I think that you should assess the risks of using such a turbine. What is the probability of the turbine failing in such a way that it cuts the fuel flow and shuts the plant down, versus the probability of any event from all possible events that can shut the plant down. I think you will find the ratio of probabilities is very small - and in any case you could just bypass the turbine with a conventional pressure reducer that operates if the turbine fails.

You can have a backup source of compressed gas to kick in if the natural gas pressure becomes unsatisfactory. For example, compressed air can be stored in cavern or depleted wells and this compressed air could be used as a back up to keep the expansion running instead of tripping.

Hope this helps answer your question.

Professor Yehia Khalil

Yale University

USA

There could be a danger of hydrate formation, because an expansion through a turbine causes a sharp cooling of the gas, if it is not completely dried.

Thak you for your answers.

Dear Mr. Khalil

The mass flow rate passes from this pressure reduction station is incredibly high (about 216000 nominal cubic meters per hour) and I think it is not possible to use any backup source for such system.

Usually turbo-expander is parallel with a throttling valve. In case of start up or joule-Thompson case, the valve will drop the pressure while the expander is out of service. In two cases, either expanding or throttling the gas, temperature will drop dramatically but not that hiring of an expander requires more fee and capital cost. How much is the pressure drop? Is using of  turbo expander instead of a valve justified? It's depending on differential pressure, amount of investment, flow of gas, maintenance cost, shut-down issues and many other parameters to be considered.

Dear Mr. Soave

To prevent the hydrate formation in this pressure reductions (caused by the sharp cooling of the gas) there are some heaters heating the gas before entering the expansion turbine (or throttle valve). It is expected the outlet gas to be about 25 C, so the inlet temperature is risen up to 105 C (based on the pressure differences).

ok, correct. But an expander causes a much greater cooling.

You could use HYSYS to calculate which should be the heating temperature in order to have 25°C after expansion.

I agree with Mr. Soave. It is unreasonable to heat up the gas then sending it to expander. In case of high flowrate and high differential pressure it is better to use gas drier.

These turbines are operating right now with mentioned operating conditions and we are using such a system in the power plant. This is not a design process. I just wanted to know how can decrease the risk of such systems. The management decided to turn the 2*8 MW expansion turbines down. I am looking for Reasonable grounds to keep the system in service.

Yes it is possible to use an expansion turbine to produce power. You should install a bypass for start up shutdown and upset conditions.  If you believe the reliability of the turbine will be low you should probably install a spare. Three 50% machines may also be a good way to spare.

  Also you should be careful to avoid vibration in this line you need very good valves to prevent vibration when there is a large pressure drop. 

It would be important for you to make a risk analysis, examining the benefits and the risks of this idea.

Using expansion turbine instead of throttle valve with the aim of power producing is a traditional idea. My suggestion is thermo-economic method. Risk of employing a turbine can be analyzed using thermo-economic method which describes that using a high quality turbine with the minimum technical problems has the economic justification or not.