In this post I want to share to you about closed drain systems and how to design them. You may know several types of drainage in process plant, which are:
- Closed drain
- Open drain
- Oil drain
- Water drain
This post will cover about closed drain system. Hopefully in the near future I share to you other types of drain.
What is closed drain system?
During maintenance activities, process liquid in the equipment should be emptied. The liquid must be collected. The volume of collected liquid can be significant, therefore must be recovered and can be reused in the process or disposed safely.
Closed drain system is facility used to collect the liquid from the bottom of vessels and their associated piping to a recovery vessel or closed drain drum or closed drain vessel. The liquid flows to the vessel or drum by gravity.
Why is it called “closed”?
The term “closed” indicates to the fact that the drainage system is not open to atmosphere. Since the liquid is hazardous or flammable, it must be piped through a closed piping system, preventing any contact with atmosphere.
Typical configuration of onshore closed drain system
This is the typical configuration of closed drain system.
Liquid from vessels and columns flows through closed drain pipes by gravity into closed drain drum. The liquid is collected in the closed drain header and to closed drain drum. From there, the liquid is pumped using a closed drain pump back to the main process or into a truck.
In the liquid drain is hydrocarbon, you may need to add gas line leaving the closed drain vessel flowing to flare to degas the liquid.
Piping Design of Closed Drain System
The drainage systems should be designed to minimize the accumulation of stagnant hydrocarbon mixtures (such as liquids, emulsions, and solid residues) on surfaces and within the collection system.
Drainage effluents should be evacuated by gravity.
Gravity pipe runs should be designed to handle the maximum expected flow with pipes that are partially full (0.9d or less).
Piping restrictions should be avoided. As a rule, 45-degree branches should be used instead of 90-degree tees to prevent clogging.
Except for siphons and seal pots (or manholes onshore), drain headers should have no pockets and should continuously slope towards a drain drum.
The required minimum gradients of horizontal piping are specified below (for gravity-type drains, non-flooded pipes).
Onshore Piping Gradients for Drainage Systems
How to design closed drain vessels
The liquid capacity of the drum should be adequate to handle the largest credible drainage volume from the single largest drum (for example liquid level below Low Liquid Level of the Largest vessel or the longest isolatable piping segment or equipment item). The liquid capacity of the closed drain drum is the internal volume between the level trip points.
If the closed drain size is significantly increased due to the drainage volume of a large process vessel, alternative drainage routes and strategies should be evaluated for the source drum.
The steps below described how to select the capacity of closed drain drum:
- Determine the liquid volume from the bottom of vessel to low low liquid level of all vessels and columns
- Determine pipe system volume
- Determine the minimal capacity of closed drain vessel based on the largest inventory among vessels, columns, and pipe system volume
- Estimate diameter and length of closed drain vessel and calculate the volume. The volume shall be larger than the capacity selected above
References:
- A Process Plant Drains Systems (https://projectcontrolsonline.com/blogs/59-project-controls-blogs/615-a-process-plant-drains-systems)
- Adnoc Isolation Vent and Drain Philosophy (AGES-PH-08-001)