Inspiration for a Steam Valve Design

Looking for inspiration for a steam valve design. Thought I would share a bit of a search of the internet, books and reference papers.

I’ve already discussed the different options for the Burrell Steam Valve principle of operation:

  1. Make a valve part of the saddle
  2. Fit a standard steam valve to the side of the saddle
  3. Fit remote standard valve and run pipes back and forth to the valve
Burrell steam valve schematic

Option 1 is the favourite as it keeps the overall design closer to the original engine.

My simple schematic of how this could work is shown to the left. This shows a top down section.

The steam comes vertically out of the boiler through the dash outline hole. At the top of this hole is the safety valve. This means I have to ensure the valve can never block it.

Burrell steam valve
The phosphor bronze saddle has two 3.3mm holes through the vertical post. Shown as the orange rectangle in the schematic.

Background to Valve Designs

spirax sarco steam valve

Steam Valve Control – a look at the different rotary and linear steam control valve designs.

Spirax Sarco work in steam control, heat recovery for a large number of industries, including: food producing to oil refining, beer making, and drug manufacturing companies.

They have some great resources freely available to learn about steam: spiraxsarco.com/learn-about-steam

One thing to note from the Spirax Sarco designs is the rounded nose of the plunger to improve the flow.

Flow Control Characteristics

flow rate versus valve opening percentage

Spirax Sarco share a few different valve design shapes and then show flow rate versus valve opening.

A linear flow percentage versus valve opening is achieved by having a valve open area that changes linearly with valve mechanical opening percentage. eg a 10% mechanical rotation of the valve gives a 10% open area, a 50% mechanical rotation gives 50% open area.

This open area has to have the same flow characteristics. If the flow changes dramatically due to shape change or flow rate then this is likely to again make this non-linear.

Corrosion

Don’t just think water and heat. When oxygen and carbon dioxide (CO2) are present they can make the hot condensate very corrosive. Therefore I need to make the valve from stainless steel, phosphor bronze and brass.

Conclusions

I think this has given me inspiration for a steam valve design for the Burrell miniature traction engine. Some points to consider are:

  1. Smooth the steam flow path.
  2. Can the valve reliably shut steam flow off completely?
  3. Does the valve provide enough control?
  4. Does the valve work at all pressures?
  5. Are all parts resistant to corrosion?
  6. Control rod must seal and still freely operate.

References

  1. Steam Valve Control – Spirax Sarco have some in-depth pages on steam and steam control.
  2. Control Valve Characteristics – a look at the valve shape and how flow rate changes versus opening. This is another great page from Spirax Sarco.
  3. Common Control Valve Problems – more about the control system and actuator, but the description of the physics is clear.
  4. Some Aspects of Steam Condensate Corrosion – article from Materials Performance that looks at all of the chemical and physical aspects of steam corrosion.

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