blowdown of pipeline 3

[Kandan]

HI all,
I have a situation in which,

we have a 36" pipeline of approx 80km length. to blow down this line in an emergency.. with a 2" line to the vent.. estimate is -33degC at the end of blow down.. but CS line design temp -29C. Can any body suggest without changing the material.. how to bring up the temp during blow down..
(study says the temp is conservative..)

 

[Kandan]

sorry.. in my previous thread..it should be 8" line

 

[epoisses]

May I dare to suggest you... not to worry about these 4 degrees? Especially since you wrote the temp is probably conservative. Explain your boss what more important jobs on your to do list are held up by this issue and hopefully he will put you off the hook.

 

[sailoday28]

Help out an ME--- What is CS?
To understand what the discharge temp is and how to bring it up, what is the fluid, source conditions and type of insulation on the blow down line?

 

[Latexman]

Kandan,

If these were adiabatic calcs, account for the heat transfer that will take place.

Good luck,
Latexman

 

[MortenA]

CS=Carbon Steel

The calculated temperature - is that equal to the gas temperature? If so: You would be surprised how much higher the steel temperature is!

I can recommend that you take a look at an article in Hydrocarbon processing, july 2004: "Material selection for low temperature applications" By S. Kumar et al. This article deals with actual metal temperature during blowdown and it is rather surpicing just how conservative you are when selecting temperature based on the gas exit temperature.

A program such as Pipeline studio can also calculate this value during a trancient depressuring analysis!

Best regards

Morten

 

[quark]

For a worst case scenario, use trace heaters to wrap up the piping.

 

[tickle]

Hi Kandan

We have been having similar thoughts and discussions on both the pressurisation and depressurisation of a pipeline.

We have discussed the problems with a metalurgist and even though the gas temperature is low the pressure is also low and the therefore brittle fracture is not likely. I could mention the pressures that have been indicated but it is specific to the actual pipe materials.

The easiest method to bring up the temperature is to control the rate of blowdown so that the gas warms up as it blows down.

 

[Kandan]

Thanks all for the reply..

1) Report indicates adiabatic case.. I understand , the blow down duration for the 80km pipe is around 7 days.. no way of prolonging the blow down..

Can quark suggest from experience how expensive would heat tracing be..

Though the pressure D/S of RO is low .. the blow down temperature can creep to the high pressure side (7 days blow down??)

Also how much low temp can CS cater to say at 60 barg..

 

[tickle]

You only have to blowdown the pipeline until the pressure is below that would give a low temperature (JT cooling). Normal procedure would be to draw down the linepack before depressuring so not to waste gas to atmosphere (EPA and the like) - do you really have problem? I guess you are considering the emergency case and then you may be able to put procedural methods in place to prevent brittle fracture of the pipe.

The temperature up stream of the orifice will not decrease because of the gas flow(?).

Have you thought about how you will pressurise the system - this has caused me lots more problems than depressurising.

 

[jsummerfield]

Consider adding a choke to take the drop; and install a stainless tail pipe on the outlet. If this is vent to a flare header and the ambient is sufficiently warm, you might provide a reasonable distance, say 20 meter SS then go back to carbon steel.

John

 

[bvi]

If it is above ground, convective heating from the ambient air should bring the metal temperature up above -29C,presuming the line is not insulated.

If it is below ground, you will get heat transfer from the surrounding earth, but I couldn't tell you it would be enough without doing heat transfer calcs. There is a constant heat flux from the earth core, and presuming ambient temperature is above -33C, you would have a heat flux from above the earth.

If it were piping rather than a pipeline (i.e. B31.3 rather than B31.8), you could have calculated a lower permissable temperature for the carbon steel based on the condition of low stress.

Finally, there is no significant difference between -33C and -29C, although I understand the need to comply with rules. Note that in Section VIII and ASME B31.3, the -50F threshold was changed to -55F, to include propylene, and one of the rationale was there was essentially no difference in the material properties between those temperatures.

 

[jay165]

bvi:

Is there an equivalent "31.3 rule" in 31.8 or API RP1111 regarding low temperature excursions when the corresponding pressure is lower? I don't have a copy of either and I was curious.

 

[bvi]

There is not an equivalent rule in B31.8 to that in B31.3, although the requirements for fracture toughness depend upon the hooop stress the piping is designed to operate at (the percent of the specified minimum yield strength).

From a blow down stand point, the ASME B31.3 approach would let you consider the stress (but also considering longitudinal stress) at the actual conditions.

 

[sailoday28]

Is it possible to inject a non reacting fluid into the blow down line? While the blowdown time will increase, the resulting mixture temperature should increase.

 

[quadtracker]

The cooling effect is related to the huge pressure drop across the blow down valve out to vent or flare. Bring down the pipeline pressure through two blowdown valves in two or more pressure stages, thus reducing a huge single pressure drop.

 

[tickle]

Quadtracker

The expansion of gas produces a cooling effect (nominally 4degC per MPa). Therefore if you expand it over two valves, you still have the same overall pressure drop and the same overall temperature decrease. The method you are proposing is only effective if you can put heat into the gas between the two valves. You can get some heat gain from the ambient and some pressurisation lines are extended in length and made of stainless.

 

[sailoday28]

What is the gas and what are the operating conditions for this process?

 

[Kandan]

The Gas is Natural gas. dehydrated.. and pressure is 80barg.

 

[vzeos]

Several years ago I wrote a VB program to analyze natural gas pipeline blowdown behavior. I ran the conditions you gave: 36” pipe, 8” stack (assuming 8” plug valve and a stack length of 35 ft), 80km long operating at 80barg. My results indicate the blowdown time is 0.7 days, the final temp is -24.4 deg C and the SPL would be about 125 at 0.2 miles from the stack.

While it is usually good to use a rule-of-thumb to estimate temp drop at lower pressures as many programs do, it should be noted the JT coefficient is not constant. It varies with pressure and this variation cannot be ignored at these pressures.