Self-flush pump

Answer—there is an error in the design of this pump. The error is that this is a self-flushed pump. The mechanical seals on centrifugal pumps require a lubricant or seal flush material to keep the seal faces from touching and rubbing. In most pumps, this seal flush fluid comes from the discharge of the pump itself. Such pumps are called self-flushed pumps. 

When a self-flushed pump is running, the space between the seal faces is filled with the seal flush fluid. When the pump is shut down, the space between the seal faces is filled with the fluid in the suction of the pump. However, if the pressure at the suction of the pump is below atmospheric pressure, then air is drawn through the seal faces and into the suction of the pump. This air displaces the water in the pump s case and, with time, causes the pump to lose its prime. 

To fix this problem in Port Arthur, I connected an external source of seal flush water to the pump, from a nearby washwater station. Pumps which have subatmospheric suction pressures, and which are not in continuous service, should not be self-flushed pumps. They should have an external source of seal-flush material connected to the mechanical seal. 

A seal should maintain its integrity for six years, provided it is operating correctly. Most often seal life is compromised by improper start-up procedures. For example, starting up a pump without venting the seal flush lines to the inner seal on a self-flushed pump may reduce the operational seal life by six months, even though the seal faces have run dry for only a few seconds. 

Out here, on our Alky Unit, most of the pumps are self-flushed pumps. That means the seal flush liquid comes from the discharge of the pump itself."... 

A pump seal ought to last about six years. Running a seal dry for a few seconds during start-up can reduce the seal life by six months. For a self-flushed pump that loses suction pressure on start-up, there will be little or no seal flush pressure until a normal pump suction pressure is established. (See section on starting NPSH requirements in Chap. 36.) Also, a pump that cavitates on start-up, due to the loss of suction pressure, will vibrate, which also contributes to the damage to the pump s mechanical seal faces. 

For self-flushed pumps, it is necessary to vent and fill the seal flush lines completely. Otherwise, on starting the pump, there will be no seal flush flow for a second or so, which will materially damage the seal. 

When a pump is not in operation, it is not necessary to have the seal flush flowing. Obviously for self-flushed pumps, this is not a problem, as the seal flush flow stops with the pump. But for externally flushed pumps, there is no sense leaving the seal flush running. For example, let s say I m operating a Delayed Coker or Visbreaker charge pump. The seal flush flow for such a pump, all of which flows back into the pump suction when the pump is idle, would be one or two... 

A self-flushed pump with a single set of mechanical seal faces can leak the process fluid into the environment. If the process fluid is hydrocarbons, this sort of leakage is no longer permitted by the EPA. The sketch I have shown in Fig. 37.1 does not represent the actual configuration of a double mechanical seal component, but does explain the function of the individual pump internal components (see reference 2 for an excellent description of the actual arrangements of the internals for an overhimg, wet sump, centrifugal pump). 

For the more common self-flushed pumps, vapors trapped inside the seal system tubing and coolers must be vented out and displaced with liquid. Otherwise, lubrication between the seal faces will be intermittent during start-up. A few seconds of running a dry seal can cause a year s worth of wear on the seal faces. Do not assume that because you have started up a pump without blowing the seal before, that no damage has been done. 

Vent the top of the pump case to fill the case with liquid. For a self-flushed pump, starting with gas in the pump case will contaminate the seal flush tubing and cooler with vapor. 

Self-flushed pumps A centrifugal pump that obtains its seal mechanical flush liquid from the pump s own discharge. 

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