Particular safety valve calculation 1

[supermax]

Hello to everyone, it seems that I post only to solve my troubles, but... I hope I will give soon my feed back!
Well, I have to size a pressure safety valve installed downstream an furnace, in wich occurs a partial vaporization of an hydrocarbons mixture. The calculation hypothesis is: outlet valve in downstram line blocked closed and total vaporization of the fluid hold up in the furnace coils, since furnace does not stop working. A conservative way to size the safety valve is to consider the mass flow given by the ratio between total heat power input from furnace and latent heat of fluid vaporization, evaluated at the relieving conditions. However, I realized that in this way the safety valve will be really enormous! So, I tried to understand this point: If I consider my fluid hold up contained in closed system (assuming coils bundle like a vessel) and I start to heat the fluid from operating conditions, in wich I have a fluid state with both liquid and vapour phases, up to relieving conditions,perhaps when the safety valve opens total fluid is already in the vapour phase! So, maybe that the valve can be sized for thermal expansion of gas, instead of total fluid vaporization. Someone can suggest me an easy way to evaluate it before sizing the safety valve? Thank you for the attention.

 

[Wilkie]

Without knowing actual process conditions, from your post it appears you will definitely need to consider two-phase flow in sizing the relief device.
Several resources can be found on the Web for relief device sizing methods (it sounds like you have a variant of the basic "fire exposure" boiling liquid/expanding gas scenario). Many larger companies have developed their own calculation equations and methods, but you can also find basic equations referenced in OSHA, API, etc. Your relief device manufacturer may also be able to provide help with your particular application.


Steve

 

[Montemayor]

supermax:

What goes in must come out. Size the PSV for the total mass rate going into the furnace coil. If the PSV is enormous, it must be because the furnace is enormous. This is not "conservative". It's the only way to protect the furnace at it's MAWP. What do you mean by "hold up"? Are you saying that you would accumulate the inlet mass rate in an over-pressurized furnace? I don't believe ASME or API agree with you on this. Read both the ASME and API 520 & API 521 recommendations.

If you accumulate mass rate in your furnace coil the pressure will increase. You are obliged -by law- to safely relieve that over-pressure at no-higher than the MAWP. Additionally, I would regard the furnace akin to a boiler - which carries another set of caveats on PSV design and conditions. This is a fired vessel - as distinguished from an un-fired (Section VIII) vessel. Your statement about designing for thermal expansion of gas doesn't make sense. Thermal expansion of gas is what is causing the over-presssure and this must be relieved safely. The "easy" way to evaluate this situation is to follow API 520 & 521.

I hope this helps to explain what is required.

Art Montemayor
Spring, TX

 

[jalvarez]

Supermax
I don't think that you can neglect the liquid vaporization.
Anyway, the size of the valve, for any scenario you can imagine, cannot be bigger than the outlet pipe of the furnace, with the exception of a simultaneous failure of the fuel gas or fuel oil valve, going to its maximum opening, and the "closed outlet" scenario you suppose. And I think that this double contingency can be eliminated. Doing the correct calculations, with the maximum thruput of your furnace and the corresponding heat flux, you will end with a maximum size equal to the outlet line, depending where you are sending the product. Remembering a case for a cracking furnace, the SV installed around the outlet valve (bypassing it), is one size smaller (6" vs 8"). It sounds reasonable due to the allowable pressure increase.
Have a safe day
J.Alvarez

 

[TD2K]

What's the purpose of this valve? Is it to prevent overpressure due to an upstream pump or other pressure source? Is it to maintain flow through the furnace tubes to avoid overheating them since you say you consider the furnace keeps firing on loss of tube flow? Are you just sizing the valve to handle the increase in pressure as you continue to heat the mixture with no flow?

Whether the relief valve has to handle only vapor or a two phase mixture depends a lot on the piping configuration as well as fluid thermodynamics. When the furnace is blocked in, you are going to continue to boil off liquid inside the tubes due to the (continuing) furnace duty. You need to look at your system considering the heat input and the phase equilibrium of your mixture to determine at relief what is the material: gas or a gas/liquid mixture. Even if inside the furnace the material is all vapor, does this get to the PSV inlet considering the two phase mixture still in the outlet piping (hence my comment about piping arrangement). You need to assure yourself if it reasonable to assume that only vapor is going to get to the inlet of the relief valve because if it has to handle a two phase mixture, a vapor only sized valve is unlikey to have enough capacity for a two phase flashing mixture of hydrocarbon.

While sizing the valve assuming the furnace does not trip is quite reasonable, I hope you have low flow trips because the inventory of liquid in the tube that is being boiled off and removing the duty from the furnace is going to be pretty limited. Once all the liquid is boiled off, you will overheat the tubes quickly and I've seen melted tubes in a furnace, it's not pretty.

 

[davefitz]

The problem is no different than that of sizing a relief valve for a boiler.

If the furnace has a drum ( or is connected to a large pressure vessel) for separation of steam and water , then you will need to size the relief valve for the max sustained heat input divided by the heat of vaporization at the lifting pressure. The damage associated with a failed large pressure vessel implies no negotiating away the sizing requirement.

If there does not exist a large pressure vessel directly connected to the furnace circuitry, then you could rationalize using the special rules in ASME sect I for once thru steam generators: safety valve capacity to a partial value of the above described value plus the remaining relief capacity is via a power actuated pressure control valve ( discharging into a lower pressure system) plus automatic rundback of the furnace heat input based on overpressure signal from the DCS. In fact, section I allows as litle as 10% safety valve capacity if you also use a special safety valve that has circuit contacts on the valve stem that is hard wired to a trip cicuti that also shutss off teh feed of fluid and the feed of fuel to the burners.

In any case, the process you described does not fall within the jurisdiction of ASME I unless the fluid is water or dowtherm; for any other fluid some other ASME code would govern.

 

[CHD01]

I probably have not studied this thread enough, but there is one variable that might be used to your advantage (at least for some non-fire Section VIII applications) in sizing a relief valve if your situation is similar to a heat exchanger with flow ceasing on one side. That is that the design rating U value will be lowered as flow stops (on the tube side for example) and this will result in a substantially lower U and relief requirement; but it requires detailed analysis of what the actual U value is under these over-pressure conditions before you can use a relief valve sized for less than the design rating.

The more you learn, the less you are certain of.

 

[krb]

For relief valve sizing see the following link:

http://www.tycovalves-na.com/ld/CROMC-0296-US.pdf

You will need to determine if you are dealing with two phase flow or not.

 

[steveen]

Supermax,

Your valve is going to be way big. A better way may be to place a small valve on the heater outlet -- set at Design pressure of the furnace tubes-- economically sized for some flowrate with a new PSV capable of handling pump minimum flow upstream of the furnace also set at Design pressure. psi. This valve will be easier to size as the fluid will be all liquid. The origianl vapor relief valve will ensure flow through the furnace so any high temperature shutdown devices in the process fluid will see the high temperature. If the furance is used for drying or regeneration, be sure the shutdown devices are in the proper location, so they will see actual flowing temperature.

 

[MortenA]

KRB

I have been looking for the Crossby PSV engineering handbook for some time now since TYCO moved it a while back.

Thank you for the reference

Best regards

Morten

 

[jalvarez]

As I'd mentioned in another thread:
If you need a piece of soft for sizing, read the above mentioned article from Tyco or, also from them, go to

http://sizing.tycovalves.com

They've also included Yarway valves.
Have a safe day
J.Alvarez