Page 519 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 8 .8 Pump Mechanical Seal Flush Best Practices

Fig 8.8.8 Seal flow control orifice

subsequent failure analysis (refer to Figure 8.8.9). It should be
noted that improper application, installation, and/or Comments Possible causes Comments/
manufacturing errors can also result in mechanical seal failures. recommendations
Seal squeal Insufficient Flush line may need to be
Seal configurations during operation amount of liquid enlarged and/or orifice
to lubricate seal size may need to be
faces increased
Mechanical seals are the predominant type of seals currently in Carbon dust Insufficient See above
use in centrifugal pumps. They are available in a variety of accumulating on amount of liquid
configurations, depending upon the application service condi- outside of seal to lubricate seal
tions and/or the user’s preference (refer to Figures 8.8.10 to area faces
8.8.13 for the most common arrangements used in refinery and Liquid film Pressure in seal chamber
petrochemical applications). vaporizing/ may be too low for seal
flashing between type
seal faces
Single mechanical seal applications Seal spits Product Corrective action is to
Single mechanical seals (refer to Figure 8.8.10) are the most and sputters in vaporizing/ provide proper liquid
widely used seal configuration, and should be used in any ap- operation flashing across environment of the
(popping)
seal faces
product at all times
plication where the liquid is non-toxic and non-flammable.
1. Increase seal
As mentioned earlier in this section, many single mechanical chamber pressure if it
seals are used with flammable and even toxic liquids, and rely can be achieved within
solely on the auxiliary seal throttle bushing to prevent leakage to operating parameters
atmosphere. Since the throttle bushing does not positively con- (maintain at
tain leakage, state and federal environmental regulations now a minimum of 175 kPa
require the use of a tandem or double seal for these applications. (25 psig) above
suction pressure)
In some plants, a dynamic type throttle bushing (‘Impro’ or 2. Check for proper seal
equal) is used to virtually eliminate leakage of the pumped fluid balance with
to atmosphere in the event of a mechanical seal failure. manufacturer
3. Change seal design to
Tandem mechanical seal applications one not requiring as
much product
Tandem mechanical seals (refer to Figure 8.8.11) are used in temperature margin ( T)
applications where the pumped fluid is toxic and/or flammable. 4. Seal flush line and/or
They consist of two (2) mechanical seals (primary and back- orifice may have to be
up). The primary seal is flushed by any selected seal flush plan. enlarged
The back-up seal is provided with a flush system incorporating 5. Increase cooling of
seal faces
a safe, low flash point liquid. There is a pressure alarm which Note: A review of seal
actuates on an increase in stuffing box pressure between the balance requires accurate
primary and back-up seal thus indicating a primary seal failure. measurement
Since the pumped product now occupies the volume between of seal chamber pressure,
the seals, failure of the back-up seal will result in leakage of the temperature and product
pumped fluid to atmosphere. In essence any time a tandem seal sample for vapor pressure
in alarm, it is actually a single seal and should be shut down determination
immediately to ensure that the toxic and/or flammable liquid
does not leak to atmosphere. Fig 8.8.9 Possible causes of seal failure

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