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]

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