Steam desuperheater

[coolcando]

I am a new process designer.I need some help in designing steam desuperheater.

High pressure steam is let down to medium pressure by a control valve. To this medium pressure steam, Boiler feed water (high pressure) is added to resuce the temperature of steam .


I have the BFW pressure and temperature.

How do I get steam flow rate ,pressure and temperature after the control valve ?

There is no direct meaurement of steam pressure,flow and temperature after the CV and before the desuperheater.

 

[tkdhwjd]

Based on your HP and MP header pressures, piping layout, and valve information (type, output, etc), you can calculated the flow and pressure. Adiabatic expansion to estimate temp after letdown.

 

[MJCronin]

cool...

What are you trying to do..???

Most of the time, there are process steam conditions to be met from a superheated steam source.

If only the steam temperature is being reduced, then you are looking for a desuperheater. These may or may not involve a control valve assembly and will reduce the steam temperature to about 5 degrees F above saturation (hitting saturation is difficult). SPX/Copes Vulcan offers mechanical atomization units..... Cheap foreign knockoffs are available.....

http://www.spxprocessequipment.com/sites/copes/products/desuperheaters/ma.asp

If you desire both a reduction in the steam temperature as well as the pressure, then you are looking for a "steam conditioning valve. CCI and Fischer are the only companies that I know who offer these

http://www.ccivalve.com/chp.shtml

Again, what are you trying to accomplish ?

-MJC

 

[coolcando]

I have 2 desuperheaters already in operation.But the reliability of them is not good.So I am asked to replace existing superheaters with new models (Most probably Fisher - DVI and DMA/AF).There is a temperature control loop that measures temperature of outlet steam and adjust control valve opening to attain the target temperature(Set Point).

1st one is a steam letdown station.HP steam is let down to MP steam by a control valve and after that desuperheated.
I dont have any direct measurement of steam pressure,flow and temperature d/s of control valve (and u/s of desuperheater).

 

[tkdhwjd]

Pressure drop across the let down lines upstream and downstream of pressure drop is relatively low. You should already know the temp and pressure of HP steam header and pressure of MP steam header. You can adiabatically expand on steam mollier chart and see what the temperature will be at MP header pressure. Flow can be calculated if you know the valve Cv to % opening correlation/chart.

 

[JimCasey]

>>There is no direct meaurement of steam pressure,flow and temperature after the CV and before the desuperheater.<<

The first thing you have to do is to find out what your goals are. Typically the desuperheated steam is controlled to a few degrees of superheat and the flow rate of the steam required is known. You have to measure the downstream pressure and temperature. Knowing the upstream pressure and temperature is really important if only from a troubleshooting standpoint.

You set up a main valve to reduce the pressure from the steam header. When the stem is throttled it stays at a constant enthalpy. Follow the constant enthalpy lines from state point 1 to state point 2 (downstream pressure) and you will almost always see that throttling steam results in superheat. Control this valve with a pressure control loop.

You spray in some liquid water to evaporate in the superheated steam to quench it down to almost saturation. Control this valve with a temperature controller. Cascade control from the pressure control valve is a very good idea if the system is expected to respond to transients. THe referenced "steam conditioning valves" have mechanical analogs to cascade control because they admit more quench flow as they open for more steam flow.

Your plant requirement for the desuperheated steam should be known. This will be Q. Look up the enthalpy for the steam. This will be Hg(d)

Look up the enthalpy for the steam upstream of the reducing station. This will be Hg(u) THe percentage of the steam at upstream enthapy is X.

Look up the enthalpy of the quench water. This will be Hf(q) The percentage of quench water is 1-X

so X x Hg(u) + (1-X)x Hf(q) = Q x Hg(d)

Running an arbitray example: If the upstream header has 500 psi steam with 100 degree superheat, and the quench water comes in at 250 degrees, and the downstream desired steam is 1 million pounds per hour at 10 degrees superheat and 125 psi(g) :

Hf(d) is 1199.01
Hg(u) is 1278.70
Hf(q) is 218.59

1278.7 (X) + 218.59 (1-X) = 1199.01

so you do the math and find that your steam from the header is 925,000 #/H, and the quench water flow is only 75,000 #/H.

So why bother desuperheating steam anyway? that 100F superheat we started with only gives up 74 btu/lb as it is cooled 100F to saturation. Once it hits saturation and begins to condense, it gives up close to 1000 BTU/lb at a constant temperature. So if you are using the steam as a heat transfer agent, it's a lot easier to control the system when you have a constant temperature process. Conversely that's why you quench the steam to a slight superheat: the temperature changes when you change the amount of water. If the steam was at saturation you might dump in a big slug of water and not change the temp. Much easier to control the desuperheating process and you get a useful uniform source of BTUs by controlling to a slight superheat.

 

[rmw]

Make sure your DSH is located in your piping properly. You want your DSH station in a pipe run with adequate upstream and downstream straight pipe distances. Depending on your steam conditions, you might need a pipe liner downstream of the DSH. Check the turndown ratio of the DSH carefully against expected flow fluctuations. If your flow is very constant, low turndown ratios are OK. If it is variable, a high turndown ratio is more desirable.

You also need to decide if you want pure mechanical atomization, steam atomization or other styles. If your straight pipe runs are short especially downstream you may need to go with steam atomization to get good mixing before the first elbow.

Make sure your temperature probe is an adequate distance downstream of the DSH. Know what the minimum distance is and increase it. Be sure to have adequate drains and traps downstream before the first elbow because no system is perfect and there will be moisture in the downstream piping at some time in the life of this gadget.

Do not oversize your water control valve. Pick a reasonable dp to design around. Remember; if you pick a DSH sensitive to dp, you might as well take a high dp across the DSH because that dp contributes to mixing of the water and the steam.

I am not going to go look, I am going to just give the tip, but I think you might find some helpful information at

http://www.graham-mfg.com

.

The thermodynamic aspects have already been addressed.

Use the search button on this site too. There are some good threads on the topic.

rmw

 

[unclesyd]

Here is a DSH that we use. It allows a smaller installation envelope than conventional DSHs's.

http://www.komax.com/desuperheater.htm

A word of caution about DSH installation from my experience. Every manufacturer will give you a minimum distance from a fitting to install their DSH. We have found this "minimum" to be a bare minimum. We have lost 2 steam headers, one SS and one CS, from differential thermal fatigue caused by the spiral flow pattern in the headers immediately after the fittings. In our case it was an elbow.

 

[CJKruger]

I strongly agree with rmw and unclesyd. Most of the desuperheaters I have seen do not work properly. Buy the best one, with the best turndown, and increase the minimum piping requirements.

Also design your downstream piping assuming the desuperheater does not work. Especially the design temperature if it can be above 185 C (to prevent mechanical from doing a service test instead of a hydro test).