9 replies to this topic

[rniemei]

Hello everyone, I am an engineering student working on a project with a local power plant.

Their goal is for us to design a system to recover heat energy from the steam they vent through their deaerators.

Currently, we are wanting to build a heat exchanger system that will recover this heat as well as some of the venting steam as condensate. However, to proceed with this we must be able to quantify their current losses, by finding out the flow rate of steam and their energy content of the vented steam.

The plant is still working to get us data, but the data I am currently working with is:

350,000lb/hr of feed water (input water is approx 170F composed of condensate and filtered make up)
50,000 lb/hr of steam (unknown temperature currently)
13-15 psi g steam inlet pressure and dome pressure
Exit temperature of the feed water is unknown, but based on deaerator design it should be within a few degrees of saturation at 13-15psi g (~245F I believe)

Beyond this we have measured the vent pipe to be 2 inches (schedule 40) at the vent exit, the venting steam is approximately 212 F and we are hoping to measure the vent steam's exit velocity soon.

What I would appreciate is any guidance that can be provided on the method to proceed with to calculate the flow rate of venting steam and the effective energy loss of this steam (i.e. how much energy is lost in the steam).

Currently I am trying to use a simple energy and mass balance system, but given what I know I have been unable to fill in enough known variables.

The vent is a simple pipe with an orifice and a bypass globe valve if that should matter for any calculations (I have also been considering using a pressure drop calculation to back solve for flow rate).

Thank you for any input and advice.

[breizh]

Hi ,
Let you show us your sketch and your understanding of the problem with the mass and heat balances , I'm sure we can help.

1) Gather all the data ( flow , temp , pressure )
2) mass in = mass out
3) Energy in = Energy out

Attached a nice spreadsheet for steam properties

Hope this helps
Breizh

[rniemei]

Hi, thank you for your advice so far.

I've attached a schematic showing the various inputs and outputs to the deaerator in question. Essentially, I have two inputs (steam and water) and two outputs (steam and water).

My balances are as follows

(S+W)=(PS+V)
(S*h+W*h)=(PS*h+V*h)

where:
S=inlet steam mass flow rate
W= Water inlet (condensate/makeup mix) mass flow
PS= Pump suction (water out to boiler) mass flow
V= venting steam mass flow
h= enthalpy of respective fluid

I believe this system is correct, my problem occurs because there are many unknowns. I currently only know S, and PS with any certainty. The plant believes they can get me data regarding W.

I also know the temperatures of S, W, and V, and from research I believe PS must be near the saturation temp of water at the operating pressure.

I also know the pressure of the inlet steam and the dome, which I assume allows me to assume that the pressure of everything inside the vessel is the same, so I believe that I know the pressures for S, W, PS and V (all at 15psig except V which is at atmospheric)

So I can find enthalpy for S, W, PS, but not V since I do not know the quality of the venting steam.

So essentially I have these knowns:

S = 50,000 lb/hr @ 13-15 psig
W= 170F
PS= 350,000lb/hr @ 13-15 psig and near saturated water temp (~245F)
V= ~212F @ atmospheric pressure

With these unkowns:

S= temperature of incoming steam (plant should have this)
W= Pressure of incoming water (should be greater than 15 psi) (plant may have this)
PS= Verify pressure and water temp
V= Quality, mass flow rate

Being that I only have two equations I can only have two unkowns. I can add a 3rd equation for quality, but I need the enthalpy of the venting steam and its flow rate. The plant does not have any data on venting steam so those must be my unknowns.

My questions are then, is this basic setup correct, and is there a way to perform these calculations with less data? I am also unsure if my assumptions regarding pressures are correct. IF the tank is at 13-15 psi as regulated by the steam input, than can I assume the W and PS water streams are also under that condition?

Thank you for any assistance. I hope that I have been clear enough to understand. If not please let me know and I will try and explain myself better.

Attached Files

•  schematic.jpg   18.55KB   158 downloads

[breizh]

Good feedback ,
Now you have to consider to do mass balances on species that will help you to determine unknown flows .

