5 replies to this topic

[ChemE2014]

I am working on cryogenic relief valve system currently. So far I understand that if you have a cryogenic liquid in a pipe system, the flow rate is based on heat leak input and heat of vaporization, since its a saturated liquid. So, Compressed Gas Association (CGA) has its on relief sizing standards.

 

I am confused with which of these will be applicable to a vacuum jacketed pipe relief valve sizing?

 

 

S-1.1 Pressure Device Standards Part 1 (circular cross-sectional storage units)

S-1.2 Pressure Relief Device Standards Part 2 (cargo and transportable)

S-1.3 Pressure Relief Device Standards Part 3 (permanently mounted units).

 

Is it S-1.1 (because pipe is like a cylinder) or S-1.3.?

Also, looking at the S-1.1 standards... they just calculate the orifice area calc based on the area of cylinder and sqrt of set point.  Your calculated flow rate from heat leak  and heat of vaporization is not being used.?

 

Also if you were to use API standards, the Thermal Hydraulic (blocked in line ) case doesn't really apply to this scenario because its a saturated liquid, it more closely resembles the fire case per API 520.

 

So if you do have  your calculated flow rate , which API equation would you use to size the relief valve.

 

 

Thanks!

 

I also understand that thermal relief valve size is going to be very small. Hell, I don't even need to calculate it and can assign the smallest orifice but it would be nice to know how to define and  solve the problem correctly and also understand which API 520 or CGA equation applies. Also, if one were to use a API 520 equations for cryogenics..is it considered wrong from legal stand point. 

Edited by ChemE2014, 05 October 2014 - 12:05 PM.

[ChemE2014]

bump!

[Art Montemayor]

ChE2014:

 

Relief systems on cryogenic fluids require close scrutiny with regards actual conditions and types of equipment.  The reason(s) for this is that you are dealing with saturated liquids in almost all cases.  This immediately implies that you can’t subject a saturated cryogen liquid to a 100% liquid-full condition in a vessel or pipe.  To do so is inviting a rupture for the obvious reason that the delta temperature drive existing between the ambient atmosphere and the cryogen is so great that latent heat amount could quickly be transferred.  Blocked-in cryogen lines are latent bombs and are to be avoided at all costs.  All cryogen vessels need a vapor space that amounts to approximately 15% of the total vessel volume – sometimes more.  The goal should be to avoid any liquid cryogen discharge from a PSV or relief device wherever possible.  The discharge is a safety hazard, sometimes a dangerous flammable, and expensive to handle and control.  The majority of cryogenic pressure relief devices are designed for vapor relief instead of liquid – whenever possible. 

 

In order to comment on how and what cryogenic PSV can be applied to secure a system, we need a detailed explanation of the system and the scenario.  An engineering sketch of the system would be the ideal way to describe the application – such as a P&ID.

 

I am presently at work and not at home where I have my CGA literature, so I can’t look up the CGA standards.  I have used CGA standards only on transporting equipment for industrial gases – high pressure cylinders and tubes, for example.  The API standards (520 and 521) are perfectly OK to use on cryogenic fluids and can, in my opinion, be applied.  Please define in detail your specific application so we can comment on it.

[oscarsender]

Hi.

 

API 521 Section 5.14 shows the discussion and equation for thermal expansion.

[ChemE2014]

it's basically a network of liquid nitrogen lines coming/originating from liquid nitrogen vessel. Its being used at the plant for the refrigeration of products. From my understanding whenever you install a valve on a cryogenic valve system, you should also install a relief valve, in case of blocked-in line. I did notice that the way the cryogenic relief valves were setup , they didn't really have an outlet pipe and inlet pipe was kind of coiled and the reasoning behind it was to make sure that the relief would be a vapor.

Edited by ChemE2014, 09 October 2014 - 09:17 AM.

[Art Montemayor]

ChemE2014:

 

As I’ve stated, you must protect any liquid-full, blocked cryogen system.  You don’t do this in a 100% gas-filled system.

 

 

The inlet LIQUID piping into a cryogen PSV is normally extended in length and has a looped, gas trap just before entering the PSV.  The reason for this setup is that you don’t want cryogenic fluid constantly in contact with the PSV and using it as a heat sink.  What would happen (besides being a constant, steady heat leak) is that the entire body of the exposed PSV would undergo cryogenic temperatures and convert itself into a huge block of water ice from the ambient humidity surrounding it.  This would, in turn, totally block the PSV outlet and mitigate any possible relief operation.  So, from a common sense and practical point, a gas trap is built into the inlet liquid piping and this creates a static, trapped gas volume directly under the PSV.  The gas – being a natural and efficient insulator – effectively allows the PSV to operate outside a cold box, exposed to the atmosphere and maintained at essentially ambient temperature.  If you check this type of installation on an air separation column, for example, you will find that the heat leak through the PSV is so minimal that you can hardly detect it by placing your hands on the PSV.  The purpose of the gas trap is not to make sure that the relief would be a vapor.  It is there to allow the PSV to operate safely 100% of the time.  The PSV will discharge liquid cryogen if it remains open long enough.