18 replies to this topic

[rxnarang]

We have a very low pressure system, namely:

While loading a truck with NGL, with large amounts of benzene, the vapors displaced need to be disposed off in a thermal oxidizer. The vapor line is 160 m long and has a check valve. The line is 4" and velocity in a 4" line is 1.5 m/s. The total pressure loss allowed in this line is about 25 milli bar. I cannot find a check valve which has less than 0.5 psi or 34 milli bar cracking pressure. Is anybody aware of a check valve which has about say, 5-10 millibar cracking pressure. One option I have explored is to reduce the check valve size to 2", so that the velocity rises to 5 m/s. Any other ideas out there?

Thanks
Rajiv

[avsp]

rxnarang,

There is one american vendor, Circle-Seal Controls Inc., who I believe supplies small size, low pressure check valves (typically in the approximate range of 5-15 millibar). Unfortunately, I do not know more than this but I had seen these valves, supplied by the above vendor, in a tank farm of a production station, when I visited that facility while working on some other project .

Cheers ,

[hduncan8943]

Why aren't you recovering the vapor instead of burning it?

[rxnarang]

No idea! I suspect this is cheaper.

[avsp]

Typically, for these the kind of systems, the amount of emissions does not justify investment in a full scale vapour recovery system. However, if there is a vapour recovery unit alrready exisiting (like the one you see in some of the refineries), then these kind of vapor emissions could be routed to that unit.

Cheers

[gvdlans]

Are you sure you don't need a detonation flame arrestor as well in the line close to truck loading station (it would be required for vapor return from oil tankers as per ISGOTT and IMO guidelines)? This would require pressure drop as well...

What about installing a blower near the truck loading station that is automatically started and stopped in case of high/low suction pressure? This would solve your pressure drop problems.

[gvdlans]

I just came across an article in Chemical Engineering Progress that addresses safety aspects of a Vent Collection and Destruction System. It can be downloaded from: http://www.cepmagazi.../pdf/050422.pdf

This article strengthens my belief that providing detonation flame arrestors is a good idea, as well as installation of an Oxygen measurement near truck loading to ensure that oxygen concentration stays below the minimum oxygen concentration (MOC).

The good news is that in this case with the low flowrate of about 0.5 Nm3/min pressure drop over a 4" flame arrestor will be quite small (see for example http://www.protego.c.../en_catalog.htm ).

[hduncan8943]

We've done vapor recovery for railcars and barges both loading and unloading. Depending on the chemical, flame arrestors play a critical role and are specifically required when doing barge loading/unloading. We've done systems for both refrigerants as well as light hydrocarbons. I'm just wondering if anyone has looked at the investment and the payout.

[rxnarang]

The vapor destruction system consists of a suction blower and a detonation arrestor at the blower discharge.

Your points are well taken.

Thanks for your help. I am still searching for a check valve. Will update when I find one.

Continuing on this topic. Initially the system consisted of activated carbon adsorbers to limit benzene emissions. This was later changed to to a thermal oxidizer when the vendor expressed inability to provide a workable adsorber system. Any idea what is the limit for using adsorbers? I suspect it will be a limit placed on loading of the bed. Something like :- kgs of benzene adsorbed per Kg of carbon. If it exceeds a certain amount , adsorbers cannot be used. Am I on the right track?

Thanks again!

[gvdlans]

Rajiv,

It is hard to believe that you have a pressure drop problem, considering that you do have a blower as well as the low velocity in a 4" pipe and the line length of 160 meters. Also I would expect the detonation flame arrestor as close as possible to the truck loading, and not in the blower discharge...

Just to have some more background:

- Is this a new installation, or a modification of an existing installation?
- Where is the blower located (at truck loading or at thermal oxidizer)?
- What delta P is blower providing? Can you consider to increase this?
- Do you have an explanation for your delta P problem? So what is giving you the high pressure drop?

It is hard to follow your remarks on the activated carbon bed. You do not seem to know why the vendor stated that this could not be used? Why don't you ask the vendor directly for this information?

If your benzene loading would be too high, I would expect that you just need a larger activated carbon bed, or you would have to regenerate the bed more frequently...

