LPG strange behaviour

[garciaf]

My issued is the following:
Premises:
Unloading from a LPG tanker
Liquid being pumped is LPG. Mixture 70 Propane/30 Butane
Pumping temperature: 15-22 celcius.
Pumping pressure: 15-16 barg
One hour before ended up the unloading, the liquid pumped was Butane.
outshore pipeline are aproximadelitely 750 bbls, the last hour of pumped butane is aproximadelitely 500 bbls.
See image attached.
after unloading, a ball valve in shore was closed.
the PIT and the TT are locate upstream the ball valve (sensing the pressure and temperature of pipeline that is underwater.
It was a rise of pressure that reached 395 psig and in order to decrease this pressure, it was opened the ball valve to equialize the pressure as outshore as inshore. the valve ball was closed again.
This pressure kept steady (130 psig), but 5 days later the pressure started to go down (40 psig). it was thought that a leakage in the outshore pipeline, because the pressure in the inshore pipeline kept normally (130 psig).
all the pipeline was checked, no leakages.
Later the pipeline recovered the pressure (250 psig) at daylight, at night 135 psig.

This strange behaviour is getting me mad. I don´t know why is this happening. ¡HELP!

 

[casflo]

What about the temperature? Change it as the pressure? The temperature of the sea wáter could change from the day to the night? If the liquid fill completely the piping before the ball valve, a little change in the temperature may provoke a high change of the pressure.

 

[LittleInch]

Please don't double post.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[LittleInch]

Just to be sure here.

You are pumping an LPG mixture (70/30 propane / Butane) then the last 500 barrels it is butane only? For about 3/4 of the pipeline volume?

I suspect something is going in with the mixing of these two liquids and if the pressure is dropping then the propane mix will boil off first, especially if it is warmer on the onshore section.

you really need to have a proper plot of temp and pressure over time, not just idle snapshots.

And it would help to know what the subsea temperature is. It won't vary much compared to onshore.
Also what is the relative length between the two sections?

How you got to 27 bar for the shut in liquid is a bit of a mystery though, but the onshore section might have got rather warm?


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Compositepro]

Sunshine on a pipe completely filled with liquid LPG can cause a great rise in pressure. LPG tanks are only filled to 80% max due to the high expansion rate. It seems such a pipe should have a thermal expansion relief valve bypassing the shut-off valve. Only a small section of pipe needs to get warm.

 

[LittleInch]

I agree with composite pro.

I think your initial over pressure was due to solar heating and is a result of liquid expansion which is often in the range 2-3 bar per deg C.

Then you unload this and the liquid in the pipe will boil off creating a vapour pocket.

Then the pressure goes up and down based on temperature ( which you don't tell us) of the onshore section of the pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[garciaf]

well outshore pipeline is aproximadetely 4 kilometers(under seawater},and 20 meters from de coast to the ball valve.
the behaviour is more normal it seems that stabilizes, the graphic of temperature vary between 77 F and 110 F, and the pressure between 216 and 260 psig. according to the termodynamics tables depending on the mixture of propane&butane this pressure do not match.
se image attached.
over this pressure it is supossed to have liquid. But the pressures do not match. I know that due to the expansion of liquid the pressure can built up.

 

[garciaf]

Here is my issue.

After unloading of LPG (70/30) mixture from LPG tanker (refrigerated). The last condition was 5 celcius and 205 psig. It is recommmeded to have a pipe section with LPG Vapor in order to protect (hoses, pipelinem, Breakaway valve) from an overpressure due to a change in temperature (pipeline underwater reach sea temperature)and the válve in the coast is close. There is a standard that state this?

 

[LittleInch]

I think this just goes under the heading of good operating practice and also complies with the requirement in your piping / pipeline code not to create a pressure higher than the design pressure.

Deciding whether this is required is simply part of the design process - to look at all aspects of the operation and determine what the pressure could be and design for the worst case.

The difficulty with anything that requires an operational procedure to overcome an overpressure scenario is how do you ensure that this happens in practice and what happens if it isn't done. Hence relief valves....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.