Mechanical seal support systems
Mechanical seal support systems
A mechanical seal support system may facilitate feeding barrier fluid to a mechanical seal. A predetermined difference in pressure between the barrier fluid at a position before and after passage through the seal is detected. The flow of barrier fluid to the seal may be reduced when the predetermined pressure difference is detected.
A seal support system comprises of a vessel or tank, which generally contains a volume of fluid. The vessel is piped to a sealing device on a pump, mixer or item of rotating equipment. Generally a return pipe is lead back to the vessel from thesealing device, hence closing the "loop". This allows the fluid, contained in the vessel, to enter and exit the sealing device. Such fluid is generally chosen so that it lubricates and cools the components within the sealing device, whilst beingcompatible with the process fluid.
The industry term for the fluid contained within the vessel, is barrier or buffer fluid.
Mechanical seals generate a tremendous amount of heat due to friction as the seal faces rub against each other. In applications where small heat loads need to be removed, it is common to use a small vessel, which has a seal supply port and aseal return port built into it.
This vessel is usually placed higher than the mechanical seal, and is piped in such a way that the seal supply port is situated at the bottom of the vessel, and the seal return port further up the vessel. This ensures that when the vessel isfull of fluid and is piped to the seal, the colder fluid is fed to the mechanical seal, and is warmed up by the seal.
This fluid then `rises` back up into the vessel, and hence a `thermal loop` is created. This effect is commonly termed the `thermosyphon effect`. The fluid in the vessel then cools to the atmosphere and the level of cooling can be increased byfactors such as surface area, the use of finned tubing, passing wind over the vessel etc.
However, in some applications the mechanical seal generates too much heat for a thermosyphon system to dissipate. This can be due to a number of factors such as: high heat transfer from the process fluid to the barrier fluid, large mechanicalseal sizes, high speed, hard seal faces and high ambient temperatures.
In this case, a forced circulation system may be employed. This type of system ensures that barrier fluid is forced through the seal from a vessel using a pump and motor set and then back into the tank where the barrier fluid cools to theatmosphere.
The AESSEAL PUMPPAC? is a system which performs this task, and has the ability to pressurise the barrier fluid being supplied to the mechanical seal, ensuing that if there is any leakage across the seal face, it is always the compatible andsafe barrier fluid into the product, rather than the product (which may be hazardous to the environment) into the atmosphere/vessel.
It is recommended to set the pressure of the barrier fluid 1 bar above that of the process fluid for this specific reason.
As the pump mechanical seal wears, the seal begins to `leak` more barrier fluid into the product, however the PUMPPAC? system ensures that the pressure of the barrier fluid supplied to the mechanical seal remains constant.
However, should a mechanical seal eventually fail the PUMPPAC? system continues supplying oil into the product, hence `watering down` or contaminating the product, which may lead to a product being wasted.
The PUMPPAC? system may also be used with a double seal, i.e. it can support both the in-board and out-board seal faces. If an in-board seal failure were to occur, this would lead to the same scenario as previously described.
The modular concept of the PUMPPAC? system ensures that a level switch may be added to the vessel, which would warn an operator of too much loss of fluid. Should the operator need to know that the mechanical seal had failed instantly, themodular concept of the PUMPPAC? system ensures that a pressure switch may be fitted, which would warn an operator of loss in pressure due to a major leak in the mechanical seal.
If an out-board seal failure were to occur, the standard PUMPPAC? system would continue pumping the fluid through the seal, but due to the damaged out-board faces, this fluid would then be ejected into the atmosphere. This would continue tooccur until the pump was either switched off, or until the vessel would be become empty.
Again, due to the modular concept of the PUMPPAC? System, level and pressure switches can be added to ensure that operators are warned of any such event.
However, warning systems which require operator input are always subject to `operator error`, and in some cases the operator may not be present at the exact time the fault occurs. Hence, an automated safety system is deemed to be advantageous.
The effects of an out-board seal failure, could potentially be a much more dangerous scenario. If the barrier fluid is for example a fluid with a low ignition temperature (the ignition (sometimes called auto-ignition) temperature of a substanceis the minimum temperature required to initiate or to cause self-sustained combustion independent of the heat source. A spark or flame is not necessary for ignition when a flammable vapour reaches its auto-ignition temperature), this type of fluid cancause a fire hazard if the fluid is leaked into the surrounding atmosphere due to an outboard seal failure. In this instance, if there is a delay in the operator shutting down the pump, a serious accident may occur, hence an automated system is evenmore advantageous.
According to a first aspect of the invention there is provided a mechanical seal support system comprising; means for feeding barrier fluid to a mechanical seal; means for detecting a predetermined difference in pressure between barrier fluid ata position before and after passage through said seal; means, responsive to said detection means, for reducing the flow of barrier fluid to said seal when said predetermined pressure difference is detected.