Because the separator stage of an evaporation unit was flooding, the technical services staff at a poultry processing plant was looking to replace an expensive pump used in its operation. The pump’s raison d’etre was to extract concentrated chicken broth from the separation vessel and push it to the next processing stage.
But with product building up in the separator, it seemed a reasonable conclusion that the extraction pump, as installed, was not working correctly. However, a call to the supplier, SPX, soon established the pump was unlikely to be the culprit and that the plant could almost certainly be restored to correct operation by appropriate maintenance.
In fact, a visit by Bob Vail, SPX commissioning engineer, had the plant up and running again in just a few hours. That saved the cost of a new pump and a purchase, which, had it been made, wouldn’t have solved the problem at hand.
Vail approached the problem using a proven troubleshooting methodology. First step was to eliminate two possible, but unlikely, possible causes of the poor process performance. These can be quickly checked and in most instances eliminated.
The first step was to check the pump itself. It is rarely discovered to be the source of the problem at this stage. The pump’s "power" is dependent on pump speed and impeller size. The pump was turning the impellor. It wasn’t connected to a variable speed drive that could have been reset. It was almost certainly operating correctly.
The second step was to eliminate any blockage. The extraction pump pulls product out of the separator vessel and pushes it into the plates of the next stage. If there is a blockage at the discharge of the pump, or if the plate orifices are blocked, the separator vessel will flood. This too, however, is very uncommon except where the product is over concentrated or pre-filtration has failed, and this is very uncommon. This possibility, too then, was eliminated.
Since there was neither a pump failure nor blockage, the probable (and most common) cause of the problem was a vacuum leak at or around the pump.
When a vacuum leak occurs on the suction side of a pump — for example, at the inlet mechanical seal — it allows air into the pump at the impeller. Depending on severity of the leak, this changes to a greater or lesser extent the pump performance.
A typical symptom of a minor leak is that liquid in the separator builds to a certain level and stays there, partially flooding. The increase in liquid height creates a positive inlet pressure to the pump that compensates for the minor vacuum leak. In extreme cases, flooding above the vapor duct from the plate pack results in violent shaking of the entire separator vessel. This is dangerous, and in the event such would happen, steam should be stopped and the vacuum broken immediately.
In this case example, there was no level control for the separator, which indicated that it was a self-leveling system and therefore should have no residual liquid level. Whether vertical or horizontal, the units are meant to be such that the system’s capacity to extract product exceeds the rate at which it enters, thus ensuring that there is no build up. Any residual level therefore normally indicates a vacuum leak somewhere below the water line.
Confirm and localize next
To confirm that the problem was a vacuum leak, Vail switched from product feed to water and shut down the steam. When the process had cooled, he turned off the vacuum pump and slowly broke the vacuum. As expected, the flooding stopped and the vessel pumped out. This showed that there was a vacuum leak at or near the flooded area. The pump was sucking air and would work fine once the net positive suction head was restored.
As a further check, water was kept circulating in the separators, without vacuum, and with the rest of the unit shut down. The feed-flow control valve was then manually closed to isolate the system on the feed side, preventing water from the balance tank from being sucked in when vacuum was applied.
Next, the vacuum was started to look for bubbles in the separator vessel discharge area caused by air being sucked in through the leak. A trail of small bubbles indicated a small leak, and in fact, a larger leak may result in violent eruptions. Note that it is important that the vessel and the water are cold, otherwise the water may boil under vacuum, giving a false result. This is indicated typically by bubbles on the entire surface of the water.
To localize the leak, all of the clamp fittings were tightened to see if the bubbles subsided, which they had.
If the leak had been from the pump’s inner mechanical seal it would be sucking air in from the service water discharge tubing along with the service water that should be going on the floor. The service water line is the ¼" poly-flow tubing coming out from between the pump head and the motor. The test is done by turning off the seal water to the pump and picking up the discharge tube to feel if air is being drawn into it. This would indicate a front seal leak. The seal should then be replaced.
As a final step, the process should always be repeated to verify that all leaks have been corrected. There may, of course, be more than one. This final step can involve breaking down all the product lines below the vessel’s water line, replacing gaskets, and maybe even replacing clamps with good quality-brand clamps. A final word of advice: It is vitally important that the lines and gaskets align without stress. Cut and weld new lines if needed. If the lines have to be pulled into position there will be problems in the future.
Based in Charlotte, North Carolina, SPX Corporation is a global, multi-industry manufacturing leader with approximately $5 billion in annual revenue, operations in more than 35 countries and over 14,000 employees. The company’s highly specialized, engineered products and technologies are concentrated in flow technology and energy infrastructure. SPX solutions play a role in helping meet rising global demand for electricity and processed foods and beverages, particularly in emerging markets. The company’s products include food processing systems for the food and beverage industry, critical flow components for oil and gas processing, power transformers for utility companies and cooling systems for power plants.