Friday, 17 January 2014

Vacuum Braking (5) Design (4)

One difficulty with Sentinels is that the 275 psi boiler pressure is above the 250 psi rating of many off-the-shelf valves. At 275 psi, water boils at 212 DegC so special materials are needed to withstand these conditions. Any valve that has to isolate a steam supply from a boiler has to be rated as such.

Three new ones are required on Sentinel 7109:

  1. A 1/2 inch type to isolate the vacuum braking supply.
  2. A 1/2 inch type to isolate the supply for a steam cleaning lance.
  3. A 1.25 inch type for the boiler's blow-down valve. (See later).
They were made in Switzerland by Valtaco and sourced from Poynton Valves. This is what they look like:
The 1.25 inch and two 1/2 inch ball valves
These are known as ball valves because the moving part of the valve is ball-shaped (but with a hole through). To begin with, investigation had sent me towards using gate valves as these are intended for isolating supplies rather than regulating a flow. However, gate valves are better suited to liquids at relatively low pressures. Ball valves can be built to be much stronger and are very effective at isolation duties.
Full bore when open
Those of you who have followed my blog for some time will recall that a blow-down valve had been obtained some time ago; however, my wariness about fitness for purpose of high temperature and pressure components has been heightened as the project has progressed.

The original blow-down valve was made from gunmetal with flange fixings. Whilst it looked the proper job, when I came to investigate the safety valve mounting flange requirements, I found that I would need a BS10 Table 'F' rated type made from Carbon steel. The Table 'F' flange is 1/2 inch thick which is somewhat more than the flanges of the blow-down valve. Add in that gunmetal is not as strong as Carbon steel and it becomes clear that the original would not be strong enough. Not having any specification to support its construction means that I really can not be sure it is good enough.

The ball valves pictured above are made from stainless steel with Carbon reinforced PTFE seals and are specified for 212 DegC and 275 psi. I thus have complete confidence that these valves will be strong enough. Also, to coin a phrase, they were 'Reassuringly Expensive!".

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