13 replies to this topic

[kybele39]

Dear All,

I am about the design an indirect fired water bath heater. This will be a dew point heater. What is the most critical design point for the dew point heaters before the gas turbines? Pressure?Hydrate formation of the natural gas? etc?

I need some highlights for the design before I started to my calcs.

Thank you to all.

Kybele

Pressure is 650psig
Temperature of the natural gas is 50C to 104C
Flow rate is 100000lb/hr

Thanks again

[Zauberberg]

Kybele,

One should avoid feeding gas turbines with liquid fuel, and I presume this is the reason for installing turbine fuel gas heater. In some plants, I have seen low-temperature tripping the turbine (interlock) if the fuel gas temperature drops too low. The possibility of condensation in the fuel gas line should be eliminated. In other plant, we used to have a knockout drum upstream of the turbines, with high-high level trip, and there was no low-temperature trip.

As for the hydrates, it might be a concern as well but this is a rare case, at least in my experience. It depends on what is the actual source of fuel gas, i.e. the composition, water content, pressure and temperature. Having 104 degC outlet temperature will certainly eliminate any hydrate formation concerns.

Fuel gas pressure of ~44 barg will be reduced down to ~10 barg (depends on the turbine type)upon entering the turbine assembly, followed by temperature drop due to expansion, and preheating the fuel gas will prevent reaching excessively low temperatures which can cause condensation or formation of solid phase.

[ankur2061]

Kybele,

Fuel Gas for Gas Turbines needs to be conditioned not only for dew-point but also for removal of the tiniest of solid contents (micron level). Many operating companies including the ones we act as consultants for, require a complete fuel gas conditioning skid for the gas feed to the turbines.

The fuel gas conditioning skid that we recommend has the following equipment sequentially:

1. Knock-Out Vessel (bulk liquid removal)

2. Filter Separator (fine liquid droplet removal and retention of ≥99.9% solid particles above 5 micron). Normal configuration is one working plus one stand-by.

3. Water bath heater for keeping the fuel gas temperature well above the dew-point. One very respected client demands the temperature to be kept a minimum of 20°C above the dew-point. Normal configuration is one working plus one stand-by.

4. Pressure Reduction Station for reducing the pressure of fuel gas to the specified pressure as required at the Gas Turbine inlet. Normal configuration is one working plus one stand-by.

The fuel gas piping is heat traced downstream of the water bath temperature to maintain the fuel gas temperature.

Hope this helps.

Regards,
Ankur.

Edited by ankur2061, 19 August 2010 - 02:43 AM.

[Zauberberg]

Have a detailed look at the source of turbine fuel gas and you can easily identify what kind of pre-treatment is required, e.g:

a. Dry, lean gas (predominantly CH4) - only the KO drum; no possibility of condensation;
b. Wet, lean gas - the KO drum plus heater/tracing;
c. Dry, rich gas (contains C3+) - the KO drum plus heater (and/or heat tracing);
d. Wet, rich gas - same as above.

In the plant where I have been for the last 6 months, a big portion of turbine fuel gas comes from the inlet reception facilities (wet, rich gas, condensation very likely) so the fuel gas line is equipped with KO drum, heater, and electrical heat tracing (including the low temperature trip). In the previous plant, fuel gas was coming from the Methane HP compressor discharge and thus being very lean and completely water free, so only the KO drum was lined up in the system (high-high level trip).

Best regards,

[kybele39]

Dear Zauberberg and Ankur,

Thank you very much such a good explanations of a fuel gas conditioning system.

Our client will build the knock out drum, filter separator and the regulating station. They only required the heater portion. However I have not received any fuel gas composition. They stated the fluid as natural gas. And they want to regulate the gas fro 650 to 450psig.

I assume they will do this regulation after the heater portion. So I need to design heater outlet temperature above dew point temperature. Based on the 90%CH4, 5%C2H6 and 5%N2 gas mix and 650psig operating pressure and 100000lb/h flow rate; when I simulated the heater inlet conditions at 50F, the results shown cricodentherm pressure 757psia and cricodentherm temperature is -98F and the dewpoint temperature is-100F. ( I used BR&E promax simulation to find out the properties of the gas fluid.

So looking at the dew point temperature -100F is the fluid is almost close the liquid due to such close pressure (650psig) to 757psia. and temperature is already lower then 50F?

The simulation results are attached.

Regards,
Kybele

Attached Files

•  heat.pdf   40.68KB   65 downloads

Edited by kybele39, 19 August 2010 - 08:38 AM.

[Zauberberg]

Kybele,

By the definition, cricondentherm/cricondenbar are the maximum temperature/pressure at which two-phase fluid can exist. At any pressure above the Cricondenbar, and at any temperature above Cricondentherm, condensation (or liquid dropout) is not possible.

If you got the result that fuel gas Cricondentherm is -98 degF, it practically means that, above -98 degF there is no chance to encounter liquid dropout from the gas, no matter what pressure you apply to the gas. And in such case, the fuel gas heater would not be required - at least not from the liquid phase dropout perspective. It may be needed to prevent hydrate formation, though.

Without knowing exact composition of gas it is difficult to give further comments, so make sure to obtain that one from the client. That way we can also know for what purpose the heater element is required.

[kybele39]

Zauberberg,

With a more realistic fuel gas composition, the Cricondentherm temperature and dew point temperatures are around 8 degree F. I really did not understand why they need a heater. As you said also, I can only think before the gas turbine they want to make sure they do not create hydrate formation for the turbine.

Another think they did not give us the gas composition but they gave us a minimum temperature span where the heater should heat the gas to outlet temperature. THat span is 62F minimum.

