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Steam & Excursion > Safety valve innards?


Date: 01/19/25 09:09
Safety valve innards?
Author: PumpkinHogger

Seen atop a locomotive (a 4-6-0 I seem to recall, didn't take a snap) at the National Railway Museum in Adelaide Australia last week.

Is this pretty much what is normally hidden under the dome? 




Date: 01/19/25 10:37
Re: Safety valve innards?
Author: wcamp1472

A very early & crude, non-typical  in American practice.
Probably a device applied early 1900s, of European origin.

Common US practice has each spring-loaded valve located inside a 
closed cylindrical casting.  American practice includes two adjustments:
A spring-tension adjustment and a lift reaction ring.

The  reaction-ring is an internally-threaded ring that allows 
adjustment  of  the re-seat pressure.   A normal re-seat pressure 
would be adjusted such that the re-seat pressure is 3 to 5 psi,
lower than the pressure that lifted the safety , to limit  boiler pressure.

If you had a safety valve that lifted at 250 psi, the open valve has a slightly 
larger lift-area, when open, and allows seating pressure very close to 
the lifting pressure ---- without an adjustment method, a safety would
quickly seat ---- and very soon would open again, beginning a cycle 
of quick, open-close chattering.

American practice includes an adjustment ring that can be set,
allowing for 3 to 10 psi re-seat pressure differential.  That way,
a lifted safety  will lower the boiler pressure enough so that it 
will not lift again, until pressure returns to the spring lifting pressure.
A  valve that opens at 250 psi, will be set to close, after boiler pressure
has  been lowered by approximately 5 psi, below it's lifting  pressure.
Eliminating open/close cycles that are too close together, and liable 
to a chattering  sequence . 

First, the lifting pressure is adjusted to the designed pressure,
then the 'reaction' ring is adjusted to allow seating, once pressure 
has been lowered by 5 to 7 psi.   

Multiple safety valves are set at successively higher pressures, and must 
limit the maximum allowed boiler pressure.   American practice includes a minimum 
of 2 safetys, per boiler and IIRC,  UP 4000s have 5 safety valves, each one set to higher
relieving pressures. and are the largest diameter valves and openings.

The reaction-ring plays a vital role in assuring smooth, re-seating pressures --- to eliminate 
too frequent openings, close to the "set" pressure.   When 'setting' multiple safety valves, on one 
loco,  gags are used to hold the lower pressure valves 'closed', while the highest pressure 
safety relieving pressure is set to it's highest pressure.  Then, the gag on the lower prsssure
 valves, are adjusted to their intended lifting pressures.  So, with 3 safety valves, you could 
have 3 lifting-pressures:  250psi, 254psi & 260psi...  or a similar stepped-sequence of limiting 
boiler pressures.

In 1970, while going around Horseshoe Curve, Pa, with High Iron Co. excursions-
with NKP Berk, #759 ( unassisted) ,  I had 3 safetys lifted, while going by the huge
crowds in attendance.

The public understands about safety valves, and I wanted to let them know where
our boiler pressure was--- THE MAX!   Even over the roar of the 3 safeties,
I could see & hear the crowd cheering!

W.

not proofed, yet.



Date: 01/19/25 11:06
Re: Safety valve innards?
Author: RBMN-ENGR

Ramsbottom safety valve. Invented by John Ramsbottom of Great Britain in 1855. There are a few videos on YouTube which explain how they work. He also invented the split metal piston ring.

Posted from Android

Chris Bost
Leesport, PA



Edited 3 time(s). Last edit at 01/19/25 11:57 by RBMN-ENGR.



Date: 01/19/25 21:10
Re: Safety valve innards?
Author: PumpkinHogger

Thanks much gents for the learned replies.



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