Eng-Tips_ Chemical Process Engineering - Water Seal for Flares

8/11/2019 Eng-Tips_ Chemical Process Engineering - Water Seal for Flares

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water seal for flaresthread798-88372

psebes (Chemical) (OP) 26

Feb04

13:52

Hello All,

Can any body pls. tell me what are the advantages and disadvantages of water seal system for flares stacks.

Thanks in advance

us1612 (Chemical) 1

Mar

04

3:53

hi psebes,

disadvantages:

- continuous water supply

- continuous waste water generation

- need of waste water treatment

- steam tracing or steam injection depending on location

advantages:

- reliable installation / proven technology

this is what i am thinking of at first . . .

but as i am not a flare specialist let me ask you, what were the alternatives to a water seal???

Nosey (Chemical) 1

Mar

04

6:38

Some additional points,

A water seal has to be sized very carefully as you don't want it to cause surges or puffing in the flare system.

Also you have to ensure that the water doesn't freeze at low temperatures (either ambient or blowdown) so you have to provide either a low

freezing temp fluid or a source of heat (i.e. steam tracing).

Flare seals do reduce the amount of purging required throughout the rest of the flare system, however many argue (and I agree) that the flare

stack itself still requires purging.

A water seal also allows the flare system to operate at a pressure above atmospheric and therefore prevent any ingress of air into the pipework.

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5 9/27/2014 6:31 PM



Again, the above comments are not exhaustive.

psebes (Chemical) (OP) 6

Mar

04

3:05

Thanks guys,

I am not a flare specilist myself, but I know there is " rapture pin seal" which can be considered instead of the water seal. My concerns are

basically as was already highlighted by both of you, Low temp freezing, waste water treatment, water carry over during depressurization andblow down. Therefore, I wonder if you guys have any actual operational experience with regard to the above concerns or operating of the

system.

In our system, we are having continuos N2 purging in addition to water seal, and on top of that our flare ignition system will be continious

sparking system, so there will be no fuel gas pilot to flare. it is just this water seal system that i am questioning.

thanks

abcmex (Mechanical) 7

Mar

04

5:04

Psebes,

sorry for answering your question with a question

which company offers the "rupture pin seal" as alternative to a water seal in flares applications?

Rupture pins are used usually as substitution of rupture disks.

RGS

Montemayor (Chemical) 7 Mar

04

12:52

psebes:

Let me address the points you bring/leave out:

1) The issue of the pros & cons of water seals is of little worth or importance when dealing with a flare system. What are you relating to or

comparing the water seal with? What I'm referring to is that the water seal does a very important job; what do you intend to substitute for it?

2) Perhaps you are not aware of what you are gaining from the employment of a water seal. The water seal functions as an important flash-

back/check valve mechanism as well as a seal and a relief device. It is simplicity in itself and has no moving parts. I defy any engineer to offer

another flow-back preventer that can handle the flowrates involved with a flare stack, with the same pressure drops and the same seal & relief

features. None exists and I don't believe you'll find anyone betting his annual salary on this.

3) What in the world are you "enraptured" with? Do you mean a buckling pin - as compared with a rupture disk? These are pressure/vacuum

relief devices only and offer no back-flow prevention or sealing capability. Please explain what you mean by "rapture pin seal" or this thread

will be inundated with engineering jokes.

Of course, having a water seal involves having to put up with a trade-off. Nothing in an engineering accomplishment comes to us like a free

lunch. Everything has a trade-off. But in this case, the options left to you are few or none if -for safety's sake- you need to incorporate positive

sealing, backflow prevention and safety pressure relief. All of this is achieved with no moving parts and practically "zilch" maintenance or

worry - which translates to maximum reliability and proven performance.

If you are seriously looking to replace or modify your water seals, consult with your flare designer and/or experts like John Zink about the

options and designs that you can substitute for it. The reliability and safety of a flare system is too important and pivotal to resolve on this

forum. Issues of safety, liability and proven expertise are key to the proper design - and this depends on your specific application. I hope this

experience helps you out.

Art Montemayor

Spring, TX

psebes (Chemical) (OP) 9 Mar

04

14:15


5 9/27/2014 6:31 PM



Hello all,

currently, we don't have any water seal system, but it is an option that we are considering for a new projet.

with regard to the " rapture pin seal", and to my knowledge, it is a physical seal working with same principle as the rapture disk for relief

system. this rapture pin is basically a fitting that is placed into your flare header. it consist of a metal barrier sitting on a thin pin, and when

the pressure in the header is reaching to a certain point, the pin bends which in turn open the header bath to flare stack. I don't know which

company offers these rapture pins, but i'll find out and let you know.

Regards to all

tkdhwjd (Chemical) 9 Mar04

15:44

us1612 and Nosey listed majority of advantages and disadvantages of water seal drum. As others pointed out, water seal is the most common

(I believe), reliable, and proven technology. Although not recommended, you have an option to go with a gas purge, or a combination of

molecular dry seal and gas purge for flashback protection. Gas purge option is pretty much self-explanatory. If you use gas purge alone, you

will have to maintain certain gas velocity at the flare stack to avoid the ingress of air into the stack. This may be attractive if purge gas can be

supplied free of charge and there are no environmental concerns. To reduce the required amount of purge, you can add a molecular dry

seal. For this option, however, the gas must be lighter than air. It basically utilizes buoyancy force of lighter gas to prevent the ingress of air

into flare headers.

