Page 499 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer
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Be st Practice 8 .1           Pump Mechanical Seal Flush Best Practices


         Best
         Best Practice 8.1Practice 8.1
         Confirm the actual seal operating conditions on the data  Lessons Learned
         sheet (PT, vapor pressure, S.G., P1 and P2) to optimize  Failure to properly specify the correct process conditions
         mechanical seal MTBFs.                             on the pump and/or mechanical seal data sheet will result
           Of all machinery components, mechanical seals are the most  in lower than optimum seal MTBFs.
         affected by the process conditions.                  Most ‘bad actor’ seals (seals with more thanone failure per year) result
           Accurate definition of the following parameters on the pump and  from improper specification of process conditions on the data sheets.
         seal data sheet will go a long way towards assuring optimum pump  Always check all operating seal process conditions against the data
         mechanical seal MTBFs:                             sheet values when investigating a ‘bad actor’ seal.
           Fluid vapor pressure                               If the proper instrumentation (pressure gauge, etc.) is not installed,
           Fluid specific gravity and viscosity              havetheappropriateinstrumentsinstalledatthetimeofsealreplacement.
           Fluid pumping temperature (PT)
           Suction pressure                                 Benchmarks
           Cooling medium temperatures (if a flush cooler will be used)
                                                            This best practice has been used since 1990 to troubleshoot
           External flush fluid pressures, temperature, vapor pressure,  mechanical seal problems and to ensure maximum mechanical seal
           viscosity and specific gravity                    MTBFs (greater than 100 months).





       B.P. 8.1. Supporting Material
                                                                Temperature gun close to seal gland – be sure to monitor off of
       Confirm process conditions are actually as stated in data sheets.  a non-reflective surface
       If the ‘bad actor’ pump seal reliability, when operating in the  Surface contact thermometer on flush line – add 5°C to
       EROE (see B.P. 2.7 for EROE details), does not significantly  measured value if pipe and use recorded value if SS tubing
       improve, a complete check of seal fluid conditions in the seal  Install a thermometer in the flush line
       chamber is required. Figure 8.1.1 shows the process variables  Use an infrared camera to record temperature and temperature
       that influence seal reliability.                          profile


                                                            Fig 8.1.2   Alternative methods to measure seal fluid temperature

           Seal chamber temperature
           Seal chamber pressure                            Pressure monitoring
           Seal fluid characteristics:
           •  Cleanliness
           •  Vapor pressure                                Unless the plant assigned machinery specialist is ‘world
           •  Viscosity                                     class’ there are usually no pressure gauges in the flush line
           •  Specific heat                                  or seal chamber. Our recommendations concerning seal
           •  Specific gravity                               chamber pressure monitoring are presented in Figure 8.1.3.
                                                            The measured seal chamber pressure must be compared to
                                                            the seal vendor’s assumed value, which should be on the
       Fig 8.1.1   Seal reliability and pump fluid conditions
                                                            seal layout drawing. If this value is not present, the seal
                                                            vendor should be consulted. Note that the seal vendor as-
                                                            sumed seal chamber pressure is the value that was used in
       Temperature monitoring
                                                            the calculation of the seal PV. If the measured value does
                                                            not agree with the data sheet, consult with operations first
       Since seal chamber or seal flush line pressure gauges are not  to see if process changes can be made. If process changes
       usually installed on new pumps, the firstplace tostartis
                                                            cannot be made, discuss this fact with the seal vendor
       with seal chamber temperature. Flush plans with coolers  representative.
       (21, 23, 41, etc.) have an option for a thermometer down-
       stream of the cooler which can be used to determine the
       seal chamber temperature. This value must be compared to
       the PT (pumping temperature) value listed on the data
       sheet. If the measured value does not agree with the data
       sheet, consult with operations first to see if process changes  Install a pressure gauge in the seal chamber
       can be made. If they cannot be made, discuss the measured  Modify the seal flush line for installation of a pressure gauge.
       temperature with the seal vendor representative. Alternative
       options for measuring seal fluid temperature are noted in  Fig 8.1.3   Seal chamber pressure monitoring guidelines for ‘bad
       Figure 8.1.2.                                        actor’ seals

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