How is velocity determined for thermowells when there is two phase flow?


E.R. Sternad, Jr.
Merrick & Company
Advanced Technologies Division
2450 S. Peoria, Aurora, CO 80014
V 303.751.0741 x1702
F 303.751.2581
E [email protected] <mailto:[email protected]>

 

How has this been answered? I have an aplication
with a Customer overseas with a erosion/corrosion problem of a thermowell, exposed to the following process gasses in an Olefin plant : Temperature 860 C. Velocity 175meters/sec. Pressure: 30 PSIg. Composition, 45-55% Ethylene, 15-20%, Propylene, 8-10% Methane, 5-8% Hydrogen & 15-20% combination of Steam, CO, CO2, C3&4's. The thermowell, protrudes vertically downward through the outlet
side of the elbow wall. The exposed end of the current thermowell is made of Stellite No. 6, welded to 316. OD=.875";ID=.260". After approximately 120 days of service, fairly excessive erosion/pitting is observed, & not satisfactory to Customer. It is improved over other materials previously used. Local supplier
recommends going to Stoody No. 1. Do you agree? Any other suggestions? We can send digital photos if needed.
Thanks & Regards, Mike Cognevich, Product Manager Gemoco Industrial Valve Products 1-985-853-5093 Direct 1-985-872-9310
Fax

 

per Mike Cognevich,

"problem of a thermowell, exposed to the following process gasses in an Olefin plant:
Temperature 860 C.
Velocity 175meters/sec.
Pressure: 30 PSIG.
Composition, 45-55% Ethylene, 15-20%, Propylene, 8-10% Methane, 5-8% Hydrogen & 15-20% combination of Steam, CO, CO2, C3&4's.

The thermowell, protrudes vertically downward through the outlet side of the elbow wall. The exposed end of the current thermowell is made of Stellite No. 6, welded to 316. OD=.875";ID=.260". After approximately 120 days of service, fairly excessive erosion/pitting is observed."

The thermowell length is not specified, but with fluid velocities of 175 m/s it is not clear that any length > 2" would be suitable. The radiation errors are likely a serious problem. The simulation is showing 2-5 Deg C error depending on the flow velocity.

At 860 Deg C, 316 ss has very little strength (maximum allowable bending stress is less than 1300 psi in a clean, low velocity service).

At these temperatures even a 7" thermowell should not be used above 15 m/s!

Alternative materials with much better hot strength and thermowell designs are available that can be made to work at the velocities reported.

It is not clear, from what you've described, if the adjacent piping is capable of standing up to the services.

Good luck,

Dave Bartran