The Cornishman Express with Clan Line: 17/06/18

Hi Alastair, priming is fundamentally when the water level in the boiler is sufficiently high (I am sure you know it fluctuates all the time), so that some water is carried over through the main steam pipe, through the superheater and valves and into the cylinders. The problem this gives is that water is non-compressible, unlike steam. If sufficient water is carried into the cylinders, it can physically damage them, or the valve chests. It is clearly undesirable, at least. Please note that not every occurrence of priming is disastrous though it should always be avoided if possible by careful management of the boiler water level. Certain classes of engine, by virtue of the design of their main steam pipe, steam collector and regulator valve, were more susceptible to priming, or to put it another way, needed more careful management of the boiler water level than others. Even if some water is carried over into the cylinders, it may a. by virtue of only a small quantity do no damage b. stress the packings that keep the cylinders steam tight so that small steam leaks are seen/heard following the priming episode or c. do more significant damage to the cylinders, valves or associated rods and valve gear.

To keep this in context, if an engine is priming this does not mean that only water is being carried over into the cylinders - there is always a mix of steam and water - and it is the composition of this mix that determines how serious a priming event has occurred. Clearly all water would be an unmitigated disaster! So the term priming is used to cover all eventualities from a tiny % of water getting into the cylinders to a large %. When an engine is running reasonably fast, let's say at 45-50 mph there is a significant volume of steam moving through the system and a low level of priming may have less physical effect on the mechanisms of the engine than at comparatively low speeds when the % of water relative to steam may be higher.

There are other spin-off problems with priming - it can make the regulator, the main steam valve admitting steam from the boiler to the cylinders, difficult to adjust/close or impossible to close in serious cases (there are a couple of well-rehearsed instances of this where engines at slow speed have become uncontrollable through the regulator - Blue Peter is probably the best known example). If priming occurs the options open to the crew are simplistically a. close the regulator, b. reduce the cut-off though this has some implications c. open the cylinder drain cocks - these are provided to clear condensed steam when the engine has been stationary and the cylinders have become or are cool.

I suspect it seems simple to think that boiler water level is a simple thing to control, and therefore priming is easily avoidable. In a simple environment this should be so, but even things like a rough shunt - when the engine stops much more abruptly than expected causing a surge in the water level, or a significant change in gradient can cause the water level in a boiler to behave unpredictably/unexpectedly.

This is not a simple subject and apologies for the length of the reply. Please imagine you are the fireman on an express engine with a significant load on an undulating road. As well as working with your mate, the driver, to make sure there is enough steam when he needs it, you have to manage the water level in the boiler by the aid of a simple sight tube with, if you're lucky on a Bulleid pacific, a light behind it. You do this by checking the rate at which water level drops as steam is produced and balance this with the use of the injectors. You do this whether it is blazing sunshine or pitch dark. As a fireman, your foremost duty is to ensure there is sufficient water in the boiler so that it is safe. You then try and balance the water that you are putting in against the steam that the driver is taking out. I hope you are getting a sense of how this is not a simple process and needs skill and experience. As previously, I am not interested in a debate over whether 35028 was or was not priming - there are clearly some very informed people on this thread. It is not clear to me whether they were on the train, lineside or have simply looked at a video but as I say, they have their opinions.

This post is my attempt to de-mystify priming as requested by Alastair. This is not meant to be the Handbook for Locomotive Engineman. I am sure all comments that add to the understanding of the priming phenomena will be welcome.

Mike

 

Many thanks, Mike, for taking the time to give an excellent explanation. Very much appreciated.

 

An excellent and comprehensive reply

 

Fun digging back through years and years of EXD-PLY runs. Many were bedevilled by pathing stops at DWW. Reason I looked was I thought Cromwell+POB+8 was the previous westbound record holder. Found my timings for the Royal Duchy, 29th September 2012, and they show EXD P6 -PLY P6 in 62m 28.56s on a proper timing day, so not just spot times. Cold start from EXD too as 70013 had been standing for 55 mins.

 

Help. Does anybody know who the engine crews were on each leg? Thanks

 

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