I've replaced all three joints from the steam end of the Sentinel 7109's cab mounted boiler feed pump; these are the ones above and below the valve chest and the failed one sealing the end cover. The valve chest ones were undamaged but I replaced them anyway to improve long term reliability.
|Light green layers replaced by the dull grey graphite of PSM150/AS|
Perhaps the check valve could stay open if the feed water and boiler water were at identical pressures? This is not likely as there is a spring to help the check valve to close. This mystery remains on the 'too difficult' pile. I checked the cleanliness of all three check valves for peace of mind and found no cause for concern.
Not only did the steam get back into the pump's water cylinders but also further upstream in the pipework towards the water tank feed. I mention this because I'd just painstakingly replaced it all (click here) after the frost damage to the original water valve. Being a water feed, I again used 'Heldite' as the thread sealant as it was well in spec for cold water and low pressure.
However, it's not designed for high pressure steam! The result was that the pipe joints were emptied of the Heldite and no longer doing their job.
I've rebuilt the pipework again; this time I've used Rocol's 'Steamseal' as the thread sealant. The name gives it away so next time it should withstand a steam ingress where it's not wanted.
The water cylinder piston rings are made of a fibrous non-metallic material. (click here for more detail). They survived steam ingress the previous time and seemingly this time also as the pump worked well when tested with compressed air.