Steam Mains Pressure Control Valve Sizing

[Pavan Kumar]

Hi All,

I need some guidance on the need for sizing the Pressure Control valve on the Steam Mains. The Steam Mains is served by two identical boilers each rated for 47000 kg/hr steam capacity at 12 bar(g). The steams mains supplies steam to users at 11 bar(g). It has been proposed to install a Pressure Control Valve on the Steam Mains that will be set at 14 bar(g). The purpose is to vent excess steam to atmosphere when the user load falls to prevent the header pressure from rising above 14 bar(g). The total capacity of the two boilers is 2*47000=94000 kg/hr. My question is that the Pressure Control Valve should be sized for the full load of 94000 kg/hr with 14 bar(g) upstream and 0.5 bar(g) downstream?. The 0.5 bar is the assumed pressure drop in the silencer (muffler) that is downstream of the Control valve before it vents to atmosphere. Or should I take credit of the Boiler PSVs capacities there by and reduce the size of the Control valve. I wanted to know what the sizing basis is normally.



Thanks and Regards,
Pavan Kumar

 

[goutam_freelance]

sizing the Pressure Control valve on the Steam Mains. T

This application is similar to Pressure Relief Valves(PRV). This can be spring-loaded (similar to safety valves) or power actuated.

So the sizing procedure should suit the requirements of PRV.

 

[Pavan Kumar]

But you should use x_choked in Cv formula also.
View attachment 3319

I see you have taken x as 0.89917 which is more than x_choked(0.711)

That was a really good catch. I have now corrected my spreadsheet. Thank You so much Mr. Goutam.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

In the interest of getting high turndown ( low flow is what this valve will be doing most of time when it is active), given this is an excess flow vent valve, sizing should be done based on max flow at say 90-95% of max Cv ( not 60-70%). However, for this Emerson selection range, given that required Cv is 530, you can only choose the Cv = 846 valve. See if there is some other style of control valve or some other manufacturer who can offer a max Cv of approx 600.

See my previous suggestion : Sizing this valve for full flow of 94t/hr is not necessary - this would be a worst case scenario which may be low frequency and you have the boiler PSV for that. Look at all the consumers and see which combination of events would lead to a normal worst case drop in demand and use this as the duty for this vent valve. This approach will give you a smaller valve, which will give you better turndown and, given a smaller actuator, a more responsive control loop.

The boiler firing rate controls will drop steam production when demand drops, but this control loop is slow acting, given the large thermal inertia in the boilers.

I dont know what you mean by " I wanted to ask again what I should do when my required CV is greater than 846." You said earlier that required Cv=530 - have you found some error in your sizing calcs ?

To minimise response lag from this control loop for sudden drops in consumption demand, given this control valve will be set up as fail close, is to install a pneumatic volume booster relay at the control valve.

Hi George,

Since we want a control valve that is robust enough for high pressure service we want to use a Globe style. I agree that Control valve's maximum flow will be the worst case that the header will see when most of the users shutdown, which may not be 94700 kg/hr. I have asked the vendor to size the control valve for Minimum and normal at 10% and 50% of 94700 kg/hr. and that 94700 kg/hr. can be passed at 90-95% opening. I also said to him that noise > 80 dBA at 100% flow of 94700 kg/hr. would not be a problem as this would be rare occurrence and could be accepted.

" I wanted to ask again what I should do when my required CV is greater than 846". What I meant by this is that 8" Globe Style's max CV is 846 and the Emerson Technical Manual lists only up to 8". In the future if I my required CV for a different application is greater than 846 how did I do my manual calculations. Yes there was an error in my calculation iin that I used XSizing in the CV formula when I should have used XChoked as in my case Xsizing > XChoked. I corrected it now and my required went up from 567 to 638 and I still have to use 8" control valve.

Thanks and Regards,
Pavan Kumar

 

[goutam_freelance]

hat I should do when my required CV is greater than 846".

Masoneilan double ported CV data

https://dam.bakerhughes.com/m/5f757373da01e3a9/original/Masoneilan-10000-Double-Ported-Globe-Valve-Tech-Spec-English.pdf

For 8 in size max Cv is 980 with quick opening. Sizes up to 16 in avalable.

 

[pierreick]

Hi,
To support your work, I encourage you to download NorriSize software from Internet (google) for sizing valves.
Pay attention to the noise level, it seems to reach 116 dBA and the velocity at the outlet (special attention is required to support the valve, the silencer)
Pierre

 

[goutam_freelance]

too big a valve will result in hunting or pulsing of the steam vent when say you only need to releive 5 kg/hr. any turndown greater than about 5 to 8: 1 will cause you serious issues.

This is important. In addition, control valves are notorious for leakage.

Even though it appears that the decision has been taken at a higher level than the OP, the decision is most likely flawed.

The cost of a 8 in CV is considerable, and there are likely to be serious operating problems (haunting, loss of control, leakages, etc.)

A better alternative will be to upgrade the boiler controls to give a faster response (this will be important to take care of fast load increase also) and use the safety valve for dumping of steam during fast load reduction.

 

[TBP]

The only situation in which I've seen the boiler safety valves lift because the burner controls couldn't react quickly enough - and the entire flywheel effect of a hard-firing boiler - is a main turbine trip in a central generating station.

 

[Pavan Kumar]

Hi,
To support your work, I encourage you to download NorriSize software from Internet (google) for sizing valves.
Pay attention to the noise level, it seems to reach 116 dBA and the velocity at the outlet (special attention is required to support the valve, the silencer)
Pierre

Hi Mr. Pierre,

I am nit able to download this software for some reason. Is there a way I can do hand-calculation to calculation the noise. Is it possible to share the PDF copy of the standard "IEC 60534-8-3: industrial-process control valves – part 8-3: noise considerations – control valve aerodynamic noise prediction method. "

Thanks and Regards,
Pavan Kumar

 

[pierreick]

Hi,
I don't have the document referenced, consider this document from Samson (link underneath);

https://www.samsongroup.com/document/w01950en.pdf

You may want to consult with the E&I engineer within your organization.
Edit:
another document from Masoneilan:

https://dam.bakerhughes.com/m/403871c7a460561b/original/Masoneilan-Noise-Control-Handbook-English.pdf

Pierre