The 2nd edition of the book Plant Design and Operations is at the printers. In it we try to provide practical and relevant information that can be used in both the design and operation of process facilities. One section of the book discusses drainage systems. These systems, like so many other utilities, have the potential to be very hazardous because they connect different parts of an operation unit to one another. (Safety Moment #10 discusses the hazards to do with utility systems.)
Drainage systems have the following purposes:
- Ensure that accidentally spilled flammable and toxic liquids flow to a safe location.
- Provide a means for safely and environmentally responsibly disposing of liquid inventory.
- Properly handle surface water, particularly rain water.
All equipment should have a valved drain unless it can be completely drained using connected piping. If the equipment contains slurries then the drain connections should be DN 25 or greater.
Many process facilities have more than one type of drain system. These could include any of the following.
- Oil-free water.
- Potentially oil-contaminated water such as might occur from a tank leak.
- Continuously oil-contaminated water such as from process equipment or maintenance activities.
- Firefighting or cooling water.
- Domestic sewage.
A facility can have as many as three different types of drain system: closed, open and domestic.
- A closed system, which may be under pressure, is designed for the safe handling of flammable and toxic materials. It is hard piped from process vessels and equipment.
- The open drain system collects surface water from sources such as rainwater, floor/deck wash-down and drip pans. The water from the open drain system is collected and any entrained oil is skimmed off.
- The domestic drain system collects water and sewage from unclassified loations and buildings.
It is critical that the systems remain separate and that there are no interconnections between them. In particular, drains from the process areas must not enter unclassified non-process locations. Otherwise flammable or toxic vapors may be released in non-classified areas. In particular, open drains should not be located close to occupied buildings. On the other hand, drainage from the non-process areas should not enter the process drains so that the oil-water sumps and treatment facilities are not overloaded with water that does not require treatment.
Drains often connect different parts of the facility with one another. Therefore they have the potential to transfer hazardous or flammable materials (which may be on fire) from one location to another. They create the possibility of unforeseen chemical reaction and, if air is mixed with hydrocarbon vapors in the drain, can create an explosive mixture. Also drains can overflow, thus leading to the possibility of hazardous chemicals entering locations where they are not expected.
For all these reasons the drain system should be designed so as to:
- Handle the worst case credible spill along with the water runoff associated with any deluge and/or firefighting activities.
- Limit the maximum horizontal spread of a spill. (Curbs or drip pans should be provided around vessels, pumps and other sources of leakage to limit the spread of small spills.)
- Prevent spilled material from accumulating under vessels or equipment.
- Not to be blocked with solid waste.
- Easy to inspect and clear as necessary.
- Slope away from storage areas and have sufficient capacity to handle large volumes of firewater.
Because drain lines are often are often low diameter and are located at grade they may be damaged by being walked on or by having wheeled equipment being rolled over them, particularly if workers need a short cut from one area to another. In such cases the drain system should be protected and/or alternative walkways provided.