Flare purge rates 1

[I2P]

In order to minimise flaring how do we calculate the minimum purge rate required for the flare header? As I understand the purge is just to ensure no flashback into the header and also to prevent a flame out of the pilot.What data,and calculations are reqd for this? thanks

 

[Nosey]

Some of the data you require for these calculations are:
- average molecular weight of purge gas
- whether or not any "seals" are present in the system i.e. molecular seal, liquid seal or velocity seal.
- Size of the flare header
- presence of hydrogen (that changes everything as the burning velocity is very very fast)
- type of flare tip (pipe, coanda or other proprietary make)
- minimum recommended purge from the flare tip manufacturer.

Unfortunately I do not have my references beside me, but you can try

http://www.johnzink.com/tech/tech_mgt/html/t_mgt_tp.htm

and look for the technical papers that John Zink have published on flare system design.

Try using the search mechanism provided on this website, if you do you'll find the following related threads

thread124-64515

thread124-40515

thread124-21178

You'll also find the webaddress for a site set up by Flareman (a forum member worthwhile listening to on flare matters). His site can be found at

http://www.geocities/flareman_xs

This is by no means exhaustive and I expect that other forum members will also be able to add useful comments.

 

[RJOSVER]

Hello, if you read the API-RP-521, they show you this equation:

Qg = (0.005xd^3)/M^0.565

Where:
Qg: Gas flow (mmscfd)
d: diameter of the flare (in)
M: Molecular weight

Regards

 

[I2P]

RJOSVER,
Thanks for your reply, could u pl describe ur equation. I know it might be sounding stupid but I would like to know the symbols used in ur equation.
Would u pl explain wether the symbol^ is for root, square root, or multiplication?
d^3 means what?
thanks

 

[quark]

d^3 is always d³ by general notation.

Believe it or not : If a person becomes invisible, he/she becomes blind as well.

 

[TD2K]

I would also suggest that you talk to your flare tip vendor to see what their recommendations are for minimum purge requirements and see how that compares to the API equation.

 

[Flareman]

I2P
Relative to the purge, basically, if you need to have the flare available all the time, you need to keep it "alive" by constantly putting gas to it in the main header.
The question is only .. how much is enough?

The flare vendor will give you a recommendation about the minimum flow but you really need to find this out for yourself as it applies to your gas and your flare tip.

The minimum flammable flow you need is that which will just keep a small visible flame at the tip and prevent burn-back. When this flow goes too small to flame will try to disappear into the top of the tip. It may make smoke because the flame inside the tip does not have enough air. If you have smoke suppression (air or steam?) turn it down (or almost off) whilst you try to find the minimum so that it doesn't overpower the flame. Then when you find the minimum put back just enough smoke suppression flow to clear up the flame and still allow it to be visible. You may need a bit more gas to stop it going back into the top of the tip. If smoke appears when you have no flame or if the smoke is a single column coming from the center of the tip, that is a sign of burn-back. Drop me an email at flareman_xs@netzero.net and I'll send you a paper which has a formula to give a first order estimate of the burn-back rate for a generic tip design.
The rate will probably be something like 0.2 fps based on the internal flow area.

Purge rates are often quoted at much lower rates than this. Those are the rates needed to prevent air from getting back into the riser, when there is no flame. My paper also covers a formula for this.

Sometimes, purge rates on plant are specified by the contractor on the basis of sweeping or sweetening the flare sub-headers. In these cases, the sub header rates may be based on a Reynolds number condition (say 2000 Re) to keep a turbulent flow. These flows often produce a much greater requirement than that actually needed by the flare itself.

Additional factors which influence the amount of purge used are

- the use of an instrumented purge input. Often this is set to keep a positive pressure at some point in the headers. I cannot argue against the logic for this as it is a plant decision intended to ensure that all leakages are outward. However, it does consume a large amount of purge gas when the gas MW is less than air because the vertical height of the stack sets the system pressure due to the weight of gas in the riser, which is probably already negative at the bottom of the flare when referenced against atmosphere in a PSL or PT.

- supplementary purges which allow for system shrinkage. If there are relief cases which heat up the header and then cool rapidly, or dramatic ambient temperature changes, or gases with a high probability of condensation in the header, the contractor may have determined the need for a supplementary purge intended to overcome the consequences of shrinkage.

These are things which need to be reviewed by the plant process engineers and addressed individually.

When all these things are considered you will turn up with a number of different rates for purge. Then, you have to decide which ones are the most important based on how you run the plant day-to-day. In a large number of cases, the day-to-day flows from vents etc. exceed the purge rates. So you may be able to settle on a permanent purge rate based on keeping air out of the system for no flame, and handle all of the other cases by operational procedure. This means watching the flame for burn-back and adjusting the flow if needed, monitoring for temperature changes and sweeping hot gases from the system before they condense, etc. according to the specific condition.

In all these cases you will continue to have permanent pilot burners. Each will consume fuel. Modern pilots only use about 50,000 - 70,000 Btu/h each, though it is a lot of gas on a yearly basis. If your pilots are the older design and consume, say 200,000 Btu/h each (a big visible flame), then you may wish to think about modernizing.

The 50,000 Btu pilot gives a fairly small flame which is often difficult to see against a bright sky. That often makes operators turn up the flow for their confidence level. Most modern pilots are fitted with thermocouple to register the flame and, in many cases also start an automatic relight sequence if the flame goes out. This overcomes the low confidence issues.


As an aside ... thank you to Nosey for your kind remarks . I do what I can!! The web site is suffering from lack of attention right now but I'll get back to it with more things soon.


David

 

[JoeNava]

I know this thread was initiated a year ago, but triyng to find the equation as proposed by RJOSVER, I could not find it in the API-521, can you tell me where (Qg = (0.005xd^3)/M^0.565)

Thanks