Efficiency of Natural Gas Transport through Pipelines 3

[jsi1612]

Hello, I am an electrical engineer employed by an electric utility. I was wondering how much energy it takes to transport natural gas through a typical size gas pipeline used in interstate transport of natural gas. Assume no gas is leaked. It is my understanding in gas transport, the energy used(losses)to transport gas is consumed by compressor stations.

Following is an example for a high voltage transmission line. I would like someone in the gas industry to provide their own example for gas transport.

Here's my example from the electrical side calculating efficiency of energy transport. These are actual.

Source: 345 kV high voltage bus (industry standard high voltage - electrical energy has been stepped-up)
Destination: 345 kV h.v. bus
Distance between bus: 100 miles
Conductor: parallel 795 kcmil aluminum conductors (industry standard conductor size)
Rate of Energy Transported: 400 megawatts
Answer(percent power lost in transmission due to resistance in conductor):
1.82%/100 miles.
Meaning, for every unit of energy put in at the source bus, 98.18% is available at the destination bus.

 

[1969grad]

As gas flows through a long pipeline the friction of the gas on the pipe wall causes the pressure to decrease. You can best understand the relationship if you look at Bernoulli's Theorem in any basic fluid flow book. Assume no elevation change (elevation head) and that the velocity change is very small or insignificant (usual case in a gas transmission line) and you will see that the change in static pressure corresponds to the friction loss.

A compressor station then boosts the pressure back up.

FYI, most cross country gas transmission pipelines operate at 700-1000 psig. The 700 psig will be the compressor suction pressure and the 1000 psig will be the compressor discharge pressure. The friction in the following pipeline segment will then cause a pressure drop from 1000 psig to 700 psig at which location another compressor station is installed.

 

[jsummerfield]

As 1969grad points out, the pipeline friction causes the losses. The compressor stations boost the pressure.

If you think of the voltage as 1000 psig, think of the current in millions of standard cubic foot of gas per day. Instead of M for mega, think of the lower case m as a Roman numeral prefix. In the gas industry the flow of 100 million standard cubic foot per day would be written as 100 mmscfd.

The small end of gathering station compressors is under 500 hp and a big distribution compressor is around 35000 hp using the same GE gas-turbine as you may have driving the generator in your peaking machines.

Natural gas has a specific gravity around 0.6. Gas plants remove the propane and other liquid components for use in the petrochemical sector. Pipeline gas is mostly methane with just enough ethane and heavier components to maintain the heating value over 1050 BTU/Hr.

John

 

[jsi1612]

1969grad and jsummerfield
Thank you for your replies

As a follow-up I consulted a midwest based natural gas pipeline company. A helpful gentleman provided this example of natural gas transmission.

Length of pipe: 100 miles
Inside diameter: 29 inches
Pressure: 1000 psi starting, once dropping to 750 psi a compressor station is needed, therefore compressor station at 50 mile mark.
Compressor station size: 11,000 horsepower
Pipeline Throughput: 747 mmscf/day

Using this info and the following relations and assumptions:
1 horsepower = 0.746 kilowatt
then 11,000 horsepower motor = 8,206 kW

Heat Rate of natural gas fired Combustion Turbine to run compressor station (typical):11,000 Btu/kWH
Natural Gas heat content: 1.029 MBtu/mscf

Then
8,206 kW * (1/1.029 mscf/MBtu)* (11,000 Btu/kWH) * (24 hr/day) =
2.105 mmscf/day

Therefore the losses in transmission from compressor are:
2.105/747 = 0.3% per 100 miles
If one assumes it really requires 2 compressor stations per 100 mile it would be 0.6 % I would request anyone in the gas business to verify, please.

Also the 747 mmscf/day is enough natural gas (rate of flow) to continously operate, approx., a 2,900 MW power plant. This is a lot of power, enough to meet the electrical energy reqirements of a city, say, with population of 0.5 - 0.75 million.

Also this shows me I must increase my transmission voltage if I want to match the efficiency of transporting energy via natural gas. An expensive proposition.

 

[jsi1612]

One other piece of info from a macro perspective:

I went to a natural gas company's website and got this info from their company fact sheet.

Their total compressor station horsepower: 840,000 hp
Their total system design deliverability: 4,500 mmscf/day

Again assuming a natural gas combustion turbine energy conversion of 11,000 Btu/kWH (for compressor stations) and natural gas heat content of 1.029 MBtu/mscf,

Then compressor stations would consume:
840,000 hp * (0.746 kW/hp) * (11,000 Btu/kWH) * (1/1.029 mscf/MBtu) * 24 h/day = 160.77 mmscf/day

Total system efficiency of delivery at system design deliverability:
160.77/4,500 mmscf/day/(&quot = 3.5%

 

[ColCol]

Hey jsi,

I'm a gas pipeline guy. 0.5% compressor fuel (called shrinkage in gas accounting) per 100 miles is not far off the mark. The 3.5% number is a bit high, but who knows what special circumstances were in that number.

You could get results from FERC form 2 submittals from the pipeline companies, if you wanted to make a real study of this question, but I have had good luck using 2% for a modern 500 mile long haul pipeline. Modern means they are using gas turbines for horsepower, which are much less efficient than the old gas engines. If they are using electric horsepower, the accounting gets messy since it is no longer shrinkage, but electric cost buried in the transportation rate.

As an alternative analysis, you may want to do all your calcs in dollars. For instance, calculate the cost to transport the gas from say, Henry hub in TX to say, Washington, DC. Then express that as a percentage of the value of the gas delivered, using large power plant sales rates (not consumer house rates, those are too high). Do the same type of dollar calcs for your electric (AC I assume?) transmission line and compare. Then do a DC link analysis (look up the Empire Connection on the web) to show your boss you are thinking outside the box!

 

[shanghaigas]

Hi,

May I contribute a rule of thumb for your reference. For large natural gas transmission pipeline built in plain terrain and without extreme climate circumstance, the compressor driven by modern gas-fired turbine normally consume 1% of its throughput gas every 400km. This is summarised from quite many projects worldwide.

The compressor capacity and the interval between each station may vary in each project even in the different phases of same project as the load profile changes. Be careful to calculate the efficiency (or energy cost) of the pipeline directly from the information of installed capacity because there are always backup and specific combination of compressors with different sizes to satisfy the operation efficiency in changing flow. The total installed capacity can be 150-200% of the actual demand. Furthermore, energy consumption, either electricity or gas, takes a great portion in the opex but not whole part. For example, the annual maintaineace cost of the compressor can equal to 3-6% of its capex figure.

Hope it will be helpful to your problem.