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A naval architect and PIPE-FLO® user recently called for technical support on a unique piping system problem he was trying to solve. He was working with a shipyard and was tasked with determining a method to pump out six ballast tanks on a ship while the ship's cargo was being loaded. The ship had to complete the deballasting operation within 12 hours to meet a tight schedule. The shipyard was having difficulty meeting this schedule and was experiencing cavitation of the Ballast Pump as the tank levels dropped.
The customer modeled the ballast system in PIPE-FLO Professional 2007 and used the PIPE-FLO Overtime module to simulate the process over the 12 hour time frame. PIPE-FLO is used to model a piping system and calculate the steady state balanced flow rates and pressures throughout the system. Overtime uses the PIPE-FLO model and a user-defined sequence of events to simulate the operation of the system over a defined time frame.
Figure 1 shows the configuration of the ballast system modeled in PIPE-FLO. Each ballast tank, three on the port side and three on the starboard side (that's the left and right side of the ship for the land lubbers out there!), start out with a liquid level of 55 ft of sea water. A Ballast Pump and flow control valve (FCV) regulate the rate of discharge from the ballast tanks to the Sea Chest. The valves and fittings (elbows, T's, and pipe reducers) are also included in the model but are not shown in Figure 1.
Figure 1. Ship Deballasting System
The pump curve for the Ballast Pump is shown in Figure 2. At the Best Efficiency Point (BEP) of 72%, the pump produces 97 ft of head at a flow rate of 4407 gpm.
Figure 2. Pump Curve for Ballast Pump
NPSHa and Cavitation
One issue that has to be addressed is the fact that as the tank levels decrease, the Net Positive Suction Head Available (NPSHa) also decreases. Another thing to consider is that the NPSH required by the pump increases at higher flow rates, as shown on the NPSHr graph of the pump curve in Figure 2. Additionally, an increase in flow rate also increases the head loss in the suction piping, resulting in a reduction of the NPSHa.
In order to prevent cavitation from occurring, the ship yard wants to maintain a NPSH margin ratio (=NPSHa/NPSHr) of 1.3 during the entire operation. In addition, to minimize maintenance problems with the ballast pump and ensure the system is operating efficiently from an energy standpoint, the shipyard wants the Ballast Pump to remain within a Preferred Operating Region (POR) of 80% to 120% of the BEP flow rate at all times. These limitations were input into the pump data in PIPE-FLO, as shown in the pump dialog box in Figure 3. The program will generate warnings if the flow rate or NPSHa fall outside of the limits.
Figure 3. Ballast Pump dialog box
Asymmetrical Design
Another issue that added to the complexity of the cavitation problem is the physical location of the Ballast Pump in relation to the tanks. The Ballast Pump is located between the #4 and #5 Ballast Tanks, so the overall system is not symmetrical. In other words, when the pump is lined up to the #4 Ballast Tank, there is less head loss in the suction piping than there would be when it is lined up to the #1 Ballast Tank. This means that the pump will have less NPSHa and therefore be more susceptible to cavitation when pulling out of the #1 Ballast Tank.
The asymmetric design also means that when two or more tanks are pumped down together, the tank levels will come down at different rates until equilibrium is reached between the driving head in each cross-connected tank and the pump suction. Once this equilibrium is reached, the flow rate from each tank will equalize and the tank levels will drop at the same rate. Figure 4 shows the flow rate trend from each tank along with the tank levels when all six tanks are lined up to the Ballast Pump at the same time. Even though the tank levels are equal at the beginning of the simulation, the flow rates from each tank are different, causing a separation in the tank level trend.
Figure 4. Flow Rates and Tank Levels with All Tanks Cross Connected
Evaluating the Current Procedure
The first thing to be determined is how the system is operating under the current deballasting procedure used by the ship's crew. The procedure starts with pulling the water out of three tanks at the same time to a level of about 6.5 feet. Next, those tanks are isolated and the other three tanks are pumped down to 6.5 feet. Finally, the six ballast tanks are individually pumped down to a minimum level of about 0.5 feet.
Initially, the crew starts out with the FCV fully open to obtain the highest flow rate from the pump. When cavitation noises are heard as the tank levels drop, the flow rate is reduced until the noise subsides. Using this procedure, the crew is able to complete the deballasting operation within about 13 to 15 hours, putting the ship behind schedule.
When this scenario was defined in Engineered Software's Overtime program and the simulation was run, the tank level trend graph in Figure 5 was developed. The #4, #5, and #6 Ballast Tanks took almost five hours to pump down to 6.5 feet (the trend for the #5 Tank is underneath the trend for the #6 Tank and is difficult to distinguish). It took another five hours to pump the #1, #2, and #3 Ballast Tanks to 6.5 feet. It then took about 25 minutes to pump down each tank to 0.5 feet, adding another 2-1/2 hours to the procedure for a total time of about 12-1/2 hours. If additional time is added to account for valve manipulations, it is easy to see how even an operator who was on top of the situation would take 13 hours to complete the deballasting operation. If the operator had other tasks to complete and could not give his full attention to the procedure, even more time would be required, placing the ship further behind schedule.
