Vapor Locking in Buffer Fluid of API Plan52 Piping

We are facing problem of vapor locking in mechanical seal buffer oilsystem(API 52) of multistage centrifugal pump handling LPG.Description of Pump: 8-stage horizontal centrifugal ,back to back arrangement of impellers, Pumpsare used to increase pressure of fluid in crosscountry pipeline to pump itto next pumping station.Pumping fluid: LPG, Max RPM=3420,Max capacity of onepump=150 Mt/hr,max discharge pr=78 kg/cm2.Flow rate is controlled with helpof variable frequency drive.Operating data: (1.)suction pressure 20-23 kg/cm2,discharge pressure: 64-65kg/cm2 @ 150MT/hr flow of LPG.Pumping temperature=45-50 deg C Mechanical seal description:double seal, tandem arrangement at both dive end and non drive end of pump,API plan 52 for secondary seal flushing with buffer fluid( mineral oil) ,API plan 11 for primary seal flushing, For API plan 11 flushing line, onetapping is taken out after fourth stage of pump, and bifurcated to both endsseal's flushing point(through orifice),and this LPG after flushing primaryseal returns back to pump through throttle bush & small balancing hole(2mm)in stuffing box.For plan 52 there is seal oil tank with required piping & seal oil finnedcooler ( natural draft air cooler), seal oil tank pressure is atmospherepressure.Buffer fluid seal oil is mineral oil.Circulation of oil ismaintaained with help of pumping ring and thermal siphon effect. Alarm arethere for seal oil level in tank and seal oil tank pressure.Problem: Vapor locking occurs frequently after 2 or 3 days in seal oil circulationlines of API plan52 (coming out of seal and going to seal from seal oiltank).what are reasons for this.We check vapor locking by touching (cold orhot) seal oil circulation pipes and rising level in tank.We are thinking in following direction:1. Pump stuffing box pressure( seal chamber pressure) is becoming somewhatnear to vapor pressure of LPG at that temperature and seal runs dry,flashing LPG vapor comes to secondary seal and mixes with oil.2. Proper heat removal of primary seal is not taking place by API plan 11flushing LPG & buffer fluid.3. Oil cooler of API plan52 which is cooled by natural circulation of air,which is not adequate to heat remove. Can we sort out vapor locking problem by ?: 1.Increasing seal chamber pressure( to create diff between vapor pressure of LPG & seal chamber pr.) byincreasing orifice diameter in API plan11 2. By replacing natural draftfinned cooler to forced draft cooler for API paln 52 for cooling buffer oil.( by installing axial fan with existing cooler). 3. Changing API plan11 to API plan 23 ( installing cooler to decreasetemperature of LPG before entering to seal faces of primary seal).4. Changing API plan52 to API plan53( pressurized fluid barrier fluid).Please comment on our direction of thinking and advice us.