11 replies to this topic

[Fatema1987]

Hi

 

 

We are facing a strange phenomena for the steam control valve to Flare tip 86FC004. It is MP steam with 20 barg and 320 C. It is injected in two different zones at flare tip as per attach. Normally, this valve operating with 0.3% - 0.5 % opening ! giving flow of 1200 Kg/Hr. We cannot maintain this valve at auto mode because of this narrow operating window of opening. Strangely, the outlet pressure of this valve is 0.5 barg!! I cannot imagine how it is reaching the tip with this very low pressure! we changed the PG and conducted pressure survey to check this low pressure value and we endup by same 0.5 barg all time! In the data sheet it should be 6 barg almost! So for resizing the valve we couldn't know which outlet pressure should be used!

 

We tried to increase this control valve FC004 opening by following steps:

 

1- Pinch off the manual block valves on downstream of the control valve to build up higher downstream pressure to the Control valve: the downstream block valve was closed gradually, there was no major change in the flow of FC004 or smoke appearance. When the valve fully closed,the flow dropped to half and pressure gauge red zero barg. We confirmed that the block valves are not passing as well as FC004. When FC004 is fully closed the flow was reading 600 Kg/hr. Then we decided to close the valve of orifice fully where this valve should be fully open normally to keep a minimum flow to flare. We noticed no change in this 600 Kg/hr with fully open or close at orifice valve. So we become sure that this small valve 2" is passing.

 

2- In next day we decided to do something else; 

• It was agreed to pinch the two globe valves (4” and 2”) at downstream of 86FC004 to increase FC004.MV to maintain the same flow. As per below pic, we tried to close 4” valve fully and closing 2” valve partially. The downstream pressure at outlet of FC004 increased from 0.5 barg into 9 barg, yet the flow reading dropped minorly <100 KG/H with the same opening of 0.3%.  Then, we fully closed FC004 and the flow dropped from 1280 to almost 800 KG/H.
• Because we didn’t notice any big difference in the steam flow after pinching these globe valves, we returned 2” valve in almost open position while we kept 4” partially close to maintain the outlet pressure at FC004 at 3.5 barg currently.
• Then, we fully closed FC004, the pressure at the PG dropped to almost zero while FC004 dropped to ~ 630 KG/H. So we confirmed there is no passing found in this valve FC004.
• Then, we closed the valve of orifice where no change was noticed in the flow whether the valve is open or close. So, susceptibly it is passing.

We found it hard to accept 0.5 barg as the outlet pressure of FC004 while the pressure at outlet valve of pilot gas is 1.5 bar.

 

What's the issue? What are we missing here?!

 

 

 

Attached Files

•  Flare.JPG   103.37KB   2 downloads

[Bobby Strain]

Well, you're missing the reality that the pressure is only .5 barg. No need to force any higher pressure.

[Sharma Varun]

We found it hard to accept 0.5 barg as the outlet pressure of FC004 while the pressure at outlet valve of pilot gas is 1.5 bar.

 

What's the issue? What are we missing here?!

Well there is no issue, just calculate back pressure required in steam line to flare tip. 

Similarly calculate back pressure in pilot gas and in this case remember to include the pressure drop across burner.

These two calculations will solve the issue for you.

Edited by Sharma Varun, 20 February 2020 - 12:23 AM.

[Fatema1987]

Well, you're missing the reality that the pressure is only .5 barg. No need to force any higher pressure.

 

Is it possible?

 

We did back hydrulic calc from flare tip to outlet of this valve, and the DP came around 5 barg !

So, how 0.5 barg MP can reach the tip ? Anything to do with MP steam type?

As well, this 0.5 barg steam can be seen at flare tip along with the pilot gas 1.5 barg !!!

[Fatema1987]

 

We found it hard to accept 0.5 barg as the outlet pressure of FC004 while the pressure at outlet valve of pilot gas is 1.5 bar.

 

What's the issue? What are we missing here?!

Well there is no issue, just calculate back pressure required in steam line to flare tip. 

Similarly calculate back pressure in pilot gas and in this case remember to include the pressure drop across burner.

These two calculations will solve the issue for you.

 

 

 

We did back hydrulic calc from flare tip to outlet of this valve, and the DP came around 5 barg ! 

So, how 0.5 barg MP can reach the tip ? Anything to do with MP steam type?

As well, this 0.5 barg steam can be seen at flare tip along with the burning pilot gas 1.5 barg !!!

 

The condensate at steam trap is huge which makes me wonder for hot MP steam 340 C and 0.5 barg.

 

Is there any side issue to increase the steam outlet pressure into 7 bargs ?!

[Bobby Strain]

You will probably search forever. What is your objective? One can describe your efforts as chasing a red herring. Move on.

 

Bobby

[latexman]

FC004 is choked.  Increasing backpressure on the outlet of this valve up to about 10 barg will not affect flow.

 

Either both PG's are wrong, or the calcs are wrong, right?  I would suspect the calcs.

[Fatema1987]

You will probably search forever. What is your objective? One can describe your efforts as chasing a red herring. Move on.

 

Bobby

 

We want to order a new valve but based on what outlet pressure?

we dont want to endup by existing issue

[Bobby Strain]

It's a simple task to calculate the control valve outlet pressure at design steam flow. And you haven't explained your current issue. You have only described conditions for a small flow. So, get to work and stop worrying. Are you the lone ranger? If you don't know how to perform the hydraulic calculation, find someone who does. Pay them if you must.

 

Bobby

[latexman]

Bobby makes a good point.  Only a small flow scenario has been discussed.  I was beginning to think you should look at a "drag" control valve, but it may not handle the design flow requirements, which have not been mentioned.

[Sharma Varun]

Learn from experience of Mr Strain & re do all hydraulics.

[ecas]

An opening of 0.3-0.5% in the control valve 4" FC-004 is practically as if the valve is closed. It seems that almost all the flow passes through the 4" globe and 2" orifice bypass valves. It is posible that the control valve internals be eroded and the valve plug stuck.

Try to close 100% the bypass valves and open the control valve to watch if the flow rate and the downstream valve pressure, increase.

I think that the soution will be to change the valve to a 2" or 3" valve, according to the design calculations in order that the valve runs with a 30-40% opening.