5 replies to this topic

[Malek]

Hello,

 

A calculation has been made to determine the min required purge rate for a flare riser:

Flare riser:

* Diameter: 50 in.

* Height: 91 m.

* Multi point sonic flare tip.

* Equipped with a velocity seal.

* Capacity: 1872 MMSCFD.

 

Purge gas

* 98% Methane.

* Min required purge rate (with seal) to prevent air infiltration as per API 521: 0.047 MMSCFD.

* Min required purge rate (without seal: more conservative) to prevent air infiltration as per API 521: 0.149 MMSCFD.

 

How ever, the operation team has refused to reduce the purge rate below 0.5 MMSCFD to avoid snuffing out especially in windy days (more than 12 m/s).

 

The question is that, is there any criteria to consider the wind speed to establish the min required purge rate?

 

Malek

[Bobby Strain]

Pilots are normally provide to assure vent ignition and a continuous flame. I suppose one can purge any amount above the minimum requirement.

 

Bobby

[ankur2061]

malek,

 

If you refer the equations (61) & (62) of API STD 521,there is a constant 'K' in those equations. This constant 'K' is a function of wind speed. With increasing wind speed 'K' increases thereby increasing the purge gas rate. In API STD 521 the value of 'K' for Nitrogen as a purge gas is given for 'no-wind' as +1.067 and with wind speed of 7 m/s is given as +1.707. No values for other purge gases related to wind speed are provided in API STD 521. 

 

From my understanding, wind-speed plays a role in determining the purge gas rate but you need to find out the exact values for the constant 'K' to determine the purge gas rate based on the gas you propose to use as a purge gas.

 

Regards,

Ankur.

Edited by ankur2061, 28 January 2014 - 12:28 AM.

[fallah]

 

The question is that, is there any criteria to consider the wind speed to establish the min required purge rate?

 

 

Malek,

 

When a molecular seal is used, purge volume rate should normally be such that to create a velocity of 0.1 ft per sec at the flare tip. In the case of not using a molecular seal the exit velocity is 1 ft per sec.

 

Obviously the purge volume rate depends on the wind velocity at the flare elevation. Above mentioned velocity criteria are based on a wind velocity of 15 mph and would vary as per the square of the wind velocity...

[Malek]

From my understanding, wind-speed plays a role in determining the purge gas rate but you need to find out the exact values for the constant 'K' to determine the purge gas rate based on the gas you propose to use as a purge gas.

 

Regards,

Ankur.

 

Exactly, I have to look around the K values and the direct relation between the purge velocity and wind speed in the case of using a velocity seal.

 

If I would find the answer, I'll update.

 

thank you Ankur and Fallah.

Edited by malek, 28 January 2014 - 02:45 PM.

[Malek]

Tried to contact the vendor but no response.

 

By the way, BP flare guidance Annex C states that "Note: It should be recognised that Husa correlation was derived under calm or no wind conditions and is generally applicable for wind speeds up to approximately 13,4 m/s (30 mph). Locations where prevailing winds can often exceed this wind speed may need somewhat higher purge rates.". to me this is fair enough.

 

Regards

Malek