Underground Piping (U/G) Piping – Part 2
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 sketch Fig 1 illustrates a typical catch basin feature and Fig 2 illustrates a typicallocation of catch basins.
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.
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 upto 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 ofPaving)
The invert level at outlet point of CRWS and OWS shall be normally 1500mm below FGL.
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.
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.
A typical cooling water and potable water system is illustrated in sketch Fig 3. Cooling water cross over piping is illustrated in sketch Fig 4.
Cooling water lines to heat exchangers are typically illustrated in sketch Fig 5.
Cooling water lines to pumps for various cooling requirements is illustrated in sketchFig 6.
2. The potable water system supplies to various units and a branch is taken to the emergency eyewash and safety shower station as illustrated in sketch Fig 7.
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 are illustrated in sketch Fig 9 and a typical fire- monitor is shown in sketch Fig 10.
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 downcomers.
- 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.
- Sketch UGP19 depicts a typical process drains – closed system.
- Sketch UGP21 illustrates a typical cross-section of a closed or chemical drain system.
- 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. Sketch Fig 8 illustrates two insulated lines A and B running below grade to a drain tank.
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 catch basin is illustrated in sketch Fig 11.
A typical sewer box or manhole is illustrated in sketch Fig 12. A typical Dyked area drain sump is illustrated in sketch Fig 13.
A typical lift station by vertical pump is illustrated in sketch Fig 14.
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.
Sketch Fig 15-16, Fig 17-18, Fig 19 illustrates the details of the subdivision of the process area into blocks with explanatory notes.
Sketch Fig 20 illustrates how to handle oily water and storm water system.
9.Closed drain system is illustrated by various sketches Fig 21-22, Fig 23, Fig 24.
10.The underground electrical and instrument cables passing under a road or paved area is takenthrough ducts embedded in lean concrete. Sketch Fig 25 shows a typical arrangement of such underground cabling details.