Meanings and Purposes of “Blowdown”

When I was on NWR project, we worked on an 11m long steam generator, where 2” continuous and intermittent blowdown lines were connected. Later, on Encana gas plants, there were blowdown lines on pressure vessels. Some designers were confused by the “blowdowns”, even thought they were the same thing.

Actually, in oil and gas industry, one term represents multiple meanings is not rare. There are different kinds of blowdowns:

1. In a boiler or a cooling tower system, blowdown is bleeding the system down to deduce the concentration of contaminants.
2. Emergency depressurization system for safety purpose.

3. Sometimes lines used for purging or cleaning are called "blowdown" lines, like compressed air lines to blow water out of a reactor jacket. It’s not a precise terminology.

I am only going to talk about the first 2 cases here.

Boiler Blowdown

In a recirculating fluid system, contaminants will build up over time as the fluid is reused. In a boiler, dissolved solids and particles in the make-up water will remain in the boiler when steam is generated. If the concentration of contaminants is too high, they will plate out inside the boiler on the heat transfer surfaces and the operation of the boiler becomes impossible. That is why boiler blowdown is used to control the salt concentration within a set operating range.

All steam boilers require an arrangement for manual (intermittent) blowdown even if they are supplied with automatic continuous blowdown systems.

Manual blowdown can be accomplished through taping at the bottom of the boiler where settled solids are removed. Continuous blowdown use a calibrated valve and a blowdown tap near the boiler water surface. Water is continuously taken from the top of the boiler at a predetermined rate.

Below is a typical boiler blowdown system.

 

In a cooling tower system, water is evaporated in the cooling tower which also leaves salts behind. A blowdown process is used to control the salt concentration in a cooling tower recirculation system.

Pressure Relieving Blowdown

Pressure relieving systems, commonly called blowdown systems, are used to ensure the safety of personnel and protect equipment in the event of emergencies such as process upset, equipment failure, and fire. Below is a sample of ESD and BD system.

 

A blowdown system consists relief valves, safety valves, manual bypass valves, blowdown header, knockout vessels and holding tanks. It can be used for liquids or vapors or both. Flares usually are considered part of blowdown system, but not the only disposal collector.

Vapor can be discharged:

- To atmosphere

- To lower pressure process vessel or system

- To closed pressure relief system and flare

- Acid gas flare

Liquid can be discharged:

- To onsite liquid blowdown drum.

- To lower pressure process vessel or system.

- To oily water sewers only if the material will not cause hazardous conditions.

- To pump suction if pump will not overheat or can withstand the expected temperature rise.

- To burning pit.

- To vaporizer.

Well, now we all know that a vessel overhead blowdown won’t be piped properly based on boiler blowdown experience.😊😍