[rxnarang]

The problem is not high pressure drop, but LOW pressure drop. The check valve will not open if the velocities are not high enough through the valve. In fact, it will, but will chatter, leading to damage of the valve. We are now designing a system where we swage down to 2" for the check valve and then swage back again to 4".

The detonation arrestor is at the blower end. The blower is near the thermal oxidizer. The detonatiuon arrestor is at the exit of the blower, because fuel gas and air is introduced after the blower, for complete combustion. As the line is closed the source of ignition can be near the blower, rather than the truck.

Hope that clarifies a few things.

I am chasing up the benzne loading parameters for the absorption bed, and will let the forum know if anything interesting turns up. I doubt that changing carbon beds frequently is the answer. We would not spend money on a thermal oxidizer, if that was the case. Engineering and installing a thermal oxidizer is not as easy as a carbon bed. This is a grass root LNG facility.

[gvdlans]

Thanks Rajiv, this certainly makes things a lot clearer...

Swaging to 2" for the checkvalve looks like a good and economical option to me.

For the activated carbon bed, I was assuming that the bed is frequently regenerated in-situ, e.g. via a pressure swing cycle. From your latest post I understand that you (were considering to) have just one bed that is replaced when it is saturated. Did you consider to contact one of the companies that provide complete VOC Recovery solutions (e.g. http://www.coolsorption.com or http://www.abbgt.com)?

I still think having a detonation arrestor near the truck as well is a good idea. Inside the truck you have flammable vapours and maybe air/oxygen as well. Preventing explosions by excluding ignition sources only is normally not considered sufficient (you only need a small spark to ignite a flammable mixture). You want to prevent that an explosion at the truck can be transfered through the pipe and vice-versa.

[rxnarang]

gvdlans,

Thanks for your inputs. I cannot disagree with your advice on the flame arrestor being nearer the tank. Let me see if I can get it changed.

The carbon bed was supposed to trucked offsite when saturated. Let me check up the sires you mentioned.

Regards

[rxnarang]

Doug,

Your observation is correct. In fact Crane also warns that cracking pressure can be controlling. In fact that was my original question, though it took some time to get around to it. We have reduced the size of check valve to 2" , in a 4 " line to give a velcoity of about 5 m/s at the check valve.

The vendor will revert back to us and I will update the forum about the final solution.

Your concerns about high cracking pressures in a low pressure line are valid. This is a much ignored area with disastrous results. I have been told that a similair situation occured in a LP steam turbine outlet. There was a 16" check valve ( line size) and when the turbine started, the valve chattered enough to physically damage the line.

I wish there was somebody out there who could throw some more light on this very important subject.
Regards

[djack77494]

Rajiv,
I look forward to your final posting. Somehow, I'm not sure how the solution to a problem involving a small acceptable pressure drop winds up being to reduce valve size from 4" to 2". I must be missing something, because I'm seeing an increase in pressure drop. Please let us know the final outcome.
Doug

[rxnarang]

As promised, here is an update on the final check valve selected.

1" swing check with a cracking pressure of 0.5 psi and a Cv ( !) of 20. In the end it was not even a special item. Just had the vendor hunt around to find a " standard" check valve which met the requirements.

[mbeychok]

Rajiv:

In your original posting (that initiated this thread), you stated "While loading a truck with NGL, with large amounts of benzene, the vapors displaced need to be disposed off in a thermal oxidizer."

If your NGL is an abbreviation for natural gas liquids, then I don't understand how you have any benzene, much less large amounts. Natural gas just simply does not contain any benzene. Could it be that you are loading into trucks which have recently been used to transport benzene and that you are displacing the residual benzene vapors from the trucks? If that is the case, can you not simply stop using trucks that have recently transported benzene? Surely, the customers to whom you are shipping the NGL would complain quite strongly if they were aware that the NGL might actually be contaminated with some benzene.

[rxnarang]

Milton,

Good observation! Well, this NGL is not coming out from a typical natural gas processing plant. This is a LNG plant and the feed gas contains about 100 ppm C6s. Considerable amount and most of it goes right down to the heavies ( NGL).

The site is not is USA, but in the Far East. Some that gas can apparantely be nasty. The customer for NGL will be aware of the high Benzenes in the product.

Hope that answers your query!

Regards