Relief Valve will not close! Steam trace? 2

[philruh]

Ethylene balanced bellow relief valves, discharging to plant flare system.
Plant wanted platforms so that they could safely access relief valves. They need access, to handle steam lances, to thaw valves after they relieve, the valves do not close, and are covered with ice. After melting the ice, the valves close.
I got the serial numbers, and contacted the manufacturer, to insure that the valves were sized correctly, AND had the proper metallurgy for the temperatures. (Ethylene boils at about -153 degrees F, process operates at about -43 degrees F)

The theory is that since the process operates below ambient temperature, that moisture condenses inside the bonnet, and when the valve relieves, the moisture freezes, and keeps the valve from closing. (Note: the valves are not insulated, ice forms in the inlet pipe upto the inlet flange)

Proposed solution is to steam trace the valve's.
[alternates considered: a. Bolt on Jackets ($4,000 a piece, and they need the valves for 8 weeks to size and manufacture) b. Nitrogen purge (expense of N2, and installation, maintenance, precedence) c. Platforms ($25,000 each)] d. Electric trace.

Questions:
1. Is the theory correct?
2. If it is correct, why would the moisture not freeze during cold weather? Would this not prevent the valve from opening, and therefore be a serious problem? (we are proposing to turn on steam during freezing weather)
3. How does one calculate the steam required to thaw the valve. (say the valve weighs 150 pounds)? [want to justify, NOT adding a trap, and piping to reclaim condensate]
4. How long will it take to thaw, with and without heat transfer cement?
5. Has anyone else seen this same problem?

Alternates:
Any other suggestions or thoughts or concerns?

 

[Rich2001]

1. Is the theory correct?
Yes, your theory is correct. This is a common problem, many cryo tanks will have two safety valves that are switched after it has releived to allow the valve to thaw and the seat material to warm up.
2. If it is correct, why would the moisture not freeze during cold weather?
For every 20F temperture difference in ambient air there is about 50% difference in the moisture content. So if the ambient temperature changes from 94F (34.4C) during the summer to 10F (-0.7C) during the winter the moisture content of the ambient air is reduced by 96%.
3. How does one calculate the steam required to thaw the valve. (say the valve weighs 150 pounds)? [want to justify, NOT adding a trap, and piping to reclaim condensate]
To calculating the energy required to melt the ice, do a lump sum analysis, estimate the mass of ice that must be removed, multiple this by the specific heat of the ice, multipled by the temperature difference then add the mass of the ice multipled by the later heat of fusion then add the mass of the valve multipled by the specific heat of the valve material mutipled by the difference in temperature.

Energy.req = m.ice*cp.ice*(delta T) + m.ice*(Latent.Heat.ice) + m.valve*cp.valve*(delta T)

Energy.req = Energy Required to Melt the ice on the valve
m.ice = Mass of the Ice
cp.ice = Specific heat of the ice
Delta T = Change in temperature required
Latent.Heat.ice = Latent Heat of Fussion of the Ice (enegry required to change phase)
m.valve = Mass of the valve
cp.valve = specific heat of the valve

(note that there are also crystalline phase change of the ice, i can no remember where these occur, but some addition latent heat is required. This may be minor.)

4. How long will it take to thaw, with and without heat transfer cement?
Depend on the amount of steam tracing, ice build-up etc.