3 replies to this topic

[engineer1985]

hi everyone,

we are producing about 22 MMSCFD of natural gas and then process it in a gas sweetning and dehydration facilities...since a part of this gas is used in a reboiler, generators as a fuel and some gas is also used to burn acid gases in a flare...i want to know the amount of gas shrinkage/consumption during the process....can anyone please tell how can i calculate the shrinkage....

[S.AHMAD]

1. Firstly, I do not fully understand the word "shrinkage". However, when you coupled it with "consumption", then i interpret that you do not have the means to quantify the portion of NG as fuel and the NG that is being flared
2. I presumed that you do not have direct measurement e.g. flow meter - that is the reason you ask.
3. We can estimate flow rate indirectly from control valve opening and the valve characteristics (Cv vs valve opening) - I believe fuel gas system should have pressure control
4. For NG to flare, if control valve is not available, probably you can estimate by assuming adiabatic choke flow system
5. I hope the above helps. Otherwise you need to tell more about your problem and the system that you have.

Edited by S.AHMAD, 01 November 2011 - 04:58 AM.

[engineer1985]

S.Ahmed,

thanks for your guidelines.

well we dont have any control valve installed anywhere at the fuel gas skid.

we only have the regulators that reduces the sales (sweet) gas pressure from 700 psig to 100 psig...

so this gas is then used in generators, reboiler and at flare as a fuel...

• i want to know how can i estimate the amount of gas that is being cnsumed at plant

[paulhorth]

Engineer1985,

i want to know how can i estimate the amount of gas that is being cnsumed at plant

If you don't have any flow measurement I don't think this is possible. You can make a partial estimate, for the gas used as fuel for the generators, thus:

• Determine the power output of the generators (volts x amps).
• Look up the vendor's data sheet which should state the typical efficiency in terms of fuel heat release per kW of power.
• Take a sample of the fuel gas and determine the compositiion. Calculate from this the nett calorific value.
• From this you can calculate how much fuel is consumed for the power output that you have, assuming the typical efficiency is valid (which it might not be).

For the gas to the reboiler (I assume you mean a fired reboiler) the best you can do is as Ahmad says by estimating the flow through the supply valve, from the supply pressure and the valve size and its percent opening. There must be a valve on the fuel supply to the reboiler and there should be a data sheet for it.
Likewise for the flow to the flare. There must be a valve - but you probably don't know its fully open size nor its actual opening.
Too bad............

Paul