Auto 1 Midterm Quiz 2

On a vessel, the automated discharge pump pressure is regulated by a PID controller. The fundamental operational principle of this controller is to:

• Continuously monitor the fluid temperature and adjust the pump speed accordingly to maintain efficiency.

• Maintain the system pressure at a consistent setpoint by calculating error and adjusting its output.

• Measure the density and viscosity of the cargo to determine the optimal pumping rate for the operation.

• Protect the pipeline system from the damaging effects of cavitation and stiction in the control valves.

A pneumatic control system on a ship, which uses air as its working medium, is generally more challenging to control than a hydraulic system because a gas's high compressibility primarily introduces:

• A significant reduction in the overall density and mass of the fluid being controlled within the system.

• Substantial time delays as pressure waves take more time to propagate and stabilize throughout the system.

• A highly nonlinear relationship that immediately causes the control valve to stick and then jump erratically.

• A dramatic increase in the fluid's viscosity, which directly restricts its flow through pipes and components.

When transferring heavy fuel oil that has not been properly heated, the high viscosity of the cold oil will most likely cause which of the following issues in the pressure control loop?

• It will cause the control valve to cavitate, creating vapor bubbles that collapse and damage the valve internals.

• It will make the fluid less dense, requiring less pump power but making the pressure control more sensitive.

• It will slow down the rate of pressure changes and signal transmission, resulting in a sluggish control response.

• It will create a perfectly linear relationship between the control valve position and the resulting system pressure.

In a ballast water pumping system, if the fluid being pumped is seawater instead of fresh water, the increased density of the seawater means that:

• The pump will require less force to move the fluid, thereby simplifying the operation.

• The pump will require more force to move the fluid, increasing the energy required for operation.

• The system will be unaffected by the change in fluid density.

• The control system will automatically adjust for the density difference without any impact on operation.

A common practice before transferring heavy fuel oil is to heat it in the settling tanks. The primary control-related reason for this procedure is that heating the oil generally:

• Significantly increases the oil's density, making it easier to separate water and contaminants from the fuel.

• Decreases the oil's viscosity, which allows it to flow more easily and improves the responsiveness of the control system.

• Makes the fuel oil more compressible, allowing pressure changes to propagate through the system much more rapidly.

• Completely eliminates all inherent time delays and nonlinearities present within the pumping and piping system.

In a long pipeline running from a cargo tank to a pump on deck, there will always be an inherent lag between a valve adjustment, and the corresponding pressure change at the sensor. This phenomenon is best described as a:

• Nonlinear system response that typically occurs only at very high or very low flow rates within the pipeline.

• Process time delay that can degrade the performance and stability of the automated PID pressure controller.

• Symptom of control valve stiction, where the valve stem sticks and then suddenly jumps to a new position.

• Direct result of fluid cavitation occurring at the pump suction due to a low net positive suction head (NPSH).

In a high-capacity cargo discharge system, the relationship between the pressure at the pump and the flow rate through the line can become nonlinear, particularly at high flow rates. This nonlinearity poses a challenge because it:

• Makes it difficult for a standard PID controller to accurately predict and manage the process behavior across its entire operating range.

• Helps the derivative action of the controller by providing a clearer signal

• Can be easily compensated for by simply increasing the proportional gain of the controller to a very high value.

• Is the main cause of time delays in the transmission of the pressure signal from the sensor back to the controller.

During the operation of a bilge water discharge system, the control valve may sometimes exhibit a behavior where it does not move and then suddenly jumps, leading to erratic pressure control. This specific mechanical issue is known as:

• Cavitation, which is the formation and subsequent implosion of vapor bubbles within the valve body.

• Stiction, which is the static friction that needs to be overcome before the valve stem can begin to move.

• A time delay, which is the inherent lag between the controller's command and the valve's physical response.

• A nonlinear response, which is a variable relationship between the valve position and the resulting flow.

A pump taking suction from a tank with a very low level might create a vacuum at the impeller eye, leading to the formation and violent collapse of vapor bubbles. This damaging phenomenon, which can also disrupt stable pressure control, is called:

• Stiction, which typically results in the control valve sticking and then releasing in a jerky manner.

• Cavitation, which generates noise, vibration, and can cause pitting damage to the pump impeller and valves.

• A time delay, which is the slow propagation of the pressure signal from the pump to the control sensor.

• A nonlinearity, which describes the unpredictable relationship between pump speed and discharge pressure.

A vessel must handle a wide variety of cargoes, from light diesel oil to heavy crude. The pressure control system for the cargo pumps must be re-tuned for different products primarily because:

• The setpoint pressure must be lowered for denser fluids to avoid over pressurizing the deck lines.

• The physical characteristics of the fluid, such as its density and viscosity, directly affect the system's pressure dynamics.

• Cavitation will always occur if the same controller settings are used for two different types of liquid cargo.

• The controller's proportional band needs to be inverted when switching from a liquid to a gaseous cargo