Blast Nozzle

[Halton TankParticipant @haltontank]

Does any one know the formula for calculating the diameter of the blast nozzle for model steam locomotives.

Regards

Luigi

[Halton TankParticipant @haltontank]

Calculation of Blast Nozzle Diameter

[michael howarth 1Participant @michaelhowarth1]

Luigi…..Henry Greenly gives it as the ratio of the cross sectional area of the cylinder to the cross sectional area of the blast pipe. In general terms he specifies a ratio of 1:30 for "very small models" and 1:25 for "larger engines". He also published this in the form of a table enabling you to read off the blast pipe size according to cylinder diameter. If you post your cylinder diameter I will tell you what he says. However, only small modification to the blast pipe diameter can make large differences to steaming so these figures should only be rgarded as a guide.

Hopefully someone with greater experience of running locos will come in and advise you further.

Mick

[Halton TankParticipant @haltontank]

Thanks Mick,

The piston diameters are 1.6 in and 1.7 in (nothing is the same on either side on this loco) and the current blast nozzle diameter is 7.6mm.

Your mentioning Henry Greenly reminded me that I was given one of his books some time ago which I have not had the time to read yet. Trouble is 'Head of Catering' has relocated the book in her tidy ups and I can't at the moment put my hands on it.

Regards

Luigi

[michael howarth 1Participant @michaelhowarth1]

Luigi…..Greenly gives the size of the blast pipe orifice for a 2 cylinder loco as 0.250" for 1.5" diameter

cylinders and 0.281" for 1.75" diameter cylinders. For a 3 cylinder loco he gives 0.312" and 0.344"

respectively. I note that your cylinders fall between 1.5" and 1.75" so I presume the smallest size will

be the obvious starting point.

Mick

[julian atkinsParticipant @julianatkins58923]

hi luigi,

the greenly formula is ok as far as it goes, but experimentation in actual use will be required. a lot depends on how freely and quickly the exhaust steam escapes from the cylinders, how big all the passages and pipes are, and the quality of the valvegear and its setting.

there is also a vital connection between the diameter of the blastpipe and the distance between the petticoat pipe 'choke', greenly's 1:3 and 1:6 rule also being applied.

a small difference in blast pipe diameter (1/64" at a time) can make a huge difference to how the loco behaves.

what is the loco you are building?

cheers,

julian

[michael howarth 1Participant @michaelhowarth1]

Julian …I am glad that you have stepped in with your advice as although I had the reference I am not well up on experience. What dimension would you use as a starting point given Luigi's set up?

Mick

[Halton TankParticipant @haltontank]

Mick and Julian,

Thanks for all help, I see this is going to turn into a suck & see exercise.

The loco is a Halton Tank (hence the user name) designed by Greenly and I bought it in 2009. It came with a litany of problems the main one being its a shy steamer. After a lot try and error I have managed to found the main reason for the poor steaming was a poor seal around the snifting valve.

During the investigations I and members of my club did check the blast nozzle height and that it lined up with the chimney – it was a little high and I lowered it as much as I could.

One problem is that loco is not exactly built to drawings and the blast nozzle is one area that does match the drawings. The drawing calls for 9/32" dia which works out at 7.14mm so it seem that 7.6mm nozzle would be a bit wide.

Currently the loco is in bits – it had only one side tank, the other being a dummy. I am converting the dummy tank in a working one and give the loco a repaint. Once the loco is running I shall experiment with size nozzles starting around 7.2mm dia.

Regards

Luigi

[julian atkinsParticipant @julianatkins58923]

hi luigi,

ive driven a halton tank many years ago on the SMLS. a big powerful loco but one that needs quite a bit of work to get it to run efficiently from the original drawings.

greenly (who designed the halton tank) used to go a bit overboard with large cylinders, so i wouldnt use the cylinder capacity as a starting point for the blast nozzle size.

the other point i would make is that the exhaust circuit needs to be generous and easy, with no restrictions. there is no point in aiming for a generous blast nozzle if there is some restriction further back down the line starting at the steam passages where they meet the cylinder bores and covers. so one needs to look at the whole picture. a generous free gas flow percentage through the boiler tubes is also required IMHO.

from a practical point of view there is a limit to how low down the blast nozzle can be and still be clear of accumulated smokebox ash. the petticoat pipe length usually needs altering and the chimney diameter to get the proportions right. a slight outward internal taper of 2 degrees above the choke is thought by many to be beneficial.

i big loco like yours ought to have at least a 5/16" dia blast nozzle. one of my locos (the GWR one in the avatar) has only 1 3/8" bore cylinders and the blast nozzle is 5/16" dia. i would be disappointed not to get a 3/8" dia blast nozzle on a halton tank. so long as the draughting proportions and comments about the exhaust circuit are complied with then 3/8" dia will give a free steaming nicely behaved loco. a free unrestricted 7/16" ID for the exhaust pipes and connections (1/2" dia pipes) will be required.

i also mill to a slot the drilled steam passageways between bore and port face.

applying the 1:3 rule for the distance between blastpipe and petticoat pipe 'choke' is easily done by making a template. the halton tank has a relatively short chimney so this proportion is crucial.

it is essential that the blastpipe should be concentric with the petticoat pipe and chimney.

although the exhaust pressure of our miniature locos is much lower than fullsize, because the boiler pressure is also lower any back pressure is just as important IMHO. what you are aiming to do is to reduce back pressure and at the same time maximise the draughting effect from the bigger blast nozzle. i try to maximise the speed of the exhaust steam with as low a back pressure as possible.

anyway that's how i approach these things.

cheers,

julian

[Halton TankParticipant @haltontank]

[snip] from a practical point of view there is a limit to how low down the blast nozzle can be and still be clear of accumulated smokebox ash. [/snip]

Julian, with a smokebox 11" long I have no worries about the blast nozzle getting buried in ash. LOL

This engine has not been built exactly to drawings, it has slide valves instead of piston and round top boiler instead of Belpair. Once I had cured the smokebox leak she steamed quite well and when warmed up she gallops along.

Next season I shall experiment with different nozzles to see if I can improve matters.

Thanks for all your help.

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