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Volume for blocked piping & Thermal RV
8

Volume for blocked piping & Thermal RV

Volume for blocked piping & Thermal RV

(OP)
I am currently working on an LNG job and a question had come up as to the the minimum volume of blocked-in pipework (between 2 isolation valves) that require the addition of a Thermal PSV.
I have some very small inventories (of the order of 5 litres) on the LNG piping; the LNG will vaporise when blocked in - operating temp is about -165 Deg.C!!.
Does anyone have an idea when I should apply a TSV (inventory?), or should ALL sections on the piping system that can be blocked in have a TSV.

Any feed back would be appreciated.

Andy Holman.

RE: Volume for blocked piping & Thermal RV

Dear Andyholman, Yes even a 300 millilitre blocked liquid such as soft atrated drinks if blocked inside a bottle without any vapor space will burst the bottle on expansion. So any blocked in liquid requires a thermal relief. Even the bombs(!!) used for collecting liquid samples are vented 20% to create a vapor space!
Best wishes

RE: Volume for blocked piping & Thermal RV

Andy:

If you go to the following NIST webpage:

http://webbook.nist.gov/cgi/fluid.cgi?Action=Load&ID=C74828&Type=SatP&;THigh=-120&TLow=-260&TInc=&RefState=DEF&TUnit=F&PUnit=psia&DUnit=lbm%2Fft3&HUnit=Btu%2Flbm&WUnit=ft%2Fs&;VisUnit=cP&STUnit=lb%2Fft

you will see the thermodynamic values of saturated LNG (actually liquid methane, but close enough) and you can calculate the liquid expansion %.   For example, if you take the difference between LNG @ 14.7 psia & -258 oF and that of LNG @ -200 oF you will see there is a 14% increase in the specific volume.  The warming up of the LNG is sufficient to create astronomical hydraulic pressures if allowed to remain blocked in.  I'm not talking about LNG "vaporizing".  There is no vaporization involved when you block-in a 100% liquid system.  The resultant rapid increase in pressure is due to hydraulics taking effect - the specific volume increases and there is no place for the volume of liquid to go to - except to overcome the yield stress of the piping containing it.

You'll never see the vapor before the piping system bursts - only afterwards.  You would be very foolish to try to contain such overwhelming hydraulic forces.  The practical and safe answer is that you can never guarantee that you will always be capable of maintaining the LNG in the pure, 100% saturated state because of constant surrounding heat leaks.   The obvious answer is that thermal relief devices are essential if a block-in situation is possible.

RE: Volume for blocked piping & Thermal RV

In chapter 6 of the Relief Systems Handbook by Cyril F. Parry, a flow scheme is shown. According to this flow scheme, a thermal relief valve is recommended for LNG (being a cryogenic liquid) if the trapped volume of liquid is more than a significant volume of 0.01 m3 (10 liters).
He further explains that cryogenic liquids should generally be provided with thermal relief providing that a significant volume of liquid has been trapped and appreciable heat gain is possible. The reference to a significant volume of liquid is to avoid stipulating thermal relief valves where it would be impractical to install it, e.g. between valves installed face to face.

RE: Volume for blocked piping & Thermal RV

3

To get an idea of the pressure developed on heating a packed liquid you may divide the absolute values of the cubic thermal expansion of the fluid
 
α = (1/ V)(∂ V/∂T)p

by its isothermal compressibility

κ = -(1/ V) (∂ V/∂ p)T

for the envisaged temperature range.

α and κ can be estimated from tables as that brought by Montemayor; α from the density changes, and κ from the sound speed in the liquid.

RE: Volume for blocked piping & Thermal RV

I would concur with Art in maintaining that NO volume of liquid may be trapped without a thermal relief valve. The ability of trapped LNG (any liquid really) to generate huge pressure upon absorbing heat can be readily calculated using the information provided above.

The solution is to avoid trapping liquified gases if at all possible. Minimize the use of block valves to those that are absolutely essential. If you use ball valves, you should use special ones that do not trap the fluid in the internal cavity. These considerations, many of which are not all that apparent on quick examination, are the reason we consult with experts before venturing into the unknown.
Doug

RE: Volume for blocked piping & Thermal RV


Once again, a premium and classic thread is born.

I cannot help but re-inforce what 25362 has shared with us:  the hydraulic pressure (force) CAN be estimated by using his clever and unique method.  (This technique should clearly be an FAQ!)  I can't help but give 25362 a well deserved star (he deserves mine).  Although he has mentioned this calculation method in past threads, it seems few people have given it the recognition it deserves.

Doug has beaten me to the punch again.  I forgot all about the terrible potential of ball and plug valves to form their own, inherent blocked flow when I wrote my response.  This is a feature that should be noticed, studied, and submitted to instant memory by every engineer working with saturated liquids - especially the liquified gases (which are usually all in the saturated state).  The characteristic that Doug describes was "discovered" in the mid-60's, shortly after ball valves became the rage and were employed in liquified gas service.  I must have replaced dozens of Teflon seat rings before it dawned on me (& others) that something was not going right.  Worcester Ball Valves caught it and started recommending a 1/8" hole be drilled normal to the flow in the valve.  The upstream face of the closed position was always used by me to furnish this relief feature.  Some ball valve manufacturers even went to furnishing another, smaller drain valve on the main ball valve's side to drain the cavity's inventory.  This latter method was a knee jerk reaction to what was suddenly perceived as a potential hazard.  I haven't seen this type of solution recently.  The application of a standard (non-self venting) ball valve in liquified gas service is rightfully considered a potential hazard - although few, in-experienced engineers would spot it.  Doug is to be congratulated with a star also for pointing this important point out.  The same applies to plug valves as well - although these are seldom considered for liquified gas service in today's applications.

These two guys have made this thread into a classic and one to be referred to in the future.   Thanks guys.

RE: Volume for blocked piping & Thermal RV

(OP)
Thanks one & all for all the useful bits, especially about the cavity inside a ball valve.

I think that I now have more than enough to continue my Design.

Thansk all again,

Andy (Englishman in France!!)

RE: Volume for blocked piping & Thermal RV

Here are two references that might be of interest as an add-on to this thread:

"Pressure buildup in a blocked-in pipe", T. Sofronas, Hydrocarbon Processing, October 2005

"Forestall Pipe Bursts", D.C. Copenhaver, L.A. Coppari, S.G. Rochelle, Chemical Engineeing Magazine, January 2001

RE: Volume for blocked piping & Thermal RV

You can also install a 1/2" tube around the block valves and place a check valve in that loop that would allow a small amount of fluid to relieve to the other side.  This is the common practice used on pipelines and on "double Block and Bleed" valves (General Twin Seal, SafTSeal, ect).

RE: Volume for blocked piping & Thermal RV

when piping up a double block and bleed on a LNG line , should the bleed piping come off the top or bottom of the header between the 2 block valves ?

RE: Volume for blocked piping & Thermal RV

The perfered is top incase a piece of trash is in the line.  If no choice the bottom will work.

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