Vertical Turbine Pump Problem (Pumping Propylene)

[nippy]

Hi all, i'm really stuck at this problem since the beginning of pump commissioning

First of all, I have various problems with my vertical turbine pump that pumps propylene from the spherical tank. There are two majors problems that we've faced which are;

1. Mechanical seal leaks and trips the pump many times.
2. Start a pump for a minute and then the pump is tripped by low current protection. In this case, the operating discharged pressure and flow cannot be built. **Noted : we used the by pass line (minimum flow line) to test the pump.

At that time, we assumed two plausible causes ; the first one is NPSHa is insufficient and the second one is the discharge vent line is not properly vented back to tank due to there are many low spots. Normally, refer to Gould's pump manual, vent piping must continuously rise back to the source with no low spots. So, we changed the vent design to comply with standard but those problems still occur!!.

Now, i've tried to calculate NPSHa by canceling out the the vapor pressure and tank pressure in the NPSHa equation. Then only hydrostatic head of liquid minus by friction loss in a suction line is left in the equation. According to my calculation, the pipe friction loss is 1 meter and the hydrostatic head is 7 meters, so, the NPSHa result is 6 meters. This pump requires only 4 meters and it seems OK with my calculation.

As i know, Propylene or liquefied gas is very sensitive with temperature change. My point here is our spherical tank is insulated but the suction line and discharge line are not. Therefore, by canceling out the vapor pressure and tank pressure is not applicable for this case right? Regarding to my investigation, the tank temperature is 28.6 degree C with pressure at top tank 11.74 kg/cm3. And the pump temperature (measured by temp transmitter at pump discharged nozzle during pump stops) is around 40 degree C with 12 kg/cm3 that measured by the suction pressure gauge.

My second question is "Are there any possibility that the suction fluid flow piping design is independent to the friction loss in the NPSHa equation and can significantly affect the pump? because now we have suspected that the suction pipe size is too small. Normally, the suction pipe size shall be larger than the suction pump nozzle 1 or 2 size with eccentric reducer at the suction flange but our suction line is 8" along the way over 40 meters to the pump nozzle which is also 8". (Actually the suction line is branching from the 16" header under the tank) Anyhow, the suction loss calculation is low.


Any help would be appreciated.

Regard.

[mskhadke]

Nippi,
Generally most mechanical seals failed because of misalignment. recommend you to check alignment and runout. Also ensure seal is properly cooled and lubricated. Consult seal vendor for specific seal type.
Have you check vibration at the type of tripping of pump on low current? If vibration level is high there must some issue with NPSHA. Check temp at pump suction. If possible share vibration level, make of the pump, flow rate, head, piping layout at pump suction.
Regards,
MSKhadke