THEA

from

Pressure Retaining Valve. HE pressure retaining valve or, for short, the "retainer," is a simple part of the air brake and differs the rest in being generally used only when descending grades requiring much use of the air brakes. Like the roof of the house in the "Arkansas Traveler," it never leaks when it is not needed, and when it is needed, there is no time to fix the leaks. Leakage has been and is yet the serious fault, yet it is rarely in the valve itself, but instead in its pipe, the brake cylinder or both.

Even down very moderate grades where little braking is required the use of a few. retainers at the head end aids in making a steady run and keeps the train bunched, in the latter decreasing break-in-two's and, where such occur back of the air, serious subsequent collisions.

Another quite common use is on the driver brake, its purpose there being to decrease the likelihood of breaking in two when releasing the brakes on a long train at low speed.

Figs. 1 and 2 show the modern 15-pound valve. Fig. 1, handle 5 down, is the normal or non-operative position and is, so far as its effect on the brake is concerned, the same as though it was not on the car.

At X the valve is screwed on to a pipe the opposite end of which is connected with the triple valve exhaust port. Consequently, every time the brake is released the brake cylinder air passes from the triple valve into this pipe, enters the valve at b, passes to the left through port

a in the key or "plug" 6 and discharges freely at port c.

The port a in the key 6 is shallow but wide, the latter dimension making up for the scant depth that causes it to appear quite small in the illustration. Two advantages of the shape of the port, as compared with the old one, which was round and formed a tee opening through the key, are that it is stronger and wasps cannot as readily get in, build a nest and thus prevent the brake from releasing. Some little trouble from this cause has been experienced in various parts of the country where cars have stood idle for a time.

In Fig. 2 the handle 5 is "turned up" as is said, meaning the horizontal position shown. The valve being located on the brake exhaust port, turning up its handle can have no effect until after the brake has been applied and the triple valve then moved to charging and release position.

The need of the retaining valve lays in the facts that the auxiliary reservoir must occasionally be recharged while descending the grade; that to do so requires moving the triple to release position; that while in this position the triple valve will let the air out of the brake cylinder very much faster than it will into the auxiliary reservoir; that, without the retainer, on reapplication an empty brake cylinder would have to be filled and that under such conditions it would not be possible to hold heavy trains down steep grades with the air brake. The pressure retaining valve reduces these disadvantages in the following manner:

When the triple valve is moved to the

release and recharging position the brake cylinder pressure entering the retaining valve at b is now turned to the right by port a in key 6 and reaches the upper portion of port b just below the stem

4

port d is so small that the fall in brake cylinder pressure is very slow. Even without any weighted valve 4 the very slow release of the brake that would take place would alone aid greatly in keeping the train in check while the auxiliary reservoir was being recharged. As a matter of fact, a series of observations made in 1899 from the rear air brake car, a caboose fitted up for the purpose, of a large number of freight trains handled, with one exception, by the regular engineers and with no preparation, everything being taken just as found, showed that in many cases the work of the retaining valve, the standard type, on this rear car was practically all done by the small port d. The auxiliary reservoir had been recharged and the triple valve brought back to either application or lap position before the cylinder pres

Fig. 1. Pressure Retaining Valve “ Turned Down,” Release Position.

and seat of weighted valve 4, held in place and protected by the valve chamber cap 3.

Assuming that the brake cylinder pressure was some 30 or 40 pounds at the time of recharging, the weighted valve 4 would be lifted and the pressure would flow into the chamber about the weight. From this space the only escape is through port d, only one-sixteenth of an inch in diameter, to the atmosphere.

Here we find the first advantages of the retaining valve and, also, the first operative point of superiority of the modern over the earlier valve of this type, many of which are yet in service. The

17 sec. 30 lbs. to 18 lbs. 11 sec. It will be seen that within reasonable limits, the ordinary disadvantages of longer piston travel are somewhat reduced when the retainers are in use.

