11 replies to this topic

[ASH25320]

How to estimate steam flow being vented from a deaerator operating at 118 DEG C temperature with steam vent size of 3" with no orifice or valve as restriction? the plume length is around 3 meters. Can somebody give a clue? what is the suggested vent rate allowed from deaerator?

did you managed to get any reply ?

[kkala]

For suggested vent rate you can look at http://www.cheresour...2129#entry52129 and its web reference from spirax-sarco http://www.spiraxsar...-deaerators.asp. It is noted there that "an automatic venting mechanism is rarely encountered on deaerator vent pipes, the task usually being accomplished by a manually adjusted ball valve, needle valve, or orifice plate".
Seeing that deaerator vent under discussion has no orifice, valve, or restriction, it seems that excess steam escapes to atmosphere, together with non condensables. Plume seen at local deaerator vents was much shorter than 3 m, yet this steam "escape" may be too small to be detected by a heat balance. It had better be measured on field, if this is possible.
Concerning a theoretical calculation of possible (max) steam escape, "steam vent.xls" is attached (for want of something more precise), trying to consider the vent pipe as a PSV of 3" opening. This is subject to comments by others. PSV sizing is based on http://books.google....0factor&f=false.
It seems that a suitable valve or orifice has to be placed on the vent pipe.

Attached Files

•  steam vent.xls   25KB   443 downloads

Edited by kkala, 13 October 2011 - 10:13 PM.

[mav9rick]

An operating temperature of 118 degC suggests that you are operating the deaerator close to 85 kPag. This means that escaping steam/air mixture stream has 85 kPa is the pressure drop through the vent. You already know the diameter of the vent so its just a matter of measuring the vent length and back-calculating the flow which yields 85 kPa pressure drop.
I would suggest that you run few sensitivity cases assuming different mix of air and steam at the inlet of the vent inside the deaerator and see if there is much variation. You may find that there teh flow is not overly dependent on teh composition anyway.

kkala, I dont think you could use the the PSV flow correlations to estimate the flowrate. Flow throuygh the PSV is sonic and is goverened by altogether different equations.

[kkala]

mav9rick is right in the sense that PSV flow correlation should be used with caution, which did not occur in the kkala's post. Sorry to have seen post No 4 at the start of 2012. Estimates on venting rate through the 3" pipe have been repeated and summarized in attached "steam vent.xls", to indicate the following.
1. Steam flow can be choked only if vent pipe is very short, less than 1 m. Assuming zero length of vent pipe and ideal gas (bizarre for steam), steam discharge to atmosphere is estimated at 5081 - 5206 kg/h.
2. Assuming zero length of vent pipe and using the PSV flow correlation by Matthews, steam discharge is estimated at 4549 kg/h, a rate more precise than above. Correlation was checked against critical mass flows given for steam in Tables and found OK.
3. Steam flow is no longer choked, if vent pipe of 3" dia is longer than about 1 m. For ~ 1 m pipe length, steam discharge to atmosphere is estimated at 4940 - 5450 kg/h.
Details can be seen in the attached spreadsheet and comments are welcomed.
At any case previous conclusions are verified. Loss of steam from vent is excessive, a needle valve (1") can be a solution (or an orifice) according to Spirax Sarco. Nornally the vent should discharge 0.5-2 kg steam per ton of steam ingoing to deaerator, see http://www.cheresour...e-calculations/.
Above estimates do not consider that air is also discharged to atmosphere through the vent, roughly at a rate of 1 m3 air per 4.4 m3 steam. So steam discharge will be less, although this does not change the conclusions.

Attached Files

•  steam vent.xls   32KB   214 downloads

Edited by kkala, 17 January 2012 - 05:37 PM.

[S.AHMAD]

1. By knowing the equivalent pipe length and neglecting elevation effect,, the flowrate in kg/h can be estimated by using following equation:

mass flow kg/h = SQRT(0.08 x Diameter^5 x Pressure drop x density/ Equivalent Length)

2. Where density is in kg/m3, pressure in bar, diameter in mm, equivalent lenght in m.
3. By using following data:
Diameter = 76 mm
Pressure drop = 0.85 bar
density = 1 kg/m3
Equivalent length = 10 m
flowrate = SQRT(0.08 x 76^5 x 0.85 x 1 / 10)) = 4152.3 kg/h

Edited by S.AHMAD, 18 January 2012 - 05:00 AM.

[kkala]

Thanks to S.AHMAD for the quick formula, assumed to be for non choked flow, a common case for deaerator vent pipe. For the conditions referred in post 3:
- Equivalent (total) length = 1 + (0.5+1)*55*0.0779 = 7.4 m
coefficient of entrance loss K= 0.5, K of exit loss = 1.0, internal pipe dia = 0.0779 m, multiplier to approximately convert Ks into equivalent length diameters=55 (for gases similar to air, Perry 5th ed, p. 5-35).
- Int diameter = 77.9 mm
- Frictional pressure drop = 0.85 bar
- steam density at deaerator / start of pipe = 1.06 kg/m3
Thus flow rate =SQRT(0.08*77.9^5*0.85*1.06/7.4) = 5286 kg/h, agreeing rather well to the corresponding case of post No 5 (case D in steam vent.xls).

[S.AHMAD]

The formula is given by fhe Spirax Sarco book entitled "The steam and condensate Loop".

Edited by S.AHMAD, 18 January 2012 - 07:37 PM.

[breizh]

Good day ,
Based on my calculation , pipe 1m long , 77.93 mm ID,pipe steel , rugosity :0.2 mm , Ki = 0.5+1
steam at 118C ( saturated 1864 mbars ) , viscosity = 0.013 cpo
Pressure drop :850 mbars

hypothesis 1 ( Isotherm ) :flow rate = 3600 kg/h
hypothesis 2 ( adiabatic) : flow rate = 3900 kg/h

Hope this helps
Breizh

Edited by breizh, 19 January 2012 - 08:24 AM.

[S.AHMAD]

Dear all
I have done similar calculation using adiabatic flow equation for non-ritical flow
Using same data as kkala and breizh I have 3,800 kg/h by assuming a vent of straight pipe of 1.5 m.
This figure agrees very well with breizh nmber which I believe more accurate than the simplified equation which is taken from the Spirax SArco Book.

Attached is an EXCEL Spreadsheet. Appreciate if someone review the spreadsheet and revert any comments. All members are allowed to use the program but the writter is not responsible for any inaccuracy of the result.

Instruction How to use the program:
1. Click on the sheet InputData
2. Change the input data e.g. pressure
3. Click RUN PROGRAM botton

Attached Files

•  SteamVentflowEstimate.xls   53KB   332 downloads

[Phanendra]

Thankyou for the solution.

[sheiko]

Hi,
There is a formula to calculate the vent rate at the adress below:
http://www.plantsupp...ad/PSE_BP_2.pdf