Before we look at the top 4 reasons the EN285 steam dryness test fails we will recap what the Steam Dryness tests is designed to test
Steam when generated in the boiler is classed as 100% Dry (Not 100% true but keep with me)
As it loses heat the steam will turn back to condensate. Some will fall out of suspension and stick to the pipe walls. Then it will be pushed along by the flow of the steam and most will settle to the bottom of the pipe. Here it can be bled out of the system using a trap of some nature. The remaining moisture will flow with the steam as a mist. All that moisture is held in the steam and travels with it to the outlets. This is what the Steam Dryness Test is designed to establish the level of.
The Dryness is the volume of condensate in liquid form against the volume of the condensed steam. E.g. if you..
- Separated The condensate (mist etc) from the steam and noted the amount Cv
- Condensed the remaining steam and noted the amount Sv
The Dryness = Sv/(Sv + Cv) and is expressed as a fraction for example:-
Cv = 1 grams
Sv = 99 grams
The result would be 0.99
The 4 Top Reasons
1 – Wrong size pitot tube.
Despite what you may have read in EN285 the pitot tube size is important and the pitot tube aperture to pressures they list are wrong. I go into detail in an article why, here You can work out the right size using our app or just pick your pressure range from this list.
1.5-2.0 barG = 1.0mm
2.1-2.7 barG = 0.9mm
2.8-3.7 barG = 0.8mm
2 – Trapping issues.
As stated above trapping will remove the excess moisture in the steam pipe. A faulty trap will allow a build up of moisture in the steam and that will be carried along and add to the condensate you end up sampling.
3 – Piping design.
While in this day and age you would think that all steam piping installations are built to the highest standards, unfortunately this is not always the case. On old plants, while they may have been installed properly to start with, over time changes will have been made to reroute a pipe that causes a pipe to fall in the wrong direction. Or pipe supports were removed or are no longer able to hold the pipe at the right fall.
4 – User Error.
When performing the Dryness test you need to be very careful when collecting the values. Pay particular attention to the temperatures and weights. You have to be meticulous and consistent. This is why we developed the SQI Test and Report APP. Used in conjunction with the SQI-TM1 you can be sure the temperature you use is right and along with the photo capture feature next to the balance value entry you can be sure you recorded the right value if you are in any doubt later.
How to resolve each of the issues
1 – The Pitot tube
This is pretty simple. Either modify an existing pitot tube to the right size or purchase another or a set of the right size. Steam Quality Innovations can supply the right size pitot tube with any fitting type you require. See our accessories page.
2 – Trapping
This is a matter of checking the local upstream traps to verify they are working. There are many ways to test the trap. The easiest is to test if the outlet is hot. If it is cold right up near the trap itself you can be pretty certain there is an issue.
3 – Bad installation
I recommend you walk the line, look for loose pipe supports, odd pipe configurations etc. It is a case of careful inspections and Sherlock style sleuthing. Below are a series of tips that will help you identify the probable causes.
4 – Fixing user errors
Just do the job better, right? Ok it sounds easy but after 20 years of steam testing when getting an odd result I still question myself if I entered the right values. This has been eliminated now using the SQI-TM1 and the SQI Test and report APP. The temperature meter records the data even after you turn the device off and transmits the last set of data to the app. When you start the test at the meter it records 10 seconds of data before you started the test to cover you starting heating the water before reading the start temperature. The SQI Test and Report APP also enables you to take photos of the balance display. This has saved me many times where fat fingers entered the wrong number.
Lastly you could consider installing a steam separator. They can work extremely well where the issue is say long pipework and the trapping is not up to the highest standard.
It probably will be a lot less work fitting a separator before the sample point than installing new traps along the existing pipe work.
Tips and tricks
You can learn a lot about the dryness of the steam by just the look and feel of the steam emitted from the pitot tube.
It should be clear with the slightest haze. If it is cloudy it will not be 100% dry. Now try placing your hand in front of the stream. Don’t fret about burning yourself. The venturi effect pulls air in and cools it. You will be able to feel the moisture content.
If the stream is constantly moist to the point it is dripping on your hand you have an issue that could be trapping or pipe sag.
Next watch the stream as a big demand is made on the steam supply. If it spurts out then there is most likely a piping issue. A pool of water is sitting in the pipe and is being whipped up and carried along by the greater velocity of steam as the extra demand is made on the supply.
I will leave you with an example of what bad steam looks like. The issue here was the trap on the steam separator mounted just before the test elbow.
