'Planet' leaving the 'New' Platform at 'MOSI' with Bev in charge
One Saturday, some time ago, I was rostered driver on the 'Planet' replica at Manchester Museum of Science and Industry ('MOSI'). I'd completed my oiling-round and daily examination of the engine and the fireman was just waiting for the boiler to develop full working pressure. I felt a bit peckish so I decided to pop across the road for a sandwich. When I returned a few minutes later, the locomotive was virtually invisible, wreathed in a thick cloud of steam. Apparently, having obtained full pressure, the fireman had tested the injector but, on shutting it down, the Clack Valve had not re-closed so the boiler was now ejecting steam through the injector overflow.
Steam locomotives use non-return valves, usually called 'Clack Valves', to allow fluid to pass in one direction only. Clack valves are most commonly inserted in the water feed to the boiler, allowing pressurised water to enter the boiler but preventing steam and hot water from escaping.
A 'Sticking Clack' can be a fairly common problem on locomotives with injectors. It's usually caused by a small piece of solid matter getting trapped in the clack valve as it closes, preventing a proper seal. The usual source of solid matter is boiler scale. Fluid flow in the vicinity of the Clack Valve can be turbulent and particles of scale can get swept into harm's way. There are a couple of techniques that can be used to rectify matters.
Vibration can be used to dislodge the offending matter. This is usually achieved by whacking the clack body. Different firemen tend to have different places on the clack to deliver the blow but I've usually found the top of the clack body is the most effective. On many old engines this preferred position is attested to by the battered appearance of the top of the clack body, caused by blows from coal picks. To prevent this sort of damage, I try to use a wooden footplate brush to thump the clack body.
Alternately, the clack can be made to 'hiccup' by shutting the steam cock feeding the injector and then opening it again. I've found that gently closing the steam cock and then immediately opening it wide can often cause the right sort of disturbance to seat and unseat the clack and hopefully, when the steam cock is closed again, the clack will seal properly.
If neither technique remedies the situation, then that feed has to be declared a 'failure'. Most locomotives have two injectors and two clacks so one failure does not prevent the crew from finishing the 'diagram', provided they're fairly confident about the other 'feed'.
The problem on 'Planet' was quickly rectified by a few sharp 'taps' and it didn't recurr. If the problem happens regularly, it suggests permant damage or scoring to either the clack or its seat.
Water Feed Pumps
Before the invention of the Injector by the Frenchman Henri Giffard in about 1859, early locomotives like 'Sans Pareil' and 'Lion' used pumps to pressurise the feed water to the boiler, allowing water to be delivered to the boiler against the internal steam pressure.
Water pump on the 'Sans Pareil' replica
Right hand water pump on 'Lion'
Clack Valves were still required to prevent steam and water escaping from the boiler when the pump was not being used to feed cold water to the boiler. The original 'Planet' (built in 1830) employed a water pump driven from the reciprocating motion of the crosshead but this method of feeding water to the boiler can only be used whilst the locomotive is in motion. The replica 'Planet' does have a pump but it's also provided with an injector (an anachronism, of course) allowing water to be fed to the boiler whilst stationary, provided there's sufficient steam pressure to operate the injector).
'Ball and Cage' Clack
'Lion' uses a 'Ball and Cage' clack design. The pressurised feed water from the pump 'unseats' a ball to allow water to enter the boiler but, when the feed pressure drops, the ball falls back onto the seat and prevents the flow from reversing. This works well if the ball and seat are 'true' but later designs of clack tended to make use of a disc valve.
A 'Ball and Cage' clack valve on 'Lion'
And what about the operation of injectors? We'd better keep this for another time.
