Steam Mains Pressure Control Valve Sizing 1

[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]

1. The 14 to 0.5 bar(g) pressure drop is more than the critical pressure drop for sonic flow. Control valve supplier literature will have specific features required and appropriate factors for calculating CV size for such situations.

2. Boiler load control should reduce the boiler flow when the steam consumption is low. This can be done by giving the steam pressure feedback to the boiler combustion control.

3. If the venting requirement is frequent it will result in loss of boiler quality water, the cost of which may be significant. If so, the feasibility of using a condenser to condense the vent steam and reuse the same in the boilers should be considered.

 

[LittleInch]

Well it's really up to you as for me this control valve is really just pre-empting the main relief valves lifting.

I'm concerned though by the proximity of the set points. I assume the "normal" operating control set point is 12 bar?

Then your excess pressure is 14 bar but your pressure relief is 15 barg which seems very close. Even with pilot relief valves those main pressure reliefs will start to "simmer" and release steam. so either 14 needs to go close rto 13 or 13.5 or the pressure reliefs need to move higher if they can be.

The flow through your control valve to minimise size etc should be the maximum you need to relieve pressure before your boilers ramp down to accommodate the reduced steam load. /only you know how fast and what the steam load demand graph looks like and how long it takes the boilers to ramp down with their thermal lag. that control valve should then be sized, with a suitable margin, to relieve the difference under the graph between steam production and steam demand over time.

This does though look like a large waste of energy / money each time this happens so you may need to look more closely at how the users signal a drop in demand or even add some sort of steam buffer vessel.

Worst case is your 94,000 kg/hr, but only you know if this is actually credible for all users to top taking steam over say 30 seconds or if there is a much lower step change.

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.

 

[georgeverghese]

It may be necessary to have to cater for 94tons/hr venting rate here given there may sudden drops in steam demand - what is the normal worst case drop in demand ? Would suggest this vent valve need only cater for this case. This will then give you more room to cater to smaller venting rates that would be much more frequent than this worst case load shed.
Remember most control valves have a Cv turndown of about 25:1, so see if you cover most of the normal venting scenarios with a single control valve. If the plant operations is such that consumption turndown requirements exceed 25:1, then you may need a split range control loop.
Agreed, right now the control setpoint for this vent controller is too close to the boiler PSV setpoint.

 

[Pavan Kumar]

Thank you very much Mr. Goutam and LittleInch for your good suggestions.

I agree with you LittleInch that the PCV set point is very close to that of the PSVs on the boiler and could cause simmering. The boilers design pressure is 16bar(g) and the piping is rated for 15 bar(g), so the pressure set of the boilers can be raised to 16 bar(g) while keeping the PCV set point at 14 bar(g). Also per Mr. Goutam's point that the boiler loading should and ( I think it will) take feedback from Main Header to control the output. Spurious releases will be huge waste of money and valuable treated water.

I did some preliminary Control Valve Sizing Calcs (spreadsheet attached) using Emerson Control Valve Manual and got a size of 8" with a required CV of 568 with a rated CV of 846. The line size would then be 10". The header is 16". I have also asked TLV to size the control valve to get the correct Control Valve based on their selected trim.

On side note Emerson Control Valve Manual has data only up to 8" size Globe valve style control valves. If size bigger than 8" is required what does one do?.

Thanks and Regards,
Pavan Kumar

 

[goutam_freelance]

If size bigger than 8" is required what does one do?.

You should investigate the use of double port valves.

 

[georgeverghese]

Cv max at 850 is too large for this service - find a smaller valve trim with Cv at 100% open of say 600 or so.

 

[goutam_freelance]

From your calculation:


From my reference (https://www.emerson.com/documents/automation/control-valve-handbook-en-3661206.pdf) the same should be:



So your X should be x_sizing

And


Please check with your reference

 

[pierreick]

Hi,
You may want to have a look at this document to comfort your calculation.
Pierre

 

[TBP]

I'm curious - why is the piping rated for slightly lower pressure than the rated pressure of the boilers? Why not have the piping rated for 16 bar to match the boilers? That way, the boilers cannot overpressure the associated piping.

Are you planning to use the proposed control valve as the overpressure protection for the piping? If so, you cannot use a back-pressure control valve in place of a safety valve.

With the limited information provided, this appears to be a very unorthodox arrangement.

Best of luck to all involved.

 

[Pavan Kumar]

Well it's really up to you as for me this control valve is really just pre-empting the main relief valves lifting.

I'm concerned though by the proximity of the set points. I assume the "normal" operating control set point is 12 bar?

Then your excess pressure is 14 bar but your pressure relief is 15 barg which seems very close. Even with pilot relief valves those main pressure reliefs will start to "simmer" and release steam. so either 14 needs to go close rto 13 or 13.5 or the pressure reliefs need to move higher if they can be.