Figure 5. Tank Level Trends with Current Procedure
Evaluate Other Options: Determine the System Line Up and Flow Rates
The shipyard wanted to evaluate other procedures to determine if the system could physically meet the time requirement of 12 hours. They came up with two feasible options:
A. Pump down all tanks together to some pre-determined level, then pull each tank down to 0.5 feet individually.
B. Pump down all tanks to 0.5 feet together, isolating each tank as it reaches its minimum level.
For both scenarios, the first thing to determine is the maximum flow rate at the start of the procedure when the ballast tanks are full. Using the POR, the maximum allowable flow rate through the pump would be 120% of BEP flow (4407 gpm), or 5288 gpm. Therefore, the FCV is intially set to 5285 gpm.
For either option, at some point as the tank levels and NPSHa decrease, the NPSH margin ratio will drop below 1.3 and place the pump in jeopardy of cavitation. At this point, the flow rate will have to be dropped to reduce the head loss in the pump suction piping and increase the NPSHa, and therefore the NPSH margin ratio. The level at which the NPSH margin is first exceeded is determined to be between 23 and 25 feet by running the simulation with a flow rate of 5285 gpm and making note of when the NPSH margin ratio warning is given. This occurs at the end of the tank level trend shown in Figure 4 above.
When the flow rate needs to be adjusted, what flow rate should we now control to? As the tank levels continuously drop, the NPSHa will continuously drop as well. For every one-foot drop in tank level, there is a one-foot drop in NPSHa. If the system is automated sufficiently to allow a cascade control scheme, the set point of the flow control valve could receive a remote set point from the output of a tank level controller. In essence, this would be a NPSHa controller!
In the absence of such a sophisticated control scheme, we must determine the necessary flow rates at each tank level that will satisfy our NPSH margin requirement. The model is calculated with the tank levels at different intervals and the flow rate adjusted at each level to determine at what point the pump will have sufficient NPSHa. Table 1 below shows the flow rate set points needed to ensure sufficient NPSH is available based on tank levels. The #4 Ballast Tank level is used as the trigger for changing the flow rate since it is the closest to the Ballast Pump and its level will come down the fastest.
Table 1. Flow Rate Set Point at Given Tank Levels to Ensure Sufficient NPSHa
#4 Tank Level (feet)
FCV Set Point (gpm)
55 to 24
5285
24 to 20
5190
20 to 16
5070
16 to 12
4930
12 to 8
4755
8 to 4
4525
4 to 0.5
Depends on Option A or B
Option A: All Tanks to 4 Feet, Then Individually to 0.5 Feet
In this option, the ballast tanks are pumped down until #4 Ballast Tanks reaches a level of 4 feet, then the other five tanks are isolated and the #4 Tank is pumped down to 0.5 feet, at which point that tank is isolated and another one pumped down to the minimum level. The process is repeated to pump down each tank individually to 0.5 feet. The flow rate when pumping down each tank is set to the minimum flow of 80% of BEP flow, or 3525 gpm.
The tank level trend in Figure 6 is obtained when this scenario is defined in Overtime and the simulation ran.
Figure 6. Tank Level Trends For Option A
The deballasting operation using Option A takes just over 12 hours to complete, so initially this looks like a good option. However, since each tank is being pulled down individually to 0.5 feet, the entire flow rate of 3525 gpm is being drawn out of one tank at a time. This results in very high fluid velocities and head losses in the suction piping from the tank to the pump, causing the NPSH margin ratio limit to be exceeded while emptying the #3, #5, and #6 Ballast Tanks (the margin ratio drops as low as 1.12). While pumping down the #2 Ballast Tank the NPSHa drops to 15.8 feet, and to 14.6 feet while pumping down #1 Ballast Tank. Since these values are below the 16.6 feet of NPSH required, the pump will be cavitating for about 15 minutes near the end of the procedure.
Option B: Pull All Tanks Down Together to the Minimum Level, Then Isolate Each Tank at 0.5 Feet
For this scenario, all six ballast tanks are pumped out together. As the tank levels drop and NPSHa decreases, the flow rate is adjusted to ensure the NPSH margin ratio is satisfied. The flow rate values in Table 1 above are used until the #4 Ballast Tank level drops to 4 feet, then the flow rate is adjusted based on which tanks are online. Since the pump is pulling out of multiple tanks, the flow rate from each tank will be lower than it would be if the pump was pulling out of one tank. This allows the overall flow rate to be higher when pumping out of multiple tanks at the same time. The flow rates in Table 2 below show when the FCV needs to be adjusted to ensure the NPSH margin is maintained.
Table 2. FCV Set Point for Option B
| #4 Ballast Tank Level (feet) | FCV Setpoint (gpm) |
| 4 to 0.5 | 4260 |
| After #4 Tank Isolated until end of procedure | 3800 |
The tank level trend in Figure 7 below, shows the results of this scenario. The procedure is completed in 11 hours and 45 minutes, meeting the 12-hour time requirement.
Figure 7. Tank Level Trends for Option B
In addition, the NPSH margin ratio of 1.3 is maintained until 11:33, at which time the NPSHa drops to 21.8 feet, compared to 16.9 feet NPSHr (margin = 1.29). At the end of the procedure, when only the #1 and #2 Ballast Tanks are online and close to 0.5 feet, the NPSHa drops to 17.5 feet, still above the 16.9 feet required by the pump. The pump will not cavitate throughout the entire procedure!
Conclusion
The naval architect was able to prove that the current system could attain the goal of deballasting the ship within the 12-hour time frame without costly modifications. In addition, he was able to provide specific guidance to the ship's crew on how to change the operating procedure, along with the flow rate set points necessary to keep the Ballast Pump operating efficiently and to prevent cavitation as the tank levels dropped.