5 replies to this topic

[Muhammad farhan]

Dear All

 

Good Day

 

We have a refinery fuel gas system in which natural gas is used as make up to maintained pressure. Natural gas is supplied from state own company and most of the time its supply pressure itself found less than the refinery fuel gas system. Hence refinery fuel gas system start to decrease and hurt the operation of furnaces specially steam methane reformer. We are evaluating to supply natural gas from discharge of natural gas compressor to refinery fuel gas system. The discharge of compressor is at 32 bar and 135 'C. Refinery fuel gas system is at 4.0 bar and 65 'C. We can install a line from discharge of compressor to fuel gas system and pressure control valve to maintain its pressure. But we found temperature after throttling is very high that is 128 'C. kindly suggest can we used this scheme  or not. Is there any draw back of using hot fuel gas in burners. What other parameters we need to consider for this plant change. 

 

Natural gas compressor is three stage piston compressor. 1st stage discharge after cooler is at 4.3 bar and at 60'C. These process conditions are more favorable than the last stage discharge of compressor. I have attached the diagram of actual and proposed plan for this modification. Please give your expert opinion weather we take off-shoot from compressor interstate stream for some other purpose or not.

 

Best Regards

Muhammad Farhan

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[PingPong]

Insufficient info to answer your question.

 

It depends on the whole control system of the compressor, as well as whether the compressor has valve unloaders and/or clearance pockets, or speed control, or .......

 

According to your sketch the compressor has only one kickback so it seems that only the first stage suction pressure is controlled while the interstage pressures are resulting.

 

If the 2nd stage suction pressure is presently 4.3 barg then taking 20 % of that gas (to send it to fuel system) will result in a drop of that 4.3 barg as the 2nd compressor stage will still take the same volumetric flowrate. as it is a piston compressor. Surely it will then not be high enough to enter the fuel gas system at 4.0 barg. The 3rd stage suction pressure will then also turn out to be lower than presently for the same reason.

 

If you would take the fuel gas not from the 2nd but from the 3rd stage suction you will have enough supply pressure even though that pressure will then be lower than presently (as explained above).

 

In any case you need to verify whether the fuel gas draw affects the compressor control system, as well as verify whether the resulting higher differential pressure over 2nd and/or 3rd compressor stage is a problem for the rods, or other compressor parts.

[thorium90]

Why not just have another compressor to bring 840kg/hr of NG from 0.65Bar to 4.3Bar?

840kg/hr is about 20% the size of the other compressor. I would think that's large enough to warrant its own compressor.

Also, if you tie in to the 1st stage suction in this manner, you link the operation of the compressor to the operation of the SMR. That means if the compressor trips, your SMR also trips. That could be a new reliability issue you have not considered. You will also now link the two maintenance schedules together since you cant shutdown one with shutting down the other.

I think its wise to consider other aspects such as reliability and maintainability of the proposed setup. If the combined tripping of the compressor and SMR will cost more in revenue losses than a new small compressor, then its probably worthwhile just to buy a new compressor.

Edited by thorium90, 19 September 2019 - 08:07 AM.

[Muhammad farhan]

Dear All 

 

Thank you for your reply, I got it will not be good to take natural gas from 2nd stage suction, please consider the first option that is taking the natural gas from discharge of compressor. we can install PCV to throttle and pressure drop while i checked after throttling temperature is not drop to much and it will remain 128 'C. Please comment is there any negative impact of high fuel gas temperature on burner and combustion. I have told my engineering team to check metallurgy of fuel gas system for handling high fuel gas temperature. 

 

The reason for this modification with minimum installation is to save time, we are facing sever problem and adding some major equipment will take time. Please suggest can we install simple pressure regulator instead of PCV for throttling and controlling pressure of fuel gas system. 

[PhilippM]

What does the vendor of the burners say about this?

In one of my previous jobs we deliberately preheated the fuel gas for the burners of a gas turbine, but that had to be done in close cooperation with Siemens (the vendor of the turbine).

 

Also, if I remember correctly, the burners might have to be changed if the density of the gas is changed too much (which it might be due to the higher temperature)

[thorium90]

Without analyzing the metallurgy of the burner, my hunch is that there is a good risk that putting hot NG to the burner will have a negative impact on the burner tips.

Would a simple fan cooler be possible?

Get a bare pipe air cooled heat exchanger. Essentially it is a multitude of small pipes (you can make it 2 pass as well) and with some fans below these bare pipes to cool down the gas in the pipes. It will also serve to "chew" up some of the pressure drop and reduce the workload of the PCV.

If you have some finned pipes, that would reduce the size. But if you are short of time, bare pipes will also work, you just need more bare pipes.

Edited by thorium90, 20 September 2019 - 04:15 AM.