What Is Viscosity Regulator?

Written by: Raunekk • Edited by: Lamar Stonecypher
Published Jul 11, 2009

Viscosity of fuel oil plays an important role in the quality of combustion inside a diesel engine. It is imperative to keep the viscosity of fuel oil in the right range in order to get the right kind of engine efficiency. Learn how viscosity regulators are used to achieve this.

Control of fuel oil viscosity explained

Fuel is the lifeblood on marine engines and hence proper characteristics of fuel need to be maintained on ships. Viscosity is one of the most important factors which directly affects combustion quality.

Controlling the viscosity of fuel oil is an important aspect of an efficient combustion. A high viscosity fuel oil leads to improper atomisation which in turn leads to incomplete combustion.High viscosity fuel prevents correct atomisation,which takes place in the fuel injectors. An efficient atomisation is the basic need for the healthy mixing of fuel and heated air, without which, no ignition or combustion can be derived. Thus, it is absolutely important to set the viscosity value in the right range.

Viscosity of high fuel oils can be reduced by raising its temperature with the help of a fuel oil heater. This can be done by using either, automatically regulated heaters in order to maintain a constant temperature or by using a manual control which can be adjusted according to the requirement.

What leads to high viscosity of fuels?

It is a usual practise on ship, to store fuels of different grades and properties into the same tank. Even if they are stored in different tanks, they finally get mix with each other when brought to the service tank. This practise of deriving fuels from different sources is the main reason for the high viscosity of fuel oil.It is for this reason that the viscosity of oil is checked and maintained within close limits, before sending it to the engine's cylinder. Generally, fuel oil viscosity is regulated between 10-15 centistrokes at 50 degree Celsius.

Viscosity regulators are used to regulate and maintain the fuel oil viscosity.

Viscosity regulator

Viscosity Regulator or Viscotherm, is a device used to measure viscosity of the fuel oil before sending it to the engine. It is generally fitted at the outlet of the fuel oil heater. The regulator is connected with the heater so that it can measure the viscosity of the oil and regulate the temperature at the same time.

Viscosity regulator is an "L" shaped instrument which consists of a small gear pump that rotates at a constant slow speed of 40 rpm. The pump takes suction from the heater discharge and is generally fitted close to it. A regulated flow of fuel is sent by the pump to a capillary tube. The capillary tube is so designed that the form of flow between the inlet and outlet of the tube generates a pressure difference, which is equal to the viscosity of oil flowing through it.

Bourdon tubes are used to monitor the viscosity at different points within the capillary tube. Thermometer is fitted on the regulator to measure the fuel temperature and maintain the final temperature accordingly. Differential pressure transmitters are also fitted to directly control the heater temperature and set the right viscosity limit.

click to enlarge

References

Diesel Engines 3rd Edition, by A.J Wharton

Image Credits

http://www.ehlersgmbh.com/de/media/images/produkte/Messgeraete/Viscotherm-sensor-V-92-electric.jpg

Jon Watson Aug 25, 2010 5:45 AM

Fuels storage

Fuel management involves segregation of fuels. Though it happens, it is not good practise to mix different fuels together of a variety of reasons including compatibility, centrifuge operation and so on.
Today, with MARPOL, it is a legal requirement of the national legislation implementing MARPOL, to segregate fuels and if necessary (dual fuel operation) to have separate settling tanks and day tanks.
Fuel may be mixed in the high pressure circuit when changing over from one fuel to another and a common problem during this operation (usually at start up or shut down when changing from HFO to MGO) is asphaltene precipitation.

Jon Watson Feb 15, 2010 7:01 AM

Viscosity

You say:
"Generally, fuel oil viscosity is regulated between 10-15 centistrokes at 50 degree Celsius."
I think you meant to omit the temperature reference.
An MAK engine, for example, has an 11 cst ste point viscosity. With a typical 380cst fuel (380cst at 50degC) the injection temperature or EVT (Equi Viscous Temperature) is typically around 141-142degC.

Jon Watson Feb 8, 2010 8:17 PM

Viscosity Regulation

The viscometer described is a twin capillary type. Originally was manufactured by Halikeinen, it was superceded by the superior VAF viscometer, pretty near the industry standard until a new vibrating element technology was introduce around 10-12 years ago.
The capillary viscometers only measured dynamic viscosity and require skilled maintenance and calibration, they are vulnerable to solids and to asphaltenes.
The modern viscometers measure kinematic viscosity and also can report the density at 15degC, viscosity at 100degC, Ignition Index (CCAI and CII) and the density at 98degC (the centrifuge temperature).
The fuel heaters are directly controlled by the viscometer signal.
The temperature signal is a fallback.
This was necessary because of the vulnerability of the capillary to block or become coated and thus report higher viscosity than was actually the case.
The engineer would necessarily keep track of the fuel temperature resulting from heater control. Usually, the fuel drawn from a full tank will be higher viscosity and density than the fuel at the top so as the fuel is used the viscosity will reduce and the temperature to maintain the target viscosity will trend lower.
Therefore, if the viscometer begins to coat or block the viscosity reported will increase and the fuel temperature will then trend up rather than down.
When this is detected, indicating a need for service and re-calibration of the viscometer the engineer would switch over to controlling based on the last good temperature value.
The modern viscometers are self cleaning and can run for years without attention. They can even handle these fuels when used in burners (where there is no fuel treatment).

ashokraj Aug 25, 2009 9:39 AM

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What Is Viscosity Regulator?

Control of fuel oil viscosity explained

What leads to high viscosity of fuels?

Viscosity regulator

References

Comments