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To ensure reliable, trouble-free operation for extended periods of time, the seal must operate in a properly controlled environment. This requires that the seal be installed correctly, so that the seal faces maintain perfect contact and alignment, and that proper lubrication and cooling be provided. A typical seal system for a simple, single, mechanical seal is comprised of the seal, stuffing box throat bushing, liquid flush system, auxiliary seal and auxiliary flush or barrier fluid (when required).
The purpose of the seal is to prevent leakage of pumped product from escaping to the atmosphere. The liquid flush (normally pumped product from the discharge) is injected into the seal chamber to provide lubrication and cooling. An auxiliary seal is sometimes fitted to the gland plate on the atmospheric side of the seal chamber. Its purpose is to create a secondary containment chamber, when handling flammable or toxic fluids that would be considered a safety hazard to personnel if they were to leak to atmosphere. A liquid (non-toxic) flush or barrier fluid, complete with a liquid reservoir and appropriate alarm devices can be used to ensure toxic fluid does not escape to the atmosphere.
Controlling flush flow to the seal
The simple seal system incorporates an orifice in the flush line from the pump discharge to the mechanical seal. Its purpose is to limit the injection flow rate to the seal and to control pressure in the seal chamber. A minimum bore diameter of 3mm (1/8” ) is normally specified (to minimize potential of blockage) and the orifice can either be installed between flanges or in an orifice nipple.
Examining some causes of seal failures
An indication of some causes of seal failures can be obtained while the seal is operating. When you consider the seal as an equivalent orifice, an examination of ‘tell tale’ symptoms can indicate causes of potential failure for which corrective action can be implemented or at least can provide direction of subsequent failure analysis. It should be noted that improper application, installation, and/or manufacturing errors can also result in mechanical seal failures.
Possible causes of seal failure
Possible causes – Insufficient amount of liquid to lubricate seal faces
Recommendations – Flush line may need to be enlarged and/or orifice size may need to be increased.
Possible causes – Insufficient amount of liquid to lubricate seal faces, liquid film vaporizing/flushing between seal faces.
Recommendations – Pressure in seal chamber may be too low for seal type
Possible causes – Product vaporizing/flashing across seal faces
Corrective action is to provide proper liquid environment of the product at all times:
1. Increase seal chamber pressure if it can be achieved within operating parameters (maintain at a minimum of 175kPa (25psig) above suction pressure)
2. Check for proper seal balance with manufacturer
3. Change seal design to one not requiring as much product temperature margin
4. Seal flush line and/or orifice may have to be enlarged
5. Increase cooling of seal faces
Note: A review of seal balance requires accurate measurement of seal chamber pressure, temperature and product sample for vapor pressure determination.
Flush line strainers
Mechanical seal reliability is significantly affected by the operational characteristics of the flush system and its components. In services where the pumped fluid can contain solids, API 610 and API 682 both offer the option of using flush line strainers or cyclone separators.
Flush line strainers can become blocked resulting in immediate seal failure. Cyclone separator effectiveness is dependent on the relative density difference between the fluid and solid particles and the flush line piping (adequate slope for the debris drain line back to the pump suction).
Using an external clean flush, if available, or a dual pressurized seal arrangement will eliminate the need for flush line strainers and cyclone separators and ensure optimum mechanical seal MTBFs.
Flush line strainers and cyclone separators have been the cause of low seal MTBFs (less than 12 months) in many applications where the seal fluid contains solid particles. Eventual modification to a clean external flush or a dual pressurized seal has significantly increased seal MTBFs (greater than 48 months).
This best practice has been used since the 1990s to thoroughly investigate, with the process engineers, the possibility of using a clean external flush source in services where solid particles were contained in the seal fluid. A contingency recommendation where a clean external flush was not available was to use a dual pressurized seal.
In many cases, the additional cost of an external flush or dual pressurized seal was justified on the basis of past plant mechanical seal history and the loss of revenue when the standby pump was under maintenance and the operating pump failed.