Mielies (aka corn on the cob)

[John Ashworth]

An old driver's trick - using the blowdown cock to cook mielies. These particular specimens belong to Marnie and are cooking nicely in 3117's blowdown cock at Cullinan on 29th March 2009.

[Steve Appleton]

I think this topic should be entitled "Corn on the Cock... Blowdown cock"!

[Derek Walker]

Ok.. now I know how to cook mielies with a steam engine.... but what is a blowdown cock really used for?

[John Ashworth]

I can't put my hands on my South African footplateman's manual at the moment, but the British one (of 1957) states that the blowdown cock is used "to keep down the concentration of soluble salts in the boiler water to minimise priming". There are water treatment chemicals in the water and, as water turns to steam, the concentration of these in the boiler rises and can cause problems. Hence some of the concentrated solution in the boiler is allowed out to be replaced by "fresh" water (ie with a weaker, "normal" concentration of chemicals) from the tender. Maybe others can correct me if I'm wrong on the details?

As firemen we were taught that the blowdown cock should not be operated while the injectors are running, as the cold water coming in at the same time as hot water is going out cools the boiler too quickly (and possibly unevenly due to convection currents?). Operating the blowdown cock is driver's work, not fireman's.

In practice nowadays in preservation it seems to be mainly used to let some water out of the boiler when it is over-filled, as the locos are only in steam for relatively short periods compared to their heyday and concentration of salts doesn't seem to be such a major concern. I've also seen it used very effectively for fire-fighting after a loco set fire to the grass alongside the track on the Cullinan branch. And cooking mielies...

On the Durango and Silverton Narrow Gauge Railroad in Colorado, USA, they deliberately blow down the boiler on a bridge spanning the gorge, which gives a very impressive sight, usually with a rainbow.

[Nathan Berelowitz]

The blow down cock, blow down valve,as it is called here, actually was operated in a different way in the early days, ranging from operating a spindle on the running board, to a pull rod in the cab. The early models operated with the pull rod was called the "everlasting blow off cock".
Other uses included filling the baffle box, as seen in the above picture, with ballast stones or empty cooldrink cans, and when approaching a road overbridge, one opened it up so that the missiles hit the ramps of the abutments and propell the missiles up and onto the bridge!!

Can also be used for dousing lineside fires or removing stubborn people that have difficulty in c;learing the tracks on approaching trains!! All unofficial of course!

[John Ashworth]

OK, I've found a South African driver's manual. I've copied it verbatim, including grammatical errors.

Blowdown cocks are fitted to a boiler for the purpose of blowing out concentrated salts (scale and mud). The Evrelasting type of blowdown cock is generally used and is connected to the lowest part of the water space.

The pipe leading to the blowdown cock is fitted at the end with a baffle drum, to break up the stream of water and steam, to prevent blowing away the ballast and to reduce noise.

The driver must see that there is sufficient water in the boiler and in the tender before opening the blowdown cock.

The driver must also satisfy himself that there are no persons or animals in the vicinity of the blowdown cock. As the discharge of hot water could cause injuries. Blowing down could also cause damage to property.

The driver must not be occupied with other duties while the boiler is being blown down. He must personally operate the blowdown cock and open it gradually until it is fully open.

The injectors must be closed while the blowdown cock is open, because the colder water entering the boiler flows to the lower portion of the boiler it is blown out as soon as it enters the lower portion of the water space.

After sufficient water has been blown out of the boiler (eg half a glass) the blowdown cock must be gradually closed so as to avoid damge to the seating of the blowdown cock valve.

If the blowdown cock cannot be closed due to an obstruction in the cock, both injectors must be put into operation and the fire must be thrown out.

Boilers must be blown down regularly to avoid priming which can cause damage to the elements and the front end.

A blowdown schedule is available at depots and boilers must be blown down accordingly.

[John Ashworth]

The above was from a recent driver's manual. I've now found my 1956 copy of SAR Handbook on the Steam Locomotive for Enginemen and Running Shed Staff.

All boilers must be fitted with one or more blowdown cocks, connected to the lowest part of the boiler, for the purpose of blowing out any deposits of mud and scale which may collect. Intelligent use of the blowdown cock can greatly reduce the tendency towards foaming caused by high concentrations of salts dissolved in the boiler water, and also protects the plates and tubes from over heating through deposits of mud. Some of the older boilers have only one blowdown cock, but the more modern boilers have two blowdown cocks, one on each side of the firebox near the front corners of the foundation ring.

Several different types of blowdown cock are userd on the S.A.R., most of them arranged so that they can be worked from the cab. The types most commonly used are the "Everlasting" and "Hopkinson", which are shown [in figures in the book].

The cock consists essentially of a chamber connected directly to the water space in the boiler, and closed by a cover, to which the blowdown pipe is attached. Inside the chamber is a lever, mounted on a spindle extending through the cover, so that the operating handle may be attached. On the lever is fitted a brass disc which slides across a face formed by in the cover at the outlet opening. When the valve is closed the boiler pressure holds the valve tight against the face, and the lever closes the opening to the boiler, but not tightly. The lever thus prevents the mud and scale filling up the chamber of the cock, but allows the boiler pressure to hold the valve tight. When the valve is opened the spindle moves the lever carrying the valve disc, and slides the disc across the face. A spring is provided between the lever and the disc to hold it against the cover when in the open position, and so prevent grit from getting between it and the face over which it slides.

The pipe leading to the blowdown cock is usually fitted at the end with a baffle drum, to break up the stream of water and steam, to prevent blowing away the ballast and to reduce noise. The baffle drum must be firmly secured, otehrwise the steam will not be properly directed.

With regard to [priming caused by foaming], reasonably frequent use of the blowdown and scum cocks will help greatly to reduce foaming, by keeping the concentration of salts down to a reasonable amount....

When using blowdown cocks, whether the engine is standing or moving, certain precautions must always be observed. These are:

1. Ensure that persons or animals are not endangered by the escaping steam or water.
2. Damage to material or structures must be avoided.
3. Where the boiler is fitted with two blowdown cocks, these should be used alternately.

Heavy blowing down for long periods is harmful to the boiler, is very rarely necessary and wastes coal and water. Blowing down for short periods once or twice on a trip is usually quite sufficient, except in areas where the water is very bad, in which case it may be necessary rather more often. It is a good practice to blow down the boiler just before the engine leaves the running shed for the traffic yard, as the concentration of salts is then normally higher than on the completion of the previous trip. Some running sheds and water stations are provided with special paved areas for blowing down, and at such sheds and stations care must be taken not to blow down boilers excelt in these areas.

It is always important to remember that the driver is solely responsible for blowing down the boiler, and he must give this operation his full attention. He should ensure that there is ample water in the boiler and in the tender, and that the injectors are shut off. The blowdown cock should first be tested by opening and closing it very slightly several times, and the lever should then be moved slowly until it is fully open. The cock should be closed slowly, as otherwise there is a danger of water hammer damaging the cock or its fastenings.

[Chris Janisch]

Interesting that in steam days some sheds would have blowdown tunnels, and all locos leaving shed were required to stop and blowdown before going out on the road. These were boxes with chimneys, so that the steam escaped safely upwards. The classic example was the double apparatus at De Aar. There were also single ones eg at Sydenham.