NIBCO Check Valve Automatic Stop F869B - Manual (Page 10)

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Generally, there are two valves between the boiler and the header. One valve is a stop-
check valve and the other valve is a stop valve, either gate, angle or globe type. The two
valves are used to make maintenance more convenient and in most cases, are required
by code for the inspectors safety while inspecting the boiler (See sketch).
Question #3 – How does a stop-check valve work?
As stated in the answer to question #1, the stop-check valve in principle works like a lift
check valve but with two very important additional features:
1. A method is provided to control the rate of ascent and descent of the disc.
2. A means is provided to close the disc positively so that the fluids may not flow in
either direction regardless of the pressure differential.
Various manufacturers provide different methods of controlling dashpot movement. Basically,
the two methods are by spring tension or dashpot arrangement.
Lunkenheimer is the only manufacturer that produces a valve with an external spring tension
adjustment to control the disc movement. Other manufacturers, including NIBCO, use the less
complicated dashpot arrangement. NIBCO is the only manufacturer who supplies a valve with
an external control of the dashpot. All other popular manufacturer’s valves must be
disassembled so that the piston rings may be added or removed to respectively slow down or
speed up automatic disc movement.
Generally, all manufacturers provide a globe valve stem arrangement to positively close the
disc. This stem merely pushes down on the disc through the linkage on the Lunkenheimer
valve, or the dashpot piston on other valves (See sketch).
The purpose of the dashpot is to control the rate of ascent and descent of the disc so that the
disc does not slam and chatter in the line. Since valves are connected to a network of pipes in
the system, this would mean that the noise of a slamming and chattering disc could be
transmitted through the piping system. This noise is distracting in office buildings, hospitals, etc,
where concentration is essential. Moreover, the slam and chatter is extremely harmful to the
disc. It should also be noted that slamming and chattering of the disc causes pulsations within
the steam line which makes it very difficult for automatic steam regulator and other steam
equipment to function properly. Therefore, the disc must be controlled by some means of
dampening which, in most cases, is the dashpot.
The disc has a tendency to slam and chatter at various times. Usually the slam and chatter will
be obvious when the boiler is coming up to operating pressure. At this time, the steam is tending
to lift the disc and as it does the pressure in the boiler drops allowing the disc to fall and make
noise. Sometimes this is in the form of a slam, or it can be in the form of a chatter if it occurs
many times a minute. The disc can also make noise at periods when the boiler is at peak
delivery. Here the disc can rise rapidly banging up against the stem end or the dashpot housing
to cause noise.
We cannot stop the disc from coming into contact with the seat or stem, but we can and do
control the rate of impact to prevent loud noises in the piping system.