In my last jottings I started to discuss boiler fittings but did not get beyond the subject of feed clacks and non return valves in general. I promised (threatened?) to continue next time, so here goes!
A few months ago I was asked how I managed to make my injector steam valves completely steam tight when closed. I was rather surprised at the question at the time because it had never occurred to me that there might be a problem, but having now thought about it I can see where difficulties might arise. The basic design of the majority of steam valves incorporated in live steam models has not changed for best part of a century and consists of what might be described as a blunt needle valve – see figure 1.
The diagram shows only the business part of the valve. We will worry about the rest of the device in a moment, but figure 1 serves to highlight the requirements for a guaranteed steam tight shut off. Clearly this will only be achieved if the conical end of the spindle locates exactly on the seating when the valve is closed. If there is any eccentricity between the axis of the thread on the spindle and the centre line of the cone on the end of the spindle or between the thread in the valve body and the valve seating a steam tight seal will not be achieved. It is, of course, possible to design valves with elastomeric sealing of some sort (the old style kitchen tap is an example) but in the small size valves we are usually dealing with it is more trouble to make such a valve then to make the traditional type properly.
Figure 2 shows details of the steam valves I have designed for my own locomotives. The dimensions shown are for a typical blower valve or injector steam valve but can obviously be modified to suit any application. The external dimensions of the body and bonnet are not shown and are not critical. If anyone would like a detail drawing a letter via Editor is all that is needed. The first and most important design feature to note is that the spindle is captive – i.e. the valve cannot be opened so far that the spindle screws out of the bonnet.
The steam valve designs promulgated by many of the famous designers of the past including such illustrious names as LBSC and Martin Evans, did not incorporate this safety feature which is now considered by most authorities as essential.
The current edition (November 2008) of the British Model Engineering Liaison Group Boiler Testing Code is not entirely satisfactory on this matter and appears to leave the requirement to the discretion of the boiler tester. Most boiler testers will, however, insist on the requirement being met. Hopefully this situation will be clarified in future editions of the Code and captive valve spindles will be a mandatory requirement. Retrospective application of such a requirement will not be easy, bearing in mind the thousands of live steam models in service incorporating valves of traditional design with non captive spindles. It is interesting to note that the Australian Miniature Boiler Safety Committee current Code (which in my opinion is far superior to the British Code) does make the requirement for captive valve spindles mandatory, but the requirement is not retrospective to boilers registered before the introduction of the Code.
The second feature to note is the use of an “O” ring seal in place of the traditional packed gland. This is a personal preference and results in a smaller, neater configuration and a more reliable seal than a packed gland.
Another important feature of the design is the securing of the operating hand wheel / handle to the valve spindle. Many designs specify the hand wheel to be screwed onto the spindle and locked in place with a nut. Indeed, many commercially available hand wheels are supplied with a tapped hole. This is NOT a sufficiently positive method of securing the hand wheel. If the valve is closed too tightly or becomes tight as the temperature is raised from cold it is possible for the hand wheel and nut to unscrew together instead of opening the valve. I have seen it happen on one of the locomotives steamed regularly on our own track.
The use of a square on the spindle engaging with a matching square hole in the wheel ensures a positive torsion drive. The nut only keeps the wheel in place. The British Boiler Test Code advises the inspector to check the security of valve hand wheels on their spindles whilst it is a mandatory requirement of the Australian Code referred to earlier that operating hand wheels and levers are positively secured to their spindles.
Turning now to the manufacture of the valve, the body is fabricated from phosphor bronze. The bonnet and seal cap may be phosphor bronze or brass and the spindle stainless steel. The need to achieve concentricity of the various features has already been mentioned.
In the case of the spindle the best way to achieve an accurate result is to screw cut the thread, finishing to size with a die or die nut. If a BA thread is employed, and in my case it usually is, screw cutting presents a problem on many lathes, including mine! If the plain outer end of the spindle is machined to a couple of thou under the root diameter of the thread it will provide an adequate guide for a die and ensure a true thread. Machine the whole spindle at one setting, including the cone on the business end, leaving a small “pip” for final parting off – there is no need for a sharp point.
The threads in the valve body and bonnet must also be cut with care but should not present a problem if sharp taps and dies are used with a tailstock die holder to present the die accurately to the job and the taps guided by a drill chuck in the tailstock. The seal cap is soft soldered to the bonnet and the “O” ring worked into it’s cavity through the spindle hole. Be careful to remove any sharp edges and burrs from the cap and the end of the spindle to avoid damage to the “O” ring during assembly. A smear of silicone grease helps. Should the “O” ring need replacement at some future date removal of the old ring is easily achieved by unsoldering the cap to facilitate removal of the debris from the old ring before re-assembly.
A set of components for a bonnet assembly are shown in Figure 3. I make my own hand wheels, machining the profile on the end of a piece of bar and drilling to rough out the space between the spokes before parting off. The spokes are finished to shape with needle files and scrapers.
Ideally the square hole would be produced with a small broach but I have never got round to making suitable tooling and do the job by hand with a square needle file. The appearance of the wheel is enhanced if it is dished slightly by pressing in the vice with suitable supports. A small hole is drilled in the rim and a suitable size piece of wire silver soldered in place and trimmed to length for the handle. Figure 4 shows opposite sides of a couple of typical 5/8” diameter wheels.
The valve body may be made as a ‘stand alone’ feature such as a blower valve screwed into the back head on the end of a hollow stay or may be made as an integral part of some other feature such as a steam turret as shown in Figure 5.
