[Dan G]

I got no responses to this post on the Relief Devices forum, so maybe it is more appropriate for Tank Blanketing and Venting - drg

We are a custom designed process equipment manufacturer. I have spent a couple of days trying to determine whether or not a proper PSV is required to vent a horizontal atmospheric expansion tank. The tank contains Class IIIB Heat Transfer Fluid (HTF), and there is a 3/4" breather vent (through a desicant filter to keep moisture out of the HTF) at the top centerline which is continuously open to atmosphere. Our typical expansion tanks range from 20gal to 300gal. I have attached a word document that shows the basic system. Note that the expansion tank has a single riser that services two process loops.

Our engineering department has recently proposed using a check valve to relieve increased pressure due to external fire. They have properly sized the orifice requirement, and argue that the probability of a check valve failing to open is orders of magnitude lower than that of a PSV. I am trying to find out if this is allowed, and if there are any requirements that we are missing. It is not clear to me whether any emergency relief is required for this situation beyond protecting the tank or connected piping from bursting due to pressure rise due to external fire.

Most sources that talk about fire venting of low pressure and atmospheric vessels reference API 2000, Appendix C. Should I purchase this standard just to have that reference? Or does it not apply to our case?


Thanks in advance,
Dan Gunderud
email address
Product Safety Manager (Ex- Electrical Engineer)
Buffalo, NY.

Attached File(s)

•  Freeze_Dryer_Main_Process.doc (73K)
  Number of downloads: 28

[djack77494]

trying to determine whether or not a proper PSV is required

Most sources that talk about fire venting of low pressure and atmospheric vessels reference API 2000, Appendix C. Should I purchase this standard just to have that reference? Or does it not apply to our case?

Dan,
I believe API 2000 fully applies to your situation, which is not altogether clear to me. You have a drum with a breather vent on it that is full of desicant. What is the pressure drop through the dryer? It must be adequate to handle maximum inflow or outflow situations as well as thermal effects. The approach is outlined in API 2000, so I'd recommend you obtain the code.

You do not necessarily need "an official PSV". You do need to provide adequate venting for normal and emergency situations. A open vent line (not acceptable in your situation), a vent line with dryer, or an open line with a check valve may be acceptable. The concept of using a check valve disturbs me some, however. You must be sure that the check valve can pass the required quantity at the prescribed differential pressure. PSV's are specified and may be tested for that. Check valves are not, and any information you may have on a check valve's expected performance does not carry the same weight as would a PSV.

Dan, I'd also urge you to be sure you have provided for all possible scenarios. When you first fill the system, you may draw HTF from this tank at the full capacity of the pump. Can you "suck in air at that rate"? It's unlikely unless the dryer/vent was designed with that in mind. Also, for a small drum like this, it is possible that it can withstand a significant pressure differential without problem. I think I'd look into that and see if it has a design pressure/vacuum or if the supplier can shed any light on this situation.

Good luck,
Doug

[proinwv]

Dan, let me add to djack's comments.

You have an atmospheric tank, which says to me that it probably was not designed to handle any pressure or vacuum. Both are possible in your situation if you, in any manner, put a mechanical vent on the tank. Even an extended vent line could cause a problem, if it had significant friction loss under a higher flow rate.

Until you determine the MAWP and the MAWV of the tank, you should not put these devices on it.

I would also be careful with a check valve. Tank P/V vents are designed for the service. Same as tank emergency vents, which would apply to fire exposure.

Definitely refer to API2000 for your case. I do not know what codes apply in your instance, but don't take this to mean that there are not any.

Last, it seems that your organization feels that there is a possible safety issue, so there most likely is. You know your entire op better than us. There is a limited amount that we can learn from anyone's schematic and a short dissertation.

Best wishes

[Dan G]

Doug, Paul.

Thank you for your responses. We have low pressure tanks made for us, which are rated for full vacuum, and we use these for either low pressure purge applications (with PSVs or burst disks) or for atmospheric applications with the desicant filter I described.

I had the mechanical eng calculate the NFPA 30 fire venting flowrate vs pressure drop of the check disk he proposes, and pressure should not rise above 2psi. We know that pressure will exceed 14psi if only the 3/4" vent with desicant filter is provided - hence the safety issue.

We understand well the filling, start-up, and normal thermal expansion/contraction venting requirements, so I am only concerned with the external fire scenario. Since it is a Class IIIB fluid, we will vent inside the building, so pipe lenght is no issue. I am just, as Paul suggests, being very careful about using the check valve.

I welcome any further comments, but it appears that the check valve will be appropriate in this case.

[proinwv]

One more comment. There are tank emergency vents for fire exposure, that are simple, inexpensive, and self-closing that will be for the application. They are weight loaded for simplicity, and generally about 1 psig set point and have been flow tested for rating.

They are generally not pipe away. I would at least look at these .


Paul