19 replies to this topic

[vibhs]

Hi all
I would like to discuss one query about the relief device in the downstream of a desuperheater.
Basic scheme of this process is that LP steam gets generated in the shell side of steam generator. Outlet from here goes to the Fired Heater which further superheat the steam and then it goes to Desuperheater, which receives LP boiler feed water and a combined stream of saturated LP steam comes out of the desuperheater and it joins the LP steam supply header of the unit.
The LP steam generator described above is already provided PSV-A in the shell side for blocked outlet scenario. Manual Isolation valves are provided between LP steam Generator and Fired Heater and also in the downstream of desuperheater. This desuperheater is also provided a PSV-B in the downstream before isolation valve for blocked outlet scenario. Boiler feed water to the desuperheater is through a temperature control valve, which is Air-Fail-Close and takes the temperature signal from the outlet of the desuperheater. PSV-A and PSV-B set pressure are same, and it matches with the design pressure of the desuperheater and the design pressure of the LP steam supply header.
PSV-A has a relief load of the maximum steam generation from the LP steam generator at the time of blocked outlet.
My query is that whether do we require PSV-B, and if yes how the relief load of PSV-B should be calculated.

[fallah]

vibhs,

You are strongly requested to send a sketch of your system. It certainly provide the conditions we can help you out regarding your query.

Fallah

[vibhs]

While I am trying to upload the sketch in .vsd format, its giving me the meaasge that I am not permitted to upload this kind of file. Plz help...

[fallah]

vibhs,

Can you upload the sketch included in Excel or PDF file? If so, you wouldn't be faced with any problem in uploading.

Fallah

Edited by fallah, 21 September 2011 - 04:40 AM.

[vibhs]

Thnx Fallah....
I have uploaded the file. Plz check and guide me.

Attached Files

•  vibhs.xlsx   58.9KB   102 downloads

[fallah]

vibhs,

Thanks for uploading. I will submit my answer to your query ASAP.

Fallah

[fallah]

vibhs,

The isolation valves aren't clear to be in CSO (or LO) position also desuperheater's type and detail isn't specified. Anyway, as far as i know each fired heater is usually equipped with a dedicated PSV for its protection against overpressure. Please clear mentioned points for better assistance.

Fallah

[vibhs]

Hi Fallah....
Thanks for your reply.
Well this is a fixed area spray mechanical atomising desuperheater. No isolation valve is either CSO or LO.

Vibhs

[vibhs]

Fallah

vibhs,

The isolation valves aren't clear to be in CSO (or LO) position also desuperheater's type and detail isn't specified. Anyway, as far as i know each fired heater is usually equipped with a dedicated PSV for its protection against overpressure. Please clear mentioned points for better assistance.

Fallah

Fallah
If you say that a fired heater is equipped with a dedicated PSV, what should be the relief load of this PSV?

[fallah]

Fallah
If you say that a fired heater is equipped with a dedicated PSV, what should be the relief load of this PSV?

vibhs,

It should be calculated based on blocked outlet scenario while fired heater is in operation.

Fallah

[fallah]

Hi Fallah....
Thanks for your reply.
Well this is a fixed area spray mechanical atomising desuperheater. No isolation valve is either CSO or LO.

Vibhs

Vibhs,

If there wouldn't be any non LO/CSO isolation valve between fired heater and desuperheater, provided that mentioned desuperheater would need overpressure protection, PSV-B can protect both fired heater and desuperheater against overpressure due to blocked outlet in downstream of desuperheater. Its relief load likely to be maximum flowrate of feed water entering the desuperheater.

Fallah

Edited by fallah, 25 September 2011 - 04:30 AM.

[vibhs]

Thank you very much Fallah
Your response has cleared my doubts.

Vibhs

[vibhs]

Hi Fallah
With your inputs I was trying to figure out the sequence of events. Well I thought this way, that when desuperheater outlet temperature increases, Boiler Feed Water TCV opens more and vice versa. Now if blocked outlet occurs then Pressure differential across control valve decreases which should decrease the flow rate across Boiler Feed Water Control Valve.
Now my queries are:
1. What Cv should I take to calculate the flow rate across valve?
2. What would be the stream in the outlet of the desuperheater, would it be still saturated steam or just Boiler Feed Water as there is no flow of steam due to blocked outlet?
Additional Information:
1. PSV-A allowable over pressure is 5% and PSV-B allowable over pressure is 3 %.
2. BFW upstream pressure is 10.9 kg/cm2.g and superheated LP steam inlet pressure is 3.5 kg/cm2.g.

[fallah]

Hi Fallah
With your inputs I was trying to figure out the sequence of events. Well I thought this way, that when desuperheater outlet temperature increases, Boiler Feed Water TCV opens more and vice versa. Now if blocked outlet occurs then Pressure differential across control valve decreases which should decrease the flow rate across Boiler Feed Water Control Valve.
Now my queries are:
1. What Cv should I take to calculate the flow rate across valve?
2. What would be the stream in the outlet of the desuperheater, would it be still saturated steam or just Boiler Feed Water as there is no flow of steam due to blocked outlet?
Additional Information:
1. PSV-A allowable over pressure is 5% and PSV-B allowable over pressure is 3 %.
2. BFW upstream pressure is 10.9 kg/cm2.g and superheated LP steam inlet pressure is 3.5 kg/cm2.g.

vibhs,

With limited information about the part of the system you mentioned, my answer to your two questions:

1-Minimum Cv of the TCV because it likely tends to close more to adjust itself with temperature decreasing in desuperheater downstream due to no steam flow while feed water injection is continued. Of course, it is better and coservative to take Cv of the TCV in normal operation.

2-I suppose you mean the fluid in upstream of the PSV-B. If i am right, it would be just boiler feed water.

It shouldn't be forgotten that there might be some effects of block outlet occurance in desuperheater downstream (back flow toward fired heater) on upstream equipment (fired heater and exchanger) overpressure, and PSV-A relieving conditions that because of lack of adequate information about the system configuration and details (check valve existing,......) can not be discussed here.

Fallah

Edited by fallah, 25 September 2011 - 05:07 AM.

[vibhs]

Thank you fallah for your inputs.....

Vibhs

[SSWBoy]

I don't understand how PSV-A and B have the same set pressure. If the system has blocked outlet and the pressure relieves then PSV-A will certainly relieve first (pressure drop through fired heater etc.) leading to a ceasation of flow through the fired heater. Obviously this may have some unfortunate consequences on the tubes which are now running dry.

Therefore in my mind the set pressure of PSV-B would have to be reduced.

[vibhs]

That was good point to highlight SSWBoy.....Thank you very much.....
After that I have checked in the fired heater datasheet, that steam superheat coils are capable of withstanding zero steam flow during normal process coil operating and design conditions.

Vibhs

[vibhs]

vibhs,

The isolation valves aren't clear to be in CSO (or LO) position also desuperheater's type and detail isn't specified. Anyway, as far as i know each fired heater is usually equipped with a dedicated PSV for its protection against overpressure. Please clear mentioned points for better assistance.

Fallah

Fallah
Can you please provide me the reference code for equipping a PSV for fired heater.

Thanx

Vibhs

[fallah]

Fallah
Can you please provide me the reference code for equipping a PSV for fired heater.

Thanx

Vibhs

Vibhs,

I haven't any code address regarding your request , but attached you will find one page from EXXON Enineering Co. may help you out.

Fallah

Attached Files

•  PSV for Fired Heater.pdf   30.85KB   103 downloads

[vibhs]

Thnx Fallah for the attachment...

Vibhs