Steam valve port size

[Eric CoxParticipant @ericcox50497]

What is the smallest width I can make the ports on a steam valve.

The loco is 3 1/2" gauge with cylinders 2" long and a bore of 0.0650"

[Eric CoxParticipant @ericcox50497]

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Eric Cox on 27/12/2016 10:04:26:

… and a bore of 0.0650"

Wow !!

[JasonBModerator @jasonb]

It's more the area than the width that will affect how much steam will pass through the port so length in the other direction needs to be known

[Eric CoxParticipant @ericcox50497]

Typo, should be 0.650", too much Christmas spirit. The length will be about 1/2".

[michael howarth 1Participant @michaelhowarth1]

This is a very interesting question as far as I am concerned. From what I have seen and read, the formulae for steam and exhaust port sizing does not seem to have changed from the days of Greenly and LBSC. Perhaps these eminent engineers were absolutely right but things do have a habit of moving on.

More recently the 16mm groups and the Gauge 1 fraternity have been using "smaller" ports consisting of just a single drilling per port with, by all accounts no discernible effect on performance. I have just completed a pair of gauge 1 cylinders where the steam port is a 1/16" drilling and the exhaust port a 1/8" drilling. Moreover the exhaust cavity in the valve is an easily cut 1/4" diameter, with a slot drill.

Drillings from steam port to cylinder also seem to have been reduced from two to a single drilling of about 2mm. This did nothing to reduce the power of my last loco.

I have previously made some calculations based on the 1/8" steam pipe from the boiler to the cylinders and in some of the older designs it seems to me that the "demand" from the traditional steam port drillings is more than the steam pipe can supply at a given pressure.

Having said that, these calculations are quite superficial and no doubt do not take account of many factors I probably haven't even heard of. I suppose if you are unsure it is best to stick with the tried and tested. It would be great to hear from someone who has looked into these issues with a more informed mind. Meanwhile wish me luck with my newly drilled "scaled down" cylinders.

Mick

[julian atkinsParticipant @julianatkins58923]

I would say that the principle consideration is the valve travel and amount of lap required on the valve. Lap is a function of the steam port width.

Perhaps an equally important consideration is the quality of the steam both incoming and as exhausted. Are superheaters going to be fitted to the loco? Will the steam passages cope with the exhaust steam, which will be of much lower pressure and will have expanded and may be wet. It will not flow as well as the incoming steam.

As Churchward once said any fool can get steam into a cylinder – it is how to get it out that matters!

For the size of Eric's cylinders I would use the port dimensions of LBSC's Juliet, assuming that Eric's valves and valve gear match with regard to lap and travel etc. Dont make the steam ports too small otherwise the loco will be constipated on the exhaust side when doing some decent work.

Cheers,

Julian

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