The On-Line Magazine of Rideable Model Railroading

  NUMBER 187


© May 20, 2012   

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The 411 on the Boiler Hydro Test


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Written by Chris Welvang
Milwaukee Light Engineering Society


For those who are not familiar with the safety practices of boilers one of the most fundamental is the "hydro" (or hydrostatic) test.  Wikipedia states that "A hydrostatic test is a way in which leaks can be found in pressure vessels. The test involves placing water in the pipe or vessel at the required pressure to ensure that it will not leak.... Testing is very important because such containers can explode if they fail when containing compressed gas."

The cold water "hydro" is conducted on an periodic basis for the inspection of a steam boiler.  The test is also used in various other applications like testing of gas cylinders and pipe lines.  Its primary function is to check the overall soundness of a pressure vessel like a boiler without using compressive pressure.  This makes it a safe process which is also very easy to achieve and control by those conducting the test.

An official or inspector will typically conduct the test annually or by a period set by the authority governing that particular vessel.   As is typically the case in boiler issues each state and local authority has codes which must be followed if applicable for boilers and inspections.  It is a good idea if not already a requirement to do the test when any repairs are made to a boiler; this includes fittings but not the throttle or appliances like injectors.  Timing for the test varies also, for instance my home state requires annual inspections for boilers covered under their rules by a state inspector. 

So what does the test really involve?   Well it’s actually pretty simple to describe, you fill the boiler entirely with water and then mechanically pressurize that water to a set pressure typically no less the 125% of the working pressure of the boiler.  It is most common to see 150% but I have seen requirements ranging from 125% to 200%.  I am not going to get into the finer points of what is right or wrong for a chosen range; I will only say that I believe 150% to be a satisfactory requirement for any boiler.  Most rules for live steam clubs, require150% as well as the ASME for their standards which many states have adopted for code vessels. 

For example my local club the Milwaukee Light Engineering Society requires an annual inspection of a boiler, being valid 365 days not on a particular day of the year.  When we do an inspection we require that the inspector (typically the safety director) has someone else whom must verify the results.  This person who witnesses the test must sign on the inspection form as such this gives some impartiality to the results of the test.  Our inspection consists of a cold water hydro,   verification of safety valve operation, and verification of steam gage operation.  We do all at once as it is convenient, we could do them separate but that would require the same amount of work 3 times and other apparatus for the gage and safety accomplishing the same thing.  This does not include the fact that these integral parts of the boiler would not be on it during the test.

A typical test only takes a few minutes, in variably it takes longer to get the fittings on the boiler then it does to fill and test it.  We have a kit of different fittings we use as to allow hook up of the pump apparatus and master gage to the members boilers.  We use a master gage which is also checked against a hydro static gage calibrator. When possible we use the steam dome for our water entrance point as to allow all air to escape through the highest point on the boiler.  When filling the boiler you need to vent it as not allow air to compress from the water pressure of your hose feed.  I like to fill the boiler to the top and then let it stand for a minute or two, usually the time it takes to attach the pump and the gage.  I visually inspect the boiler at that point, accomplishing two things. One is to make sure there are no leaks at this point already, two to get a frame of reference of the shape of the boiler at 0 pressure.

After hooking up the rest of the apparatus, which includes a hand pump if the engine is not so equipped already, the first stage begins with only one of the two safety valves mounted. Depending upon your pump setup the water pressure in the line could instantly bring up the boiler to upwards of 80 psi, saving some work on the hand pump.  Pumping gently the pressure is brought up on the master gage to working pressure of the boiler.  The gages are checked against one another, making sure the boilers gage is functioning and reading very close to the master.  The pressure is held for a short time and the boiler is inspected visually, noting any deformations, leaks or noises while getting to this point. Should any of that happen it is now time to investigate that item and see if it is alright to proceed. 

Next the pressure is brought up to the set safety valve pressure if not done so already, noting the exact pressure that the valve relieves at.  The above is repeated for the other valve after slowly dropping the boiler pressure down. It is hard in the live steam size equipment to get the valve exactly set using a hand pump; it may be effort better spent just finding the pressure they relieve at setting them later.  If you are not familiar with setting safety valves refer to club member or vendor who is for your valve.  The safeties ideally should be relieving with about a 5 psi differential between them, the effect of temperature on them does not always make them cooperate with the intended settings though.  A difference of more than 10 psi renders one inoperable; the higher should be adjusted down to the lower of the two until they can be more thoroughly adjusted assuming that the boiler gage is reading correctly. 