So, with a 8degree F dewpoint temperature, I heat the 50degree F fuel gas to 112degree F at 650psig pressure. Then I regulate the fuel gas to 430psig. and the gas temperature drop to 99degree F with JT effect.

So I guess I am in good condition as the heater design.

I appreciate your inputs.

Kybele

[Zauberberg]

If you can get the gas composition, everything would be quite easy to evaluate. Or at least to obtain phase envelope of the turbine fuel gas. It's difficult to understand that someone wants from you to design a piece of equipment, but he doesn't supply you with all necessary data. It's like shooting in the dark.

However, with such a huge margin (112 degF) you should really be away from any liquid/water dropout, and certainly away from hydrate formation (hydrates form very rarely above 70-75 degF). As a rule of thumb, for each 2 bar (29.4 psi) pressure drop across the valve, you get 1 degC temperature decrease (or delta T = 1.8 degF) of the fluid. Your results are somewhere in the same ball park, and with a 98 degF fuel gas entering the turbine skid I would say you are very much on the safe side.

Is the gas saturated with water, and can you at least get the gas composition (rich gas) for the worst case scenario?

[Erwin APRIANDI]

dear kybele39,

You have to gain the required fuel spec from the gas turbine vendor, since different
manufacture has different spec and sometimes the different is quite drastic

so before you design the fueal gas superheater system, better you get the spec first
so that you don't have to redesign the system later

I ve done this kind of system 3 times, and with different vendors

[Zauberberg]

The composition of fuel gas doesn't really come into play, as far as the turbine performance is concerned - unless we are speaking about drastic differences in heating values of two fuel gas streams. This is to be expected since turbine combustors operate with usually more than 250% excess air.

For example, consider Frame 5 gas turbine combustion process with 280% excess air and the combustion mixture temperature of 335 degC. Operating with fuel gas consisting of 50% Nitrogen and 50% Methane (quite low LHV) will result in 965 degC adiabatic flame temperature. Having fuel gas with 80% Propane and 20% Methane (quite high LHV) results in 993 degC adiabatic flame temperature - having all other things equal (inlet temperatures, flow, air flow). Now, the 30 degC difference is not insignificant, but this is the biggest span you will see when operating with extremely low and extremely high heating value gas. In practice, this difference rarely exceeds 5-8 degC.

Gas composition is important in this case to assess hydrate formation and the potential for liquid dropout.

[Erwin APRIANDI]

Dear kybele39 and Zauberberg

The fuel spec that I refer is not regarding to the composition of the fuel gas

But it is mostly the fuel spec will be the required difference between the fuel gas inlet temperature and the HC dew point, Rule of thumb it is mostly 50 deg F

And based on the turbine type, they'll recommend the minimum pressure required for the turbine

[kybele39]

Dear All,


First I thank you very much for your inputs to my topic. This is really benefits to all I suppose who reads this forum.It helped me to understand the heater run process.

Since I do not have fuel composition and I assumed it and calculate the DewPT as 8degree Fahrenheit. However, when I look at the inquiry again, the client states the dewpoint temperature as 41F(5C) and they gave the span temperature as minimum 60F. They have also two differetn operating temperature 650 and 700psig. The turbine needs 450psig operating pressure for this reason they need to regulate the fuel gas to 450psig from 650 or 700psig.

So in this case, I should not need the fuel gas composition since they gave me 60F span temperature. I the designed heater will heat the fuel gas from 50F to 110F.at 650 psig. then this will be feed to compressor system for gas pressure regulating. At the 450psig, the gas temperature drops to 100F. At 700psig operating pressure the gas temperature comes down to 96F after the pressure regulating. So looking at the 41F dewpoint temperature (clients recommends) I should be ok with my design.

If all these right, the heater needs to be operate for the 104F gas inlet temperature, too beside the 50F inlet temp. So when I run the 104 degree F inlet gas temperature with the designed heater, should I add the same span temperature (60F) or should I rate the heater for same outlet temperature as 110F.

Let me rephrase it, If I designed a heater that will heat the fuel gas from 50 to 110 (50+span (60)=110) degree F and provide ~10MMbtu/hr duty.
Should the same heater deliver to same heat duty for the 104 degree F. inlet and (104+(span,60)=164) degree F. Or this heater should work for the same outlet temperature with the different inlet temperatures.

If the heater runs in different outlet temperatures that time heater design will be really oversized. So, how this heater should be designed? is it for the same outlet temp or different outlet temp?
Is the turbine can work for this two differtn temperature 110 and 164F?

Thanks
Kybele.

[Zauberberg]

Kybele,

Let's go back to the basic question: what is the purpose of heater? To ensure that no liquid (or hydrate) can form anywhere in the system, particularly downstream of the pressure regulating station. For doing so, you consider 62 degF temperature span (or gas temperature increase) considering the worst case scenario, i.e. when the fuel gas has the lowest inlet temperature of 50 degF. Having 62 degF temperature rise across the heater, you provide a robust solution in combating against liquid/hydrate formation. In other words, you want gas temperature of 112 degF upstream of the pressure regulator regardless of what was the gas temperature at the heater inlet.

Now, whenever the gas is hotter than 50 degF, required heat duty of the heater will decrease, so as to maintain the same gas outlet temperature. By designing the heater for 62 degF gas temperature rise, you have already considered the worst case = the highest heater duty requirement. If there would be a fuel gas stream available at let's say 80 degF, you would need to heat it less than you have to do it in the design case, since only 32 degF temperature increase would be required.

In other words, the heater designed for 50+62=112 degF case will cover all other cases as well, when the fuel gas inlet temperature is higher than 50 degF.

[kybele39]

Dear Zauberberg,

Thank you very much. I thought the same thing but I was not sure. Thank you again.

Best Regards,
Kybele