16 replies to this topic

[farid.k]

Hye all,

 

Typically in venting design for overpressure, In P&ID normally we just indicate “to safe location” for the outlet of the venting. But how piping designer want to route the piping? I mean how they know the location is safe or not? Any rules? How to know when dispersion study need to be done and not necessary? Currently there is a pipe (venting for gas) very close to the platform and seems like not safe to operator. Appreciate to advice for the design.

[Bobby Strain]

There are a lot of considerations for a safe location. Especially if the vented material is flammable. Someone like you will determine a safe location and convey the information to the pipers. But there should be some responsible entity set forth in work procedures so that nothing gets overlooked. But I have seen some real nightmares that actually got installed. Engineered by several major E&C companies. Noise can be a hazard, too. Any flammable vent should be located such that it is not a problem when it ignites, as it surely will.

 

Bobby

[Zauberberg]

There is no universal recipe for venting at safe location. As Bobby has mentioned, each case is specific and it needs to be addressed by considering all relevant facts. My suggestion is to get an experienced Process and/or Safety Engineer on board when making decisions on routing of potentially hazardous vents to atmosphere.

 

Some of the things that should be considered are:

1. Is the stream flammable/toxic?
2. Is the stream lighter or heavier than air?
3. What is the temperature of the vented stream? Can physical injuries to personnel or damage to surrounding facilities occur?
4. Is there a possibility for combustible liquid entrainment to the vent line?
5. What is the maximum duration of venting?
6. Are there any ignition sources around the vent, and at what distance?
7. Are there any ambient/climate effects that would adversely affect safe dispersion of the fluid?

 

Venting inert or non-hazardous fluids (e.g. air, steam, Nitrogen) usually does not require such extensive evaluation, however in cases when there are flammable and/or toxic fluids involved, more detailed study may be required. This normally involves dispersion modeling, or even a quantitative risk assessment if venting is envisaged on a continuous basis.

[shan]

API 14C

C.2.2.3 Discharge Point

The final discharge point for atmospheric gas may be through a vertical, cantilevered, or underwater pipe.  In some cases the discharge point may remote from the platform.  The following should be considered in selecting a safe discharge point:

a. Personnel safety.

b. The discharge volume.

c. The location in relation to other equipment, particularly fired vessels or other ignition sources, personnel quarters, fresh arir intake systems, and helicopter and boat approaches.

d. Prevailing wind direction and, in the case of underwater discharges, the prevailing current.

[farid.k]

Thanks bobby and zauberberg for the sharing

 

My case is venting 78% H2 gas, others component are CO, CO2, and traceable C1. So I consider the venting is flammable. Currently the “safe location” that has been done by pipe designer is 2.5 meters above the access platform and at the end of venting piping there is goose neck in order to avoid the rain. So when the venting open (venting controlled by control valve and pressure transmitter), it will pointing the gas downwards and if there are operators on the platform, I am pretty sure that the platform is not a safe area. However, the set point for the venting valve to open is quite small which is .3 barg. The maximum possible flow rate is about 340 kg/h.

 

I did proposed to re-route the outlet piping for at least outside of the access platform and far from possible ignition source, any roatating equipment and make the height 4 meters from the platform (as some typical standard did mention minimum height is 3 meter).

 

Kindly comment my suggestion

[Zauberberg]

2.5 meters riser height and downward orientation of the vent do not appear to be a safe solution because they do create a hazardous area on the platform itself. Secondly, in case of accidental ignition, this arrangement probably creates an excessive radiation area on the platform around the vent pipe.

 

I agree with you and would recommend the height of at least 5m above grade (or at least 4m above the platform, whichever is greater), and sufficiently far away from any possible ignition sources. If avoiding accidental ignition is not possible, then make sure the radiation levels are acceptable - this should be covered by the recommended vent height.

 

As discussed above, you need to make sure that dispersed gas in the platform area does not exceed prescribed limits (e.g. 10-20% LEL) and there are no other hazards associated with the installation.

[breizh]

Farid ,

 

Don't forget lightning in your risk assessment !

 

my 2 cents

 

Breizh

[fallah]

Currently the “safe location” that has been done by pipe designer is 2.5 meters above the access platform and at the end of venting piping there is goose neck in order to avoid the rain.

 

farid.k,

 

No need to consider goose neck at the end of venting piping. In fact, 2.5 m above access platform appears to be ok but it should be also routed around 1 m horizontally to be away from the platform and instead of considering goose neck at the end of venting it's adequate chconsidering a drip hole (with few mm diameter) in low point of horizontal section for draining any condensate or rain droplets...

[shan]

Please clarify what is your guys basis to determine 2.5 m, 3 m, or 5 m elevation is the "safe location".  Dispersion (25% LEL)? Radiation (2000 BTU/(Hr-FT2) ?  You don't even have the gas discharge velocity (no vent pipe diameter specified), environment conditions (no wind speed or wind direction indicated), and structure around (no helideck or no crane located).

[farid.k]

Please clarify what is your guys basis to determine 2.5 m, 3 m, or 5 m elevation is the "safe location". Dispersion (25% LEL)? Radiation (2000 BTU/(Hr-FT2) ? You don't even have the gas discharge velocity (no vent pipe diameter specified), environment conditions (no wind speed or wind direction indicated), and structure around (no helideck or no crane located).

Dear shan, Zauberberg, fallah and breizh,

Thanks so much for the sharing.

Breizh, yea, i was thinking about that too. But does it look weird to put flame arrestor at the outlet venting? I said weird because other existing typical venting outlet didn't equipped with flame arrestor. What say you to eliminate the lightning spark risk?

