Emerson Exchange 365

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What happen when an 67CF fails?

if you have a 67CF on a control valve, and if it fails in such a way, so that instrument air (IA) supply pressure, 7 bar, is supplied to the positioner.

Would this mean that when the positioner got to the end of its travel, that when it kicked over to full pressure on the diaphragm, that the actuator could see the full IA pressure on the actuator.

For the 657 and 667 actuators, this would exceed the maximum allowable diaphragm pressure, for example,  657 size 46/46i is only allowed to 3.4bar.
There are notes associated with it, that discuss that the maximum allowable diaphragm pressure  "Its purpose is to allow for typical regulator supply settings and/or relief valve tolerances."

The thing is that I've never seen relief valve between the airset and the positioner. I know that there is the 67CR, but the relief on it doesn't seem to be such to prevent supply pressure on the positioner.

Gerard Nagle

• 24 Jul 2018 8:03 PM
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1 Reply

• Frans Martens, owner of The Other Ways consultancy

  

  • 25 Jul 2018 10:11 AM

  When overpressurised, a diaphragm actuator is not likely to 'fly apart'. Something will give way, the diaphragm may internally tear or the diaphragm bead between the upper and lower half of the shell may pop out, causing a leak.
  In general, providing a relief valve on a diaphragm actuator is not required.
  That is different for a piston actuator, regardless of whether it is a casting or made out of pipe.
  In case of overpressure those cylinders may fly apart, potentially causing harm.
  Luckily, the design pressure of most cylinders can be selected to be higher than the maximum air supply pressure. Then no relief valve is required in those cases.

  Frans Martens

  ‘The Other Ways’

  Consultant Field Instrumentation and Systems

  The Netherlands

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