The 888 monitors the pressure differential between the process and the sample aspirator and automatically adjusts the flow rate relative to the process pressure. During start up, shut down, turn down, plugged rundowns and any time there is entrained sulfur leaving the final condenser, the flow rate can be minimized which reduces sulfur mist take up and optimizes the demister function. This can be monitored and adjusted using the front panel or the remote interface.
Four Pressures are monitored to auto adjust the gas flow
- Cell Pressure
- Manifold Pressure
- Aspirator Drive air Pressure
- “Differential” Cell Pressure – this is the difference in the cell pressure as measured with the aspirator
Probe Design Overview
The sample probe is designed to extend to near the center portion of the process pipe, ensuring that a representative sample of the process gas is extracted (i.e. wall effects are avoided). After passing through the analyzer, the sample gas and motive air are exhausted into the process pipe near the wall, and are rapidly dissipated by the process flow.
Aspirator Sample Flow Diagram
The sample transport is driven by a heated-air operated aspirator. Motive air, moving through the venturi, creates a reduced pressure on the sample inlet which pulls the process gas through the demister and into the measurement gas cell. The sample gas, which has passed through the analyzer, is combined with the aspirator motive air and exhausted back into the process pipe. Sample flow is continuous and reliable.
Automatic Blow Back Prevents Plugging
Air blow-back is achieved by diverting the aspirator drive air through the cell, demister, and out the sample inlet. Blow-back is automatically initiated when the demister temperature, cell temperature, cell pressure, or air pressure fall outside the preset limits. The analyzer restarts automatically, after the fault condition has cleared. At start-up and on power failure, the system automatically maintains air blow-back, until the cell temperature reaches the proper operating range.
When ammonia is present in the Claus feed gas, any residual will react with the sulfur compounds to form ammonia salts, which can plug the demister. These salts are not removed by air blow-back, but they are water soluble. Steam blow-back provides a steam propelled blast of hot water to dissolve any salts that have accumulated within the demister.
The heart of the sample handling is an improved sulfur demister. The Model 888 uses the proven sample handling technique of a “reflux” demister with double demister pads (316 SS top and PTFE bottom) the essential solution for the worst of process conditions. The demister pads are held in place on a shaft with retention rings to withstand the hot condensate “steam blowback” for ammonia salts (hence only one quality of steam is now required). The demister is connected with Vacuum Compression O-ring (VCO) (face-to-face union) fittings, easily accessed, minutes to disassemble and service while being assured of zero egress of process gas. Features…embedded demister pads…face-to-face fittings make it easy to “slide it out” for maintenance.