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
Types 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. The earth 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 up to the terminal points, the space shall be kept free of piping, equipment or associated maintenance access. While developing &preparing the underground piping layout there are different components and terms that would be used and referred in the drawings. Some of the common terms used are explained below: 1. Catch Basin This device is used to collect surface drainage with an outlet liquid seal and sediment trap. The maximum area coverage of a catch basin is approx 150 sqm. ie. 12m x 12m or15mx10m. The area covered by a catch basin should be of square configuration, as far as possible. The catch basin should be located in the middle of the area as far as possible. Slope of pavement 1 in 100, e.g. HPP (Ridge of catch basin area = 100.00) and LPP (at catch basin peripheral drain = 99.85) No catch basins or manholes should be located within 15m radius of heaters. No vent pipes should be located within 15m of the heaters. Area drainage around heater areas shall be done by pavement sloping towards open ditches. Each catch basin shall be connected to manhole and shall be provided with fire seal. Fig 1. Catch Basin Fig 2. Plot subdivided into drainage areas
2. Sealed Manholes Sealed manholes shall be provided at a. Unit battery limit and b. Junction of sewers and at change in size of main header. These devices are provided so that the unit area is isolated from any fire in offsite area/sewer. Within the unit area, sealed manholes in main headers should be provided in such locations so that each sub-unit within the unit is isolated from the other areas. In case, this demarcation is difficult, one sealed manhole for every 30m length of main sewer in the unit shall be provided. Sealed manhole with bent pipes seal type shall be used for carbon steel pipes up to size16″ NB and for greater than dia 16″ NB and for all diameters of RCC pipes double compartment type manholes should be used.
3. Invert elevation This term, usually associated with any underground line, refers to the elevation of the inside bottom of the line. Because of the wide range of materials used in the underground piping system / drainage system with varying wall thickness, it is the constant that is used to set the elevation on construction drawings. The starting invert level of CRWS shall be normally 750mm below HPP (High Point of Paving) The invert level at outlet point of CRWS and OWS shall be normally 1500mm below FGL.
4. Cleanout A cleanout is a piping connection in a sewer system that is located at grade level for inspections or for cleaning the system.
5. Vent Pipes Vent pipes shall be located along piperack columns or building columns and should be taken 2m above the building parapet or last layer of pipes on a piperack.
6. Valve Pit / Maintenance pit for flanges and instruments.
When the underground system needs valves for isolation and instruments for control, the normal practice is to enclose these valves and instruments in a RCC pit with cover. These valves and instruments in a pit can be operated as well as maintenance work can be done with ease.
Piping arrangement-Underground Under piping are generally arranged based on the location of the consumers and the also depends upon the depth for the soil surface. Some of the guidelines for routing of the underground piping are stated below: 1. The overall Plot Plan allocates the space for the major underground services in the beginning of the project. The cooling water supply from cooling water pump discharge to the various units as well as the cooling water return from the various units to the top of cooling tower is routed in a simple, straight orientation at a suitable depth avoiding any major road crossings. 2. The potable water system supplies to various units and a branch is taken to the emergency eyewash and safety shower station. 3. Fire water system protects each piece of equipment by providing water through hydrants, monitors or deluge spray systems. Each process unit will have its own underground firewater piping loop system. 4. A typical hydrant and monitor installations and a typical fire- monitor installed. 5. Normally, chemical process units will have multiple drain systems designed to collect all corrosive or toxic chemical waste as well as surface drainage around the equipment. Drain / sewer system in a plant can be categorized as: Uncontaminated storm water Contaminated storm water Oily water sewer Chemical and process sewer Sanitary sewer Blowdown system Uncontaminated storm water system generally collects all service water from equipment areas, access ways, roadways to equipment. This collection is done through area drains, catch basins, roof rain water down comers. Contaminated storm water system collects surface drainage from areas containing hydrocarbon processing equipment. This system water must pass through a treatment facility before being discharged into an uncontaminated system or natural body of water viz. river or a stream connected to a river. Oily water sewer system collects waste, drips, leaks from equipment and piping in non-corrosive process equipment area. The designer should identify all the specific drain points in consultation with the process engineer. Chemical and process sewer system recovers acid or chemical drains from equipment / piping as well as surface drainage by providing curbing and drain sump around such equipment. Sanitary sewer system collects raw waste from lavatories and is either connected to the municipal battery limit or routed to a septic tank. Blowdown system picks up drains around boilers and steam drums and is run as a separate system. It is permissible to connect the blowdown system to a sewer box in oily water sewer system downstream of drainage from a furnace.
6. Trench Piping Occasionally, drain piping or process piping should be run below grade but not buried. The top of the trench is covered with grating but could be covered with RCC slab depending on the traffic load estimated in the area. The width of trench should allow adequate clearance to valves and drains as required.
7. A typical sewer box or manhole and A typical Dyke area drain sump is installed. 8. A process area for the purpose underground drainage is subdivided into block areas with high point ridge and low point catch basins / pits. The low point catch pits are connected to manholes. The manholes are interconnected by sloping piping and led to the battery limit valve pit and finally discharged into the treatment pond.
9.Closed drain system is installed.
10.The underground electrical and instrument cables passing under a road or paved area is taken through ducts embedded in lean concrete.