The most repetitive and common maintenance task on a steam boiler is the blowdown. Few people really understand why they have to do it or what makes it so important.
So, what is a boiler blowdown anyway? Every steam boiler has valves connected to the pressure vessel which are opened on a regular basis to blow out water to drain. This process of opening the valves for a short time and closing them is what it means to blow down a boiler.
Bottom and Control Valves
There are generally two types of blowdown valves on a boiler: bottom and controls. Bottom blowdown valves are connected on the bottom of the pressure vessel and are designed to remove the solids that build up in the bottom of the boiler. A controls blowdown valve is usually on piping attached to the boiler that contains a device related to the boiler water level—most commonly a pump control/ low water cutoff assembly. The water in the low water cutoff assembly will build up with solids and by blowing down that control, it removes the solids from the control and replaces them with fresh boiler water.
Quick and Slow Opening Bottom Valves
High-pressure steam boilers (operating over 15 PSI) are required to have two bottom blowdown valves; a quick and a slow opening. The two valves are in series on the piping so that both must be opened for water to flow and drain.
The first valve coming off the piping connected to the pressure vessel will be a quick opening valve, commonly a rotating disk valve with a lever or a ball valve. The second valve is the slow opening valve which, as its name suggests, is slower to open fully than the quick open valve. It’s usually a rotating disk valve with a wheel handle or a wye valve. To note, you may see a slow opening valve with another slow opening valve after it, but there cannot be two quick opening valves or a slow then quick opening valve.
How to Perform a Blowdown
Here’s how to properly blowdown a high-pressure boiler:
- Gather the required PPE. This typically includes heat resistant gloves, safety glasses, hearing protection, and perhaps a face shield.
- Inspect the bottom blowdown piping. It should be in decent outside physical condition with no leaks, and properly supported to not move when stressed.
- Verify that the boiler is operating safely with the water level normal and near operating pressure.
- Verify that both valves, the quick and the slow, are closed.
- Open the quick opening valve. This will allow water to flow to the slow opening valve.
- Open the slow opening valve. As the valve is opened, the flow of the water will increase through it.
- When you have opened the valve for the proper time period (generally 10 seconds), close the slow opening valve.
- Close the quick opening valve.
- Open the slow opening valve to relieve the pressure between the two valves and then close the valve.
- Make sure both valves are seated tight and that there is no water continuing to flow from the bottom blowdown piping.
- Slowly open the blowdown valve(s) on the water level controls of the boiler. The number of valves will depend on the size and type of boiler, but it is usually just one or two valves. Make sure the boiler shuts down when the valve(s) is opened 100%, as this is simulating a low water condition. If the boiler does not shutdown, secure the boiler so that it cannot operate, and have the water level control operation verified by a boiler operator/contractor.
- Record that the blowdown on the boiler was performed. There should be a chart on each boiler for tracking blowdown intervals.
Consistent and proper blowdown of a boiler helps to maintain a safe operating environment, and prioritizing this basic maintenance is vital to avoid water quality issues that will affect the boiler operation and the steam quality of the plant. Contact Boiler Specialists Inc. if you have any questions about the blowdown procedure of your boiler. We’re here if you need us to make a training program for you, or if you just need some blowdown valves for your boiler, as we stock many varieties and sizes.
Thank you to Everlasting Valve Company for permission to use the photos.