6 replies to this topic

[arkae]

hi everyone,

 

I am designing dew point heater(electric heater type). The process flow of the natural gas sytem is as follows:

 

KO drum - Natural Gas Metering station - Dew point heater - pressure reducing station - absolute seperator - performance heater - scrubber - Gas Turbine

 

The interface point from our client is after Natural Gas Metering Station.

 

The interface Natural Gas conditions are:

 

Pressure : 29 bar A to 73 bar A

Temperature : 10 to 45 deg C

flow: 31220 kg/hr

 

The fuel gas composition is as follows:

 

methane(90.066%), ethane(4.566%),Propane(0.236%), n-Butane(0.018%),Isobutane(0.018%),n-Pentane(0.007%),Isopentane(0.007%),n-Hexane(0.002%),Nitrogen(5.025%),Carbon Dioxide(0.055%),HHV (kJ/kg)-50,629,LHV (kJ/kg)-45,694,Specific Gravity-0.6

The above composition is volume basis.

 

The pressure required at the inlet of Gas Turbine(as per gas turbine vendor) is 33 bar A.

 

We have to size only the Dew Point heater.

 

The temperature after performance heater is fixed. It is 152 deg.C, during normal operation.

 

 Gas Turbine vendor states that " the natural gas temperature shall be atleast 28 deg.C above dew point temperature after performance heater, to avoid condensate formation".

 

During normal operation, there is no problem. As the temperature after performance heater is 152 degC, the Gas turbine vendor requirement is satisfied.

 

But during startup, the performance heater will be bypassed. Hence during startup dewpoint heater is required to fulfill the requirement of Gas Turbine vendor(i.e gas temperature atleast 28 degC above the dew point)

 

Considering maximum Natural Gas pressure(73 bar A) at the inerface, the pressure is to be reduced to 33 bar A.(as required by the Gas turbine vendor) by pressure reducing station.

For every 1 bar A decrease in pressure, the temperature decreases by 0.6 deg.C(as given by Gas Turbine vendor), hence the temperature decreases by 24 deg.C.

Considering dew point of 0 deg.c(not known only assumption), the temperature at the outlet of dew point heater shall be 52 degC(0 degC + 28 deg C+ 24 degC). This is only during startup when performance heater is bypassed.

 

During normal operation, dew point heater can be bypassed.

 

My questions are

 

1. How can we calculate the dew point of natural gas. Iam using hysis but donot know how to calculate dew point. Or is there another way of calculating dewpoint.

 

2.Is there any problem if we place dewpoint heater after absolute seperator, please tell the advantages
 of placing dew point after absolute seperator.

 

 

 

Thanks in advance.

 

[Art Montemayor]

Arkae:

Judging from your explanation, there are somethings that are not fully explained or understood by you.

Before making any decision on installing a heater before a turbine gas meter, you better be very sure about what your gas dew point is. First, WHAT DEW POINT are you referring to?  There are two (2) basic dew points involved in compressed natural gas:

1. The dew point of the natural gas with respect to its WATER CONTENT; and
2. The dew point of the natural gas with respect to its HYDROCARBON LIQUIDS CONTENT.

The concern of a turbine meter manufacturer is that you have liquid water or liquid hydrocarbons present in your natural gas at the flowing temperature and pressure.  The turbine simply cannot tolerate liquids within the gas flow in order to make an accurate measurement as well as the erosion that takes place together with the loss of lubricity for the turbine’s bearings.  All these effects are detrimental to the turbine meter.

 

Therefore, the manufacturer is insisting that the gas be dry and devoid of liquids.  The measurement employed to determine when water moisture or liquid hydrocarbons are present is the DEW POINTS for each of these components AT THE FLOWING CONDITIONS.  If your gas is devoid of these potential liquids at the flowing conditions, you should have no worries or need for a heater upstream of the meter.  All the heater does is increase the temperature and thus reduce the possibility of liquid formation.

 

Note that I state that the dew point should be identified as that “AT THE FLOWING CONDITIONS”.  The Dew Point is normally taken (and referred to) at atmospheric conditions.  Therefore, the measured dew point often is the temperature where the first droplets of liquid (water or hydrocarbon) are formed AT ATMPOSPHERIC PRESSURE.  Since this is often the case, the recorded dew point has to be converted to that at flowing conditions (flowing pressure).

 

You are very confused when you ask how the dew point is calculated with a simulation program like HySys.  No simulation program (or equation) can calculate the dew point. You have to measure it with an instrument called a Dew Point Meter.  The meter has the ability to measure the temperature at which the first formation of liquids is detected.

First find out what your dew point readings are for your flow stream and then determine if the dew point is so high that you have to use a heater.  I would not subject a turbine meter to 152 ^oC.  I am afraid this would damage the instruments bearings.

From your analysis I suspect you would have no problem using the meter at the flowing conditions of  73 bara and 45 ^oC.  But I would have this confirmed with the manufacturer together with the gas analysis.

 

[arkae]

Art Montemayor

 

thank you very much for your reply.

 

By dew point i mean hydrocarbon dew point. Since the fuel composition has no water component i assume that water dew point has little significane. Please correct me if i am wrong.

[Pan Nata]

Arkae,

 

after review your process scheme, i think you have to put much concern about "pressure reducing station", because i see the main reason to install dew point heater is to avoid condensation at outlet of pressure reducing station. You have to investigate hydrocarbon dew point temperature after pressure reducing station first then you can make good engineering judgement to sizing your dew point heater.

 

I dont know what a kind of handbook you used during your college but i suggest you to read Gas Processing and Conditioning Vol.1 (Campbell) chapter 5 for better understanding about dew point before you use any software or make hand calculation to determine it.

 

good luck for you.

 

Rgds,

 

Wild

[thorium90]

Hi arkae,

 

The following might be helpful.

http://www.slideshar...ium-using-hysys

 

Your composition has about 90% methane and 5% N2 and very little heavier fractions. I did a quick plot of a likely PT curve.

Attached Files

•  NG BP DP Chart using PR.jpg   95.51KB   24 downloads

Edited by thorium90, 15 February 2013 - 04:38 AM.

[arkae]

hi thorium 90,

thankyou very much for your reply.

From your PT curve, i understand that at a pressure of 30 bar and vapor fraction 1,
the dew point for this mixture is -66 deg.c.

Even i tried with hysis using peng robinson equation. And i found out the dew point
at 30 bar is approx. 64 deg.C.


As per Gas Turbine manufacturer requirement, the degree of superheat shall be atleast 28 deg C above
the dew point temperature.
In this case i think there is no need of using electric heater to fulfill Gas turbine manufacturer's requirement,
as the dew point is high.

The above dew point is hydrocarbon dew point. since water vapor is not present in the mixture, i think
there is no need to consider it. please correct me if iam wrong.

[thorium90]

The thing with simulators is that the vapor fraction doesnt distinguish water or hydrocarbon dew point. If it says 1, it just means it is all vapor. Dont go too cold though, at certain temperatures CO2 comes as a solid.