Saturday, 18 January 2014

Vacuum Braking (6) Testing (1)

I have to admit to being mildly excited! I'd previously been unable to include an article on Sentinel 7109's Jet Pump (Vacuum Exhauster) device itself and only realised today when I'd completed a highly sophisticated (i.e. garage based!) compressed air test of it.

So here is the little sucker:
Penberthy GL-1 Jet Pump
It's not very big as can be seen from the picture (hopefully it will be big enough!). Steam (or air) enters from the right and exits on the left. It draws a vacuum through the sideways port.

For quite some time, I've had to be satisfied with the calculations I'd done in selecting this particular Jet Pump; however, it's not a technology I've been previously familiar with and have also been at variance from others who have fitted vacuum exhausters to Sentinels before. So my R&D has not been without some risk as to its success.

My first attempt at testing the jet pump was to connect it to an air compressor. However, this produced a poor result which I put down to the narrow bore of the air feed pipe being less than the bore of the jet pump itself. Thus it was never going to be able to feed sufficient air at the required pressure (around 60 psi).

By a stroke of luck, I found a scrap air cylinder at Midsomer Norton Station that had a 1/2" bore outlet and a valve. I considered that if I could pump this up with the compressor, I could then use it to feed the jet pump from its 1/2" bore outlet, at least for a short time anyway.

So here's the test rig in all its sophistication:
Jet Pump Test Rig
The air cylinder is on the left with the compressed air inlet at the top via the orange pipe fitting. There is also a non-return valve on the inlet. The valve controlled outlet is by the black air pressure gauge. The jet pump is above with an Ex-BR vacuum gauge on its suction port. The jet pump outlet is skywards.

This time I was much more successful (which led to my excitement that what I'd calculated actually worked!).
Vacuum gauge showing 21" Hg with about 66 psi air pressure
(It's easier to read on its side!)
I'm not concerned at the 66 rather than 60 psi. Neither gauge has been calibrated so the figures cannot be relied upon absolutely; however, I'm satisfied they are good enough to show the principle.

Here it is in video form: (also on YouTube).

Jet Pump Test

Some years ago, I visited the USA when printed 'T' shirts were becoming popular. I bought one with the message: "Engineer - Individual who turns abstractions into malfunctions". Hopefully not always!

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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