A thermocompressor is a steam control device that uses high-pressure steam (motive steam) to induce flow from a lower pressure steam source (suction steam) and discharge the mixture at an intermediate pressure. The high pressure is used to create a high velocity jet that mixes with and accelerates the suction steam. The velocity of the mixture is exchanged for increased pressure in the diffuser. A Kadant Johnson thermocompressor is shown below.
Thermocompressors can be installed in any orientation, but directing the discharge horizontally or downward is preferred. A thermocompressor should be independently supported. Using the unit to support piping can impose excessive loads and cause bending and misalignment.
Suction and Discharge Piping
Suction piping must be independently supported. It should be full size to match the suction connection on the thermocompressor. Avoid filters, valves and other fittings that cause pressure loss in the suction line that were not considered in the original design specification. Use low-pressure drop non-return valves and full bore ball or gate type isolation valves in all locations to minimize pressure losses. Avoid low points or loops that might accumulate condensate. A steam pressure gauge with an isolation valve should be located as close to the low-pressure inlet as possible.
Discharge piping should be the same diameter as the discharge connection on the thermocompressor. Discharge piping must be independently supported. Care should be taken to avoid placing restrictions or undue obstructions that will increase the discharge pressure above the design point. A minimum length of 10 pipe diameters is recommended before an elbow to Tee. A steam pressure gauge with an isolation valve should be located as close to the discharge connection as possible.
The line size should be determined based on the maximum design flow for the thermocompressor. Dry steam is a basic requirement for good performance and wet steam is extremely detrimental to both the performance and the parts of a thermocompressor. Motive pipe runs longer than 10 feet and should have a drip leg and trap to remove condensate from the piping before the motive steam enters the thermocompressor. High-flow losses in the supply lines should be avoided. As the motive pressure falls, the amount of steam required increases. A steam pressure gauge with an isolation valve should be located as close to the motive connection as possible.
A thermocompressor in the fully open or closed position during run conditions is usually a problem. Accuracy of the instrumentation and controls should be verified.
Substandard performance can usually be traced to either external or internal causes. Substandard performance can also be classified as either sudden or gradual. A gradual deterioration in performance, usually a loss of recompression, invariably suggests either erosion or corrosion, whereas a sudden loss of compression will usually suggest an external cause.
Since the external causes of trouble are usually easier to check, they should be investigated first.
When a fault is investigated, it is prudent to treat as suspect all the gauges fitted, especially Bourdon Tube type dial gauges. These gauges should, whenever it is possible, be recalibrated.
External Causes of Poor Performance
- Low motive steam pressure
- Wet motive steam
- Differential pressure too high
- Incorrect discharge pressure
- Attempting to operate outside the design envelope
- Excessive superheat
- Blockage in the suction piping (check valves, isolation valves, etc.)
A thermocompressor will operate efficiently with a motive pressure higher than the design point, but not lower. Low pressure reduces the available motive flow through the nozzle and reduces the jet velocity. Entrainment will decrease as the motive pressure falls below the design point. Excessive superheat reduces the available motive flow and will reduce entrainment as well. If especially severe, superheat can also damage the device.
Wet steam will cause poor performance and, in addition, will erode the internal components which will cause further loss in performance.
High differential pressure will reduce entrainment.
If the discharge pressure is too high, entrainment will be reduced. If it is too low, it’s possible the flow will choke.
Thermocompressor geometry is optimized for the set of conditions in the design specification.
Properly sized and functioning thermocompressors will operate in the approximate range of 20 to 80%. Operating outside of this range is likely to result from changes to the operating conditions.
If no external causes have been identified, check for internal causes.
Internal Causes of Poor Performance
- Eroded or corroded internal parts
- Blocked nozzles and / or diffusers
- Cracked or worn parts
- Internal leakage of steam from the high-pressure section to the low pressure section
- Misaligned nozzle or spindle assembly
Disassemble the unit and inspect the steam nozzle and spindle for damage. Inspect the body and nozzle for steam cutting at their juncture. Inspect suction line valves. Have new gaskets available for the reassembly process.
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