Thursday, 7 November 2013

Vacuum Braking (4) Design (3)

Having settled on the GL-1 Jet Pump ejector running at 60 psi in Vacuum Braking (3) Design (2), here are the next three challenges:

1. How to connect the Ejector's American NPT threads to UK BSP threads?

2. Selecting a Pressure Reducing Valve to take the 275 psi boiler pressure down to 60 psi for the ejector.

3. Selecting a Vacuum Relief Valve for making sure that the Jet Pump does not draw more than 21" Hg of vacuum.

1. The GL-1 Jet Pump Ejector has a 3/4" steam inlet pipe thread and 1" exhaust and suction pipe threads. The trouble is that the American NPT and British BSP threads are not generally compatible and Sentinel 7109 uses BSP threads.

There are two challenges here: the pipe threads themselves and the need to be able to assemble and disassemble the ejector from the rest of the pipework. Initially, I'd started to look for simple female NPT to female BSP pipe couplings. However, it occurred to me that, if I could find a pipe fitting supplier with both NPT and BSP threaded unions, the thread linking the union halves might be the same. Thus it would be possible to create a union with NPT thread at one end and BSP at the other.
Mixed NPT-BSP Union halves
After a lot of internet searching, I eventually found Nero Pipeline Connections Ltd who seemed to have what I was looking for in stainless steel. I rang Nero and a very helpful Daryl offered to go and actually try a NPT and BSP union together. He rang back proclaiming a success with a proviso that I might have to do a little fitting to ensure a good seal between the dissimilar union halves. This was the sort of service I needed and have been able to obtain what I needed as in the photo above. Having tried the dissimilar halves together, there does not seem to be any need to persuade them to fit with each other.

2. The Pressure Reducing Valve not only has to drop the boiler pressure from 275 to 60 psi to suit the ejector but it also has to be able to let enough steam through for the ejector to do its job (and possibly a bigger ejector if ever needed).

I'd been guided towards Spirax Sarco as a suitable supplier partly because Gervase was already using one successfully but also because another Sentinel had a different type which had the persistent habit of blowing a continuous Raspberry! This was not a particularly attractive feature and one which was worth avoiding if possible (I'll diplomatically not say which Sentinel has this feature!).
Spirax Sarco BRV2S rated at 276 psi and 212 Deg C.
with Orange hat.
The beast is in the photo above. It arrived very quickly from BSS in Gloucester, UK.
Two optional features had to be chosen:
(1) To ensure sufficient steam flow was possible, I chose a 1/2" type easily capable of supporting a GL-1 ejector and even a much bigger GL-2 if found to be needed later.
(2) To be able to set the 60 psi outlet pressure, I chose an 'orange' rated spring allowing a range from 3.5 to 8.6 bar (60 psi = 4.1 bar).

The full specification of the BRV2S can be found here.

3. The Vacuum Relief Valve has to let air into the vacuum pipework when the 21" Hg level is reached. It also has to be able to let more air in than the ejector can pump out so there is a size factor too.

It took me a long time to find a supplier of a suitable device. Eventually, I found Flowstar of Kingston upon Hull, UK, who distribute products made in Hamburg, Germany, by Niezgodka GmbH. The Type 91, size 1 with a Viton seal and 3/4" male thread fitting is the chosen one.
The Niezgodka VRV data sheet has most of the detail while an additional data sheet covers the discharge capacity (2nd column under '18'). Note: 1 cu metre = 35.3 cu feet).
The size 1 type is good for 50 cu metres/hour = 29.6 cu feet/minute. (I enquired about the empty cells in the discharge capacity table and, for the size 1, 50 cu metres/hour also applies at greater vacuum levels than -0.6 bar). 29.6 cu feet/minute is plenty to overcome the suction possible from a GL-1 ejector or a GL-2 should it ever be necessary. So at least I won't have to replace all the parts should I find I need a larger ejector after all!
Niezgodka Type 91, size 1
Note: the Penberthy Technical Data Manual (page 9) shows a graph which indicates that a GL-1.5 ejector running on 60 psi is capable of 13.5 cu feet/minute at a suction pressure of 21" Hg gauge (= 9" Hg Abs.). Therefore it is safe to assume that a GL-1's capacity will be less than 13.5 because it is smaller. A GL-2 is not twice the size of a GL-1.5 so the 29.6 cu feet/minute of the VRV will be more than a GL-2 can remove.

Next, I'll look at the boiler's isolation valve, the reason why a curvaceous syphon pipe is used with a steam pressure gauge and possibly at the driver's brake valve.

No comments:

Post a Comment

Related Posts Plugin for WordPress, Blogger...