What is a slug?
Slug is a bulk quantity of either liquid or gas that flows through a pipeline, causing problems in the handling capacity of the production unit. Slug catcher are traditionally used in all plants to identify and eliminate slugs.
Most pipelines in petroleum and refinery industries handle both liquid and gas inflows. In many cases, both the liquid and the gas components are essential and have to be retained and extracted separately. The liquid and gas components flow together, and this is called a two-phase flow. Ideally, the pipe should transport both these components at the same rate. However, due to certain abnormalities, a large amount of just liquid or just gas flows through the pipe, leading to the formation of slugs. These slugs compromise the plant’s functioning and can contaminate the final product. They can also lead to equipment damage.
What Causes a Slug?
Three conditions lead to the formation of slugs.
- Terrain Slugging
When the pipeline follows uneven terrain, and there is a sudden elevation, liquid components can get accumulated in the low level. Over time, the amount of liquid in the low-level increases and reaches saturation. The pressure here increases and pushes out the fluids all at once, and this causes a slug. Terrain slugs cannot be prevented and happen periodically.
- Hydrodynamic Slugging
When the gas flowing through a pipeline flows at a much faster rate than the liquid, the gas ends up disturbing the liquid surface and forming ripples or waves. Due to the waves, the liquid starts rising up, causing a blockage to the gas outflow. As a result, slugs are formed. This very commonly happens when gas and liquid flows from an elevated surface to ground level. Hydrodynamic slugging is also common and happens periodically and cannot be prevented entirely.
- Pigging Slugs
Pigging is the practice of using inspection devices to check the insides of the pipeline division. These devices are called pigs, and they travel inside the pipes, checking for faults and pushing out sediments and other clogs without interrupting the pipeline activity. These pigs can lead to the formation of slugs. However, in most cases, the equipment handler expects a slug when these pigging slugs are used.
Slugs always don’t have to cause problems. In some cases, these slugs may grow smaller as they travel and disappear before reaching the outlet. In other cases, they may grow bigger and lead to bursts and severe damage to the pipeline and other equipment.
What is a Slug Catcher?
A slug catcher is a unit added to the outlet pipes to catch these slugs before entering the next stage. Usually, this is the first equipment the liquid and gas are sent to before being processed further. These catcher are enormous and can capture and hold back the largest slugs from the outlet pipes. They act as buffer devices, and the large volume of the slug is slowly sent out to the next processing unit so that the next equipment doesn’t have to bear the high pressure of the slug.
Usually, it has to be emptied after it captures a slug to be prepared to handle the next bulk of slug effectively. This equipment is a permanent fixture between the outlet pipes and the processing unit in many industries. In other cases, they can be bypassed when not needed and connected only when there is a need or a risk for slugs.
Types of Slug Catchers
There are five common types of slug catchers identified based on their design.
- Horizontal Vessel Type Slug Catcher
This is used when there is more liquid in the slug when compared to gas. The vessel uses an impingement baffle to direct gas to the outlet and lets the liquid collect at the bottom of the vessel, thanks to gravity. There is a separate outlet to collect the liquid.
2. Vertical Vessel Type Slug Catcher
This is a similar design but is vertical and is used when there is more gas component in the slug than liquid. This also uses the impingement baffle design to separate liquid and gas.
Both horizontal and vertical slug catcher are slightly expensive and large equipment.
3. Finger Type Slug Catcher
This is a very popular model of slug catcher and is also referred to as the harp slug catcher. Here, several individual pipes that resemble fingers are connected to one another, and they buffer the slug and reduce the velocity of flow. This is a relatively straightforward design, but the space occupied by the finger slug catcher may be expansive, and this can’t work in compact areas.
4. Parking Loop Type Slug Catcher
This is a combination of both the vessel type and the finger type slugs. The initial separation of the gas and liquid happens in the vessel, and the liquid volume flow is then reduced and stored in the finger-styled pipes. This model is also called the stored loop type slug catcher.
5. Hybrid Slug Catcher
A hybrid model of slug catcher also works in several industries. Here, the finger pipes are used to store, and the vessel type is used for separation.
