Mechanical seals operating under the correct conditions can provide years of trouble-free service. Most mechanical seals are designed to be surrounded by liquid to provide cooling and to avoid running dry. In pumps used for liquids, it is a common requirement that they must not run dry.
It is often assumed that if the pump is pumping liquid, then the seal must be surrounded by liquid. In a horizontal pump, this is almost always the case. In a vertical shaft pump, it should not be taken for granted. A simple way to illustrate this is to push an empty cup with the open end down into a container filled with water. Air is trapped in the cup because it has nowhere to go, and if a sealed shaft passed through the cup the air would remain trapped.
If we turn the cup horizontally, the air escapes from the cup’s open end like it would in a horizontal pump. In a vertical pump with a mechanical seal, this principle must be considered: If air is trapped in the seal chamber, the seal is running dry.
A wastewater plant in Amherst, New York, uses several vertical shaft end suction pumps to return activated sludge and raw sewage. These pumps were originally braided packing sealed pumps. They had a history of requiring frequent gland adjustments and shaft sleeve wear that required rebuilding the pumps.
The plant decided that the energy savings and reduced downtime of split mechanical seals were worth the investment. The seals were installed with a restrictor bushing in the stuffing box to reduce flush water usage and limit solids entering the seal. The seals were flushed with process water and vented at startup. Pressure in the seal housing is maintained as recommended by manufacturers and industrial practice. The seals were vented by releasing the air through a vent before startup.
But there was a problem: The seals were prematurely failing on a regular basis, despite being built to high quality standards.
One day a plant manager noticed the air release valves on the plant’s hot water heating system were called “high vents”—they release air from the heating lines, and they are located at the high points in the system. Even though the seals were vented before starting the pump, the flush water carries entrained air that is probably collecting in the seals. Because the seal is the highest point and is working properly, over time enough air collects to create a dry pocket around the seal faces. The flush water is going through the air pocket but not displacing it. (The faces were getting splashed but not surrounded with water.)
In order for this valve to work, the seal chamber or housing pressure must be high enough to push water up into the valve. The manager looked into the specifications of the air release valve used on the heating system, a float and needle valve type similar to a carburetor float—only air passes through the needle valve, so it is unlikely to clog.
Plant personnel attached the valve to the pump frame above the seal and placed a shutoff valve below it so it could be closed in the event the valve requires repair. Plastic tubing connected it to the vent port on the seal housing (opposite the flush port). The seal is now constantly vented, and the seals operate as they should. The plant modified all similar pumps.
In a pump with double mechanical seals (horizontal or vertical), the flush water is deadheaded between the seals. Venting at start-up should be adequate. In any case the vent should be the higher port and the flush the lower; this removes the most air possible from the seal.
In the situation described in this article, a pump designed to be operated horizontally was installed vertically. The seal chambers may or may not have flush and vent ports installed. Even though it is equipped with two single seals, they are naturally vented and flushed by the pumped liquid when used horizontally.
Once the pump is installed vertically, the upper seal chamber acts like the cup with the open end down. The upper seal runs dry. In this case, the pump is pumping clear cooling water. The suction pressure is probably lower than in the seal chamber. If no port is provided, the plant operator recommends drilling and tapping a hole in the seal chamber as high as possible.
The operator also would vent the seal and check the seal chamber pressure with a gauge, and confirm it is higher than that in the suction line.
If this is the case, connect the seal chamber vent that was just made with a similarly drilled and tapped hole in the suction line. Connecting them creates flow from seal to suction line, commonly called a suction recirculation system. The clear cooling water is unlikely to plug the line. The higher the pressure differential, the greater the flow. By doing so, it is drawing air and water from the seal chamber continuously
If the pump were in a flooded suction condition with the suction line pressure higher than the seal chamber, the air release valve may be an option. This does not address every possible cause of seal failure, but it eliminates an environmental problem created by operating a pump designed for horizontal use in a vertical orientation. Proper venting of the seal chamber is critical, particularly in a vertical shaft pump.