General Guideline for Drainage Philosophy

This document provides general design guidelines for classifying and designing various plant drainage systems based on their source, properties, and quantities.

Drain System Classification

Broadly, plant drainage systems are categorized into two main types:

1. Closed Drain System: A hard-piped system for collecting hydrocarbon drains from pressurized equipment.
2. Open Drain System: An atmospheric system, often gravity-fed, for handling water with potential traces of effluents.

Closed Drain System (CBD)

The Closed Blowdown (CBD) system collects drains from pressurized equipment, typically for maintenance. It is not intended for continuous discharge.

General Principles

• • Drains are routed via headers to a CBD drum, preferably by gravity.
• • The CBD drum's vent is typically routed to the flare system.
• • Equipment should be depressurized to the flare system before being manually drained to the CBD to prevent over-pressurization.
• • Material is typically carbon steel, but this must be reviewed for special services (e.g., cryogenic, chemical).
• • Drainpipes must be sloped to ensure gravity flow and prevent liquid from accumulating in pockets (e.g., 1:500 slope, or 1:250 for congealing service).

Separate closed drain systems should be considered for equipment handling incompatible chemicals (e.g., Amines, TEG), subzero temperature fluids, or systems with a design pressure lower than the main CBD drum.

Closed Drain (CBD) Drum

• • Sizing: The drum is typically sized to accommodate the liquid volume from the largest connected process vessel (from its low-low level). This volume is held between the pump start (Low) and pump stop (High) levels.
• • Location: Often installed underground to allow for gravity drainage from the plant.
• • Design Pressure: As the drum is floating on the flare system, its design pressure is normally the same as the flare design pressure.
• • Design Temperature: Typically ~80-90°C, assuming contents are cooled before draining. This must be higher if steam-out connections are included.
• • Special Service (Congealing): For congealing fluids, the drum must be designed at a much higher temperature (e.g., ~200-250°C) and be equipped with heating coils. The associated piping should be steam traced and run in sand-filled trenches to allow for thermal movement.

Closed Drain (CBD) Pump

• • Type: Vertical submersed centrifugal pumps are common, mounted on top of the drum to avoid NPSH issues.
• • Configuration: 2x100% (one operating, one spare) is typical.
• • Operation: Pumps typically operate on auto-start based on On-Off level control in the drum.
• • Capacity: Sized to evacuate the drum in a set time (e.g., 30-60 minutes), based on the downstream system's handling capacity.

Open Drain Systems

Open drains handle atmospheric drains, such as surface water, that may contain trace effluents. These are segregated by contamination type to route fluids to the correct treatment or disposal.

Storm Water Drainage (SWD)

• • Purpose: Collects surface storm, ground, and firefighting water that is normally free from contamination.
• • Sources: Building roofs, roadways, unpaved areas, and paved utility areas.
• • Destination: Typically collected via channels and discharged to a storm water pond without treatment.

Oily Water Drainage System (OWS)

• • Purpose: Collects wastewater that is continuously or likely contaminated with hydrocarbons.
• • Sources: Equipment wash-down water, sample collection drains, drips/spillage from pumps, unloading/loading areas, and flare seal water.
• • Design:
  • Open funnels are provided with a water seal (e.g., 150mm turndown elbow) to prevent vapors from escaping the header.
  • Underground piping velocity should be low (e.g., < 0.9 m/s) to prevent emulsification.
  • Effluent temperature should be kept low (e.g., < 45°C) to prevent flashing.
• • OWS Sump: An underground sump, often an RCC pit, sized to accommodate wash water. It is typically covered and vented to a safe location with a flame arrestor. A hydrocarbon skim-off facility is common.
• • OWS Pump: Vertical submersed pumps (2x100%) are used to pump the skimmed hydrocarbon to the closed drain system and the wastewater to an effluent treatment plant (ETP).

Contaminated Rain-Water Sewer System (CRWS)

• • Purpose: Collects normally clean surface water that may accidentally become contaminated (also called Accidentally Oil Contaminated, or AOC, water).
• • Sources: Pipe racks, concrete paved process areas, LPG storage areas, and tank farm dykes.
• • Operation: The system is typically sized to capture the "first flush" (e.g., first 15 minutes) of a heavy rain or firewater event. This collected water is sent to the ETP. Subsequent water, which is assumed to be clean, is routed to the SWD.

In some facilities, the OWS and CRWS systems may be combined into a single system that handles both continuous process drains and accidental surface run-off.

Chemical Water Drainage (CD)

• • Purpose: Collects concentrated chemical waste.
• • Sources: Acid/alkali bunds, BFW dosing chemicals, demineralized water plant drains, lab sinks, and lubricant handling areas.
• • Destination: Routed to a neutralization pit before discharge to the ETP. Multiple, independent sewer systems may be required to keep incompatible effluents separate.
• • Design: Pits and drains must be lined with material resistant to the specific chemicals (e.g., protective tiling for acid/alkali).

Sanitary Drainage (SD)

• • Purpose: Collects domestic sewage.
• • Destination: Routed to a dedicated sewer treatment plant.

Overall Drain System Classification Summary