Guidoo (Chemical) 9 Mar

0416:09

psebes:

Let's end this silly discussion about 'rapture pins'! As already pointed out by abcmex and Art Montemayor, you are clearly talking about rupture

pins (often called buckling pins). In API RP 520/521 it is shown how they work. They can for example be obtained from BS&B Safety Systems

(www.bsbsystems.com).

I don't have all the details here with me, but I know for sure that they are used by some operating companies i.s.o. water seals in flare

systems. This is especially so on offshore platforms, where a water seal requires expensive plot space.

Montemayor (Chemical) 9 Mar

0416:23

psebes:

I seem to be having a communications problem on this subject. I repeat: The water seal functions as an important flash-back/check valve

mechanism as well as a seal and a relief device. What you are now describing is a rUpture pin (as opposed to rapture) or - as it is also known -

a buckling pin.

As I pointed out previously, the rupture pin (by the way, this is a misnomer, since the pin doesn't "rupture" - it "buckles" in compliance with

Euler's Law) only gives you pressure (or vacuum) relief protection. It does not prevent backflow protection nor does it function as a seal. It

contributes nothing to the ability of nitrogen purging.

Buckling pins are a great contribution to pressure/vacuum relief. I highly endorse them and have ceased to apply rupture disks and only

recommend buckling pins. The best information on buckling pins is at these two site:

http://www.rupturepin.com/

http://www.protectoseal.com

Protectoseal is a distributor of the technology developed by Rupture Pin Technology of Oklahoma. I would certainly apply buckling pins to the

flare system; but this still leaves a need for a water seal or its substitute. One thing that I don't accept is the argument that the water seal

causes a need for waste water treatment. This statement is simply not true. If your flare gases (& liquids) were clean & burnable, there

wouldn't be any need to clean up the water. The fact is that most flare systems are forced to handle 2-phase flow and a lot of bad and toxic

compounds. This will continue to happen whether you use a water seal or not. And you will have to handle/transfer/treat/eliminate their

accumulation in the flare system - whether you use a water seal or not.

I hope I've succeeded in explaining myself as well as my recommendations and that you succeed in your quest for a safe and dependable flare

system.

Art MontemayorSpring, TX

Art Montemayor

Spring, TX


5 9/27/2014 6:31 PM



Guidoo (Chemical) 9 Mar

04

16:59

Art:

Again, I am quite sure that some (renowned) operating companies use buckling pins in a configuration that replaces a water seal, especially on

offshore platforms for plotspace reasons.

From what I remember it consists of a pressure controller with a pressure control valve that maintains a constant pressure in the flare header.

This pressure is slightly above atmospheric to prevent air ingress. Note that there is still a purge provided. In a bypass around the control valve,there is a buckling pin device. When there is a relief, normally the pressure controller will open the pressure control valve. If the pressure

controller and/or the valve fails, the pin will buckle so there is still a safe relief.

Unfortunately, I don't have my papers here with me and can't give more details, but this is what I remember.

abcmex (Mechanical) 9 Mar

04

20:06

Guidoo,

There are different applications for a water seal:

1. there are water seals that work like pressure/vacuum relief valve, they look like a dip pipe in a receiver with a controlled water level.

2. there are water seals that work like pressure relief valve and a flame arrester combinated (look at www.protego.com , catalog, section 10,type TS/P).

I suppose you can substitute a water seal acting as a pressure relief valve with a rupture pin (I agree it buckles ). If you can explain me how

to use the rupture pin for the second application I will be glad.

RGS

Guidoo (Chemical) 10

Mar

04

3:34

abcmex,

I am not talking about a buckling pin device replacing a relief valve but about a system consisting of a pressure controller with a buckling pin

device in parallel that is used i.s.o. a water seal in a flare system. I already tried to explain how it worked in my previous post.


Mar

04

4:36

Now I have some papers with me, and they show that system consists of a rupture disk (could also be a bucklin pin device), with a high

pressure trip valve and a (smaller) pressure control valve in parallel. The PCV is capable to handle the purge gas and minor vents. In case of a

relief, normally the high pressure trip valve opens. In case this valve fails, the rupture disk opens. This system is used i.s.o. a water seal drum.

For anyone thinking that this is a kind of novel development: I have a reference list here showing that these systems are used by manydifferent operating companies (list contains more than 20, including the large ones). Earliest reference is from 1989.

abcmex (Mechanical) 11

Mar

04

5:31

Guidoo

Your description sounds to me like a low pressure relief system. I agree (see description No. 1 in my comment above) that this can be done

with a water seal (dip pipe), rupture disk/pin or pressure relief valve. Does this company has a www-site?

Regards


Mar


5 9/27/2014 6:31 PM



04

8:57

abcmex

Which company do you mean? I did not mention a specific company anywhere...

pardal (Automotive) 16

Mar

04

19:04

Please apologize me fo a so silli question?

Where can I see a such flare water seal???.

Not at an ofshore tower, of course, there is any web site to see it.?

Just as matter of curiosity.

Pardal

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