The third stage involves removal of the boilers safeties, if the owner would like removal of the boiler gage (I recommend it), and shutting off and draining the sight glass if not removing and plugging it.  Again the boiler is brought up to working pressure, and gently brought up to %150(or required pressure) of the working pressure of the boiler.  This time the pressure is held as close to that particular pressure as possible, a really sound boiler and good pump will not leak down.  If any pressure drop, leaks, noises, or deformation occurs it is now investigated.  Trained eyes and ears can with a light tap of a hammer discover broken stay bolts while doing hydro, notice bad welds, seeping fittings, and cracks in sheets and tubes and places that need to be caulked.  Pressure in my view should be able to be held around one minute, which typically is about half as long as it takes to actually look at the boiler for any of the above flaws.  This can be a two person affair one person monitoring the pump/gages and the other doing the inspection.  Ascertaining any and all leaks is important; a simple throttle leaking will make the pressure drop quickly without monitoring during the inspection, resulting in other missed leaks.   Leaking is common from throttles and other valves which tend to do so at lower temperature, the exact amount is little hard to pin down to words.  I would only say reasonable leakage from them, steam cutting of valves can happen so it is up to the inspector to judge this on each case.   

Some things that are not considered a proper hydro test for boilers.  One, you should never use an air compressor and fill up a boiler with air until after the boiler has been given a proper hydro.  This is a common mistake in the live steam community.  It could be a dangerous one to. Another is doing the air over water hydro, again, not real safe and very hard to control and relive the pressure.  Water should be used which is near to the coldest possible, not ever hotter than 120 degrees.  Avoid very cold or very hot water. Ideally the water should be near the ambient air temperature while running the test to prevent changes in density.  In theory, water does not compress. In practice, it compresses very very little. It does expand greatly the hotter it gets above 40 degrees F.  Changing the temperature of the water even a few degrees can give a rapid rise in pressure in a full boiler.  So do not ever leave a boiler full of water unattended out in the sun while it is closed up.  The same applies to adding heat to a full boiler (like a torch), it is a quick way to destroy hard work not to mention it’s just hard on the metal.  I have seen the term “warm water hydro” used in particular outside the USA but I would warn against trying to warm the water, for the above reasons. 

Some people have given strong statements to me in the past that indicate that doing a hydro test is detrimental to the boiler and therefore should not be done to the pressures above working pressure.  In a strictly mechanical sense, every time we fire up a boiler it is detrimental to its existence; pressures are induced upon it that are of course not natural.  Heat, pressure, water, chemicals reactions, all take their toll on boilers as we use them, they are finite. In comparison to the full size engines that we duplicate in live steam they are considerably safer just by size of the materials we are using.   Hydro testing is not a catch all, nor is going to make your boiler explode because you did it either.  I will say that my one personal opinion on the subject is that there is never a need for a steel boiler to be tested above 150% working pressure for any reason. 

 I have personally witnessed a scale boiler which was given a pass on a hydro test and then a year later have a screwdriver pushed through its firebox side sheets with a slight hammer tap. The rust was literally holding it just below skin surface of the little good metal that was left. In that case it was determined that the boiler was more than likely not built with the proper materials even though it had passed hydro's for over a decade through multiple owners.  It is possible that boiler could have passed its hydro again, or failed spectacularly; given the owner at the time a person who I would consider very knowledgeable they had the foresight to check the soundness of the boiler even before the official test. Boiler safety is not just passing a test, but due diligence in all aspects involved including water treatment, operator training, metallurgy, engineering data, and construction methods even if you are not the owner of the engine. 

Chris Welvang  served as a volunteer mechanic and fireman for the Kettle Moraine Railway for 6 years until its close, and has been an active member in the Milwaukee Light Engineering Society since 2001.



Written by Chris Welvang
Milwaukee Light Engineering Society


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