The piping system is taken underground generally for the utility services like cooling water supply to various units and cooling water return to cooling tower for line sizes normally 18 inch NB and above, other water services with big pipeline sizes, big oil supply lines and various sewer systems in the process units of the chemical, petrochemical and refinery type of plants. The term “underground” applies to the piping – both buried or in trenches. The underground system consist of gravity flow drainage system carrying process waste, spillage, reclaimable hydrocarbons, sanitary and storm water, firewater and drinking water system.
Good engineering practice, local code / regulations, specific client requirements shall govern the design of the underground piping system in the plant.
The following are the common underground services in a chemical / petrochemical / refinery plants.
- Cooling water (line size normally ≥18″ NB).
- Fire Water.
- Contaminated Rain Water Sewer from process catchment area.(CRWS).
- Oily Water Sewer (OWS).
- Liquid Effluent to the Effluent Treatment Plant.
- Closed Blow Down system (CBD).
- Sanitary system.
- Storm Water.
- Equipment drainage to slop tank.
- Electrical cables.
- Instrument cables
Type of Underground System
Depending on the service and the material that each system handles there are different system defined in the Underground system.Various underground systems can be described in the following way both for Utility system and sewer system.
Cooling Water System (CWS & CWR)
This is generally a buried system with protective wrapping and coating or with cathodic protection or both.
Any valve for isolation of a part of the cooling water system shall be enclosed in a valve pit. The normal compacted earth cover shall be 1200 mm over the top of the pipeline. Theearth cover over pipe (back-fill) shall be compacted to at least 95% Proctor compaction Index to protect the pipe from aboveground loadings as per ASTM D-698.
Oily Water Sewer (OWS)
This system collects waste, drips and leaks from equipment and piping in areas that contain process equipment in non-corrosive services. The layout engineer should consult the process engineer to fully identify all such equipment and provide a drain hub at each item.
The piping engineer should locate the oily water drain hubs using the above ground piping studies, setting each invert elevation and routing the line with relevant components / fittings.
Oily water sewer shall collect oily waste / drain from pump, equipment through funnel points and shall run with separate headers and manholes in the units. The regular oil contamination areas shall also be segregated and discharge from these areas shall be collected in catch basins to be joined in oily water sewer. Oily water sewer shall consist of carbon steel sewer, funnel points, clean outs, RCC catch basins, RCC manholes, vent pipes, flame arrestor etc.
Contaminated Rainwater Sewer (CRWS)
The areas which are contaminated due to floor wash drains etc. inside unit boundaries shall be demarcated. Contaminated areas collected in catch basin shall be drained through CRWS while un-contaminated areas, normally at periphery of the units shall be drained through ditches covered with grating.
CRWS shall consist of underground carbon steel sewer with corrosion protection, funnel points, clean outs, RCC catch basins, RCC manholes, vent pipes, flame arrestor etc.
Open ditches of units should have a bypass either to the CRWS or to storm water, drains of offsite.
This system collects surface drainage from areas containing hydrocarbon – bearing equipment. This water must pass through a treatment facility before being discharged into an uncontaminated system or natural body of water e.g. river or a stream.
Closed Blow Down (CBD) sewer
This system picks up drains around boilers and steam drums and is run as a separate system preferably to the battery limit.
The system shall be designed as per P&ID and the effluent collected from equipment through funnel points and underground piping system shall be connected to the underground CBD drum.
Amine Blow Down (ABD) sewer
The amine blow down system (ABD) shall be designed as per P&ID.
The effluent shall be collected from equipment through above ground points into close funnels connected to underground system. The main header shall be connected to the underground Amine sump / drum.
Fire Water System
This system consists of a fire hydrant network around a process unit or equipment, with branches as required for hydrants or monitors to protect the unit in case of fire.
This is a close loop system starting from Firewater storage and pump to the specific location of hydrants and monitors. This is always kept under a predetermined working pressure level.
Potable Water System
This water is used for drinking, emergency eye washes and safety shower facilities.
Sanitary Sewer System
Sanitary sewer system collects waste from all toilet facilities provided in various plants and non-plant buildings and shall be discharged to WWTP (Waste Water Treatment Plant)
Acids, caustics, hydrocarbons, rainwater or other chemical waste shall not be discharged to this system.
This system is routed to normally to a septic tank.
Underground Electrical and Instrument ducts
In the beginning of a project, the decision to route the major electrical and instrument conduits – above ground in the piperack or buried below grade shall be taken.
In case underground route is selected, electrical and instrument engineers shall be consulted for the optimum layout of ducts by the plant layout engineer.
Where conduits enter the unit through a pull box and cables come above ground for routing upto the terminal points, the space shall be kept free of piping, equipment or associated maintenance access.