In last month’s column (June 2019), I shared my concept of what constituted a “pump twilight zone,” and I offered five mystery examples.
The preliminary reader reaction was good, so this month I am adding four more examples.
1. The case of the hidden factor
At a large paper mill in the Midwest, they use 8-inch self-primer trash pumps throughout the facility to transfer waste sump influent. Each sump has two installed pumps, one for main duty and the other is a backup spare. On a particularly troublesome sump, one of the pumps was having difficulty keeping up with the flow rate and was consequently replaced with a new pump from a different manufacturer. The new pump performed slightly better than the previous one, but not as well as the other pumps in the plant. It also could not keep up with the required flow rate.
The plant personnel contacted the manufacturer to have a representative visit the site for troubleshooting assistance. The manufacturer’s technician checked the pump’s mechanical settings and conducted all the normal performance tests, including a check for vacuum on the suction side with the suction isolation valve closed. All checkpoints on the pump were satisfactory and yet it would not perform properly in service.
The technician called his “pump-phone-a-friend” (aka, the old curmudgeon pump engineer for the manufacturer) and explained the situation. The suction lift was an easy 16 feet, the fluid is ambient temperature water and the submergence was more than adequate. The sump design is of adequate size and presented a laminar flow profile with little to no air entrainment at the pump suction. The engineer told the technician to look for an air leak in the suction line, but none were found. Some people suggested a new pump from a different manufacturer.
The technician knew air leaks (ingress) in self-priming pump suction lines can be difficult to find because ambient air is leaking in—where most people are incorrectly looking for the fluid to be leaking out. He tried several methods involving leak detection equipment and the old and simple (but tried and true) method of duct tape and plastic wrap, all to no avail. The customer made a decision to live with the issue, add a temporary backup system and a plan to replace the “bad actor” pump during the next major outage. Months later, during the outage to replace the pump, they decided at the last minute to also replace the suction pipe. It was only then they found the hidden air leak in the suction pipe. It was in a transition piece that passed through the concrete floor above the sump, where it was inaccessible.
2. The magical exploding pump
At a surface mine near the previously mentioned paper mill, a rubber-lined slurry pump exploded during normal operation. When the pump exploded, the casing shrapnel damaged peripheral equipment, electrical enclosures and support structures. The good news was that no personnel were injured. The manufacturer was called and the engineer flew to the site to investigate. The customer insisted the issue was simply a bad casing (poor casting quality), and it was a straightforward warranty issue to replace the pump. The engineer did not believe in magic and went looking for the cause.
The engineer knew that pumps do not explode for supernatural reasons. He determined the forces required could only originate from a change of phase in the pumped fluid; that is, the liquid had changed to steam inside the pump. The volumetric expansion from water to steam is highly exponential and sudden, like an explosion. The suction and discharge valves for the pump remained open during the incident, so trapped fluid in the operating casing did not seem to be the cause.
Further investigation revealed the customer had added a variable speed drive to the pump to ostensibly control a sump level farther down the process line. The designer for the revised system did not factor the critical carrying velocity of the slurry into the calculations. A brief explanation is that solids will drop out of suspension if the velocity in the pipe drops below a certain level.
As the sump approached a full level, the newly added controls told the pump to slow down. It slowed down several times and so sufficiently that the solids in the slurry dropped out of suspension and formed a block on both the suction and discharge sides of the pump as if the suction and discharge valves were closed. Yet the pump was still running (but now with a blocked suction and discharge). The motor horsepower was simply being converted to heat in the liquid captured in the pump casing. We do not know exactly how long it took, but the liquid transformed to steam and the pump exploded; not as if by magic, but from the simple laws of physics in a PVT for liquids relationship (where PVT = pressure/volume/temperature). To understand and calculate liquid temperature rise in a pump casing, see my two articles on this subject in the November 2015 and January 2018 issues.
3. Electric bill increases 50 percent in three months