Information Notice No. 82-56: Robertshaw Thermostatic Flow Control Valves
SINNS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
December 30, 1982
Information Notice No. 82-56: ROBERTSHAW THERMOSTATIC FLOW CONTROL
All nuclear power reactor facilities holding an operating license (OL) or
construction permit (CP).
This information notice is provided as a notification of a potentially
significant problem pertaining to Robertshaw Thermostatic Flow Control
Valves. No specific action is required in response to this information
notice, but the manufacturer in 10 CFR Part 21 correspondence with the NRC
has requested that the NRC advise the nuclear industry of a potential
problem with application of three versions of flow adjustment subassemblies,
since substantially identical units are sold commercially and may have been
purchased through distributors without identification that the end use is in
a nuclear power plant.
Description of Circumstances:
Robertshaw Thermostatic Flow Control Valves are widely used for temperature
control in fluid systems. Among other uses, Robertshaw has supplied Model
1284 and Model 1285 valves to manufacturers of diesel engines which power
emergency generators for nuclear power plants. Details of the systems may
vary, but in general, the engine cooling fluid flow path is either directed
through a cooler or diverted around it by modulating action of the valve in
response to coolant temperature sensed at the valve inlet. The valve stroke
is set by a nut on a threaded stem. If that nut backs off, the temperature
setpoint is significantly lowered. Running a diesel engine with excessively
low coolant temperature may lead to engine damage.
The valve stem nut in the original design was staked in place at the factory
on assembly. In 1977 a customer reported that the nut on one assembly had
completely disengaged; subsequently, the same customer reported a similar
occurrence on another assembly. Manufacturing records showed that both
assemblies had been made on the same day by a single mechanic. To provide
better assurance that the nut would stay in place, the manufacturer changed
the assembly procedure to lock the nut by soldering with lead-tin solder.
Recently, the manufacturer became aware that some "corrosion inhibitors"
used in closed cooling, water systems aggressively attack lead-tin solders
(IE Circular 80-11). Based on this information, the manufacturer further
revised assembly procedures to use of a castellated nut and cotter pin,
eliminating use of lead-tin solder.
December 30, 1982
Page 2 of 3
Since there has been only the one reported failure, none of the above
described assembly modifications were considered by the manufacturer to be
of sufficient significance to warrant production retrofit, except for valves
specifically identified as destined for use on emergency diesel generators
for nuclear power plants. The manufacturer believes that many valves in
service have been assembled by one of the now-obsolete techniques. Because
of the wide distribution and use of these valves, some may be in use in
safety-significant applications. This in itself may not be a problem since,
depending on use, the postulated failure mode may be acceptably "fail safe."
Another design feature of the subject valves is the use of a modified "O"
ring to control bypass leakage within the valve when the valve is positioned
to divert flow from the cooler. A cylindrical valve plug controls the fluid
flow out of one or both of the discharge ports by moving axially. The "O"
ring floats in a groove in the valve body web and is around the valve plug.
It limits bypass leakage to about 1 percent of full flow.
Thermal element (valve actuator) power available to position the valve plug
is small, and the dimensions of the "O" ring and the groove are such that
there is no preload on the "O" ring. Instead, the "O" ring has been cut and
a small section of the ring has been removed to assure that the ends do not
make contact. In one instance, contamination of the coolant by diesel fuel,
caused by a faulty fuel injector, resulted in swelling of the "O" ring,
which prevented normal movement of the valve plug. Evaluation of this event
resulted in the manufacturer recommending elimination of the "O" ring for
emergency diesels in nuclear power. The basis for this recommendation is the
stated practice of maintaining the engine at an elevated temperature at all
times that it is in a standby condition. Therefore, the slightly increased
bypass flow through the cooler is considered to be acceptable during the
startup transient. As soon as the engine picks up load and begins to reject
heat, the valve opens to allow partial flow through the cooler, and the
"bypass" flow becomes unimportant. The validity of this analysis has been
confirmed by the engine manufacturer.
A similar situation exists with other applications of the subject valve.
Depending on circumstances a licensee may or may not decide that prudence
dictates replacement of the stroke adjustment assembly and/or removal of the
split "O" ring.
December 30, 1982
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If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC Regional Office, or this office.
Edward L. Jordan, Director
Division of Engineering and
Office of Inspection and Enforcement
Technical Contact: J. B. Henderson
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