Pumps with mechanical seals that seal rotating shafts are used in many industries, starting from pumps in home heating and food industry to petrochemical and nuclear plants. Since a mechanical seal is a device with wearing parts, unless it is serviced from time to time, it will ultimately fail.
A failure of a mechanical seal will most often mean leakage of the pumped fluid outside. The consequences may vary. Sometimes it is OK that a pump will leak until someone notices it. But sometimes it will mean a possible fire for example if a pump is leaking gasoline. As one of our customers, a petrochemical plant, likes to say, "What we are running here is not a chocolate factory!" - meaning that if something bad happens the fluid coming out will be not hot chocolate but some very hazardous hydrocarbons.
This is why seals are very often used together with the so called seal systems – to alert personnel about a problem and probably shut down the pump for repair, and to prevent or minimize leakage to the atmosphere even if a seal has failed. Now, how is it possible?
There are several possible flush plans as they are called in the API 682 Standard.
Plan 65 seal system for a single mechanical seal (first image from top) will collect leakage in a small reservoir which is connected with a specialized drain. The reservoir will have an orifice in the outlet. Small acceptable leakage will pass through the orifice and go to drain. If leakage rate is big enough, the orifice will create an obstruction, and the level in the reservoir will start to go up, sending an alarm to a control panel.
Plan 52 seal system for a tandem mechanical seal (second image from top) will provide a bigger reservoir, somewhere between 2 and 5 gallons, which is filled with buffer fluid. The buffer fluid is often mineral oil or alcohol solution. A tandem seal has a “stand by” outboard pair of seal faces which take the pressure if the primary inboard seal starts to leak. But before that, buffer fluid is needed to lubricate the outboard seal. The buffer fluid is filled up to a certain level and is usually kept under atmospheric pressure. If the primary seal fails the pumped product will leak into the buffer fluid loop, raising level and / or pressure in the reservoir. (This will happen of course if pressure in seal chamber is greater than atmospheric pressure.) A change in pressure or level is easily detected visually or by transmitters. And what is very important, unless the outboard seal fails, too, the product will never leak into the atmosphere.
Plan 53 seal system for a double mechanical seal is similar to Plan 52, but the system is pressurized above the pressure in the seal chamber. So in the event of seal failure barrier fluid leaks into the pump. Barrier fluid may leak into the atmosphere if outboard seal leaks but this fluid is selected not to cause any harm.
Plan 53 has several varieties, 53A, 53B (third image from top), and 53C. 53A is the most simple. 53B is getting more popular these days. Let’s compare them:
Plan 53B vs Plan 53A
Plan 53B is recommended when barrier fluid pressure is above 10 bars to avoid the problem of nitrogen dissolving into the barrier fluid. In Plan 53B barrier fluid and nitrogen are physically separated by an elastomeric bladder.
While a Plan 53A system is many times cheaper and is used a lot at lower pressure applications, Plan 53B is a must at higher pressures. If an automatic top up unit is added to the 53B system it becomes very similar to Plan 54 system.
Plan 54 seal system for a double mechanical seal is functionally similar to Plan 53 but a mechanical seal does not have an impeller. Barrier fluid is circulated by the seal system pump. A typical application example would be a number of mixers / agitators with double seals and one centralized Plan 54 system. A mixer shaft may rotate very slowly so barrier fluid circulation depends on the circulating pump rather than the seal impeller.
There are other types of seal systems as well. We would recommend to choose the simplest seal system that will do the job because today seal systems tend to get overcomplicated and to cost more than mechanical seals themselves.
See also: Seal systems offered by TREM Engineering