The flow through your control valve to minimise size etc should be the maximum you need to relieve pressure before your boilers ramp down to accommodate the reduced steam load. /only you know how fast and what the steam load demand graph looks like and how long it takes the boilers to ramp down with their thermal lag. that control valve should then be sized, with a suitable margin, to relieve the difference under the graph between steam production and steam demand over time.

This does though look like a large waste of energy / money each time this happens so you may need to look more closely at how the users signal a drop in demand or even add some sort of steam buffer vessel.

Worst case is your 94,000 kg/hr, but only you know if this is actually credible for all users to top taking steam over say 30 seconds or if there is a much lower step change.

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.

Hi LittleInch,

I am not sure at this project(Basic Engineering) , I am aware of the worst case flow that I can relieve nor would I get any answers at this point. The best I can do is to size the control valve for the full boiler load and put a note in the specification sheet that this sizing is to re-evaluated after determining the worst case relieving rate at the Detailed Engineering phase. As George is saying above that most control valves have 25:1 turn down ratio, which means my control valve sized for a maximum flow case of 94000 kg/hr ( say between 60-80% opening) can flow 3760 kg/hr (=94000/25) at say 20% opening. I will check the Control Valves characteristic curve when I get it from TLV rep.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Cv max at 850 is too large for this service - find a smaller valve trim with Cv at 100% open of say 600 or so.

Hi Mr. George,

The 8" Globe Style control valve 100% CV of 846 is based on my preliminary calculation using Emerson Control Valve Technical Manual( same as the one shared by Mr. Pierre).
I will verify with TLV rep when I get the data sheet from them for the max open CV. As I said in my previous post Emerson Manual provides, CV, XT values only to 8" size. For me to do manual sizing I can only go up to 8" Globe Style Valve. See CV table below from Emerson Control Valve Technical Manual. I wanted to ask again what I should do when my required CV is greater than 846. Mr. Goutam suggested using Double port valves but I am not sure how to use that in my calculation spreadsheet.



Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

Hi,
You may want to have a look at this document to comfort your calculation.
Pierre

Hi Mr. Pierre,

I have used the same document for my calculations. I am grateful to you for sharing much needed documents in all these years.

Thanks and Regards,
Pavan Kumar

 

[Pavan Kumar]

I'm curious - why is the piping rated for slightly lower pressure than the rated pressure of the boilers? Why not have the piping rated for 16 bar to match the boilers? That way, the boilers cannot overpressure the associated piping.

Are you planning to use the proposed control valve as the overpressure protection for the piping? If so, you cannot use a back-pressure control valve in place of a safety valve.

With the limited information provided, this appears to be a very unorthodox arrangement.

Best of luck to all involved.

Hi Mr. TBP,

I have already raised this question with my Project Lead and he did not have a proper answer for this. It does not make sense to me also. Also the pipe spec has put a design pressure quite loosely as 15 bar(g) for a temperature range of 5 - 200 Deg C. The PCV is not there to protect the piping but only to prevent the pressure from going beyond 14 barg due to user consumption drops. I agree with you a PSV is needed in addition to a PCV. I will talk to my Project lead about all this once again.

Thanks and Regards,
Pavan Kumar

 

[TBP]

I don't have the background information on this project that you do, but as an earlier poster mentioned, the boiler burner controls should be able to run the firing rate back to compensate for a drop in steam load. The steam header should have a pressure transmitter mounted on it, tied-in to the burner management system. Note that the header will typically be about 25 PSIG (1.5 to 2 bar) lower pressure that the boiler operating pressure, at full load.

 

[Pavan Kumar]

From your calculation:
View attachment 3230

From my reference (https://www.emerson.com/documents/automation/control-valve-handbook-en-3661206.pdf) the same should be:

View attachment 3231

So your X should be x_sizing

And
View attachment 3232

Please check with your reference

Hi Mr. Goutam,

I have taken care of this requirement as the minimum value of expansion factor Y is 0.667. In my case XSizing(= 0.899) is greater than Fk*XTP (=0.919*0.774=0.711), so I have equated XSizing to Xchoked(=Fk*XTP) to get Y=0.667.

Thanks and Regards,
Pavan Kumar

 

[LittleInch]

George has a different view to me and that's fine, but I would normally not look at anything as high as 25:1 turndown for a control valve. If you get 10: 1 to work effectively without constantly hunting, especially at the low flows, then you're doing well IME.

You normally look to operate between 20% to maybe 75 to 80% open for a control valve.

 

[georgeverghese]

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.

 

[TBP]

Instead of having one large valve, I'd look at using a pair of half-capacity valves, and stagger the setpoints slightly. This will give you much better turndown, is often significantly cheaper in total, and the smaller valves typically have a shorter delivery time.

 

[goutam_freelance]

ave equated XSizing to Xchoked(=Fk*XTP) to get Y=0.667.

But you should use x_choked in Cv formula also.


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