Benefits of a Slug Catchers
Here are some of the top benefits of using a slug catcher.
- Manage fluctuating volumes of gas and liquids in refineries and petroleum industries
- Prevent damage to expensive equipment
- Separate liquid and gas and send them out to different processing units
- Protect processing units from getting affected by downstream input volumes
- Temporarily store liquids
- Bring down the rate of inflow of liquids and gas
How to Choose the Right Slug Catcher?
There are different factors that need to be analyzed to choose the right slug catcher. Some of the vital points are discussed here.
- Storage Capacity
The first thing to consider is the storage capacity. Usually, the storage capacity is calculated as barrel sizes. If your input quantity is going to be less than 1200 BBLs, then consider using the vessel type slug catcher. These don’t occupy a lot of space and can handle such input load effectively. If your input is going to be more than 1200 BBLs, then you may have to pick finger-type catcher. These are the best options to store huge slug volumes.
- Available Space
This is another significant factor to consider. A finger-type slug catcher may be very effective, but it occupies a lot of floor space. Trying to install a finger-type catcher in a plot size of less than 100 ft may not serve the right purpose. If you have limited space, pick up vessel models. The vertical vessel types occupy lesser space than the horizontal ones.
If you can afford a large floor space, then finger-type models work the best.
- Lead Time
Lead time is the time required for any manufacturing unit to design your slug catcher. Do remember that some manufacturers take even up to a year to customize these slug catcher for you. If you can make do with the standard sizes, then you can get your product delivered quickly. If you plan to install a slug catcher, consider the lead time before planning other operations.
- Cost of Shipping and Installation
There is no doubt that these slug catcher are expensive equipment. The larger the equipment, the more costly freight charges are going to be too. It will also take considerable money to install these slug catcher.
Slug Catcher Design Steps
Your manufacturer will design the perfect slug catcher for you based on intensive fieldwork and requirement collection. Different factors will be used to design these devices. Few important ones are mentioned below:
- Where the slug catcher will be used
- The average kind and volume of input that would be fed to the pipeline, which will determine the slug size
- The floor space available
- Available budget
Once all this information is collected, the manufacturer will do a feasibility study. This will help understand the kind of device that will work for your setup.
The manufacturer checks the rate and frequency of pigging, the ramp-up rate, and the liquid handling rate too.
Working of a Slug Catcher
The working of a slug catcher is quite straightforward. When extracted, natural gas is taken out from the ground at very high temperatures and pressures. As a result, the gas often comes with some liquid additives. In most cases, the liquid additives are useful too, and they need to be separated from the gas.
The mix of gas and liquid is sent from the ground to the inlet pipe. This flows at high speed towards the outlet.
Slugs form, at this point, somewhere inside the pipe. The slug is usually a mix of both natural gas and liquid. This gets pushed into the slug catcher.
The slug catcher now does multiple jobs. Firstly, it reduces the speed and bulk of the slug, thereby protecting the processing equipment. Secondly, it uses gravitational force to collect the liquid separately at the bottom of the tank, and the gas stays on the top. The gas and the liquid can now be sent to different processing units as required.
Some slug catcher may be compact, about 50 meters in length. Others can be really expansive, about 200-300 meters in length. However, even the smallest of these devices take considerable space in an industrial setting. Some plants may need multiple such slug catcher for the load they process.
It may be a very vital component in a refinery or an oil and natural gas plant. Without it, precious liquid and gas may go to waste, and the processing equipment may also have to handle load fluctuations and get damaged with time. If slugs are a regular problem in your industry, then the only way to handle them is to invest in slug catcher.
With over 50 years of experience in this industry, Multitex helps clients understand the need for slug catcher and design and install them precisely in the industrial setting. Our experts start with a careful analysis of the requirements and available resources and then get back to you with a working plan.
Get in touch with our team to know how exactly a slug catcher can improve your operations. Multitex is your best bet to find engineering, design, and manufacturing equipment and solutions at the most competitive prices.