Pressure reducing and desuperheating steam velocity and straight length pipe requirements

[snoffs]

I currently have a project I'm working on which is an existing power plant designed to asme section 1 that has some letdown valves that have desuperheating capabilities. The pressure on the high side is 1500 psig and the low side is 425 psig. Boiler feed water is introduced within the valve to reduce the temperature of the steam to 700 deg F. The power plant is existing and I'm assuming the design velocity upstream and downstream of the valve was 200 ft/sec. I'm working on trying to increase the capacity through the valves which would require increasing the line size to maintain the 200 ft/sec velocity. My question is what is the velocity range you should keep the flow downstream of the let down valve for the boiler feed water to properly reduce the temperature? Also, is this in a standard or a guideline somewhere? One more thing; I'm also concerned about the straight length of pipe required as well as whether a reducer impedes the de-superheating.

 

[quark]

Your steam is already wet downstream.

 

[snoffs]

quark, will you please explain what you mean by "your steam is already wet downstream"?
Maybe i didn't explain this well in my original post, but what I'm trying to do is increase the mass flow rate through the valve. The system is existing and since my mass flow is increased now my velocity is going to increase downstream, and I'm concerned that the velocity will be too fast for the water droplets to evaporate properly to cool the steam. That is why i asked if there is a velocity range you need to keep the steam at for the boiler feed water to perform the de-superheating, and what straight length of pipe will be required for the de-superheating. Is it based on the velocity (i.e. .1 sec * velocity (ft/sec)). If the velocity is indeed too fast then i will need to replace the existing piping with larger pipe to keep the velocity below the max level.

One more thing to mention. These are turbine bypass valves. This is medium pressure supply steam to the plant.

 

[quark]

Assuming 1500 psig steam is at saturation, the enthalpy is 1168.19 Btu/lb. 425 psig steam at 1168.19 Btu/lb enthalpy (pressure reduction is isenthalpic) is with a temperature of 453.98 F, where as saturation temperature is also same but DF is about 95%. There is a difference of approximately 216 Btu/lb between latent heats of steam at 1500 and 425 psi.

 

[snoffs]

Quark, I apologize i forgot to mention the HP steam is superheated to 1000 deg F on the upstream side of the valve.

 

[MJCronin]

1000 degrees superheat ??? This is kind of an extreme case ..... at the very limits of the design of desuperheaters.

In my opinion, I would go with a seasoned DS vendor and take his recommendation for the installation.

There are trade-offs on design: pressure drop, price, erosion resistance, turn-down etc. etc.

https://www.s-k.com/pdf/6_desuperheater_bulletins_brochure.pdf

http://www.graham-mfg.com/usr/Product Manuals/IOM-DSH-0908.pdf

http://www.spiraxsarco.com/Resource...superheating/basic-desuperheating-theory.aspx

You also may want to consider a change to a higher alloy piping material downstream of the DS..... This is an area prone to erosion and cracking.

There is a lot to consider here ...





MJCronin
Sr. Process Engineer

 

[quark]

As suggested by MJC, this is quite a task. For desuperheating the steam you need water at about 34% of steam flowrate. My experience is with medium and low pressure steam and the general guidance is 4 meter straight portion and temperature sensor at 12 meter distance from point of injection.

 

[Latexman]

Is there a misunderstanding here? Snoffs did not say "1000 degrees superheat", they said "superheated to 1000 deg F". There's a big difference. It has 400 deg F of superheat.

Good luck,
Latexman

To a ChE, the glass is always full - 1/2 air and 1/2 water.

 

[snoffs]

Thank you Latexman for the clarification. You are correct and I apologize for the confusion.
Thank you MJCronin for the reference links. I will review the information. The existing piping downstream of the valve is currently P91 and if we were to increase the line size we would replace the material in kind.
Thank you also Quark for your responses.

 

[quark]

No confusion. I considered 500 psig and 1000F steam at inlet, for calculation. I am cautios due to higher quantity of water required for desuperheating. Further, specific volume is approximately half of initial condition and 34% extra mass flow rate.

 

[georgeverghese]

You'd be better off with a standard desuperheater nozzle which is fitted with individual connections for BFW and superheated steam. At 60m/sec, you'd have noise and hgh velocity erosion issues, and the latter will rear its head later especially if the BFW O2 content is not to spec. To address response lags in the TC scheme for BFW injection, go for a thin wall thermowell with a thermocouple, and not a thick walled TW with an RTD.