Hi,

Can anyone explain why a brake pipe leakage test is undertaken after making an approx 100kpa reduction and cutting out the feed valve rather than cutting out the feed valve when the brake pipe is fully charged at 500kpa?

Thanks.

Hi,

Can anyone explain why a brake pipe leakage test is undertaken after making an approx 100kpa reduction and cutting out the feed valve rather than cutting out the feed valve when the brake pipe is fully charged at 500kpa?

Thanks.

If you consider a train coming down a mountain, recharge time is a critical factor(the time it takes to get from 400 to 500 kpa) hence you are more concerned with the integrity of the brake pipe, when the brakes are applied.

Hi,

Can anyone explain why a brake pipe leakage test is undertaken after making an approx 100kpa reduction and cutting out the feed valve rather than cutting out the feed valve when the brake pipe is fully charged at 500kpa?

Thanks.

Its been a long time since I worked as a driver.  Two aspects need to be considered and they both relate to the type of BV fitted to the loco, but both were similar with the prospect of a different result.  

With 26L types, if a brake pipe leakage was severe, and was not picked up with the cutting out of the BV going down a grade with the cut out valve in the normal position, the brakes could release on you as the pressures changed.  That's a very rough description of what could happen and did happen as the 26L came into operation with the 47 & 422classes and a vast amount of old Triple valve and early relay valves were in operation.  

With the BV cut in, the over riding features meant the BP gauge would remain constant, and you would know of a brake release on the vehicles until the trains momentum changed and you started to pick up speed, which meant some quick remedial application was needed and closing the valve. Thankfully I never had a problem, but had many a trip that had vehicles cut off owing to leaks.

The older brake valves though, while the handles were in lap position after a reduction was made, if there were leaks on the train, you would see the gauges go down, and that tended to not so much give you harder braking but it meant on long grades that you may have to bring the train to a stand, try to hold it on the engine brakes, until the train started pushing you, then release the engine brakes and wait for the pump/compressors to charge the train.

The introduction of the MR hoses to the trains meant it was very rare to ever not have a train fully charged owing to the extra air available to recharge the auxiliary reservoirs, which on a BP only train/vehicles was done through the feeding from the Brake Pipe.

On steam there was a lot of skill needed, and to a lesser extent on non working Dynamic Brake diesels. Dyno's working gave you a faster compressor charge which was a bit help.

Hi,

Can anyone explain why a brake pipe leakage test is undertaken after making an approx 100kpa reduction and cutting out the feed valve rather than cutting out the feed valve when the brake pipe is fully charged at 500kpa?

Thanks.

The simple answer.

The brakes are applied i.e. bp reduction, so that everything is under pressure. Brake cylinders, relay valves (if fitted), reservoirs. pipework, everything is under pressure, so if there is a leak it will be obvious.  With the brakes released, only the brake pipe, triple and reservoirs are under pressure.

The feed valve is cut out so that there is no chance that any air loss is replenished particularly with maintaining brake valves (26L and later).  You're testing the train for leaks not the loco, that's should be a separate test.

Because the "relay" equipment on modern trailing rail vehicles uses the BP as a constant supply of air to maintain against a BC leakage on each vehicle if there is a leak. In other words, once the BP is reduced, the BP is only acting as a set air supply to maintain air to the Aux Res if air is leaking from the cylinders on the wagons.

And more other words - if you dump the air from a train with loco attached, the wagon brakes will leak off and not be maintained. If you give a set BP application of only 100, the BP will maintain the wagons against BC leakage and the brakes will stay on as long as the loco compressor works.

Because the "relay" equipment on modern trailing rail vehicles uses the BP as a constant supply of air to maintain against a BC leakage on each vehicle if there is a leak. In other words, once the BP is reduced, the BP is only acting as a set air supply to maintain air to the Aux Res if air is leaking from the cylinders on the wagons.

And more other words - if you dump the air from a train with loco attached, the wagon brakes will leak off and not be maintained. If you give a set BP application of only 100, the BP will maintain the wagons against BC leakage and the brakes will stay on as long as the loco compressor works.

But only on loco's with a pressure maintaining feature, so basically those carrying 26L or newer equipment. The 48's and other's with a B7 without the PM functionality require handbrakes to be applied if detained on grade beyond 10 minutes.

And I thought the leakage was traditionally tested with a FS application, though my new mob uses 100kpa too...

modern trailing rail vehicles uses the BP as a constant supply of air to maintain against a BC leakage on each vehicle

Modern vehicles use main reservoir as a constant supply.

modern trailing rail vehicles uses the BP as a constant supply of air to maintain against a BC leakage on each vehicle

Modern vehicles use main reservoir as a constant supply.

Most, but not all. I work with some brand-new (well, less than 18 months old, now) wagons and they are BP-only. Even supposed "two-pipe" wagon's do not always have a suitable connection between the BP and MR, as was found in the El Zorro runaway down the Illawarra a few years ago.

modern trailing rail vehicles uses the BP as a constant supply of air to maintain against a BC leakage on each vehicle

Modern vehicles use main reservoir as a constant supply.

Depends, because older modern vehicles (for EG, those long retired) have supplementary reservoirs and MR pipes. Newer modern vehicles have no MR pipes, just a BP that provides air at a set pressure (IE, 500KPA) for the control valve on the wagon to to do what ever it is programmed to do (ECP). And other modern vehicles that have "Relayed equipment" will require a BV with a Maintaining Feature (as mentioned) to supply selected BP air pressure to alleviate cylinder or pipework leaks on those wagons.