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.

Friday, 1 November 2013

Vacuum Braking (3) Design (2)

Having laid down the requirements and played with some design ideas, now it's time to start making some decisions and begin the shopping.

The decision from which follows all the other design choices is the size and capacity of the Vacuum Ejector (Jet Pump).

I've decided to use a bronze Penberthy GL-1 Jet Pump operating at 60 psi and which has a vacuum pipe of 1" nominal inside diameter. (see pages 8, 9 of the link)

This is why:
Extracted from the Penberthy Jet Pump Technical Data manual, this is the sequence of activities to determine the best item for the job.
So step 1 converts our requirement for four Mk1 carriages (16 cu ft/30 secs) into minutes/100 cu ft/min. which equates to 3 minutes/100 cu ft.

For steps 2 & 3, have a look at the table below (also extracted from the Penberthy Jet Pump Technical Data manual (click it to make it readable)):
Jet Pump Selection Chart
This next table of conversions is also needed in step 4 to determine the steam consumption (right hand column above) for Jet Pumps other than a 1.5" size.
Conversion of table figures for non-1.5" Jet Pumps
In the first table, I've highlighted the GL-1 column and the 60 psi 10" Hg Abs. row. (From the table, the GL jet pumps seem to be most efficient at around 60 psi. 10" Hg Abs. is equivalent to 20" Hg gauge which is the nearest figure to the 21" Hg gauge vacuum level to be achieved).
Two columns to the right of my highlight, is a figure of 3.7 for a GL-1.25 jet pump. Although the figure is fairly close to the 3 minutes/100 cu ft required, the GL-1.25 uses 2.9% of the boiler capacity (See Vacuum Braking (2) Design (1)). Whilst 2.9% is OK (135 lbs/hour), a GL-1 uses only 76 lbs/hour or 1.6% of boiler capacity. With these figures, it's worth having a look again at the original requirements.

(The lbs/hour figures come from the right hand column of the first table multiplied by the capacity factor for the size of jet pump in the second table. The first table is normalised for a 1.5" jet pump but converted for other sizes by the capacity factor to avoid having to produce a separate table for each jet pump size, e.g. a GL-1.5 at 60 psi working to give 10" Hg abs. vacuum uses 221 lbs/hour. A smaller GL-1 uses 0.344 x 221 = 76 lbs/hour - only 1.6% of the boiler's 4600 lbs/hour capacity).

Requirement 1 was for four Mk1 carriages, i.e. 16 cu ft of evacuation space in 30 seconds. In fact, 16 cu ft only needs to be evacuated fully when the system has been full of air. Once evacuated and the brakes applied, because the carriage vacuum cylinders are still partly evacuated, the full 16 cu ft volume does not need to be evacuated. Hence the requirement can be relaxed to some extent.

Requirement 1 assumed four Mk1 carriages. At Midsomer Norton, it is very unlikely that as many as four would be involved and two carriages or occasionally three would be nearer the mark.

Thus if two carriages are involved, a smaller GL-1 should be able to evacuate 8 cu ft in 30 seconds and reduce the steam consumption accordingly.

Arguably this is cheating; however, without an original requirement figure, it is very difficult to know what to aim for. Having gone round the calculation loop once, the implications of the requirement have now become apparent and show that there is a good pragmatic case for using a smaller Ejector - particularly if it is being run continuously.

Compared with the Penberthy No. 3 size water lifting ejector having a 1" suction pipe size, the GL-1 has the same suction pipe size but is specially designed for sucking air and not water. So, intuitively, the GL-1 is a similar size to the No. 3 type used on other Sentinels but should perform more efficiently.

Recalculating the figures for two Mk1 carriages with a GL-1 Jet Pump gives a time of around 0.5 minutes to evacuate two carriages whilst using 7.6 gallons of water per hour.

Well! That's the theory anyway.

These Penberthy Jet Pumps are imported by Jenex Ltd of Great Yarmouth, UK. I'm very grateful to Mark Collins of Jenex for helping me to order the right item.
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