Thanks fallah, yes, I agreed for at least to reroute outside of the platform. Agreed too to put the drip hole.just that plant didn't practise it. They preferred to consider the goose neck. Basis is, they don't want the accumulated rain droplet going down after raining. However other plant is common to have that drip hole.

Dear shan, I trust that as there is no standard rigid value for the riser length. I trust that it based on common sense based on quick evaluation which is the setting point of the vent (.34 barg), the flowrate (max 340kg/h) and the pipe size is 3"(sorry for not telling earlier). For me, in order to know the riser length is just by doing the dispersion study.form the study the minimum height of the venting will be known. But yea it is not easy to do that study and I have an experience that, the study always give over conservative value of the pipe rise (10m and above) which is for me quite long and the pipe support will be the problem too.

[breizh]

farid ,

 

lightning rod grounded to floor.

 

Breizh

[shan]

API 521, 7.3.4 Vent stacks

7.3.4.1  Sizing

Where the atmospheric vent handles combustible vapors, the outlet from the vent should be elevated approximately 3 m (10 ft) above any adjacent equipment, building, chimney or other structure (see 6.3 for additional discussion).  Provisions should be made for drainage of each vent pipe so that liquid cannot accumulate in the vent.

7.3.4.2  Design Details

The height of the vent stack is selected so that the concentration vapor at a point of interest is well bellow the lower flammable limit of the vapor.  Flammability consideration can be satisfied with 0.1 times to 0.5 times the lower flammable limit.  Toxicity consideration can require much lower concentrations of certain applications and is, therefore, the controlling factor.

Edited by shan, 29 June 2015 - 10:08 AM.

[farid.k]

API 521, 7.3.4 Vent stacks

7.3.4.1  Sizing

Where the atmospheric vent handles combustible vapors, the outlet from the vent should be elevated approximately 3 m (10 ft) above any adjacent equipment, building, chimney or other structure (see 6.3 for additional discussion).  Provisions should be made for drainage of each vent pipe so that liquid cannot accumulate in the vent.

7.3.4.2  Design Details

The height of the vent stack is selected so that the concentration vapor at a point of interest is well bellow the lower flammable limit of the vapor.  Flammability consideration can be satisfied with 0.1 times to 0.5 times the lower flammable limit.  Toxicity consideration can require much lower concentrations of certain applications and is, therefore, the controlling factor.

dear shan,

 

what say you regarding to the snuffing steam/nitrogen? do we required it? based on API 521 5th ed, it say "should be install". what if i didnt install it?

[shan]

What if your released gas is ignited?

[farid.k]

What if your released gas is ignited?

Dear shan,

Thanks so much for the response

Yea, the answer is the venting gas will be ignited. And fire/ potential explosion to happen is possible. Just that the thing that play in my mind, why API didn't make it as a compulsory/mandatory requirements to have snuffing at the every outlet for flammable venting?

I did refer to other typical standard, it didn't make compulsory to have snuffing..
And existing typical venting facility in my plant also didn’t have snuffing too..

If new venting has snuffing, the others existing venting will be questionable.

Appreciate for your opinion.

[shan]

Please notice 78% H2 concentration in the released stream.  H2 has very broad flammability rang 4%-74% in air and it requires only 0.02 millijoules of energy to ignite H2/Air mixture, which is less that 7 percent of the energy needed to ignite natural gas.  Also, CO is harmful to human body even in ppm concentration levels.

If I were you, I would connect the release line to the nearby LP flare header instead of struggling for a "Safe Location" to avoid the concerns on the vent stack height, rain protection, snuffing, and possible purge gas.

Edited by shan, 30 June 2015 - 09:12 AM.

[Lai.CY]

Hello Farid,

 

Firstly, I would like to apologize jumping into this discussion from no where. From past experience, some old platforms have its cold vent routed minimum 2 meters from the edge of platform, which indeed is questionable. Some of the later platform, has its vent tip size reduced so that the gas will be jetted away. The tip is rotatable depending on the prevailing wind. I agree with Fallah that the low point section can be drilled with holes (weep holes) to drain off any rain water accumulation. However, sometimes, clients will raise their concern that at a cold rainy night, gas will condense and dripped from the weep hole; regardless how unlikely or insignificant the amount. Well, then the vent header will need to slope backwards with a low point pipe connected to a transparent segment to see any visible liquid, and manual draining. Good design needs to be consider so that no excessive rain water is able to enter the vent header, which is a tough one to design. The few obvious points will be that the vent shall not be near stairways, away from boat landing, and it'll be great to do a dispersion study vs prevailing wind sweeping downwards towards the process topsides. The outcome of the study will tell you the required stack height.

While the composition is 78% H2, I hope that the overall molecular weight is lighter than air and goes up easily.

 

When it comes to flame arrestors, please correct me if I am wrong. Flame arrestors can be considered IF the gas is clean, since it is very easy for a flame arrestor to get clogged. Secondly, if venting is intermittent, in other words, if it is continuous venting, you do not need a flame arrestor. But you will need an automatic snuffing system (which is good for 3 attempts incase the tip is so hot, it auto re-ignite the vent gas). Thirdly, if PSVs are tied onto the vent header, then Flame Arrestors is a potential obstruction which is against API 521 code. This can be solved by installing a buckling pin. But if the buckling pin has buckled due to flame arrestor partial blockage, operator attendance is required to reset the seat manually. At that duration, if there is lightning and the vent gas caught fire, we're back at square one. I'm never a big fan of flame arrestors. Flame arrestors can be installed, but frequent inspection is required. Also note if the flame arrestor is installed horizontally, there will be an additional drain point requirement.

 

Lastly, I agree with Shan as well that it is better connecting the vent gas to the LP flare header (if available... I know some areas still allows continuous venting).

 

Best of luck!