To reduce the operating cost of the ship, ships are bound to use lower grades of heavy oil fuel. These fuels are at much higher in viscosity than the viscosity required for burning. Therefore every ship has been provided with viscotherm to control the viscosity of these fuels. It is one of the most neglected machinery onboard because the fuel oil temperature is considered more important than viscosity. But the temperature of the fuel is raised to achieve the desired viscosity of the fuel. And in case of any malfunction in viscotherm, it is bypassed with manual heating of fuel. This may lead to a major problem when the different grade of fuel is bunkered and temperature settings are not changed. So, let’s try to understand the working of viscotherm and why it is required onboard?
Viscosity & Viscosity Index
It is defined as the property of a fluid that resists the force tending to be a cause to flow. This happens because of the internal friction of its molecule. A fluid having low viscosity flows easily e.g. water, marine diesel oil, marine gas oil. Whereas high viscous oil flow is very slow e.g. heavy oil. The unit for the viscosity is centistoke(cst).
While selecting any Fuel or lube oil for machinery, the viscosity is a major property of them which take into consideration. For fuel oil, it is very important to maintain a proper viscosity range for higher efficiency of engine and boiler. Mostly two types of marine bunker HFO used onboard, 180 cst and 380 cst. In some cases, anything between these two will also work. The viscosity of heavy fuel should always be maintained in between 13 cst to 15 cst at the time of injection. The exact viscosity value may vary for different engines.
Please do read about Heavy fuel oil properties.
The viscosity of heavy fuel not only determine the combustion process of an engine but also affects the wear of associated machinery parts. If the viscosity of fuel is high the atomisation of fuel will not be proper during injection, it will form a large drop which results in improper combustion and pitting due to fuel on linear and piston crown. Whereas if viscosity is too low fuel will burn rapidly, and chances of early ignition and knocking increases, which is not desired. All these factors will increase the wear of liner, cylinder head and injector nozzle(in short combustion chamber). High viscous fuel will also cause a major damage to the pumps and piping of fuel oil system onboard. All fuel pumps are positive displacement pumps mostly gear or screw pump. The wear of screw or gear of pumps reduces the efficiency of pumps and leads to loss of desired fuel pressure. The time between overhaul for pumps increases. For reducing fuel viscosity to 14 cst heating is required by means of steam or an electrical heater.
How much fuel should be heated to achieve the desired viscosity depends upon the viscosity index of the fuel. Viscosity index is a number with no unit that represents how the viscosity of the fluids changes with temperature. The greater the viscosity index of the fuel smaller the change in fuel(fluid) viscosity for any given change in temperature and vice versa. If any fluid is having lower viscosity index the change in viscosity of fuel will be higher as its temperature changes. Hence higher viscosity index fuel is less affected to temperature change as compared to the lower VI fuels.
Viscosity control on board
The viscosity of fuel is maintained by heating with steam or an electrical heater. The steam supply is proportional to the viscosity difference between measured value and the desired value. When the measured value is more than set value the controller valve will open more and increase the supply of steam to the heater, which will increase the fuel temperature and decrease its viscosity. And if the measured viscosity of fuel is less than the set viscosity, steam will be reduced by the control valve to reduce the temperature of the fuel, which results in an increase of viscosity of the fuel. At stable condition when measured value and the set value are same, then control valve will hold its position.
This image shows a typical example of viscosity control system onboard having pneumatic control. It includes viscotherm (with DP control), fuel oil heater, steam control valve and other control systems.
- Viscotherm: It measures the viscosity of the fuel. If there is any difference between the set value of viscosity and present value, the attached Differential Pressure cell and transmitter gives a signal to the pneumatic valve to increase or decrease the steam supply to fuel heater.
- Fuel oil heater: fuel is heated to decrease the viscosity. The amount of steam to the heater is controlled by the pneumatic control valve.
- Steam control valve: The function of this valve is to control the amount of steam as commanded by the viscotherm and DP cell.
- Air Filter Regulator: This supply air to control system and viscotherm. The air to this unit is supplied from service air, it reduces the air pressure to 1.4 bar(generally all pneumatic control system works between 1 to 2 bar).
- Control station: It takes manual input from a human for viscosity that should be maintained before injection of fuel. This value can also be temperature, but the setting of this temperature should be as according to the fuel property(as per BDN of fuel).
- Starter Box: It supply power to viscotherm.
- Analog Viscosity Indicator: It shows the viscosity at viscotherm.
- Alarm Switch: This will give an alarm when the viscosity of fuel deviate by a large margin from the set value.
- Recorder: It records the viscosity of the fuel.
The viscotherm is an equipment which is used to control the viscosity of the fuel. A continuous sample of the fuel is drawn into it at a constant rate through a fine capillary tube with help of a small gear pump. Two tapping points are provided, one at the capillary tube and other at main flow. The oil passing through the main unit may have turbulent flow but oil which passes through the capillary tube is always having laminar flow. The laminar flow of the sample through the tube shows a pressure difference across the tube which is directly proportional to the viscosity of the sample. The attached differential pressure cell calculate the pressure difference in terms of viscosity of fuel and send a signal to the controller.
The viscotherm get choked because of heavy oil, whenever heating or recirculation of the heavy oil is shut. This happens usually when ship come out of drydock. To prevent choking of pressure differential tubes, they are filled with glycol or glycerine because of its specific gravity (0.63) and anti-freezing property. Some time chambers are also provided, and they need to be drained at regular interval. The gear pump is driven by a small electric motor, with reduction gear or magnetic coupling at very less RPM(usually 40). The magnetic coupling has an advantage over the other as it prevents FO leakage and damage to the electric motor in case of pump get struck because of cold heavy oil. The capillary of this is made of either glass or steal to prevent it from corrosion. This tube should always be cleaned at a regular interval, otherwise, the laminar flow of the fuel will get disturbed and reading will not be correct.
It works on piezoelectric and turning fork technology. It is directly mounted on the flow pipe. The fork inside the pipe is rotated with help of piezo effect when live current is given. The rotation of the fork is restricted by fuel oil. Higher the viscosity of FO, higher will be the restriction. This reduction in the rotation is sensed, and viscosity is calculated. The reduction is directly proportional to the viscosity of FO.
The viscosence gives many advantages over the viscotherm. These includes:
- It is directly mounted on FO pipe, therefore space required for it is less.
- The accuracy is higher than the viscotherm.
- The transmitter is incorporated within it, therefore no need of differential pressure cell.
- It doesn’t involve electric motor, so, the problem of getting struck and leakage of FO is avoided.
- The temperature sensor is also attached, no need for an external thermometer.
In spite of these major advantages over viscotherm, ship owners prefer viscotherm. Because of its high-cost installation. And if any malfunction occurs to it, the whole system is to be replaced by a new one.