There’s vital information to keep in mind when it comes to compressor rod packing. Learn more below.
Reciprocating compressors used extensively within the natural gas industry generally leak during regular day-to-day operations. The areas known for frequent high leaks include:
- Valves
- Flanges
- Fittings located on compressors
However, the highest volume of natural gas leakage is linked with the piston rod packing systems.
According to research, over 51,000 reciprocating compressors operate within the natural gas industry in the United States. Each of these compressors functions optimally with at least 4 cylinders on average. This represents more than 200,000 piston rod packing systems in service.
These systems contribute methane emissions massively – more than 2.4 Bcf per annum – to the atmosphere. This makes it one of the largest sources of methane emissions at natural gas compressor stations worldwide.
Under normal circumstances, every packing system leaks. However, the amount of leakage depends significantly on the fitting as well as alignment of the packing components, cylinder pressure, and amount of wear and tear on the rod shaft and rings.
On average, a properly aligned and fitted brand-new packing system can lose anything from 11 to 12 scfh (standard cubic feet per hour). But as the system ages, the leak rate will increase accordingly, usually from wear on the piston rod and the packing rings.
The Need for Packing Systems
Packing systems are generally employed for maintaining tight seals around the piston rod. Their primary goal is to prevent gases compressed to incredibly high pressures within compressor cylinders from leaking while permitting the rod to move freely.
What You Need to Know About Compressor Rod Packing
Compressor rod packing is made up of a series of highly flexible rings. These flexible rings fit remarkably well around the shaft in order to create a tight-enough skill against leakage.
Each packing ring is lubricated with circulating oil in order to minimize wear, eliminate heat, and seal the unit. Water jacketing, air cooling, as well as circulating coolants within the packing box, are other methods employed.
A set of packing cups, one for every pair of rings, secure the packing rings. A surrounding spring holds the rings tightly against the shaft. The number of rings and cups may vary, depending significantly on the pressures of the compression chamber.
There’s a mechanism called the ‘nose gasket’ located at the end of the packing case. Its primary assignment is to prevent leaks around the packing cups.
Under perfect conditions, brand-new packing systems correctly installed on a well-aligned, smooth shaft are estimated to leak at least 11.5 scfh. If the leak rates are higher, it is attributed to 3 factors:
- Alignment of the packing parts
- Fit
- Wear
Leakage usually occurs from 4 distinct areas:
- Between the shaft and rings
- Around the packing case via the nose gasket
- Around the rings as a result of slight movements in the cup groove as the rod travels back and forth
- Between the packing cups typically mounted against one another, metal-to-metal
Leaking gases vents into the atmosphere via packing vents on the flange. However, proper monitoring can minimize leakage through a cost-effective schedule for replacing piston rods and packing rings.
New designs and ring materials for packing cases that will help reduce emissions are emerging soon.
Maintenance of Compressor Rod Packing Systems
Efficient monitoring and regular replacement of compressor rod packing systems can significantly minimize methane emissions to the atmosphere, thereby saving money.
Bronze-metallic packing rings wear out over time. These rings need to be replaced at least every 3 to 5 years. But then, when the packing deteriorates, leak rates increase astronomically. It will get to a level that necessitates a more frequent replacement of packing rings.
Replacing compressor rod packing more frequently can prolong the life of the compressor rod. When this happens, here are the benefits you will derive:
- Extended life span or service life of the compressor rods
- Minimized methane emissions
- Significant reduction of methane or natural gas emissions
Steps to Economic Packing/Piston Rod Replacement
Here are the steps to follow that help determine a cost-efficient replacement schedule:
- Monitor and Record Piston Rod Wear and Baseline Packing Leakage
The vent port on the packing case flange offers a medium for gas leaks to escape into the atmosphere. But gas can also flow from the gasket located at the end of the packing case or along the rod, easily avoiding the packing cup vent and entering the distance piece.
Therefore, measurements should encompass emissions from the distance piece and packing cup vent, where possible.
The packing vent system should be checked before initiating measurements. Measurements should also be undertaken right after installing new seals and rods. Leakage rates, as well as related operating conditions – e.g., lubrication, pressure, temperatures, etc. – should be monitored and recorded routinely after installing the rings and over the entire lifespan of the packing rings.
- Determine The Leak Reduction Expected by Comparing the Current Leak Rate to The Initial Leak Rate
The baseline emission measurement should be evaluated and compared to the current leak rate. This helps determine whether or not the current leak requires replacing the packing or rod.
For accurate analysis, the leak reduction expected (LRE) – which refers to the savings expected to occur after installing new equipment – is calculated. The calculation must be done separately for piston rods and packing seals.
When determining the leak reduction expected after installing new rings only, you should assume that leakages have increased all-round since ring wear is responsible for replacing the last set of rings.
But when determining the leak reduction expected for replacing the rings and piston rod, make use of the initial leakage measurement taken immediately after the last rod replacement on this particular compressor.
If historical data on individual compressors are unavailable, data from similar compressors functioning under similar operating conditions should be obtained and used for establishing initial leak (baseline) values.
Other steps that may be taken include:
- Assessing the costs of replacements
- Determining the economic replacement threshold
- Replacing all packing when cost-efficient
Conclusion
Reciprocating compressors operate almost like perpetual motion machines in the natural gas industry. As a result, methane emissions to the atmosphere occur in distinct areas, usually around fittings, valves, and flanges. But piston rod packing systems record the highest volume of gas losses.
Therefore, it is essential to ensure packing systems have tight seals which minimize natural gas emissions to the atmosphere. The suggestions made above would greatly help in this regard.