In

The effect of the larger volume had with 10, 12 and 14-inch cylinders is quite noticeable with passenger trains. order to prevent too much slowing down and consequent loss in time the recharge must be commenced before the sharp curve or let up is reached. It is not the fifteen pounds that makes the train drag if the grade is steep, but the higher pressure which reduces slower the larger the cylinder.

In the older retaining valve previously referred to there are two large ports in place of the small one d in the modern valve. As a result the pressure falls very rapidly with such.

A peculiar action of the modern valve is the puffing that occurs while the discharge is taking place. When the weighted valve 4 raises, the pressure flows into

the space about it faster than it can escape at the small discharge port d. The consequent accumulation of pressure in the cap 3 sufficiently balances that below: the valve that the latter seats and cuts off the flow. The immediate fall in pressure in the cap 3 causes the weighted valve 4 to quickly rise again and the rapid succession of this action results in the puffing sound.

If the triple valve is allowed to remain in the release and charging position long enough the brake cylinder pressure will fall to the closing point of the weighted valve 4, being about 15 pounds. Here is the second operative improvement over the old valve, as it closed at 10 pounds. While the 10-pound valve answered fairly well in its day, its generally worn and defective condition and greater train tonnage warrant at least repairs and change to the 1-16-inch retaining exhaust port. The better plan is to replace it with the 15-pound valve.

There often exists the wrong impres sion that the large car calls for a heavier than 15-pound retainer. The brake shoe pressure is greater with the large than with the small car and this increases proportionately the effectiveness of the same retainer. The 15-pound valve is the heaviest and only type made.

It may appear to some that 15 pounds ought to do little holding. That this is an error will be seen from the fact that 15 pounds is one-fourth of the total brake cylinder pressure possible from 70 pounds train pipe pressure, and an emergency application with quick action triple valves.

The use of the retaining valve not only reduces greatly the amount of air needed to keep auxiliaries recharged, thus saving pump labor, but gives about 6 pounds more in full application than would otherwise be got from the same auxiliary pres

sure.

It is not expected that these valves will hold in check the speed of a heavy train on a steep grade, but they will render sufficient aid in this line to enable the auxiliaries to be recharged before the speed has seriously increased. Of course, this is assuming that the brakes are in fair or good shape, that engine brake equipment is what it should be and that the engineer exercises a reasonable amount of good judgment.

A very common supposition is that retaining valves hold better when a train is standing than when it is running, yet the

opposite is the fact. When a car is quiet the weighted valve coming to its seat gradually may cock slightly and let all the pressure escape, yet a slight jolt would send it to its seat. The jar of the moving train has this latter effect.

From the construction of the valve it will be seen that it can only act properly when in a vertical position.

The small size of the retaining position exhaust port d permits of it being more easily stopped up than the old and larger one. The effect of this is apparent only when the retaining valve is in use, and is, where there is little or no leakage in its

Testing Retainers.

To test the retainer on a standing car, turn up the retainer handle, charge the auxiliary to 70 pounds, make a reduction of 20 pounds, move the triple to re lease and recharging position, wait at least a minute, but not over two, then

see if the brake shoe can be moved sidewise on the wheel by a push with one's foot on the end of the brake beam and note the strength of the exhaust on turning down the retainer handle. If a pressure of 15 pounds is held the brake shoe cannot be moved and the exhaust will

16-8

+

-6-81

16/10 +

Station

=

Stationa

20

Train Pipe Chart No. 25.

pipe or the brake cylinder, to retain practically all of the pressure. Overheated, cracked and slid wheels on that car may result.

The overheating is the warning that should call the trainman's attention. At the next recharge he should observe whether or not a discharge takes place and, if not, should open the port with a pin or fine piece of wire.

A triple valve which blows at the exhaust port (retaining valve) can also cause too much pressure to be held and result similarly; but this is no fault of the retaining valve.

be strong when the handle is turned down.

Two minutes should be ample time for recharging a train. If it is not, some serious conditions need correcting.

Speed and Grain Pipe Charts.

Two speed and two train pipe charts selected from the record of the tests before referred to are here reproduced. Though interesting and instructive in many ways, particular attention is called to the effect of the long as compared with the short holds, and using full release