Steam & Excursion > Whazzit? (44) Cock / Valve

In railroad terminology, what's the difference, if any, between a cock and a valve? Why, if any, is it important to know?

-LD



Edited 1 time(s). Last edit at 10/22/20 14:44 by LarryDoyle.

To use an electronic analogy, the distinction is binary vs. analog
A cock, a mechanical device like a cock, can be massaged into a prtialy open state...
but it is designed to only be in one of two possible states: open or closed....like the
familiar cylinder COCK...drain valves...

Valves are traditionally made of two styles "globe valves" and "Gate valves" .
Gate valves are also intended to be opened fully. And not used as a 'regulating device'.
The gate valve is analogous to the Guillotine ---- when closed its 'blade' closes off the round
hole, connected to the two pipes.  The gate valve uses a screw fastened to the round handle ...
the screw move the gate across the round opening.   The gate valve is not designed. To
be used to regulate tge tgrough it...it should be either fully opened or fully closed.

Its body is more slender, and the gate ( moveable partition) is the moving part...
Gate valves are less expensive than the similar Globe valves..

Globe valves are designed to REGULATE the flow rate through the valve body.  The regulating 
element is a tapered-plug that is on the end of the screw-thread valve stem.  It IS desiged to
be a flow-regularuing device.

Typically, a globe valve is at its maximum opening in about 3 turns,
CCW, from “closed”..

That means the plug has fully withdrawn from the circular flow-port ( hole)--- thus, once the internal
valve port is fully uncovered, & turning the screw stem further, CCW, does NOT make the opening any wider. The flow is limited by the area of circular port, NOT the ‘movable plug’ attached to the threaded valve stem & handle...

Globe valves are excellent regulating devices.  A unique application is using two different size 
globe valves in a parallel arrangement .   The 'boost valve'' matches the pipe diameter.
Like an 1 1/4" globe valve will connected in parallel to a 3/4" globe valve ---- a typical arrangement 
for the fireman's stoker engine, speed-control valves.

Often times you need high pressure to bust up lump coal, and the larger valve is used to send
increased pressure to the pistons of the Standard Stoker engines.  Typically the smaller globe valve
gives more precise speed control to the stoke engine.

A white hot fire, and a steady draft up the stack will more thoroughly burn tge very slow rate
of feed of the crushed coal fuel.

Remember, the GLOBE valve will be opened to its maximum port opening in about
3 complete turns of the valve handle.

One more caution, when opening gate or globe valves, NEVER jam the valve stem at
the rear ( OPEN). stop.  Thermal changes can seize the sream in the body ... and making 
virtually impossible to determine if the valve is in the opened OR closed position.
Never jam, or "Back-stop", the valve stems of gate or globe valves.  If the handle seems
too loose, tighren or replace rye valve stem packing to add resistance to a too-loose
valve stem...

​W.

(to be proofed yet)



Globe valves use 



Edited 3 time(s). Last edit at 10/23/20 10:58 by wcamp1472.

I may be wrong but always thought that a cock indicated a drain function, whereas valves do what Wes explained.

Not necessarily....see the use of the 'angle cock' at the ends of the car,
in the brake pipe.

"Angle cocks" have the straight-inlet connection screwed onto the brake pipe,
The angled portion has the brake hose screwed into it..

Cocks are intended to be fully opened, or full closed, typically with a 90-degree
turn of the handle.

However, using cocks as you describe, is a perfectly suitable application.
And they are much less expensive ( compared to globe and Gate valves)... to manufacture
and to market.  
The use of cocks can be a wise and economic choice ..

Wes C.

 



Edited 3 time(s). Last edit at 10/22/20 16:33 by wcamp1472.

wcamp1472 Wrote:
-------------------------------------------------------
> To use an electronic analogy, the distinction is
> binary vs. analog

That nails it. A cock turns a flow or device "on" or "off". A valve regulates a flow. For example, a brake valve regulates the flow of air to or from the brake pipe, while a cut out cock enables or disables the brakestand.

-LD

Wonderful, you both passed basic hydraulics.

Out 

May be  more applicable to the field of  "Fluid Dynamics"?

W.

Nah, we teach not to use a cock for metering purposes very early in both basics and troubleshooting. Then we get on to even though a gate valve may have a
"v" closue it's still not a knife edged orifice, 

Out 

Agreed....
Different purpose intended for common gate valve applications.

(Additionally, typical gate valve installations exclude common gate valves from compressed-air applications...)

W.

Posted from iPhone

It's been said that "Nothin' in railroading is like anything else."  This axiom especially applies to cocks. 

Railroad standard for the position of the handle of a cock is to set the handle parallel to the pipe to stop flow thru the controlled device, and set the handle perpendicular to the the pipe to permit flow.

BUT!  There is one exception to this rule:  Air brake angle cocks on the ends of cars and locomotives are parallel to the pipe when open, and perpendicular to the pipe when closed.

BUT!  (Again)  There is one exception to that exception, which can sneak up and bite you!  The cock on the front of locomotives may be set up to be closed with the handle parallel to the pipe.

Got that?

-LD

I had learned that the original reason for the angle cock handle at 90-degrees
was so that anything ( while a moving train) that may strike the handle,
would tend to open the handle and set the train brakes...

But more modern variants use a positive-intent, lock-out handle.
Some current angle-cock handles must be raised to clear a cast-block, that the handle
must be cleared before allowing rotation of the handle  ---- exhausting trainline air,
& applying the automatic air brake.

W.