What You Should Know About Compressor Blowdown
Compressors are machines across a wide range of industries for several processes. They work for several hours daily, while others work non-stop for days.
Since they work long hours, compressors need to be taken off-line at regular intervals for maintenance, emergency shutdown testing, and operational stand-by. However, something else occurs when compressors are taken off-line: methane is inadvertently released into the atmosphere via several sources. This can be catastrophic in the long run unless adequate steps are taken.
What is Referred to as ‘Compressor Blowdown’?
When operators shut down compressor units, it is not uncommon for high-pressure gas to still be inside the compressors. Therefore, the operators usually vent the high-pressure gas within the compressor as well as the related piping between isolation valves to a flare or the atmosphere. If the high-pressure gases are vented into the atmosphere, the process is referred to as ‘blowdown.’
However, the danger is not limited to the blowdown that occurs as a result of shutting down compressor units. Besides blowdown emissions, depressurized systems usually continue leaking gas from improperly sealed or faulty unit isolation valves.
Compressor Blowdown at Natural Gas Pipelines
About 1,650 compressor stations are situated within the United States transmission sector. Each station contains up to 9,000 fully functional compressors.
But then, approximately 50 billion cubic feet of Bcf is lost from compressor fugitives per annum. This doesn’t include the usually sizable lube oil that is also discharged.
Take note that these discharges are transmission line compressor emissions. An extra 40,000 compressors are used extensively in oil and gas production, and the processing phase often precedes the storage/transmission state.
Compressor Blowdown: The Technological Background
Before discussing how to alleviate compressor blowdown, let’s look at what happens behind the scenes.
As mentioned earlier, all compressors used within the natural gas system are cycled on-line and off-line in order to meet the ever-fluctuating demand of consumers for gas. In addition, emergency and maintenance shutdowns are usually taken off-line for proper analysis and regular maintenance checks or repairs.
The standard or usual practice also includes venting or blowing down all high-pressure gases left in the compressors when taken off-line. Although compressors can be depressurized, it doesn’t prevent leakages, especially from the unit isolation valves. According to rough estimates, these valves leak at 1.4 Mcf per hour on average.
A fully pressurized compressor still leaks methane from the compressor rod packings as well as the closed blowdown valve. However, the leakage rate from such compressors is somewhat smaller than a depressurized system, i.e., 0.45 Mcf per hour.
The operating mode of a compressor significantly determines the number of times it is taken off-line for emergency or maintenance purposes. For instance, base load compressors function almost all the time and are only taken off-line very few times each year.
On average, the downtime for compressors designated as the base load is 500 hours per annum. Some compressors function only for peak load service and only come on-line when there is a considerable increase in gas demand, and extra pipeline volumes are required.
These units eventually shut down or drop off the system as soon as market demand decreases. Peak load compressors operate for up to 4,000 hours, which is typically less than 50 percent of the year. However, they may cycle on-line and off-line for up to 40 times per annum.
Largest Source of Methane Emissions
The most significant source of methane emissions linked with taking compressors off-line is depressurizing the system by venting the remaining gas within the piping connected with the compressor and the compressor itself.
The volume of gases released during compressor blowdowns depends on several factors, which include:
- The pipeline pressure
- The overall size of the compressor
- The pipe volume is contained between the unit isolation valves.
On average, a single compressor blowdown releases about 15,000 standard cubic feet of gas into the atmosphere.
Bear in mind that every option discussed thus far is about a compressor blowdown that occurs when the compressor is off-line. The primary differences between maintaining a depressurized compressor as well as blowing it down on shutdown and the presented options are the volume of the blowdown and its timing.
Another source of methane emissions from depressurized, off-line compressors is the unit isolation valves. Large unit valves are often employed to isolate the compressor from the pipeline. Unfortunately, they are prone to leaking considerable amounts of methane into the atmosphere.
Unit isolation valves usually come with an acceptable range of leakages specified via design tolerances for these valves. Therefore, they are often maintained periodically in order to minimize leakage. However, accessing these valves all the time can be pretty challenging. This limited accessibility often results in increased leakage of methane in-between scheduled maintenance. The rate of a typical leak for unit isolation valves is 1.4 Mcf/hour.
How to Reduce Methane Emissions or Compressor Blowdown
To minimize methane emissions from compressors taken off-line due to emergency shutdowns or maintenance, changes in operating procedures and maintenance must be implemented.
Here are a few recommended practices from experts:
- Install A Static Seal On The Compressor Rods While Keeping The Compressor At Pipeline Pressure
Installing a static seal on the pressure rods makes it possible to do away with rod packing leaks during compressor shutdown periods while the machine is still pressurized.
A static seal is installed on every rod shaft beyond the conventional packing. An automatic controller readily activates as soon as the compressor shuts down in order to wedge a gas-tight seal around the shaft. The controller deactivates the seal as soon as the compressor comes on-line, i.e., on startup.
By installing the static seal, leakage only occurs from the closed blowdown valve at approximately 0.15 Mcf/hour when the system is at high pressure. The new leakage rate represents a reduction of up to 89 percent of the emissions that would typically have occurred if the compressor remained off-line and depressurized.
- Maintain The Compressor At Fuel Gas Pressure And Connect It To The Fuel Gas System
Connecting the flare lines or blowdown vent to the fuel gas system allows the purged gas from a compressor taken off-line to be routed directly to a useful outlet.
The pressure of a compressor taken off-line matches that of the fuel line gas pressure – typically from 100 to 150 pounds per square inch or PSI. At lower pressure, total leakage from the entire compressor system drastically reduces by over 90 percent, especially compared to the leakage that may occur via the unit isolation valve if the compressor were depressurized off-line to about 0.125 Mcf per hour from the compressor rod packing.
Leakage originating from the unit isolation valves into the compressor travels to feed the fuel system through the vent connection instead of flaring within the fully depressurized system or venting to the atmosphere.
- Install An Ejector
An ejector is a venture – i.e., a tube with a constriction used to control fluid flow – that uses high-pressure gas as the motive fluid to draw suction right into a lower-pressure gas source that discharges into an intermediate-pressure gas stream.
Technicians can install the ejector on vent connections between the suction and discharge of a compressor which generates the necessary pressure differential or up and downstream of a partly closed valve.
The motive gas and the captured gas will then be routed to the fuel gas system or compressor suction.
Compressors need to be shut down and taken off-line from time to time for operational, stand-by, or maintenance. In each instance, methane is emitted into the atmosphere from several sources. Locating these sources and minimizing the emissions helps preserve the environment and save money in the long run.
Implementing the suggestions above helps minimize compressor blowdown significantly.
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