Two drive-rated propane unloading pumps rated at 30 horsepower (hp) consistently operate at high flow rates in excess of the design rated capacity of 110 gallons per minute (gpm).During normal unloading, the pump is running at 190 gpm, which is outside the pump curve.The pump is operating at the 160% Best Efficiency Point (BEP), which is unacceptable.Based on operating history, a pump runs twice a week with an average run time of one hour per run.In addition, the pump underwent a major overhaul after six years of operation.The approximate runtime between major repairs is about 1 month, which is very short.These pumps are considered to have low reliability, especially since the process liquid is considered clean with no suspended solids.Propane unloading pumps are important to maintain safe propane levels for reliable natural gas liquids (NGL) operation.Applying improvements and pump protection mitigations will prevent damage.
To determine the cause of high flow operation, recalculate the friction losses of the piping system to determine if the pump is overdesigned.Therefore, all relevant isometric drawings are required.By reviewing the piping and instrumentation diagrams (P&IDs), the required piping isometrics were determined to help calculate friction losses.A complete suction line isometric view of the pump is provided.Isometric views of some discharge lines are missing.Therefore, a conservative approximation of pump discharge line friction was determined based on current pump operating parameters.Therefore, the unit B suction line is considered in the calculation, as shown in green in Figure 1.
To determine the equivalent piping friction length of the discharge piping, actual pump operating parameters were used (Figure 2).Since both the truck and the destination vessel have pressure equalization lines, this means that the sole task of the pump can be split into two.The first task is to lift the liquid from the truck level to the container level, while the second task is to overcome friction in the pipes connecting the two.
The first step is to determine the equivalent friction tube length to calculate the total head (ΔHtotal) from the data received.
Since the total head is the sum of the friction head and the elevation head, the friction head can be determined by Equation 3.
where Hfr is considered to be the friction head (frictional losses) of the entire system (ie suction and discharge lines).
By looking at Figure 1, the friction losses calculated for the suction line of Unit B are shown in Figure 4 (190 gpm) and Figure 5 (110 gpm).
Filter friction needs to be considered in the calculation.The normal for a filter without mesh in this case is 1 pound per square inch (psi), which is equivalent to 3 feet (ft).Also, consider the friction loss of the hose, which is about 3 feet.
In summary, the suction line friction losses at 190 gpm and pump rated flow (110 gpm) are in Equations 4 and 5.
In summary, the friction losses in the discharge line can be determined by subtracting the total system friction Hfr from the suction line friction, as shown in Equation 6.
Since the friction loss of the discharge line is calculated, the equivalent frictional length of the discharge line can be approximated based on the known pipe diameter and flow velocity in the pipe.Using these two inputs in any pipe friction software, the friction for 100 feet of 4″ pipe at 190 gpm is calculated to be 7.2 feet.Therefore, the equivalent friction length of the discharge line can be calculated according to Equation 7.
Using the equivalent length of the discharge pipe above, the discharge pipe friction at any flow rate can be calculated using any pipe fraction software.
Since the factory performance of the pump provided by the supplier did not reach 190 gpm flow, extrapolation was made to determine the pump performance under existing high flow operation.To determine the exact curve, the original manufacturing performance curve needs to be plotted and obtained using the LINEST equation in Excel.The equation representing the pump head curve can be approximated by a third order polynomial.Equation 8 shows the most suitable polynomial for factory testing.
Figure 7 shows the manufacturing curve (green) and resistance curve (red) for the current conditions in the field with the bleed valve fully open.Remember that the pump has four stages.
Additionally, the blue line shows the system curve, assuming the discharge shut-off valve is partially closed.The approximate differential pressure across the valve is 234 feet.For existing valves, this is a large differential pressure and cannot meet the requirements.
Figure 8 shows the ideal situation when the pump is downgraded from four to two impellers (light green).
Additionally, the blue line shows the system curve when the pump is stopped and the discharge shut-off valve is partially closed.The approximate differential pressure across the valve is 85 feet.See the original calculation in Figure 9.
Investigation of the process design revealed an overestimation of the required differential head due to incorrect design, missing the presence of a gas/vapor balance line between the top of the truck and the top of the vessel.According to process data, propane vapor pressure varies significantly from winter to summer.So the original design appears to be done with the lowest vapor pressure in the truck (winter) and the highest vapor pressure in the container (summer) in mind, which is incorrect.Since the two are always connected using a balanced line, the change in vapor pressure will be insignificant and should not be considered in pump differential head sizing.
It is recommended to downgrade the pump from four to two impellers and throttle the discharge valve by approximately 85 feet.Determine that the valve should be throttled until the flow reaches 110 gpm.Also determined that the valve is designed for continuous throttling to ensure there is no internal damage.If the valve inner coating is not designed for such situations, the factory will need to consider further action.In order to stop, the first impeller must remain.
Wesam Khalaf Allah has eight years of experience at Saudi Aramco.He specializes in pumps and mechanical seals and was involved in the commissioning and start-up of Shaybah NGL as a reliability engineer.
Amer Al-Dhafiri is an engineering specialist with over 20 years of experience in pumps and mechanical seals for Saudi Aramco.For more information, visit aramco.com.
Post time: Feb-21-2022
