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Posted by Tony Conner on October 14, 2001 at 08:20:05:

In Reply to: SPIRAX-SARCO TEMP CONTROLLER posted by MIKE on October 14, 2001 at 06:52:33:

Is this a steam to water heat exchanger (HX)? Does the water continuously circulate over the temperature sensor for the TCV?

On many systems, the sensor for the TCV is located just after the HX. If there isn't a circulating pump for the water (or the pump shuts down when the demand is met), the water temp surrounding the sensor gradually falls. This tells the steam valve to open (just like it's supposed to), resulting in heat being put into the system when there is no load. The water pressure increases, and the relief pops. Another way to deal with this is a flow switch in the water flow, connected to the TCV. Unless there's a water flow, the valve won't open.

On domestic hot water systems, this situation can be pretty nasty, as you can have a pocket of very hot water in the heat exchanger.

Another thing to look at is the TCV itself. Is it correctly sized for the load? A huge percentage of steam components are GROSSLY oversized, and this causes more problems than undersizing.

Is it just an old valve, and in need of an overhaul or replacement?

Is there a drip leg and trap installed just upstream of the TCV station? If not, you'll likely be getting a slug of condensate forming when the valve is shut. As soon as there's a call for steam, this pocket of water gets slammed through your valve. Worst case is water hammering the valve to death, but most often this regular diet of water causes the valve to wear out long before it's time.

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