For example , if you consider the liquid flow , some impurities will concentrate in the liquid and won't be distillated . As an example CL- or CAO or others ( minerals) .


Hope this helps

Breizh

[kkala]

Attached "steam.xls" roughly estimates (pegging) steam consumption out of given data. It is noted that incoming steam has usually higher temperature than this of outgoing water (here assumed same).
Vent steam is neglected in the above balance, being less than 0.1% of outgoing water in usual conditions, that is 378 kg/h (max).
According to balance above, incoming steam requirement is 27870 lb/h, versus 50000 lg/h reported. So venting steam might be 22 ton/h versus 0.38 ton/h needed (max). If so, effort should be made to reduce steam venting to what is necessary (without much excess).
But check again measured quantities. Measure as many streams as possible, then check them through a mass and heat balance to realize how accurate measurements have been. This patient procedure can give precise results to proceed.
For deaerators and venting steam see
www.altairequipment.com/technical_files/DA_work.pdf, as well as
www.swagelokenergy.com/download/PSE_BP_2.pdf.
(cannot be noted as live site references, even though [URL]...[\URL] is written)

Attached Files

•  steam.xls   17.5KB   525 downloads

Edited by kkala, 27 February 2011 - 12:26 AM.

[rniemei]

kkala:

Thank you for your assistance.

In this case I would prefer not to neglect the venting steam because I am trying to find out exactly what is venting currently. I was given a 2% vent rate (2% of total steam used) and am seeking to verify. I've been told that plants often do not measure the steam flow into the deaerator and a third equation specifying the vent rate in terms of the inlet steam such as:

V=x*S

where x is the percentage of the inlet steam that vents to atmosphere. I would prefer not to use this method as the vent rate percentage is not something that we want to control. We only want to vent what is necessary and then to capture the latent heat in the venting steam. However, I don't know any other way to get the system down to two unknowns without knowing all three other stream flow rates.

I am now incorporating the dissolved oxygen and CO2 into the mass/energy balance. I've attached another schematic with revised and additional information about the streams. At this point I am unsure if the 330,000 lb/hr of water is inlet or outlet. I am scheduled to meet with the plant tomorrow and hopefully will be able to verify the flow rates for steam in, water in, water out.

I will hopefully provide more information tomorrow and be able to solve for the venting flow rate as desired.

Attached Files

•  deaerator_schematic.jpg   38.86KB   136 downloads

[breizh]

Hi ,
As I said let you consider to measure the Cl- or Ca2+ content of the liquid ( In and Out) , then you will access to the flowrate of water leaving the deaerator !
PS = Cl-(w)/CL-(PS) *W
to validate the balance , do the same for Ca2+ or other minerals like SiO2 for example .

Breizh

[kkala]

I have used the method of concentrations mentioned by Breizh by injecting a KCl solution into the water stream using a metering pump. Dilution was strong in that case, which increased precision, also depending on how stable the flow rate of metering pump was. K+ concentrations of metering pump fluid and of water were measured in the plant laboratory (by flame photometry, if I remember well).
It is efficient to measure all ingoing and outgoing flows and temperatures in one deaerator, then check results through mass and heat balance to realize precision. In this way you will certainly know the situation, which may be somehow different to what is assumed now. And you can apply suitable measures for energy conservation and saving.
It may not be hard to measure flows, even by installing restriction orifice.
Concerning water, you may also send flow out to a tank to measure time between two levels (by a chronometer). Or you can use a pot 3 times bigger than a barrel to measure flow (be careful of hot water).
Common industrial thermometers can be also used to measure temperatures.

Edited by kkala, 02 March 2011 - 02:05 PM.

kkala,

do you have any literature which state Vent steam flow rate being less than 0.1% ?

this will be useful to attach to my calculation note.

[kkala]

You can look at http://www.spiraxsar...-deaerators.asp of Spiral Sarco document, end of paragraph about "deaerator air venting capacity". Mentioned 0.5-2 kg / ton concerns steam and non condensables going out.
The figure of 0.1% concerning steam is information from boiler suppliers.