10 replies to this topic

[arkae]

hi everyone,

 

i just wanted to know the temperature drop across a natural gas pipe line.

 

The inlet conditions of the natural gas are:

 

Pressure:39-49 bara

Temperature:10-45 degC

 

Climate conditions are:

Minimum:-6 degC

Normal:17.6 degC

Maximum:40 degC

 

The pipeline is having a length of nearly 150 m from the natural gas suppliers terminal point to Gas Turbine and the pipeline is in a trench.

 

Please let me know how to calculate the temperature drop in the pipeline.

 

Also i would like to know can we calculate the temperature drop in natural gas pipeline in hysis. If so please please let me know how to do it.

 

 

Thanks in advance

 

 

[ankur2061]

arkae,

 

Any temperature drop in a gas line occurs due to gas expansion from pressure drop. However, such a temperature drop would require a significant pressure drop. For a length of 150 m the pressure drop would be very insignificant and as such the temperature drop would also be negligible.

 

However, if you are feeding natural gas to a gas turbine, you need to maintain the gas temperature at least 20 deg C above the hydrocarbon dewpoint of the natural gas (often a gas temperature of 60 deg C is maintained). This could be done using a gas heater (water bath / electric). In fact upstream of the gas turbine, a gas conditioning skid is provided which consists of a Knock Out Drum, Filter Coalescer and a Gas Heater in sequence. The gas conditioning skid ensures that the gas being fed to the gas turbine is absolutely dry, free of any fine solids and at a temperature well above its hydrocarbon dewpoint.

 

Regards,

Ankur.

[thorium90]

Hi,

 

Thought I might add on the following.

Attached Files

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•  2.png   218.25KB   48 downloads

[arkae]

Dear Ankur sir,

 

Thankyou very much for your reply.

 

First of all i would like to describe you our system

 

knockout drum-metering station-Electric Gas heater-Pressure Reducing station-Absolute separator-performance heater-scrubber-Gas turbine.

 

Our scope starts after Metering Station. The length of the pipe line from Metering station till Electric Gas heater is 150 m. .After Electric Gas heater there is no problem. But from Metering Station till Electric Gas heater, since the minimum temperature of the gas is 10 degC and the minimum ambient temperature is -6 degC, are there any chances of the gas temperature dropping. If the temperature drops significantly because of surroundings temperature what can be done?( will providing insulation be sufficient?)

 

And also we received comment from our owner that "waterbath heater is more efficient for continuous operation compared to electric heater". What is your opinion?

 

 

thanks in advance.

[arkae]

hi Mr.thorium90,

 

thankyou very much for your reply.

 

I have gone through your attachments. But the Hysis output is for a buried pipe. But in my case, some part of the pipe is in trench(not buried) and some part of the pipe is on sleepers(above ground).

Can we still calculate for this case as well using Hysis?

 

Thanks in advance.

[ankur2061]

Arkae,

 

The purpose of the electric gas heater is to heat up the gas to maintain temperature sufficiently above the HC dewpoint. Your electric gas heater duty must be specified for the minimum gas temperature or in other words the maximum delta T based on minimum gas temperature.

If you do this, you don't need to be concerned about the temperature drop of gas from the metering station to the electric gas heater.

 

The practice that I have seen being followed downstream of the gas heater is to provide electric heat tracing of the line up to the gas turbine to prevent the temperature dropping again. Insulation could also be provided with due care to quality of insulation in terms of having good weathering resistance.

 

As far as water bath heaters is concerned, they work absolutely fine and as such I haven't encountered any problems. If water is not a problem then in my opinion a water bath heater is a better choice compared to an electric heater.

 

Hope this helps.

 

Regards,

Ankur.

[thorium90]

hi Mr.thorium90,

 

thankyou very much for your reply.

 

I have gone through your attachments. But the Hysis output is for a buried pipe. But in my case, some part of the pipe is in trench(not buried) and some part of the pipe is on sleepers(above ground).

Can we still calculate for this case as well using Hysis?

 

Thanks in advance.

 

Thats why "User specified" was selected. Although i'm not sure what values would be suitable in your case. Probably some past data will be helpful. For the parts above ground, go to "insert segment", then modify the length and elevation change. Or, you could just add another "PIPE-101" segment.

Edited by thorium90, 10 March 2013 - 04:29 AM.

[SenOil]

Dear all,

 

I want to add another question to this forum,

a month ago during testing a natural gas well by the test engineer we noticed that from the wellhead to the gathering pipeline system we saw our equipment ( pipeline, valve from the wellhead and daniel metering being frozen )

what could be the reason or explanation of this phenomenon??

I were thinking about a big pressure drop betwenn the bottom hole pressue and the wellhead pressure due to a small choke size select.

 

many thanks in advance.

[S.AHMAD]

SenOil

1. Yes I agree when you said that large pressure drop can reduces the outlet temperature according to adiabatic expansion.

2, However, normally a significant temperature drop down to below ambient temperature is affected by "flashing of liquid", the process we normally referred to as "auto-refrigeration".

3. Therefore, I suspect that the natural gas upstream contains liquid hydrocarbons at high pressure.

Edited by S.AHMAD, 19 March 2013 - 07:40 PM.

[ankur2061]

Senoil,

 

You have a typical problem that is seen from gas wells due to pressure reduction by choke. This as mentioned by Ahmed is due to the free expansion of the fluid across the choke and the reduction of the fluid temperature leading to formation of soild gas hydrates which are ice-like crystalline structures. This problem gets exacerbated if you are starting the gas well during low ambient temperature conditions.

 

To counter this a hydrate inhibitor injection system is provided upstream of the choke. Typically during start-up of a gas well you inject a hydrate inhibitor such as methanol into the flowline along with gradually allowing flow of well fluid into the flowline. This does not require an elaborate fixed methanol injection system and can be done using a portable methanol injection system skid consisting of a small tank and methanol injection pump. The methanol would be injected into the flowline using injection nozzle assembly pre-fitted on the flowline upstream of the choke.

 

Since methanol depresses the hydrate formation temperature the possibility of soild gas hydrates is reduced substantially. Once flow is ramped up to the well normal output and steady flow conditions have been achieved and the fluid temepratures in the flowline are well above the gas hydrate formation tempertaure the methanol injection can be discontinued.

 

Hope this helps.

 

Regards,

Ankur.

[SenOil]

Thank you very much Ahmad and Ankur for these helpful contribution