[Steam-engine] relief valves

[Bruce & Marlene Babcock]

Ken,

    Per your request a few days ago, I will offer a few of my "opinions" on
the subject of safety valves.

    First, in the 1920's and through the 1940's, when boiler manufacturers
changed from hand-fired boilers to automatically-fired boilers the
requirements for safety valves changed at the same time.  It seems that by
the 1950s or 1960s, the bottom-guided valves with either beveled seats or
wide flat seats were replaced by top guided valves with narrow "optically
flat" seats.

    In their 1909 Transactions ASME specified, "The function of the safety
valve is two fold: (a) it gives notice of the highest pressure permissible;
(b) it gives the alarm that more water or less fuel is needed."  The
Transactions also state that the purpose of the safety valve is to "...
simply limit the working pressure under ordinary conditions."

    In the following 90+ years the definition changed.

    In their 2001 catalog Anderson Greenwood Crosby Company clearly and
simply state the purpose of a modern safety valve:  "A PRV (pressure relief
valve) is a safety device intended to protect life and property if all other
safety measures fail."

    Secondly, From 1914 to 1924 ASME specified that blowdown could not be
more than 6 psi.  At 150 psi this would be 4%.  From 1924 until some time
between 1983 and 2001 ASME specified that blowdown could not be more than 4%
of the set point.   Today, the ASME Boiler Code allows blowdown to be as
much as 6% of the setpoint.   It is my understanding that if the blowdown is
not specified, in most cases, safety valves will be set for 6%.

    Over the last year I have been asking engine owners and operators
whether or not they are satisfied with their safety valves and how much
their valves blow down.   It is my "opinion" that owners of safety valves
that blow down 4% are happier with their valves than are owners of valves
that blowdown 6%.

    Another opinion.  I suspect that 6% blowdown is more likely to cause
problems with stays and tubes than is 4%.

    One last opinion.  It is good practice to have your gauge reset before
you install a new safety valve.  Settings of both blowdown and set point
have been found to vary from what is stamped on new valves.

Bruce E. Babcock

----- Original Message ----- 
From: "Ken Hough " <k4sb at niia.net>
To: "Steam-engine mailing list" <steam-engine at lists.stationary-engine.com>
Sent: Thursday, September 16, 2004 9:13 PM
Subject: Re: [Steam-engine] relief valves


> Considering the poor mechanical construction and condition of a NEW
> safety valve
> I would rather lift it as a test than not lift at all. However, we
> have all discussed that a new pop valve nowadays seems to be a one
> time use device. If it blows it will never seat properly again!
> Bruce Babcock? Opinions? I think you and I at Wauseon were discussing
> this.
> Ken
>
>
>
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