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Underground gravity piping

In this blog I will review underground gravity pipes – oily water, potentially oily water and storm water. Please do not get confused with terminology since other plants might have other terms for the same services. For example some plants will use term contaminated rainwater sewer for potentially oily water.
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Oily water
Oily water comes from equipment and it is contaminated water with oil. Good example is pump. When pumps need to be drain, water must be collected into one system and then run to the oil separators or water treatment plant. Always refers to PDF, since each plant has its own waste water management system. In the Fig.1 , oily water is represented by blue colour. Manhole (MH2) must be close to battery limit and must be water sealed. Equipment drain must also be water sealed with “P’ traps. Typical equipment drain is shown on the figure below.
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Cleanouts (CLH) are represented by magenta colour. Cleanout is required for pipe cleaning and most common mistake is omitting it. Cleanot is required when underground pipe change direction for 90 degree. Please see the figure below.
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Typical cleanout detail is shown on the figure below.
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Typical oily water sewer is shown on the figure below.
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Pipe specification for oily water service is yellow jacketed (or protective wrapping) steel pipe A53. Engineering calculation is not required, and we normally use 4″ pipe with 1% slope.
Potentially oily water
This water is clean water that comes from rain, but it can be contaminated under certain circumstances such as equipment failure, fire fighting, etc. Potentially oily water is rain water that will be collecting off the slab into the catch basins and then run to water treatment plant. Potentially oily water service is shown in red colour. Manhole must be water sealed and installed close to battery limit. If catch basin (CB1) is connected to manhole (MN1) than catch basin does not has to be water sealed. If connecting catch basing to the manhole is not possible, solution might be to connect the catch basin directly to the main pipeline. If that is the case, than the catch basin (CB2) must be water sealed. Typical detail of the catch basin (CB1) is shown on the figure below.
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Typical detail for the water sealed catch basin (CB2) is show on the figure below.
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Pipe specification for oily water service is yellow jacketed (or protective wrapping) steel pipe A53. Usually, 8″ pipe will be satisfactory, installed at 1% slope. One catch basin can serve maximum 450 sq. meters of slab (drainage area). Pipe size, drainage area and slope must be determined by drainage analysis. Engineering calculation is required.
Storm water
This type of water is clean water that comes from rain and it is represented by black colour. Rain water out of paved (slab) areas will be collected into ditch and than thru the manhole (MH3) and the underground pipe (cyan colour) will be run off site. Normally, for this type of service 36″ concrete pipe is used. Pipe size and slope will be determined by rainfall runoff analysis. Engineering calculation is required. Manholes are required on each 100 or 150 meters, depending of company standard. These manholes must be water sealed to prevent fire spreading thru underground system on other units.
Bedding
Bedding and soil classification is complex subject and if you are interested in learning, there are many good sources of information on internet. Typical bedding for steel pipe is shown on the figure below. For choosing right type of bedding always refers to company and client standards and relevant state code. Always take 95% to 98% (standard proctor) soil compaction, since 100% soil compaction is hard to achieve.
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All underground pipes must be installed bellow frost line. In the cold environment like Canada, the frost line might be ranging from 4′ in Ontario to 8′ in Saskatchewan, and installing underground pipe bellow frost line might not be possible in all situations. If that is the case, underground pipe can be installed above frost line but styrofoam insulation must be used. Please see the figure above. Insulation thickness must be calculated conducting heat transfer analysis. Usually, 2″ thickness will satisfy heat requirements. Model of heat transfer is shown on the figure below.
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Figure 1 is just illustration to show basic design principles. In real life, underground piping system can be very complex and below is the snapshot of underground system taken from one of my projects that I was working on.
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Note that some older plants might have a clay pipe for oily and potentially oily service. This type of pipe requires more work on bedding and they can easy brake during construction. In the case that you need to connect different type of pipes, dresser coupling is good solution.
Please see the figure below.
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Lift station
Since underground pipe is sloped and can be very long, we need to provide lift station to avoid going too deep. The lift station with two pumps is shown in the figure below.
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Normally, one pump is installed, but if water level can vary due to process requirements, it might be economical to install two pumps. One pump is for normal operation (in the case of rain) and another pump will be started when water level is higher. This is applicable to all underground piping systems.
When you work on gravity pipes in refineries, always refer to design specification (criteria) or customer standard since there are many rules that must be met, such as fire and explosion hazard, minimum sewer size, etc. Last, but not the least. Start working on underground piping system as soon as equipment is modeled and positioned. It is easy to change layout or move equipment to accommodate underground piping when you are in study phase or early development stage. It is going to be problem if your model is in 60% development phase and you realize that layout must be changed to accommodate underground piping. Keep in mind that besides gravity lines you will have cooling water supply and return, firewater, and your underground piping system will be very complicated. A good source of information related to underground piping can be found on Piping- engineering site Underground piping part 1 and Underground piping part 2.