
what would be the correct approximation for determining P2?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pressure reduction during contraction
Using the ideal gas law would lead to the formula P2=(rho2/rho1)*P1 and not P2=(rho1/rho2)*P1 ...
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
Then you can use Joule-Thomson coeeficients to get the temperature change to calculate the probable change in density. Finally you can use your equation.
Typically Bernoulli is close enough for this sort of calculation, but if you need more precision this will get you there.
David
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
The harder I work, the luckier I seem
RE: Pressure reduction during contraction
Could you please tell me where does the relationship P2=(rho1/rho2)*P1 come from (different from ideal gas law which wuold lead to P2=(rho2/rho1)*P1...)?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
David
RE: Pressure reduction during contraction
Steam tables show that below 40 barg, the saturated steam line shows that the products P.v are about constant (with an error of less than 5%) down to, say, 10 Bar.
where P are absolute pressures and v specific volumes = 1/ρ of saturated steam.
P.v = P/ρ
Is this sheiko's intention ?
RE: Pressure reduction during contraction
However your relationship still leads to P2/rho2=P1/rho1 while some engineers use the relationship P2/rho1=P1/rho2 to estimate the pressure reduction during contraction...
So:
1/ Where does this expression come from?
2/ In the case i mentionned:
State 1 is saturated steam at 40 barg (rho1=19.46 kg/m3)
state 2 is saturated liquid (rho2=799.36 kg/m3)
Is this expression valid?
Please.
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
T1=Tsat=250°C, Z1=0.872 and
rho2 is obtained from specific volume in steam tables at same temperature T2=T1=Tsat=250°C (maybe it is wrong...)
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
Sheiko, what actually are you trying to do?
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pressure reduction during contraction
I'm trying to check if the minimum design pressure of a 40 barg steam pipe is low enough. So i considered the formula that i presented you in order to figure out the pressure reduction during condensation.
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
You may well find that your steam traps will work as vacuum breakers. Depends what type you are using and what your condensate piping looks like.
Off topic - but if you are talking saturated 40 barg steam your density is wrong.
Harvey
Katmar Software
Engineering & Risk Analysis Software
http://katmarsoftware.com
RE: Pressure reduction during contraction
rho1=4100000*18/(0.872*8314.5*(250+273.15))=19.46 with real gas law and rho1=19.95 using steam tables at 250°C.
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
I2I
RE: Pressure reduction during contraction
If it's a closed system, there is no change in density.
I2I
RE: Pressure reduction during contraction
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
I2I
RE: Pressure reduction during contraction
Reminder:
Initial state is saturated steam at 40 barg (and T=250ºC)
Final state is saturated liquid. What is the final T?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
Think of a 200 mile long gas pipeline that is at 80F and 1,400 psig at the head and 55F and 600 psig at the foot. The density of 0.6 SG gas at the head is 4.2346 lbm/ft^3 and 1.9247 lbm/ft^3 at the foot--less than half.
Mass flow rate is constant for the two points. This is accomplished in the face of declining density by increasing velocity.
David
RE: Pressure reduction during contraction
sheiko:
1st approach (blocked in line, initially filled with steam)
- look up the specific volume corresponding to the initial state
- follow the line of constant specific volume corresponding to the initial state specific volume
- from this line, you can determine the quality and pressure of the mixture at the temperatures of interest
- how much the temperature changes is a function of the ambient temperature and heat transfer coefficients
- if left sufficiently long, the pipe will equalize in temperature with the surroundings
- to determine the pressure, look up the pressure value corresponding to the initial (and final) state specific volume and ambient temperature
2nd approach (flowing process line with heat transfer)
- the pressure difference will equal the two-phase frictional flow losses, several correlations have been published for this case
I2I
RE: Pressure reduction during contraction
The practical answer is: Can the line withstand total vacuum without collapsing, if not, install a vacuum breaker set at a pressure above the collapse pressure.
RE: Pressure reduction during contraction
The question only makes sense if the line is flowing.
David
RE: Pressure reduction during contraction
My conclusion is that, as i want to check the pipe against vacuum, maybe i should account for full vacuum, as even with a final temperature close to room temp, the vapour pressure will be substantially zero.
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
Suppose, you have 1cu.mtr of pipe volume filled with steam at 40barg and it started condensing. At 40barg the density is about 20.6078kg/cu.mtr. If 10 kgs of steam is condensed then you have about 10.6078kgs of steam left to occupy a volume of approximately 1cu.mtr (actually, water specific volume at 40barg is 0.00125cu.mtr/kg, so 10kgs of water occupies 0.0125cu.mtr or 10.6078kgs of steam occupies 0.9875cu.mtr and density should be 10.6078/0.9875 = 10.74kg/cu.mtr)
Now, get a steam table program like steam tab and go use the saturation temperature corresponding to 40barg and go on varying the pressure till you end up with 10.74kg/cu.mtr density. This will give you the pressure of 23.75bara.
The important point is to first calculate the heat loss and then use that data to check the rate of condensation and then using the above method, you can check the pressure at intervals.
RE: Pressure reduction during contraction
Could you please clarify/explain your last sentence "The important point is to first calculate the heat loss and then use that data to check the rate of condensation and then using the above method, you can check the pressure at intervals. ", because i don't really see how to perform that...
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
1/ Why the fact that the entire steam is condensed invalidates your method?
2/ Why did you make all your calcs assuming steam and water at 40 barg while the pressure is suppose to vary?
"We don't believe things because they are true, things are true because we believe them."
"Small people talk about others, average people talk about things, smart people talk about ideas and legends never talk."
RE: Pressure reduction during contraction
RE: Pressure reduction during contraction
When the entire steam condenses then it will be the vapor pressure at the condensate temperature.
I just showed one example. You can better split the process into no. of intervals and do the calcs as your time permits.
However, like earlier suggested, if the steam quantity trapped is insignificant compared with the exposed area and if it happens for a longer periods of time then full vacuum is a better assumption.